miles
away but a hydrofoil is a measure of
progress
that has come at last to stay.”
-Dr. Leopoldo Rodriquez
M.D.,
Cantiere Naval-Technical
Shipyard
[Italy]
1.
The word unconventional conjures up in ones mind various things, from
long hair and a bohemian lifestyle to avant-grade architecture. But what, if anything, does it imply when applied to the maritime
scene? The conventional in this environment
may be construed as merchant vessels plying serenely for trade, warships going
about their business, submarines, perhaps, engaged in their mysterious tasks
and aircraft flying about in execution of their chores. Conversely, the unconventional would can note
the family of surface skimmers like hovercraft, hydrofoils, SWATH ships, ram
wing craft and the like or by a stretch of imagination; mammoth submarine
bulk tankers [if these could become a reality].
2.
In the same way that the super tankers, at their inception, may have
been thought of as unconventional, but are now quite commonplace, similarly,
a number of innovations like the helicopter, have gone the same way. In the case of surface skimmers, however, the
state of the art is still sufficiently novel and exclusive, to a few, for
these craft to be called unconventional, within the meaning of the term as
specified in the Oxford English Dictionary.
3.
The hydrofoil has been conceived through a marriage of the aerospace
and marine technologies [1]At
the present moment this craft as well as the other such innovation – the hovercraft
– have a bright future in numerous applications in the military and civilian
fields.[2]
4.
The maritime environment today is fraught with a high threat factor
where large warships are a definite liability.
Aircraft carriers, for instance, require protective screens of escort
ships, which are out of proportion to the cost effectiveness of the Task Force.
Even so, some justification for carrier task forces can be found if
one is to argue, for the preservation of a global stake, like the USA and
more recently the USSR are doing. For smaller developing nations with a regional
stake this argument is a fallacy. What
is required by them are small ships and craft with a sufficient punch, that
brooks no mischief. It is for this
reason that today, small fast craft, armed with immense firepower, are cheaper
by the dozen.
5.
A stage has been reached, when an edge in this high threat environment,
can be obtained only by him who exploits the one capability which gives a
significant advantage – speed. For
conventional displacement craft the speed barrier of 35 knots or so cannot
be economically crossed. This barrier
needs to be crossed, however, for the advantage, to accrue. The rapidly blooming skimmer technology has not only enabled this
speed barrier to be broken, but sights are now set, on the attainment of a
hitherto inconceivable speed at sea – 100 knots.
6. It is the aim of this paper to examine the feasibility of adapting the hydrofoil craft to the Indian environment. The aspects studied would be the need for such a craft and the capability of a nation like ours to imbibe the emerging technology and put it to practical use. The associated effectiveness of the craft, cost and otherwise, as against conventional displacement vessels like frigates, for the roles envisaged, would also be examined.
7.
India is a nation, as yet, developing, which can ill afford the exorbitant
cost of defence in general, and warship production in particular, but which
nonetheless has an important role to play in the region.
To give credibility to this status it is imperative, that the Indian
Navy has a credibility at sea. With
our Research and Development [R & D] and industrial infrastructure it
is more than possible for us to adapt to the basic skimmer technology required
for hydrofoil craft, and use its advantages to achieve, in some measure, this
credibility. Additionally, hydrofoil technology is sufficiently widely dispersed
for its acquisition, to be possible, from more than one source.
8.
Through the application of this technology it is possible to convert
the fast patrol boats [FPB], in the inventory from merely fast craft to craft
with immensely improved capabilities. The
OSAs, for instance, if given a shot in the arm with this technology, could
be transformed, into well-nigh unbeatable adversaries, at sea.
9. For the study, accurate figures of the cost or time required to design a modified diesel, for the hydrofoil craft; have not been readily available. However, the annual costs for running such craft, the cost for modifying existing assembly lines and developing designs, have been extrapolated from information, regarding similar projects, carried out elsewhere, allowing for factors such as inflation, existing know-how and the possibility of collaboration with foreign firms in the field.
10.
Understanding the principles of hydrofoil operation requires knowledge
of some specialized terms. These have
been placed in the glossary at Appendix
A.
11.
The
study will examine the hydrofoil craft under the following headings:
[a] History of Military Hydrofoils.
[b] Principle of Operation.
[c] Military hydrofoils – their roles and advantages over other
craft.
[d] Current state of the Art and Futuristic Projections.
[e] Do we need the hydrofoil.
[f] The possible options.
[g] The Financial Burden.
Sources of Data
12.
The source matter available in the College Library, on which this study
is primarily based are the monthly editions of Hovercraft and Hydrofoils,
the annual publications of Jane’s Surface Skimmers and data available on ship
building in India. The Bibliography
has been placed at Appendix B.
13.
No discussion on the pioneering design and development efforts, in
the field of hydrofoils, can begin without a mention of Baron Hans Von Schertel. In the period, from 1927 to 1936, he designed
eight experimental boats, despite the negative attitude and discouraging prejudice,
against the concept, of the conservative ship owners of the old school, in
the Germany of that day. He was however,
convinced of the feasibility of the idea, of flying on foils in the water,
to reduce drag and thus attain extremely high speeds, whilst improving sea
worthiness. This steadfastness and
some financial help from the pragmatic management of the Rhine Shipping Company
enabled him to demonstrate his seventh experimental boat to Professor Kampf
of the Hamburg Experimental Institute.
14.
Professor Kampf was so taken up by the concept, that with help from
the Institute, Baron Von Schertel, proved the utility of this craft, when
he did a trip, in his eighth boat, on the Rhine, from Mainz to Koln and back,
a total distance of 370 kilometer in unfavorable weather conditions.
This first success induced Herr Gothard Sachsenberg to form with him
the Schertel-Sachsenberg Syndicate, in October, 1936, which granted a license
to the Gebrunder-Schsenberg Shipyard, in Germany.
This Shipyard subsequently produced a 17-ton minelayer [VS-6] and other
hydrofoil craft for the German Navy.
15.
In the meanwhile Professor Tietjens, in the United States had tested
a one-seater hydrofoil craft using a monoplane configuration, for which he
filed a patent in 1934. The Vertens
Shipyard in Germany obtained a license from him, in 1940, and also constructed
a 17-ton minelayer. The problem of maintaining flight in a seaway
was not sufficiently solved, however, and the craft crashed several times
due to foil broaching. Although the
boat was considerably faster than the Gebsuder-Sachsenberg VS-6, its turning
ability and longitudinal stability were not satisfactory. The Tietjens concept never came into prominence
and development was eventually stopped.
16.
Two other personalities who bear mention as pioneers, in the field
of hydrofoil technology, were Grunberg who had migrated from Paris to USA
in 1939 and Christopher Hook, an American.
Both of them were instrumental in developing the fully submerged foil
system and the associated control system.
17.
Construction of the first vessel for the German Navy was started in
June 1940 and completed in 1941 at the Gebruder-Sachsemberg Shipyard at Dessau-Rosslau. This hydrofoil craft, called the VS-6, was
equipped with a vee-foil system in a tandem configuration. The craft attained a speed of 47 knots on two
1,500 h.p. diesel engines, each running its own propeller.
18.
A month after starting construction of the VS-6, a series of six smaller
craft was started. The propulsion
system of the last of these is interesting as an attempt was made to avoid
the use of long propeller shafts and their brackets by employing a double
bevel gear or a “Z” drive, for the first time.
This craft was less thoroughly tested than the VS-6, as the Russian
Army occupied Dessau-Rossiau just after completion of the craft.
19.
