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Barring some completely unexpected difficulties, sh»ps riding on a cushion of air will be joining the within the next decade. After more than 50 years °f only trivial increases in the speed of ships, a tripling 0r even quadrupling of the maximum speed is within U. S. Navy’s grasp.
Two precursors of this new "100-knot navy” are in 'he later stages of their trials programs. The SES-iooA, huilt by the Aerojet General Corporation, and the ^S-ioob, by the Bell Aerospace Company, are testcraft Signed as "proof of principle” articles for the U. S. h^vy. Resistance measurements at speeds up to 70 knots have thus far confirmed the design predictions.
The two testcraft are of about 100 tons gross weight ^h and, although comparatively small as ships go, expected to be capable of speeds in excess of 80 knots. There appears to be no fundamental limit to 'he size of ships using the air cushion or surface effect Principle and, in general, performance increases with Sl*e. Speeds up to 200 knots may one day be realized "'ith surface effect ships (SES) larger than perhaps 20,OOO tons.
The type of surface effect ship currently receiving 'he lion’s share of attention is the "hard-sidewall” or raptured air bubble” configuration. Air is forced into 'he cushion by powerful fans. This air cushion is reined by flexible seals fore and aft and by rigid catama- 'an hulls along the sides. This air layer separates the hull plating from the water, effecting an enormous 'Auction in frictional resistance. The air cushion vehi- ck rides high enough to allow most waves to pass ^impeded beneath her. Only her side hulls pierce the SUrface. The end seals either ride clear on a thin film escaping air or plane on the surface complying with 'he waves as they pass beneath. Leakage is replaced by 'he fans. In waves, the cushion also attenuates the ffcets of what otherwise would prove to be a very Unapy ride.
Although an SES can go very fast, her lift-to-drag 'J'io is inferior to that of a displacement ship at speeds
below perhaps 50 knots. Weight, therefore, is important, and many design practices approach those used in aircraft.
SES will probably be highly automated with most machinery automatically or remotely controlled. High speed tends to minimize mission durations. This fact, coupled with automation, tends to make the crew small, with a minimum of "hotel” facilities. The crew quarters of the 100-ton testcraft resemble those of airplanes rather than displacement ships. The Bell craft has no below-deck accommodations. Her controls closely resemble those of a large multi-engined airplane. Crew members have only airline-type reclining seats. The Aerojet testcraft has somewhat more elaborate quarters with four bunks, a galley, and mess spaces below.
The hulls of the two 100-ton testcraft are made largely of marine aluminum, and the future may see materials of higher strength ratio extensively employed.
Propulsion and lift engines, for the foreseeable future, will be marine versions of aircraft gas turbines which alone adequately combine high power with light weight. The Bell testcraft uses Pratt & Whitney FT12 engines; the Aerojet uses AVCO-TF35S.
There are three candidate propulsors: waterjets, super- cavitating propellers, and ducted fans. The SES-iooA uses waterjets; the SES-iooB uses semi-submerged, supercavi- tating propellers.
The propellers of the Bell craft are not like those found on displacement vessels. Normally, a propeller produces thrust by an increase in pressure on the face of the blade, and a decrease in pressure on the back. This latter pressure becomes lower and lower as relative speed is increased until at some point (usually near the leading edge on the back face) the pressure is reduced to the vapor pressure of water. At this point the water boils. No further decrease in pressure is possible, and the contribution to the thrust made by the backside of the propeller ceases to increase. Worse, when the vapor bubbles reach a region of higher pressure, they
implode with great violence. If they are in close proximity to any solid surface, very high stresses are induced, and the surface may disintegrate.
Cavitation, as the phenomenon is called, normally places a limit on propeller speed, since impingement of the collapsing bubbles on blades or structure is difficult to avoid under all operating conditions. Some special blade shapes (super-cavitating propellers) encourage the formation of a vapor cavity in a controlled fashion. It is then possible, at the design point at least, to avoid bubble implosion near any solid surface. Further alleviation of the problem can be obtained by ensuring that low pressure areas are filled with air before vapor pressure is reached. With such ventilation, violent collapse of the bubbles is avoided. Appropriate design is required to ensure that all cavities created are properly ventilated.
Another problem with propellers is the drag created by the hub and by supporting struts. The solution to this problem adopted for the SES-iooB is semi- submerged propellers, only the blades of which contact the water.
Waterjets, produced by a diffuser-pump nozzle combination, are another solution to the problem of highspeed ship propulsion. Cavitation is avoided by slowing the water down from ship speed, converting some of the dynamic head (velocity) into static pressure before it meets the energy-imparting blades. It is possible to keep the pressure well above that of water vapor, especially at high speed. Of course, the relative velocity of diffuser and the water is equal to the ship speed and, thus, care is required to avoid excessive lowering of the pressure before adequate slowing down takes place.
