The US Navy's daring new ship: Will six be enough?

The ships are on probation. Sitting pierside, each looks more like a snowplow than a warship. Their steel hydrofoils glisten in the sun against the blue water of south Florida.

Ounce for ounce, they are the most expensive vessels in the whole US Navy. Only six were built, and the production line was shut down as soon as they were finished. Key officers were skeptical that there would be a place in the fleet for them, even though the Russians have had similar ships for years.

But when ''airborne'' at more than 50 knots , these awkward-looking vessels represent the Navy's latest attempt to wage guerrilla warfare at sea. They may be the most innovative ships in the fleet.

Their small size and high speed allow them to move as clandestinely on the surface of the water at night as nuclear submarines do beneath it. They represent a tactical 180-degree turn from the huge aircraft carriers with their escorts that have dominated the surface Navy's shipbuilding since World War II.

But still to be decided is whether this first Patrol Hydrofoil Missile (PHM) squadron, based here at Key West, will also be the last. Before the Navy commits any more money to them, it wants to know what they can do.

Dubbed ''Pegasus class'' (the name comes from the flying horse of Greek mythology), the PHMs are built for antisubmarine warfare, minesweeping, and support of carrier battle groups.

When used in ''limited'' waters, such as the Caribbean and the Mediterranean, they are effective platforms against other, larger warships.

''We carry as big a punch'' says Capt. Frank Horn, squadron commander, referring to the eight Harpoon antiship missiles on board.

''They [the PHM vessels] can be a better use of resources,'' enthuses retired Capt. Gil Slonim, now president of the Oceanic Education Foundation in Falls Church, Va. ''Should the Navy assign a 3,000- to 10,000-ton ship costing $500 million plus to carry out a task which could be accomplished by a 250-ton ship costing $100 million and with far fewer people? In that context, there is a place for hydrofoils.

''You can't just think single-purpose ships. You have to think mission, the total fleet mission of controlling the seas and projecting power overseas for our island nation,'' Mr. Slonim continues. ''Hydrofoils point the Navy toward 21 st century technology.''

One advantage that the Navy - and such proponents as US Sen. Gary Hart and Adm. Stansfield Turner, former director of the CIA - has emphasized is that PHMs don't require a lot of men to run them. With a crew of only 23 men per ship (plus a mobile logistics support group) total squadron personnel are fewer than those required to man a single destroyer.

The crew of 19 enlisted men and four officers have an endurance time of five days at sea and two days in port. Under special conditions, this cycle can be extended to a longer at-sea patrol.

Even on the shorter patrols, however, the PHM crewmen have the sympathies of sailors aboard more conventional ships. Living quarters are cramped. A king-size mattress would not fit inside the captain's suite, the only single-occupant room on board. Officers and crew share the same mess.

In the engine room, tall sailors ought to qualify for hazardous-duty pay. Its compactness rivals a Volkswagen for red-knuckle mechanics.

But they are fast.

''We can reach out anywhere in the Caribbean,'' says Captain Horn. ''We can 'fly' in a 750-mile radius at 40-plus knots.

These vessels are so intimidating in action that when the Navy uses them to assist the Coast Guard in the offshore drug war, crewmen of suspected narcotics-carrying vessels ''think it's the end of the world when we come flying up to them,'' the captain chuckles.

In fleet operations, the Reagan administration's show of power off the coast of Nicaragua offers an ideal situation in which to test the PHMs. One Navy spokesman says of the Central American mission: ''There is a learning curve going on down there now on just how hydrofoils can support a major action. We will be looking very closely at the results.''

The Navy is testing the PHMs individually and in squadron and fleet maneuvers. How to fire at targets at various speeds, from various positions, and in single-ship or group attacks is still being determined.

With their low radar profile, PHMs are hard to detect. They show up just like a fishing boat. This enables them, according to Captain Horn, to ''perform special operations, like the fast transport of light-armed personnel.''

The Navy will not comment on whether its top-secret special forces, called SEALs, are participating in the Caribbean maneuvers. But, translated from the military lexicon, SEALs are what ''light-armed personnel'' most often refers to.