The wartime hydrofoil craft which deserves the most attention was the
80-ton VS-8. Just as the speed of
the VS-6 was unchallenged for a number of years the VS-8 remained the largest
craft for over two decades. It was
initially designed as a 70-ton fast transporter working on two super charged
2,500 h.p. Mercedes-Benz diesels. The
craft was designed to carry tanks from Sicily to North Africa. For this purpose, the after quarter of the VS-8 could be flooded,
so that a pontoon raft with a 20-ton Army tank could float in. After unloading the tank and withdrawal of
the raft, the ballast water was pumped out and the craft could take off. Due to the supply of normal 1,830 h.p. diesels
and weight additions during construction, the craft could attain only 37 knots
in six-foot waves as against the designed 45 knots. The dimensions and performance of the VS-8
were as follows:
[a] Length overall 105 ft.
[b] Beam over deck 25 ft. 6 ins.
[c] Width over front foil 33 ft.
8 ins.
[d] Draught hull borne 14 ft.
[e] Draught foil borne 6 ft.
6¾ ins.
[f] Displacement 80 tons.
[g] Relative Payload 33%
[h] Power 2
x 1,830 h.p. diesels
[i] Max speed 40 knots
[j] Material hull: aluminum
20.
Another major wartime construction was the 46 ton, VS-10, torpedo boat
which was designed to use four 1,500 h.p. diesel engines and be capable of
60 knots. The outcome of this venture was unknown as
a day before the scheduled launching of the boat, it was completely destroyed
in an air raid. Another fast hydrofoil
to be tested was a single seat 30-ton torpedo boat. During tests in the lakes of Berlin this craft had attained 52 knots.
Tests stopped at the end of the war and were not completed.
21.
Concluding the line of wartime boats one particularly unusual craft
was the four-ton Pioneer workboat built for the German army’s Pioneer Corporations. The requirement was for the boat to be able
to beach. This was achieved by providing
the forward hull of the craft with floats with the forward foil between the
floats. The rear, foil was attached
to the transom by struts. As the craft
approached the beach the foils were retracted and the craft settled onto the
floats maintaining a constant draught of two feet six inches. With the foils fully retracted the craft could
run ashore bow on and remain steady on the floats for unloading or loading.
22.
Switzerland: Baron Von Schertel moved for good to Switzerland
in 1952, for the foundation of Supramer AG,[3]a
concern which today produces one of the widest range of hydrofoil craft. He took with him all the scientific works;
results of tests and trials carried out before and during the war, as well
as construction plans of various hydrofoil craft built during the war, in
other words the technical know-how of two decades.
These form the basis on which Supramer craft are built today. Baron Schertel is presently one of the directors
of Supramar AG.
23.
Soviet Union: The Soviets received their first knowledge
of hydrofoil technology from studying the craft which fell into their hands,
when the Russian army occupied Dessau-Rosslau.[4]
This was a 6.3-ton coastal surveillance hydrofoil in which
the Z-drive had been used for the first time.
This Schertel-Sachsemberg vessel formed the basis, for the Soviet passenger
craft Myr and Strela and the military version of the Strela, called the Pchela,
which are in widespread use today, in the USSR.
The Russians subsequently developed their own semi-submerged foil system,
for their boats operating in relatively calm shallow rivers.
24.
Italy: In 1953 one of the principals of the Leopold Rodriquez Shipyard,
in Italy, obtained a construction license from Supramar AG. The Rodriquez Yard which had earlier been a
repair yard for locomotives and railway carriages was transformed into a
25.
United States: Apart from an examination of the possibility
of constructing foil borne landing craft interest in naval hydrofoils, generally,
was not displayed in the United States, till after the Second World War.
After some preliminary comparative tests, the Department of the Navy,
decided to abandon the surface piercing foil system and employ fully submerged
foils. This was, possibly, due to
the influence of Grunberg and Christopher Hook in this field.
The fully submerged foil was, however, found to be inherently unstable. To achieve stability an automatic control system was necessary to
vary the lift of the foils according to their submergence. Grunberg did toy with the idea of developing
an autopilot based on sensing accelerometers on the principle, enunciated
by Nicolai Miniovsky. This principle
is the foundation, on which all modern autopilots used in aircraft, inertial
platforms and spacecraft, are based[5]. The practical application of
this principle was hampered, however, by the lack of precision engineering
and complex electronic control mechanisms in those days.
He thereafter, devised the Grunberg system, which though of simple
design and a high drag and gave a rough uncomfortable performance in waves.
Subsequently the Gibbs and Cox design and an autopilot developed at
the Massachusetts Institute of Technology were tested.
In 1959, test runs were carried out on a DUKN amphibian when the vehicle
was also to attain 35 knots when equipped with fully submerged controlled
foils and a gas turbine. In 1961 the programme was expanded to include
a design of a LVHX-2 [Landing Vehicle Hydrofoil] which also attained 35 Knots
with a 1,000 h.p. gas turbine and fully submerged foils. In 1960 super caviating foils were tested for
the first time, on a 50 feet amphibious experimental craft, constructed by
Grumann, called Sea Wings.
26.
Canada: In early 1961 De Havilland Aircraft of Canada, was contracted by
the Department of Defence, to construct the FHE 400, to establish the feasibility
of an ocean going ASW hydrofoil. The
first foil borne trails took place in April 1969. The craft was a super caviating steerable bow
foil and sub caviating main foil in a canard configuration. The main foil is a combination of the fully submerged and surface piercing foil types
and has a unique design made for excellent sea keeping qualities. The FHE 400 has been commissioned into the
Canadian Navy as the Bras d’ Or.
27.
The concept of the hydrofoil craft was born from the need to break
the speed barrier, caused in the case of displacement vessels, due to the
drag caused by wave resistance. Sheer
increase in power did not provide the answer as it was realized that after
a certain stage, normally reached at speeds of approximately 22 to 24 knots,
an inordinately high increase of power was required to generate even a marginal
increase in speed. Thus the speed
of displacement vessels is restricted to about 30 knots for larger vessels
and about 35 knots for small patrol craft.
28.
The use of a foil section to gain lift had been demonstrated, by the
Wright brothers, at the turn of the century.
It was not long before the aerofoil spawned the hydrofoil concept,
designed to overcome the speed barrier. A
significant aspect of this -concept is that since water has a density 815
times that of air the same lift as an aeroplane wing, is obtained for only
1/815 of the corresponding wing or foil area; for a given speed.
29.
Construction: The hydrofoil craft is essentially a lightweight
hull similar to that developed for FPBs i.e., it is a hard chine hull section.
The hull is supported on foils which are mounted on struts forward
and aft. With the increase in speed the foils generate lift until at the
take-off speed the entire hull rises out of the water. The percentage of hull weight supported by
the forward or aft foil, when the craft is flying, depends on the configuration.
There are three types of foils employed currently namely the surface
piercing or vee foil, the fully submerged foils and the shallow draught submerged
foil invented by the Russians, for their craft plying in calm shallow river. All the foils can be made retractable, to decrease
the hull borne draught. Among these
the fully submerged foils being inherently unstable require an automatic control
system and is the most expensive to install.
30.
Propulsion System: There are two methods by which hydrofoil craft
are propelled. Firstly by propellers
which are mounted on shafts inclined between 10° to 15° to ensure submergence
of the propellers when the craft is flying and secondly by water jets.
In some craft there are separate propulsion system for hull borne and
foil borne operation to increase the endurance of the craft and provide an
emergency “get home” capability. In either case, whichever method of propulsion
is used the main engines can be either gas turbines or diesel. Examples of the various types of propulsion
systems are available in craft currently in operation such as:
[a] High Point: This
is a hydrofoil craft built by Boeing Aerospace Company for the US Navy.
The foil borne propulsion is provided by two gas turbines coupled to
a pair of contra-rotating super caviating five bladed propellers whilst the
hull borne propulsion is provided by a single diesel driving a propeller.
[b] Flagstaff: This
hydrofoil gun boat built by Grumann Aerospace Corporation is an example where
a single gas turbine with a controllable pitch propeller provides foil borne
propulsion and two diesels driving one feet four and half inch wide water
jets, equipped with + 35 degree steering and reversing nozzles, provide
hull borne propulsion.