Stability and control offer special problems. Surface-effect ships tend to have a high center of gravity. Because the wetted surface must be minimized, buoyancy forces cannot provide any great measure of stability. Hydrodynamic, aerostatic, or jet reaction forces must be used instead. Appropriate shaping of the side- hulls or special foils can give adequate stability, but drag considerations militate against indiscriminate use of such surfaces. Only the area required at any given moment should be exposed to the water. The rough nature of the ocean surface makes such controlled immersion difficult.
about a year. During this time, the performance
Hydrodynamic surfaces or jet reaction must also be used for directional stability and control. Design of such surfaces is further complicated by the cavitation problem. Not only does cavitation lead to material damage, but also it gives rise to unsteady and rather unpredictable changes in the forces produced by the control and stabilization surfaces.
In spite of severe structural problems in regard to
space and weight, despite the difficulty of maintaining an air bubble with acceptable leakage while traversing a rough surface, and notwithstanding the destructi'c effects of cavitation, acceptable design solutions app^j possible. How well the first generation of SES has solve these problems will be known shortly.
The two testcraft have many similarities and tn»nl differences besides the difference in propulsors preVI' ously mentioned. The rakish SES-iooA shows morC attention to aerodynamic drag. The forward position of her cabin gives better visibility and, being partial submerged in the hull, she looks more like an airplane
The SES-iooB on the other hand has her crew quart#* aft and entirely above the maindeck. The aft position reduces the accelerations resulting from pitching m0' tion and may offer a more comfortable ride. The sy®' metrical outline was selected to minimize the tooM required.
Steering concepts for the two ships differ sharpl) The Bell craft uses a more conventional system, rudders providing the turning moment. The centrifug^ force of the turn is balanced by yawing the sidehulb to generate "lift.” The sidehulls are supplemented b) ventral fins to provide enough wetted area.
Since the waterjet inlets on the Aerojet craft cann°[ tolerate appreciable angles of sideslip without cav‘" taring, the SES-iooA uses center of gravity steering-
To make a turn, hydrofoils are extended out the side by hydraulic rams, producing a side force which goCS very nearly through the center of gravity.
The forward seals of the two craft are quite different Bell uses a combination of bow bag and individual fingers. The fingers accommodate to small waves locally without causing the entire seal to deflect.
The Aerojet’s forward seal is essentially a fabrlC membrane stiffened with fishpole-like fiberglass rod* Hinges and air springs provide adjustable compliant The rear seal is similar to the forward seal but a hold' down bag is added to counterbalance the cushion pt#’ sure. The Bell stern seal also has a hold-down bag hut does not have stiffeners.
Both ships are heavily instrumented. Several hundred measurements are taken and recorded. Many are moo1- tored on a real-time basis.
The Aerojet craft, the SES-iooA, is being tested the convenient variety of sea states available in Pugct Sound. The Bell boat (SES-iooB) is being tested fa1 on Lake Ponchartrain in the New Orleans area, 3(1 later in the Gulf of Mexico just off Panama City, usifle the Naval Coastal Systems Laboratory as a base.
The two craft have been tested by their builders f°r
oft*
ships was measured in a wide variety of conditions and
Surface Effect Ships in the Surface Navy 53
these measurements compared with the theoretical predictions. If necessary, the design technique for future ships will be modified to conform with actual results.
The test programs include determination of speed, range, maneuverability, seakeeping characteristics, and ride characteristics or "habitability.” Any special problems of maintenance and operation peculiar to this type of vessel will also be noted.
After the builders’ tests have been completed, the Navy will take delivery of the craft for an extended period of engineering and simulated operational evaluation. For this period, a special facility has been constructed at Patuxent River, Maryland, adjacent to the West Seaplane Basin, originally built to accommodate large seaplanes. Here Navy crews and government engineers will put both craft through their paces and in general apply naval experience to the understanding and exploitation of this new capability.
Unless some major surprises are encountered, we may expect to see a progressive conversion of the surface Navy to an all-SES Fleet, for there is no major class of naval vessel that cannot profit greatly from the unique advantages of these air cushion ships. Except for garage scows and a few auxiliaries, all naval ships should t>e fast. Speed not only allows getting there "fustest with the mostest” but once at the scene of the action, gives tremendous tactical advantages.
SESs, because of their high speed and shallow draft, ate virtually invulnerable to torpedoes, and they present a much more difficult target for aircraft and missiles.
Except for hydrofoil ships, SESs are the only surface vessels that cannot be outrun by a modern submarine. So great is an SES’s speed that she can stop dead in the water, listen, and still maintain a rate of advance greater 'ban any surface or subsurface ship.
The SES, while not as fast as airplanes or helicopters, have a loiter capability comparable with conventional displacement ships.
Speed increases use. Many SES trips can be made in [be time required for one by a conventional ship. Speed allows one task force to cover simultaneous potential 'hreats in widely dispersed areas, and minimizes the re<piirement for America to maintain advanced bases all over the world.