(SEALs are trained for reconnaissance missions like those carried out by members of the Royal Navy's Special Boat Squadron, who landed on South Georgia Island days before it was recaptured by the British in the Falkland Islands war. The first American military adviser to die in the line of duty in El Salvador was a Navy SEAL.)

Hydrofoils were conceived as part of a proposal for Italian, West German, and US participation in NATO. But it took more than 12 years for construction of the six-ship US squadron. West Germany dropped out along the way, when the Carter administration sought to discontinue funding. Congress overode the Carter cutbacks.

Three hydrofoils cost the same as one of the next-least-expensive combat vessels - frigates - that the Navy has on order.

Still, the entire production line - the PHMs were built by Boeing Marine Systems in Seattle - has been closed pending the Navy's decision to order more. In many ways the hydrofoil represents what critics of US military procurement denounce as the on-again, off-again cycles that lead to the cost overuns that have nagged ship construction. Should the Navy decide to order more PHMs, start-up costs will be duplicated.

The Navy counters by saying until it knows what the new ships can do - and how they can be integrated into the fleet - it would be playing loose with taxpayer money to order any more.

Each PHM weighs 245 tons, is 131.2 feet long, and has a beam of 28.2 feet. Diesel engines push two streams of water out the rear when the hull is lying in the water. A single waterjet driven by a gas turbine engine (the same engine as on a DC-10 airplane) takes over when foilborne.

In addition to the eight ship-to-ship missiles (with a range of more than 60 miles) that can be nuclear-tipped, PHMs carry a 76-mm rapid-fire gun and a system that shoots out strips of metal and aluminum as a decoy for incoming missiles.

Proponents say this firepower provides ''more bang for the buck'' when used in the closed waters of the Caribbean and the Mediterranean and can secure narrow choke points that major ships of the fleet must steam through, like the Straits of Malacca or Gibraltar and the Panama and Suez Canals.

Dead in the water, a hydrofoil looks like any other boat, except for the ''snowplows'' attached to the bow and stern. But once the foils are lowered, like the landing gear on an airplane, their aeronautical design forces the boat up out of the water as speed increases. The ship begins to ''climb,'' and rises slowly several feet out of the water. Soon only the bottom struts have contact with the water.

So when Lt. Comdr. C.E. Weiscopf, skipper of PHM Aries, talks about going airborne, he means just that. ''Cockpit, . . . foilborne, . . . banking . . .'': His descriptions sound more like the talk in a ready room for carrier pilots.

The ride is smooth enough to make landlubbers feel comfortable. When airborne , there are no waves to buck, no rock and sway.

''When we first trained on them, we had to learn to maneuver the same as an airplane,'' says Commander Weiscopf.

Officially the Navy maintains that the speed of the PHMs is ''40-plus knots.'' But on the bridge of the USS Ticonderoga, a conventional Navy missile cruiser, during an exercise in the Florida Straits with three hydrofoils, this reporter saw one PHM being tracked well in excess of 40 knots on the radar screen.

For all its speed, however, there is one area of concern about the hyrdofoil that is still to be tested: The exhaust from its jet engine may make it as vulnerable to attack from a heat-seeking missile as are aircraft.

According to Navy sources, the Soviet Union has transferred two of its hydrofoils every January for the past four years to Cuba. The Turya-class vessels are thought to be slightly slower than the PHMs and are not equipped with missiles. But their very presence, admits Captain Horn ''is one of the major reasons we're based here in Key West.''

The Soviet Navy has the world's largest hydrofoil fleet, probably because the number of closed seas bordering the USSR warrant it. And, according to the authoritative Jane's Surface Skimmers, it plans to build more than 200 by the end of the next decade.

Admiral of the Fleet Sergei Gorshkov, longtime commander-in-chief of the Soviet Navy , wrote in ''Sea Power of the State'' that ''Scientific and technical progress has provided a real opportunity to produce fundamentally new surface ships with a number of tactical features which cannot be achieved in displacement ships.''

Will there be more PHMs - or other ships like them?

It's still too early to tell. But if the guerrilla warfare in Central America should spill over into the surrounding waters, or the USSR continues to feed hydrofoils to the Cubans, the US Navy will need a ''ship in place'' to help stabilize the Caribbean.

That ship, PHM proponents say, is already down there - and the more of them, the better.