[c] Swordfish: This
is a missile equipped hydrofoil craft built for the Italian Navy, which uses
a single gas turbine driven water jet for foil borne and single diesel with
propeller or hill borne propulsion.
[d] Soviet Hydrofoils: These
craft seem to favour diesel engines for both foil borne and hull borne propulsion
possibly due to the economy of operation.
31.
Comparative Study: Despite the major drawback of low efficiency
of water jet propulsion it has specific advantages over conventional propellers
namely:
[a] It allows speeds in excess of 50 knots as beyond this speed the drag of appendages like propellers hubs and brackets is too high.
[b] Besides the ship auxiliary and lubricating
machinery, it does not require any additional units as everything is already
integrated in the system. Water jet
propulsion systems normally require a bow thruster to enable control at slow
speeds.
32.
Operation: For take off the foil depth is set [in case
of fully submerged foils only] and the throttles are advanced. As speed increases the hull clears the water.
Landing is accomplished by reducing the throttle setting.
Speed diminishes from 45 knots to 15 knots in as little as 30 seconds. There is a height command lever provided in
some craft to make emergency landings. In
this case since the foils are not retracted a minimum depth of five fathoms
is required.
THE ROLES AND ADVANTAGES OF NAVEL HYDROFOILS OVER CONVENTIONAL WARSHIPS
33.
Although military hydrofoils had been built during World War II, it
is only recently that they have found acceptance, among NATO nations, as instruments
of naval warfare – some 30 years after they had first proved their feasibility. Unlike the NATO nations, the Warsaw Pact countries
were quick to recognize the suitability of hydrofoils for military roles and
have been employing them on surveillance and other roles for many years [6][6].
34.
If the world fuel shortage is taken into consideration, as well as,
the pollution problems which strangle today’s economy and life, the hydrofoil
is tailor made to overcome these difficulties.
A hydrofoil files clean and requires only 50 per cent of the power
of a displacement vessel of comparable size, for a given speed. The advanced military hydrofoil is superior
to conventional warships, due to its superior speed, which can be maintained
even in a seaway and due to better sea keeping ability. In fact, hydrofoils do not need to be large
[multi-thousand ton size] to provide an all weather capability, which is in
stark contrast to conventional warships.
35.
Various comparative studies have been undertaken to ascertain the validity
of better sea keeping qualities in a hydrofoil as follows:
[a] A study conducted by Boeing, for the Royal
Navy and presented before the Royal Institute of Naval Architects, showed
that the Boeing Jet foil [a 110 ton craft with fully submerged foils controlled
by a sonic control system] was capable of operation in the North Sea 96 per
cent of the year [at speeds greater than 38 knots] [7][7]. This corresponds to wave heights
of four meters [see state 6].
[b] In November 1977, a study undertaken by
the Cantiere Naval Technica [Italy] showed a far lesser roll and pitch in
a surface piercing foil craft in sea state 6 compared to a conventional ship
at 20 knots in sea state 5.
[c] An all weather; high speed, smooth ride capability is illustrated in the graph opposite which was obtained as a result of a study, conducted by another leading Italian hydrofoil manufacturer; the Cantiere Navli Riunitis [CNR]; the makers of the “Sparviero Swordfish” missile hydrofoil, for the Italian Navy.
36. It is doubtless that larger warships due to their greater size and lesser maneuverability, are targets rather more vulnerable to tactical offensive weapons, like missiles. They are also more attractive both in terms of value and prestige. Therefore, employment of even a modern frigate, like the Leander, in a high threat environment is too hazardous, to be cost effective. Another consideration is the crew size of larger ships. This is quite a remarkable factor because of the high level of specific and qualified training required, for personnel to run the complex and sophisticated modern equipment.
37. All these considerations, of course to the advantage of the small ship, normally find compensation, in the employment limitations, which they have to face in rough sea conditions. The hydrofoil craft has overcome this handicap, and guarantees to a small craft the ability to put to sea in safety, have good platform stability, high speed and maneuverability and an all weather capability. It has been conclusively proved that the speed advantage passes from a hovercraft to a hydrofoil craft beyond sea state 3 and to conventional craft beyond sea state 6.[8]
38. When considering hydrofoil craft in general, and naval hydrofoils in particular, it is easy to get an impression that these are highly specialized vessels. This idea promotes the belief that they need to be supported by highly specialized workshops, specially trained and highly skilled technicians and extensive logistics. As a result, the idea develops that exceptional costs are involved, not to mention the high technical risk.
39. Any modern equipped FPB yard, however, would also be able to construct and support naval hydrofoils if a certain design philosophy is respected. When operational and maintenance requirements of a modern FPB are compared with those of a naval hydrofoil, it will be seen that the problems involved are identical Garden Reach Shipbuilders and Engineers [GRSE], Calcutta which has hitherto constructed FPBs for the Indian Navy could therefore be ideally suited for the purpose.
40. The lightweight form of construction, developed for hydrofoils, is in demand, increasingly, for a modern FPB’s. However, as the weight of marine engines is reduced,
so the need for this lightweight construction for naval hydrofoils is decreased. This enables the project engineer to specify heavier materials and use less expensive approaches, with lesser demands for a high level of metallurgical engineering. The marked similarity between well proven FPBs and hydrofoils currently operational is evident from the table below. Another thing brought out vividly in the table, is the power required, for the same speed, for a FPB [MT 250] and as hydrofoil [MZ 50G] of comparable displacement.
|
|
|
FPBs |
|
|
Hydrofoils |
|
|
|
|
SA'AR |
OSA |
MT 250 |
MT 504 |
FHE
400 |
TURYA |
PEGASUS |
|
|
|
|
|
|
|
|
|
|
Max
Displacement |
240 |
210 |
250 |
250 |
212 |
190 |
235 |
|
|
|
|
|
|
|
|
|
|
Max
Power [h.p] |
13,500 |
12,000 |
25,000 |
14,000 |
22,000 |
13,000 |
-- |
|
|
|
|
|
|
|
|
|
|
Max
continuous |
30 |
27 |
53 |
53 |
50 |
40 |
40 |
|
Speed
[knots] |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Max
dash |
40 |
35 |
60 |
60 |
60 |
-- |
45 |
|
Speed
[knots] |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
SSM |
Gabriel |
Styz |
MM 38 |
MM 38 |
ASW |
ASW |
Harpoon |
|
|
|
|
|
|
Version |
Version |
|
|
|
|
|
|
|
|
|
|
|
Guns |
3x40 mm. |
2x2.30mm. |
1x76 mm. |
1x76 mm. |
-- |
1x1.5 mm. |
1x76 mm. |
|
|
|
|
|
|
|
1x2.25
mm. |
|
|
|
|
|
|
|
|
|
|
|
Torpedoes |
2 tubes |
-- |
-- |
-- |
Yes |
Yes |
-- |
|
|
|
|
|
|
|
|
|
41.
Today the makers of marine engines are capable of offering systems
which enable the project engineer to install the required engine power within
the weight and space limitations. Also,
compared to conventional FPB, the increase in total weight when adding the
foils, can be limited to a few percent, by using lightweight metals. As far as the basic design of the naval hydrofoil
is concerned the hulls of numerous existing FPB’s could be readily equipped
with foils.
42.
As a consequence, to the preceding examination, of the advantages and
adaptability of the hydrofoil craft and concept, the naval hydrofoil can be
viewed in an entirely different light. It
is not for instance, such a sophisticated and expensive proposition as to
be unacceptable. Another significant
factor is that naval hydrofoils can be integrated with a force of conventional
warships without difficulty and will contribute appreciably to its fighting
capability.
43. The military missions that can be undertaken by hydrofoil craft are:
[a]
Anti-ship strike.
[b]
Decoy, picket duties
[c]
Surveillance, interdiction, mine laying
[d]
Screening coastal convoys
[e]
Recce, clandestine operations
[f]
Medical evacuation
[g]
Amphibious operations
[h]
ASW.