SES have two additional characteristics that should allow them to be superb aircraft carriers. First, their speed allows wind-over-the-deck to approach aircraft takeoff and landing speeds. Deck size requirements, and tNe need for catapults and arresting gear should be greatly reduced. Landing techniques could be similar to those used on the airships Akron and Macon, where 'be plane is caught by a trapeze-like device and pulled inside the hull. Numbers of such devices might be installed, allowing simultaneous takeoff and landing of
several aircraft. Second, the broad beam, which is characteristic of SES, lends itself well to formation landings and launchings.
The impact of surface effect ships on the Navy will be extensive. It will involve both pain and pleasure. On one hand, their new capabilities will provide an intellectual and professional challenge that will infuse a new enthusiasm into younger officers. At present, our Navy is only beginning to understand how best to exploit SES characteristics. New strategies, new tactics, new maintenance, and new operating philosophies will be needed.
On the other hand, new habits must displace old, cherished totems. With smaller crews, even big ships will not represent the "command” they used to. A captain’s energy and effort will be concentrated more on machinery than on people. There will be fewer seagoing billets and thus, the shore establishment will grow. Navy yards will change radically, evolving into something of a cross between what they are now and an aircraft overhaul and repair (O&R) establishment. Drydocking facilities must change. And although the crews will spend more time on the beach than at sea, seagoing time will be more confining and more physically exhausting. This means there will be even fewer sea duty billets for older, more senior officers. All these pains must be borne, however, for the new level of effectiveness the SES will give to the Navy is a power that, for the good of both the Navy and our country, is inarguable.
There will be a transition period. The SES will not replace the conventional ship overnight. Neither iron hulls nor steam power elbowed their predecessors aside immediately despite a superiority that is now obvious. Technological problems alone will force us to climb the growth ladder a rung at a time. We need bigger engines. Those engines must be developed especially for SES. The airplane industry has, for the moment at least, reached a size plateau, and larger spinoff engines are not likely, for some time, to be available for SES. Big gas turbines cost money—as do other technological developments required for full exploitation of the potential inherent in SES. We will need SES-oriented weaponry, if not initially, then ultimately, for full effectiveness. We will have to reorient our thinking about research and development (R&D) costs for ships. We accept R&D costs for airplanes that are orders of magnitude greater than the cost of a single plane, yet R&D for a ship class usually runs to only a fraction of the cost of one ship. For displacement vessels, this ratio was probably justified. Such ships represent a mature technology, where the payoff from R&D is small regardless of the sums spent.
With the advent of the air cushion principle, however, whole new horizons are presented. We shall not realize the bright promises if the problems are approached timidly. Boldness of both an administrative and financial nature is required. Some naval officers will have to stick their necks out, perhaps even lay their careers on the line, if both the engineering and opera
tional problems are to be solved, and solved with i speed that will allow the U. S. Navy to take maximuff advantage of the higher effectiveness the air cushiot principle allows.
Most of us realize that there is no ultimate weapon A new discovery, properly exploited, gives a few days months, or years of advantage to the nation that fits recognizes and uses it. Every weapon, sooner or later is either countered or encountered in all the majo powers’ arsenals. Thus time is of the essence, in peao as well as in war. What we gain from all our militar] R&D expenditures is but a transitory advantage ove our enemies or potential enemies. Still, such technica advantages can often spell the difference between vie tory and defeat. Moreover, in time of peace, they cat mean the difference between peace and war. A Scarlet O’Hara—"I’ll think about it tomorrow”—attitude witl any new development of potential military significant simply fritters away all or part of the weapon’s benefit
Finally, in time of peace, the absolute secrecy neces sary to achieving technological surprise is next to im possible in our open society. It is primarily by takinj financial risks that this country can maintain the tech nological superiority of its weapons. Financial risk followed by both administrative efficiency and bash technical know-how in converting dollars to weaponry is the sine qua non of the weapons game. The surfac effect ship is not the kind of doomsday weapon th nuclear bomb was and is. It might be possible to wii a war without SES or lose one with them, but thes ships represent a new and important capability tha must be taken advantage of—and quickly. Withou ships we do not have a Navy. If the choice had n be made between a somewhat improved airplane am the beginning of a revolution in our surface ships, w ought to give priority to the latter. Sometime sod this choice may have to be made, because the SE revolution will not be brought to a successful conclu sion with half-hearted measures.
Captain Truax is a graduate of the U. S. Naval Academy, Class of 1935 He was designated a naval aviator in 1943 and AF.DO the same year. of his naval career was spent in Navy R&D, in rocket propulsion andguiEp missiles. In 1955, he was loaned to the U. S. Air Force ballistic miss'l program, where he initiated the Thor and Reconnaissance Satellite progra® After retirement in 1959, he worked for private industry for nine ’-■'r returning to the government as a civilian in 1968. He is presently Assistar Project Manager for Development, Surface Effect Ship Project Offic (PM-17).