Advance Propulsion Technology Demonstrator

Chris Brown dropped the throttles then spun the 40-foot monohull in a tight circle in the narrow channel. "It turns on a dime," said Brown, a naval architect with the Naval Surface Warfare's Carderock Division. "Even in rough water."

Formally known as the U.S. Navy's Advanced Propulsion Technology Demonstrator (APTD), the craft may some day carry teams of SEALs and other special forces into combat.

"The Department of Defense has mandated that all gasoline must be removed from Navy craft by 2010," Brown explained. "But the problem is that most Navy high-speed craft use stern drives that just can't handle the torque of a diesel-powered engine."

Enter Vectorworks, a Titusville, Fla.,-based manufacturer that has designed and built everything from high-speed passenger ferries to high-speed oil-rig support boats.

The prototype costal assault boat, which is designed to be dropped from a C-130 aircraft, can carry three crew members, two six-man SEAL tams and two inflatable "raiding craft" with outboard engines. The APTD measures 40-feet long and has two 440-horsepower Yanmar diesel engines and can hit 50 mph.

"The APTD has a fuel capacity of 350 gallon, and at a cruising speed of 40 miles per hour, the engines will burn about 60 gallons an hour," said William Kulenguski, the Vectorworks engineer who spearheaded the project. "That is very economical for what they are doing."

Vectorworks had to adhere to strict specification in designing the craft, which not only needed the capability to be air-dropped, but also to be picked up by a helicopter or crane and loaded back onto a mother ship.

Current Navy assault craft use gasoline-powered stern drives or gasoline-powered jet drives.

"If you are in a jet drive and you have to come along side another boat, or something happens and you have to maneuver, your natural reaction is to let off the power," he said. "But the problem is that if you do that in a jet drive you will lose your steering. And that isn't good."

The key to the APTD, which was seen by the public for the first time in June at the sixth annual Multi-Agency Craft Conference (MACC) at the Naval Amphibious Base Little Creek in Norfolk, Va., is its unique propulsion system.

The POWER-VENT system uses a surface-piercing propeller in a tunnel beneath the boat. The surface-piercing propeller, like those used on race boats, are necessary because a regular inboard propeller will begin to cavitate at speeds of 40 knots and higher.

Surface-piercing propellers are designed to have the top half of the propeller exposed so each blade entrains air as it re-enters the water. This results in positive pressure on the blade surface, whereas a conventional propeller is subject to cavitation because of the low pressure on the submerged blade.

Clockwise from top left: POWER-VENT uses propeller pockets and a plenum in the center to introduce air to the surface-piercing propellers. The plenum also evacuates exhaust into the prop wash. The prop shafts are fixed, so the driver doesn't have to mess with trim in combat situations. The system works well enough so that the boat can turn sharply - even in rough water. And you thought that you had full instrumentation.

POWER-VENT works because it feeds the right amount of air, in the right proportion, at the right time, to keep the propeller disc half submerged.

"It does this by creating an air plenum at the ransom of the boat that is connected to the propeller tunnel by a hole located just forward of the propeller," Kulenguski explained. "As the propeller turns and starts to pull water through the tunnel, air is sucked into the air plenum and hence into the propeller disc."

The air also enters the tunnel by itself, without the operator doing anything. And the vessel also can be operated at any speed, instead of having to get over "the hump" to trim out the boat.

"POWER-VENT" acts like a torque converter," Kulenguski said. "The propeller's inertia keeps it rotating when the boat reenters the water and filling the prop tunnel with water, and the rotating propeller immediately draws air from the plenum, since it is the path of least resistance. So POWER-VENT enables the propeller to automatically self-regulate the amount of air flow to keep the propeller half submerged."

Another advantage is that the exhaust is dumped into the air plenum, which in turn gets sucked into the propeller and gets mixed into the prop wash. This reduces the thermal and noise signature of the boat, two important benefits for military use.

The open transom also allows for divers to operate at the stern and the propeller configuration permits shallow-water operation.

POWER-VENT technology has been in use for several years in the recreational sector. The propulsion system is a key part of Don Smith's high-end custom powerboats built in Pompano Beach, Fla. An architect of POWER-VENT technology, Smith worked as a consultant on the APTD project.

Modeled after the Smith 42, the boat was tested in January and the Navy is currently evaluating whether more will be ordered. Price tag for a combat-ready craft: about $800,000. -Terry Tomalin