44.
A hydrofoil craft equipped for the anti-ship strike role, with missiles
and small caliber guns, would be able to undertake all except the last two
roles and would be the most suitable configuration for a test craft, for purposes
of evaluation.
45.
The United States: The US Navy has become the forerunner in the
development of hydrofoil craft. Today,
it has a number of such craft, which are suitable for open sea operation. These have all been primarily designed by the
aerospace industry, particularly Boeing and Grunmann. Fully submerged foils are used exclusively,
which are controlled by autopilots. The
ability of these craft to maintain speed in high sea states has been proven.
The first vessel called the “High point was a 120 ton craft designed
for 45 knots and could operate in severe seas.
Subsequently, the Flagstaff, the Super Flagstaff and the Tuenmcari
were built, the former and latter of which were tested in surveillance operations
during Operation Market Time, in Vietnam, for several months.
Presently, the 230 on patrol missile hydrofoil craft, the Pegasus [PHM-1]
is under extensive trials, for examining all aspects of operation. This has been designed to meet specific NATO
requirements, the primary emphasis being on ship performance including a mission
time of five days at sea, with substantial hull borne time. The PHM has during trials come up with various
problems. A brief look at them would
enable the state of the art to be more fully appreciated. The major problem areas identified after some
9,000 hours of underway time [more than 7,500 hours foil borne] were as follows:
[a] Sea Water System: There
was a difficulty encountered in the operation of the auxiliary seawater pumps,
when landing after foil borne operation. The pump motor was improved to withstand salt
water and this resulted in a substantial reduction in failure rates.
[b] Piping System: The
PHM uses glass fiber reinforced piping systems which suffered from leaky joints
caused by faulty manufacture. After
the factory made joints were repaired on board with more refined techniques
a dramatic reduction in leak rate occurred.
[c] Height Sensor: The
acoustic height sensor suffered from spurious returns, which signaled the
automatic control system that the craft was flying too high and the ship would
inadvertently, land. This occurrence
was associated with operations in high sea states or large swells.
Transmitter receiver gams and gating relationships were changed, water
traps and drains were added at the transducer face and structural deflectors
for secondary reflected signals were added.
Thereafter, the vessel operated repeatedly in sea state 5 without any
recurrence of the problem. A radar height sensor has been tested as a
potential alternative.
[d] Hydraulic System: The
PHM hydraulic pumps and system have a high noise level which is, presently,
being treated to achieve reduction.
[e] Frequency Converters:
Faulty parts and workmanship produced high failure rates in the frequency
converters which were later rectified.
[f] Main Propulsion Gearbox: The main propulsion gearbox life needs to be improved as this was
the greatest contributor to ship down time.
It will be noticed that most of these problems do not relate to the hydrofoil concept but to everyday aspects found on other FPB’s as well. It is, therefore, possible to achieve trouble free service from a craft with the correct engineering techniques and expertise, of the type available, even in India.
46.
Swiss Hydrofoils: Supramar AG designed a test craft for weapon
systems, called the KTS 160, in the late sixties. It had a hybrid configuration consisting of surface piercing bow
foils retracted sideways and a fully submerged retractable foil after. The hull was of plywood sheathed in glass reinforced
plastic. Presently, a design concept
for a patrol missile hydrofoil, MT 250G, capable of all weather operation
exists. This craft uses an air stabilized
fully submerged foil system in a canard configuration.
47.
Italy: A 64-ton missile-carrying hydrofoil has been built by the Cantiere
Alnarc Shipyard. This craft uses auto
controlled, fully submerged foils, in a canard configuration and uses water
jet propulsion. For such a small craft
it is surprisingly well equipped. The
craft has two exocet launchers, one twin 76 mm. Oto Melara; IFF, search and
navigation radar; HF SSB radio and
an up-to-date combat information center. It is designed for a continuous speed of 45 knots in sea State 3
and 40 knots in sea State 4 with an endurance of five days. The Italians have also designed heavier missile
carrying hydrofoils, namely, the 84-ton Maifus 300 and the 1127-ton Maifus
600, both with a speed of 37 knots and a range of 500 nautical miles.
Both these craft use diesel engines with controllable pitch propellers.
48.
USSR: The Russians have, in addition, to a series of craft for passenger
ferry, a 46 ton, 35 knot craft, for the KGB frontier police, carrying depth
charges and machine guns. The latest
hydrofoil to enter service in the Soviet Navy is the Turya class. This craft is based on an OSA hull, equipped
with are fixed surface piercing foil, set back approximately a third of its
length from the bow. At 20 to 23 knots
in relatively calm conditions the foil system generates sufficient lift to
raise the greater part of the hull clear of the water, providing a ‘sprint
speed’ of 45 knots. In addition to
improving the maximum speed, the foil reduces the vessels wave impact response
thus enhancing its performance as a weapon platform. The craft has a pocket size VDS aft with four 24-inch single AS
torpedo tubes, as well as, 57 mm. and 25 mm. twin gun mounts. A missile-carrying version of this craft is
also likely to go into production [9]. The craft is powered by three 4,330 h.p. diesels.
The Soviets are also constructing an extremely formidable 300-ton craft
with a hybrid foil system carrying the latest SSMs and SAM’s.
The craft is designed to do 55 knots.
49.
Canada: One remarkably advanced concept, is the 200 ton HMCS Bras d’or [FHE
400] in the Canadian Navy designed for ASW operations in the open ocean.
The surface piercing non-retractable foil system differs considerably
from the usual configuration. It has a steerable bow foil carrying 10 per
cent of the weight. The main rear
foil uses sub caviating sections and is fully submerged. During trials wave heights of 25 feet were successfully negotiated.
A calm water speed of 63 knots has been achieved by the craft, on one
22,000 h.p. gas turbine. A speed of 40 knots in 15 feet waves was also
possible. Other models are now under
development.
50.
Before an attempt is made to anticipate, the probable size, speed and
character, of the next generation of naval hydrofoils, it must be accepted
that today, no such craft has
51.
Although the US Navy has decided to adopt fully submerged foils it
should not be taken for granted that future naval hydrofoils will adopt this
system indefinitely. The vee-foil
offers the advantage of a higher potential for remaining foil borne in a seaway,
a wider range of foil borne speeds and a less sophisticated solution combined
with higher reliability. Another solution
is the new partly revealed Supramar system which largely combines the advantages
of the two systems.
52.
The DEH: The US Advanced Naval Vehicles Concept and
Evaluation Study has concluded, based on a design study carried out by Boeing
that the present state of the art permits the construction of a 1,300 ton
Destroyer Escort Hydrofoil [DEH], with endurance to cross the Atlantic foil
borne, without refueling. The design
study revealed that the craft would have a foil borne range of 2,500 to 3,700
nautical miles at 50 knots. There
exists a general opinion, that the hydrofoil does not lend itself to exploitation,
into the domain of a full blown open ocean escort, with all the attendant
implications of long endurance, self maintenance and integration into existing
logistic systems. In the opinion of
the study, the often discussed ‘size barrier’ ascribed to the hydrofoil has
not been adequately challenged, in the context of a real design based on contemporary
data. The DEH is designed to have
a far greater speed and wave height capability in a seaway, as illustrated
in Figure 1 and 2, than a conventional ship. Figure 3 shows the speed capabilities of several
types of craft as a function of the sea state. It will be noted that a very large conventional
ship, like a nuclear carrier, suffers only a slight degradation of usable
speed until the significant wave height is 20 to 25 feet. Conventional destroyers escorts begin to have
degraded speeds in 10-foot seas and for significant wave heights of 20 feet
can be optimistically credited with an average speed of 15 knots. The large surface Effect Ship with calm water
capabilities of 80 knots, must give up much of this performance, in high sea
states, for the same reasons as displacement hulls. The 50 knot DEH though not entire immune to
rough seas retains its speed because the main hull is decoupled from the sea
surface and its seaway response is governed by the design length of the struts
and the specific type of dynamic control system employed. There are numerous tactical benefits of the
DEH, most notably the fact, that as speed increases, the number of ships required
for most tasks decreases, and the number of escorts per task
53.
One wonders why, with so many advantages; the hydrofoil craft has not
proliferated more rapidly than it has. Baron
Von Schertel, in an interview, on 3rd December, 1977, gave the
following factors as limiting development of future hydrofoils:
[a] Size and Speed: It
is extremely difficult to avoid cavitation of foils, above 50 knots. This is the region where air fed foils with
super cavitating foil sections and a low co-efficient must be used, which
in both cases is associated with a high drag.
[b] New Hydrofoil Systems: Although no fundamentally new foil systems are likely, air feeding
of foils developed at Supramar promises to lead to substantial improvements
in foil systems. Measurements have
shown that in certain flow conditions drag can be reduced by air feeding,
to 75 per cent. Lift control can also
be achieved, by air feed, with lower drag than would occur with flap control. Hereby, the change of lift is brought about
without the energy input required for the actuation of flaps or incident control.
[c] Super Cavitating Foils: A grave hindrance to greater speeds is the onset of cavitation.
The base ventilated foil, super cavitating foil sections and transiting
foils are some innovations which could enable prevention of cavitation or
at any rate a smooth transition from sub cavitating to super cavitating speeds.
DO WE NEED THE HYDROFOIL
54. Having discussed the hydrofoil craft, it is now necessary to examine whether we can afford such craft and more importantly, whether we can afford not to make the investment. To arrive at the correct perspective a brief survey of the present geo-political scenario must be made.
55. India is undoubtedly the major power in South Asia and on her shoulders falls the responsibility of maintaining stability in this region. Our destiny is inextricably linked with the sea, as by all counts we are a maritime nation. There are a number of countries
near and not so near us, whose interests are inimical to ours. To do justice to the responsibility, we have an imperative of obtaining for ourselves, sea power which is credible.
56.
In the concept of sea power, as enunciated by Mahan, it has two distinct
elements, namely; sea power and sea force.
Sea power is built up of the nations merchant marine, bases, geographic
location, people and natural wealth. In
these terms India is on a reasonably firm base.
On 1st January, 1978 the GRT of Indian Shipping was 5.35
million. Although this is sufficient
to carry, only about eight to nine per cent of our total overseas trade, Indian
tankers in 1976-77 carried 99 per cent of our coastal cargo and 70 per cent
of our overseas oil cargo. Another
significant fact is that 60 per cent of our vessels, are below 10 years of
age and more are on the assembly line.
57.
The picture is less rosy with regard to sea force.
Although naval power is not the end product of sea power, it is an
important adjunct of it. For the twin
missions of sea denial and sea control, we must be capable of projecting our
naval power, in the seas surrounding us, when we want it, where we want it.
Without a doubt the maritime environment has today become more an more
dangerous, with most of our neighbours having acquired a hard kill capability. The threats as perceived could be from various sources.
58.
Pakistan: Pakistan has always been hostile towards us.
Although in terms of a wholly conventional Navy, Pakistan may be somewhat
weaker, its latest acquisitions of Atlantic LRMP aircraft, with the AM 39
missiles, Komar class missile boats and Hu Chwan class hydrofoil craft from
China; has given it a fairly decent punch which can be rapidly deployed, over
vast areas. It is also known that the fledgling Pakistani Coast Guard, operates
a few BH 7 Wellington hovercraft, which could be used on naval missions of
interdiction. The trend them is on
fast small non-targetable craft, which are none the less formidable, due to
their excessive speeds, for which we may not have an answer.
59.
Iran: In the present situation, and for some time in the future, no direct
confrontation is likely to occur. The
recently proclaimed Islamic Republic could, however, view with favour the
fact, that the Zia in Islamabad is waxing eloquent on the Islamic theme.
The possibility of a transfusion of naval arms, and financing of purchases,
of naval vessels, in the near future therefore exists.
60.
Bangladesh: The threat from Bangladesh is its weakness.
The hostility and resentment of Bangladesh, towards India, despite
our help in its formation and massive aid thereafter, is well known.
Presently Bangladesh is increasingly coming, in the orbit of the Chinese
grand design and is heavily influenced by China.
An agreement in the future, for base facilities in Bangladesh, for
the Chinese, is not at all inconceivable and would substantially increase
the risk in our eastern theatre. With
the Karakoram Highway, on the one hand, and base facilities available to China
in Bangladesh on the other, the specter of a two front war, at least at sea,
becomes a reality.
61.
Indonesia: Indonesia has a very large Navy, which for
the last decade has been mothballed, due to the abrupt cutting off, of spares,
by the Soviet Union, after the termination of their honeymoon, in the sixties.
The political scene is also unstable, or at least not sufficiently
stable for anything to be done about this.
However, the Indonesian island of Sumatra is a mere 20 miles from the
southern most island in the Nicobars, whilst it is more than 750 miles, from
our mainland. Indonesia, being in the OPEC is in today’s world a ‘have’. Added to this are reports that there are personalities
in the Indonesian armed forces, with the ability to pull the country out of
the morass and put it back on the rails. At the best of times the relations between
India and Indonesia have been cordial and the fact that it is the most populous
Muslim state, does not help matters. Over
navy must therefore be capable to rapid deployment in the islands and operation
from forward unprepared bases within the islands.
62. In the last few years the Indian Ocean has caught the eye of the Super Powers and hectic activity is under way by each of them to gain access and basses in this region. The US base at Diego Garcia and a Russian base, also in the Chagos Archipelago, capable of accommodating an entire fleet in the lagoon anchorage, are some indications of the fact, that they mean business. The Russians have reportedly placed under-surface tanks with associated Single Buoy moorings in a number of locations in these islands to facilitate refueling of ships. The reported US plans, to station the planned Fifth Fleet, in the Indian Ocean and expand Diego Garcia are other pointers of the increasing activity. The upshot of this would inevitably be pressures on littoral states, of various kinds. India being a major power in this region, it devolves upon us to ensure that Super Power rivalry does not engulf the Arabian sea and Bay of Bengal.
Our Tasks
63. Sea Control/Denial: The primary mission of our Navy, as for any other navy, is to keep our sea lines of communication open, whilst preventing the use of the same, by our enemy. To achieve this our Navy must have a rapid deployment capability, which is commensurate with the requirements of operating, in a high threat maritime environment.
64.
Defence of Our Islands: The most vulnerable of our maritime possessions
are our island territories. The Lacadives
Islands, being within 150 miles from our mainland, would not pose a major
problem with regard to maritime safeguarding. The Bay Islands, however, are a different kettle
of fish. The shortest distance from
the mainland is 750 miles. The islands,
themselves are flung over 500 miles, from North to South, with a base existing
only in Port Blair. Although our Navy
has some amphibious capability it is incomplete. Our capability for rapid deployment is negligible.
The Capability of our Navy
65. At the moment our significant naval strength, consists of one carrier [with obsolete or insufficient aircraft, six frigates [two with Styx SSMs], 10 patrol vessels, two tankers and eight submarines. This force is, by no stretch of imagination, sufficient to project credibility, in the ocean area, of consequence to us. In terms of cost-effectiveness too the situation is bleak. For instance a Leander class frigate required a capital cost of approximately 60 crores. The running costs, in terms of fuel bills, crew costs and upkeep can be extrapolated from a recent study carried out by Boeing for the Royal Navy [10]. These annual costs, on a comparative basis were calculated, for 10 x 19 knot ships, 12.5 x 15 knot ships and five hydrofoils, as the time on task, for these three categories, was the same when used in the EEZ patrolling role. The results were as follows:
Crew
Costs Fuel
[a]
Five Hydrofoils
£ 0.45 £
1.35 [in millions]
[b] 12.5 x 15 knotters £ 1.9 £ 2.0 [in millions]
[c] 10 x 19 knotters £ 1.55
£ 3.3 [in millions]
66.
The significant factor with regard to these costs is that after allowing
for down time for refits and maintenance each frigate is available for 66
per cent of the time, only; i.e., to keep a two frigate force on task the
year round a minimum of four vessels would
be required. After
all is said and done, the question which requires to be answered urgently,
is can we afford to lose one such vessel? If we are to face facts, we cannot. If this is the tale, for a lowly frigate, what of the carrier? How then can we best achieve credibility without
making holes in our pocket, especially in a high threat environment, which
is bound to prevail in any maritime conflict, in future. The answer is undoubtedly in small craft.
No wonder them that small fast craft, such as missile equipped boats,
with a formidable punch are so sought after.
67.
Even these, however, lack or to put it more accurately, do not possess,
a sufficiently high degree of tactical flexibility, derived from a rapid deployment
capability. If one can imagine such
a missile boat with a speed of 50 knots plus, this flexibility could be attributed
to it. From imagination, to translation
into practical reality, is fortunately a short step today.
That step is to apply hydrofoil technology, to the already existing
characteristics of craft, such as the OSA class missile boat.
68.
There is yet another scenario, which is ideal for the application of
hydrofoil technology – amphibious warfare.
Our Navy possess six LST to Polish origin.
All these vessels are rather large, and therefore do not guarantee,
that our troops will come ashore dry-shod.
Then again the LSTs are not primarily designed to carry troops. In fact, they would form the second echelon,
in any amphibious operation, when troops have secured a beachhead and armour
can safely made ashore, without any immediate opposing fire. Our capability to put the first few waves of
foot soldiers ashore is negligible, as this requires small sized LCA [Landing
Craft Assault] and amphibians like the DUKW and LCVX, referred to earlier
in the study.
69.
Any amphibious assault can be expected to be opposed, by shore based
missiles and heavy accurate fire. It
is therefore imperative for the assault force, to spend the shortest possible
time, in their most vulnerable phase – that of getting to the beach from the
troop carrying ships. Every nation
dabbling in amphibious warfare, has realized the need for rapid envelopment. The concepts of vertical envelopment, using
helicopters and use of hovercraft to transit the short run up to the beach
are manifestations of this realization. Just
as a DUKW and a LCVX can be given enhanced speed using hydrofoil technology,
the Indian Army, amphibian APCs, such as Topaz and Skot, could also be provided
with comparable capabilities. Additionally
LCAs equipped with foils could be produced, within the country, at a shipyard
like Garden Reach Calcutta; which has experience in building small fast patrol
boats.
70. Hydrofoil technology is still recent enough, for the hardware to run to exorbitant costs. This, however, is the case only with the ultra sophisticated craft, being built for the US Navy, by the Boeing Aircraft Corporation. A recent purchase of a single jet foil of the Pageus type, but minus the missiles, acquired by the Royal Navy cost £ 7.5 million [11]. India cannot certainly go in for such a buy, even though this is less than a quarter of the 60 crores cost, of one of our Type 16 frigates, being built in Mazagaon Docks. What we require is a cheap acquisition, which will have an enhanced value, due its cost effectiveness.
71. The Soviet Union, which has been operating hydrofoil craft for a much longer time, could provide us with an alternative. The possible options are either outright purchase or an adoption of the Soviet idea, of conversion of the FP 8 hulls, by adding struts and foils.
72. Outright Purchase: With our present rapport with the Russians, at least where purchase of warships are concerned, outright purchase of a hydrofoil craft may not prove to be too difficult. The only shortcoming in outright purchase of warships, from foreign countries, is the tie up due to spares. Having purchased a number of craft from foreign countries, the spares problems attendant to these acquisitions, are only too well known to require elaboration. This then may not be the best solution.
73. New Indigenous Design: Hydrofoil technology is nothing but an offshoot of aerofoil design. The Hindustan Aeronautics Limited has acquired sufficient expertise in this field, to produce an excellent airframe for the Marut aircraft flown by the Air Force. With minimal R & D effort, it is conceivable, that they could come up with a feasible hydrofoil design. From there to production, would be a fairly easy and not too costly step, given our industrial infrastructure. As regards the hull of the hydrofoil craft, Garden Reach Calcutta, has for long been involved in the construction of small fast patrol craft, of the Ajay and Akshay class, and should have no difficulty in designing a hull if the correct QRs’ are provided to them. The marine diesel propulsion engine technology is still not sufficiently developed to provide the main propulsion system. Therefore, this and the “on board” equipment, especially the navigation and fire control software and perhaps, the anti-submarine equipment [if this role is envisaged], will be the imported components, in the craft.
Conversions
74. Another option is the conversion of some of our small craft into hydrofoil-equipped craft. The Indian Navy possesses many of these craft. These craft are ideally suited for our tests, to ascertain the feasibility of this type of conversion, if it requires proving. The Soviets have converted an OSA hull with hydrofoils [the Turya Class] and have proved that it is possible. The requirements them are two fold. Firstly, to design a workable strut and foil assembly and secondly, to design a propulsion system.
75. Strut Foil Assembly: The indigenous design of a strut-foil assembly will involve the design of the hydrofoil section and a strut structure which can withstand the stress. Most of these assemblies in craft being built abroad are made of stainless steel which may have to be imported. Even if Hindustan Aeronautics Limited can produce a design the testing of the first prototype will have to be done abroad as we do not possess the towing and manoeuvering tanks and cavitation tunnels required for the purpose. The design and testing of the prototype abroad would cost approximately Rs. 2 crores, if the exorbitant charges of hiring testing facilities, is taken into account. Since fully submerged foils require an associated auto control system, the best thing would be best the vee foil, as the fairly good sea keeping ability of this type of foil is proven by the Canadian FHE-400. A collaboration agreement is likely to reduce this cost somewhat as also the time required to manufacture the assembly.
76. Propulsion: When a hydrofoil is foil borne since the entire hull is out of the water the propeller shaft has to be inclined by twelve to fifteen degrees downwards to ensure propeller immersion. This is a problem mainly requiring a modification to the transmission of the engine output through a V or Z drive and is likely to be well within the capacity of the design departments of technical institutions, like IIT Kharakpur or possibly even of the Directorate of Naval Design. The existing diesels on the OSA’s may be suitable for this modification.
77. Auxiliary Systems: Since in the foil borne mode the hull of the craft is out of the water the existing sea water systems would require modification to enable operation in this mode. Perhaps an intake via the strut and foil assembly is the answer and would not involve a very heavy financial outlay.
THE
FINANCIAL BURDEN
Outright
Purchase
78. As brought out earlier, a sophisticated, fully submerged foil craft, built by Boeing costs £ 7.5 million. Although crafts manufactured by Italian and Swiss yards, may be cheaper, a fully submerged foil craft would be expensive, due to the need for an automatic control system and the associated software. The option of a vee-foil craft therefore is more attractive. At present price levels even a vee-foil craft purchased from Italy, USA or Switzerland would still be around £ 5 million or about Rs. 8.5 crores.
79.
Hull: The design of a FPB hull is already available with the Garden Reach
Calcutta and would need some expenditure to improve structural strength to
cater for non-displacement operation. By
using the existing OSAs in our inventory even this cost may be reduced.
80.
Strut and Foil Assembly: The design, development, prototype manufacture
and testing of a stainless steel strut and foil assembly would cost approximately
Rs. 2 crores. If subsequently assembly
line production commences, each unit would cost approximately Rs. 2 lakhs
or less. The acquisition of the strut
and foil design by technology transfer would be cheaper and faster.
81.
Propulsion System:
[a] New Indigenous Design: The cost of designing a suitable marine diesel, with inclined shafting
and adaptation of the existing assembly line, at Garden Reach Calcutta, to
manufacture these engines would be in the region of Rs. 8 crores.
The time factor involved in the process would be around five to six
years.
[b] Collaboration: With
the already existing collaboration with MAN [West Germany], the Garden Reach
Shipyard or Kirloskars should not find it difficult to negotiate a collaboration
for smaller high speed diesel of the type required for hydrofoil craft.
The cost of transferring such technology would be in the region of
Rs. 3 crores with a time factor of about two years before production could
start in India.
[c] Redesigning Gearbox/Drive: As discussed earlier the redesigning of the gear box and drive train
for the existing engines in the OSA class missile boats would involve even
lesser expense and is estimated to be about Rs. One crore.
Total Outlay
82.
Conversion: The cost of conversion or modification of the
existing OSAs would involve four distinct parts namely hull, strut and foil
assembly, main drive and gearbox and auxiliary systems. The total financial outlay for the entire modification
in each case is estimated to be as follows:
____________________________________________________________________
Item
Cost [Crores of Rs.]
________________________________________________________________________
[a] Hull [strengthening of structural etc.]
0.5
[b] Strut and Foils 2.0
[c] Main drive/gear box
1.0
[d] Auxiliary systems
0.5
________________________________________________________________________
Total ..
.. .. .. 4.0
Crores
________________________________________________________________________
83.
Logistic Infrastructure: In addition to the above expenditure there
will be a requirement of converting existing assembly lines to cater for the
further conversion and repair and maintenance of these craft. With the already existing assembly lines in
Mazgaon Docks, and Garden Reach, this would take up to Rs. 6 cores.
84.
The Pay Off: The pay off of this venture when considered
in its long-term effect is remarkable. If
the first two test craft as envisaged are successful, the other OSA’s could
also be converted, missiles et.al., into hydrofoil craft. Although the weight factor with four Styx missiles
would need looking into, it is estimated that at least two of the missiles
could be retained. It is not inconceivable
also, that a foil assembly could be designed and manufactured, which could
lift the craft, with all four missiles. The pay off would then be the availability, to the Navy, of a dozen
missile hydrofoils, for an initial cost of approximately Rs. 10 crores and
a retrofit cost of about Rs. 2.5 to 3 lakhs each i.e., for a total outlay
of less than Rs. 11 crores, twelve of our small craft could be converted. In fact, with the retention of the existing
conventional diesel, the craft could be given and emergency “get home” capability,
along with enhanced mission times. It
is also possible that the present 500-mile radius of action would not be degraded
and could well improve, since for a given speed, a hydrofoil requires only
50 per cent of the power, as a displacement vessel.
85.
The advanced naval concepts for the navies of the late eighties and
nineties invariably center around surface skimmer technology. The effort and expense spared by most developed nations on this
field is evident, from the variations of this technology, that are being tested.
Even the fledgling air cushion vehicle and hydrofoil technology is
in danger of being regarded as rather conventional.
Ram Wing craft, wing in ground [WIG] effect craft, small water plane
area twin hull ships [SWATH] and others are today common terms.
The Indian Navy cannot ignore this trend, particularly when viewed
with the cost effectiveness of these craft.
86.
Closer home we see the need for these craft even more. Our most antagonistic neighbour, Pakistan has invested in Atlantique
MR aircraft with an anti ship missile capability, improved Daphne submarines
and ASM armed Sea Kings. This amounts
to Pakistan acquiring a capability to effectively neutralize our substantial
advantage in surface combatants. One question which is exercising the mind of
every naval tactician is how to counter the Atlantique and the Daphne. The loss of even one of our highly priced Leanders
is almost unbearable to contemplate.
87.
The Indian Navy requires a potent system to counter these threats and
which does not strain the financial back.
With the conversion of the missile boats into hydrofoil craft, within
the realms of possibility, this system is ours for the asking – a system which
is virtually immune to both the threats looming so ominously large on the
tactical horizon.
88.
An offshoot of this conversion is the availability of a method for
operating these converted craft in our islands from froward in prepared bases. A Logistic Support Mobile Train consisting
of six modular units a d capable of providing support to three converted craft
could be acquired for a nominal cost. This
logistic support mobile train would be on “the same lines as being put in
the market by the Italian firm Cantiere Navali Riuniti.[12]
89.
The acquisition of expertise, in hydrofoil technology would also open
up new vistas in water transportation. The
immediate result could be the conversion of the amphibians in the Army to
fulfil the role of an LCA [Land Craft Assault] so solely
90.
This study after examining the hydrofoil technology in its applicability
to existing Indian conditions has arrived at a few recommendations which could
be the first step in harnessing the advantages of this unique concept for
the enhancement of the creditability of the Indian Navy.
91.
Recommendations: The following recommendations are made in order of priority:
[a] Hindustan Aeronautics Limited should be contracted to study and develop a design for a hydrofoil section.
[b] IIT Kharakpur, and the DND, NHQ should be asked to study the
modification of the existing diesels on the OSAs to achieve inclined shafting
through a V or Z drive.
[c] Failing this, Garden Reach Workshop [GRW] Calcutta or Kirloskars
should be asked to arrange for technology transfer of suitable small sized
marine diesels from a suitable foreign firm. The collaboration arrangement should also include the technology
transfer of inclined shafting.
[d] If feasibility is proved all our small craft could be retrofitted
with this.
[e] A logistic support mobile train based on the Cantiere Navali
Riuniti Shipyards [Italy] [10] concept should be developed, by the Naval Dockyard
Vishakhapatnam, to enable forward operation, from unprepared bases, by these
craft.
Aeroplane Foil System: Also
called the conventional foil system it is a foil arrangement in which the
main foil is located forward of the center of gravity to support 75 per cent
to 85 per cent of the load, and the auxiliary foil, supporting the remainder,
is located aft as a tail assembly. The
forward and aft foils may be retractable to reduce hull borne draught.
Angle of Attack The angle made by the mean chord
line of an aero or hydrofoil with the flow.
Angle of Incidence: The angle made
by the mean chord line of a hydrofoil in relation to the fixed struts or hull.
Automatic Control System [ACS]:
Although
fully submerged foils are the most versatile in operation they are inherently
unstable and are workable only with an automatic control system. There are three types of control systems namely
sonic, mechanical and pneumatic. The
ACS has to stabilize the craft from take off to touch down in heave and all
the three axes – roll, pitch and yaw. It
must also see that the craft makes coordinated banked turns in heavy seas
to reduce the side loads on the foil struts, ensure that vertical and lateral
acceleration are kept within limits in order to prevent excessive loads on
the structure and finally ensure a smooth ride by maintaining the foils at
the correct depth.
Base Ventilated Foil: A
system of forced ventilation designed to overcome the reduction in lift/drag
ratio of a foil at super cavitating speeds.
Air is fed continuously to the upper surface of the foil unwetting
the surface and preventing the formation of critical areas of decreased pressure. Alternatively air may be fed into the cavity
formed behind a square trailing edge.
Canard Configuration: In
this arrangement the main foil of wide span is located near the stern, aft
of the center of gravity and bears about 65% of the weight while a small central
foil is placed at the bow. The fore
and aft foils could be made retractable to reduce hull borne draught.
Cavitation: This is the
formation of vapour bubbles due to the pressure decrease on the upper surface
of the foil, or the back of propeller blades at high speeds, and falls into
two categories, unstable and stable. Non-stable
cavities or cavitation bubbles of aqueous vapour form near the leading edge
of the foil and extend down stream expanding and collapsing. At points of collapse positive pressure peaks
may rise to as high as 20,000 psi. These
cause erosion and pitting of metal. Cavitation also causes an unstable flow of water flow over the foils
which results in abrupt changes in lift and therefore discomfort to those
abroad the craft. Foil sections have
now been developed which wither delay the onset of cavitation by reduced camber,
thinner sections or sweepback, or if the craft is required to operate at super
cavitating speeds, stabilize cavitation to provide a smooth transition between
sub cavitating and super cavitating speeds.
Foil Flaps:
Foils are frequently
fitted with [a] trailing edge flaps for lift augmentation during take-off
and to provide control forces [b] upper and lower flaps to raise the cavitation
boundary.
Foil Systems: There are three
foil system current in use:
[a] Surface Piercing Foils: Surface piercing foils are more often than not vee shaped and are
also called vee foils. The upper part
of the foils from the tip of the vee and pierce the surface on either side
of the craft. The forces restoring
the normal trim are provided by the area of the foil that is submerged. A roll to one side means the immersion is increased
on that side, thus generating greater lift to counter the roll, restoring
the craft to even keel. Similarly
a pitching movement generates greater lift either in forward or aft foil.
This system is also called the emerging foil system as a vee foil craft
increases speed greater lift is generated raising the craft and the foil with
it further out of the water. This results in decreased lift. The hull therefore rides at a predetermined
height above the water.
[b] Shallow Draught Submerged Foil: This foil system is used for craft plying along
long calm stretches of water like rivers and are used widely in hydrofoil
craft designed in the Soviet Union. The
system is also known as the immersion depth effect system and was invented
by Dr. Alenyev. It carries two equal
area foils forward and aft which are submerged.
These foils lose their lift gradually as the foil approaches the surface
from one chord [distance between leading and trailing edge of foil] submergence,
which prevents, it from rising completely to the surface. Means therefore have to be provided to assist
take-off and to prevent the vessel from sinking back to the displacement mode.
Two planning sub foils are therefore located in the vicinity of the
forward strut so that when they are touching the water surface the main foils
are submerged at a depth of approximately one chord.
[c] Submerged Foils: This
system consists of forward and rear foils which are fully submerged well below
the surface. Modern constructions
generally prefer this system as the surface piercing foil suffers from the
following disadvantages.
i] The inability of vee foil craft without
control surfaces to cope with the downward orbital velocity at wave crests
when overtaking waves in a following sea; a condition which decreases the
foil angle of attack causing reduction of lift resulting in hull slamming
or a stall. Submerged foils, on the
other hand are capable of platforming, contouring or an intermediate response,
when automatically controlled.
ii] On large craft, the weight and size of
vee foils is considerably larger than a corresponding submerged foil system.
iii] Since restoring forces in the vee foil
system have to pass above the center of gravity of the craft, the foils have
to be placed only a short distance beneath the hull. This means a low wave clearance thus making to vee foil unsuitable
for rough weather operations. Crafts
like the FHE 400 have however proved that this is not strictly correct.
iv] Vee foil systems are susceptible to waves
and provide a rough ride whereas the fully submerged foil system being below
water is immune to wave disturbance providing a smoother ride.
The greatest disadvantage of the submerged foil system is that these foils are inherently unstable and require on ACS to control the depth of the foils. These auto control systems add to the cost of this system. The various types of auto control systems are:
i] Sonic System: This system is probably the most expensive
and sophisticated. The key element
of a typical acoustic electronic autopilot is the sonic height sensor located
at the bow. Craft motion input is
received from dual sonic ranging devices which sense the height above the
water of the bow in relation to a fixed reference; from three rate gyros which
measure yaw, pitch and roll; from forward and aft accelerometers which sense
vertical accelerations fore and aft and from a vertical tyro which senses
the angular position of the craft in both roll and pitch.
The information is processed by a computer and fed to hydraulic actuators
of the foil control surfaces which then move to maintain the craft stability
through wave action and during turns.
ii]
Mechanical Incidence Control:
The most successful purely mechanically operate incidence control system
was developed by Christopher Hook who pioneered the development of submerged
foils. The control surfaces are generated either by
deflecting flaps at the trailing edge of the foil or varying the angle of
incidence of the entire foil surface. To
achieve this a fixed high riding crash preventer plane is mounted ahead of
and beneath the bow. The fixed plane,
which is only immersed when the craft is in the displacement mode, is also
used as a platform for mounting a lightweight pitch control sensor, which
is hinged to the rear. The sensor
rides the waves and transits continuously their shape through a connecting
linkage to vary the angle of incidence of the main foils, as necessary to
maintain the required depth. A filter
system ensures that the craft ignores all waves with a height below that of
the keel above the water. The additional
sensors on either side provide roll control.
The pilot of the craft has overriding control through a joystick control.
iii] Air Stabilization Control: This is a system designed by Baron Hans Von
Schertel. Air from the free atmosphere
is fed through air exits to the foil upper surface and under certain conditions
the lower surface also [into the low pressure regions]. The airflow decreases the lift, and deflects
the flow away from the foil section; with an effect similar to deflected flaps,
the air cavities extending out behind producing a virtual lengthening of the
foil profile. Life is reduced and
varied by the quantity of air admitted, this being controlled by a valve actuated
by a damped pendulum and rate gyro. The
pendulum causes righting moments at static heeling angles. If exposed to a centrifugal force in turning,
it causes a moment, which is directed towards the center of the turning circle,
thereby avoiding outside banking. The rate gyro responds to angular velocity and acts dynamically
to damped rolling motion.
Grunberg Foil System: This system
has a main mid-ship foil which carries about 90 per cent of the load and small
planning foils, set forward, carrying the remainder.
An adaptation of this system is the Aquavion System, which has the
main foil located slightly abaft the center of gravity carrying 85 per cent
of the load. An aft stabilizer foils carries about 10 per
cent while the remainder is carried by a pair of bow planning sub foils. These sub foils give a variable response to
wave shapes and turn the angle of the hull, in order to correct the angle
of attack of the main foil.
Inclined Shaft: A marine drive shaft used in small vee foil and
shallow-draught submerged foil craft, with keels only a limited height above
the water level. The shaft is generally
short and inclined at about 12 to 14 degrees to the horizontal. On larger craft,
Tandem Configuration: This configuration consists
of equal area foils fore and aft balancing the loading between them.
Thruster: Controlled
aperture through which air can be expelled to assist control at low speeds.
Transcavitating and Super Cavitating
Foils: Since at very high speeds foils
cannot avoid cavitation, sections are being designed which induce the onset
of cavitation from the leading edge and cause the cavities to proceed downstream
and beyond, the trailing edge before collapsing.
These are called super cavitating foils. Lift and drag of these foils is determined by the shape of the leading
edge and under-surface. Foils which
are required to operate at super cavitating speeds also have to cater for
a smooth transition from sub-cavitating to super cavitating speeds. Foil sections have been designed to achieve
this. By loading the tip of the foil
more highly than its root, cavitation is first induced at the tip and then
extends span wise over the foil to the roots as speed increases. These are known as Transcavitating or Transiting
foils.
[1]
Michael
Coster, “Introducing PCOs Jetfoil Service” p. 25, Hovercraft and Hydrofoil,
October 1977, Vol. 17, No. 1.
[2]
Roy
McLeavy “Forward”, Janes Surface Skimmers, 1974-75.
[3] An interview with Baron Von Scherkel – Interviewed by the International Hydrofoil Society, p. 12, Hovercraft and Hydrofoil, February/March, 1978
[4] Ibid.
[5] Waldemar Graig – Grunberg Hydrofoil Pioneer Interviewed by Mark Thornton. P. 7, Hovercraft and Hydrofoil, November, 1977.
[6] Hanns Von Schertel, Egon Faber and Eugen Schette, Military Hydrofoils, p. 301, Janes Surface Skimmers, 197273.
[7]
R.C.
Macgregor, Small Fast Warships and Security Vessels, p. 27, Hovercraft and
Hydrofoil.
[8] Ibid
[9] Roy McLeavy James, Surface Skimmers, 1975-76.
[10] Anthony Preston, The Royal Navy’s New Jetfoil Hovercraft and Hydrofoil, April 1979.
[11] Ibid.
[12] Francesco Cao Sparviero “Swordfish” Type Multi Role Combat Hydrofoil, Hovercraft and Hydrofoil, May, June 1978