For high-performance yachts, there is no question in my mind that waterjet propulsion systems hold many advantages over propellers," says naval architect Jon Bannenberg about the merits of jet propulsion for boats. "The absence of underwater gear is clearly a huge advantage, especially when operating in shallow waters, as is the lack of vibration so common to normally propelled vessels, especially when going astern. Having run a 157-foot waterjet for some time now, I see no disadvantages and find maneuverability no problem at all .. . At present I have seven waterjets under construction and I hope to see many more in the future.''

And I agree. They can turn on a dime and accelerate like a porpoise. I know - I tried one back in the early '70s on Biscayne Bay when I accepted an invitation from Jack Laskey of Stewart &. Stevenson to test one of their new Dieseljet propulsion units in a 23-foot Sea Craft that could operate in 17 inches of water. We were able to get 29.5 mph from the 150-hp Detroit Diesel engine.

The idea of waterjet propulsion has been around since long before Stewart & Stevenson put their unit on the market. But the popularity of jets has recently taken off, launched not only from the drawing board of Jon Bannenberg, but also from such yards as Denison, Lydia, Pacifica, Oceanfast, Palmer Johnson and Derecktor.

The driving principles behind today's areo-jet engines and marine waterjets are practically the same - except one sucks in air and the other water. Stripped down to essentials, thrust is generated by adding momentum to water coming in the front door and accelerating it out the back door. The front door is a grid-inlet duct in the bottom plating of a boat's hull; water is sucked in through a tube by a heavy duty stainless steel impeller in an inboard pump mounted at the transom, and the water pressure is then increased. This increased pressure is converted into high velocity as the water passes through an outlet nozzle and is ejected astern.

The forward thrust depends on the rate of flow of the water. To understand the effect, think of a water hose: If you're holding it loosely in your hand and the water is suddenly turned on, the hose end will leap in the opposite direction to the flow of water. The more forcefully the water flows, the more energy it imparts to the hose. Now stick the hose under water and do the same thing. There's much less effect on the hose because water flowing out into water generates less thrust than water flowing out into air. This explains why jets function best up on plane, above the waterline.

The chief advantage of waterjet propulsion is the absence of protruding parts below the hull. Props are unnecessary, and so are rudders. Steering is accomplished by swiveling the nozzle directing the jet (30 degrees each way). To back up, a portion of the jet stream can be intercepted by a "reversing bucket" that deflects it forward rather than aft. A stepless control (usually hydraulic and/or electronic) of the reversing bucket permits variable speeds ahead or astern.

Jet propulsion in high speed planing hulls was originated by C.W.F. Hamilton in New Zealand around 1960. Its popularity spread to the U.S. during the Vietnam War era when the U.S. military used a Jacuzzi waterjet coupled to Detroit diesels in hundreds of small coastal and river patrol boats. It was not an efficient marriage and was plagued with problems. But the project did advance the state of the art of jet propulsion enormously and also helped popularize it when it arrived on the U.S. pleasure boat scene in the early 1970s.

This popularity was short lived however and the concept almost died until KaMeWa, a Swedish firm specializing in fixed and controllable-pitch propellers, developed a successful waterjet system in the mid to late '60s. Today KaMeWa water jets in the 250 to 14,000 horsepower range are powering several hundred yachts and commercial vessels throughout the world.

Proof of the acceptance of KaMeWa jet propulsion is best illustrated by the fact that over 86 large yachts or commercial craft with KaMeWas have been launched since the introduction of the system in 1981. Last year alone 18 of these jets were installed in high-performance megayachts.

One of the first of these KaMeWa installations was on the King of Spain's 98-fool Fortuna. Utilizing KaMeWa waterjets driven by 12-cylinder, 1330-hp MTU wing diesel engines, this exotic megayacht achieves a 24-knot speed. Then, when you kick in the centerline straight-thruster waterjet/gas turbine combination, Fortuna jumps up to 52 knots.

In 1983, the 150-foot Shergar was launched with a single centerline KaMeWa waterjet driven by twin Allison gas turbines delivering a total shaft horsepower of 14,000 for a speed of 45.5 knots, making this waterjet propulsion unit the largest and most powerful in the world. Twin KaMeWa wing jets, each with MTU 1500-hp diesels add another nine knots to Shergar's speed. They also provide steering and reversing.

KaMeWa waterjets are an excellent alternative to conventional propulsion on medium or high-speed yachts, particularly those that have to operate in shallow-water situations or in waters contaminated with floating debris. Their assets are excellent maneuverability, good acceleration and deceleration, lower noise and vibration levels, fuel savings (nine to ten percent is claimed) over conventional propeller systems, especially above the 20-knot speed range.

ON AND OFF WITH EASE

Engine overloading is practically impossible, thereby reducing engine wear and maintenance. Another advantage is that these jets are designed and constructed so they can be dismounted and remounted from outside the yacht without any drydocking.

In the U.S., Denison Marine in Dania, Fla. has been in the forefront of marine jet propulsion. Joe Langlois, Denison's naval architect, has accumulated a wealth of practical engineering knowledge on waterjet yachts. Langlois has designed all four of the 100-foot-plus Denison yachts. Looking them over from a perch in the huge boatyard shed, Langlois called them his kamikaze fleet. The slowest should clock over 38 knots and the fastest about 50-plus.

Last year Denison launched the sleek, 100-foot For Your Eyes Only, the first high-performance megayacht constructed in the U.S. to have KaMeWa waterjet drives. Owner John Staluppi, a former powerboat racing champion who loves speed and high performance from his boats, has high praise for its performance.

Powered with a pair of 1960-bhp MTU 12V396 TB93 diesels coupled to KaMeWa jets, lhe $3.2 million yacht was clocked at over 35 knots and cruises at 30 knots.

All four of the big Denisons utilize the MTU/KaMeWa combination. The next to be launched (Oct.- Nov.) will be Thunderbolt, a 102-footer with an expected speed in excess of 50 knots from her twin 3500-bhp MTUs.

KaMeWa water jets can be operated in either of two modes, separate or together. In separate settings each jet can be operated independently of the other. This individual vectoring of thrust allows for movement and control unequalled by yachts with flxed propller shaft lines. When the jets are operated together, they operate and give control the same as any outdrive or surface drive units.

Acceleration on For Your Eyes Only from stop to maximum is ten seconds then back to stop in 13 seconds; it can also lean into turns at full speed. "With kidnappings, terrorism and piracy a way of life today, the ability of high priced megayachts to quickly maneuver and make a fast getaway is a must in today's world," says Joe Langlois.

The problem of marine growth on jets is minimal: A study by the University of Miami Marine Laboratory indicated that "as the aluminum inlet waterjet tube and its components are not exposed to sunlight, and if the jets are used regularly, any marine growth will be negligible and will be quickly removed by the high velocity of the water stream passing through the tube.'' The reduction gearboxes presently being used have reverse capability for the back flushing of both the jet units and the inlet tubes, as well.

Langlois also points out that the thick stainless steel impellers in the pump are capable of grinding up any coconuts or other small water debris and then spitting them out the discharge end without any damage to the pump or its impeller blades.

DONUTS AND FIGURE EIGHTS

Parts VI, a 153-foot Oceanfast 4000, was christened for Gary Blonder of Blonder Marine and chartered by the commodore of the Royal Perth Yacht Club for use as the committee boat for the America's Cup races in Perth. "I have owned some very fast yachts, but the overall speed and performance of Parts VI resulting from her KaMeWa jets is astounding. The ability of a 153-foot yacht to do figure eights and donuts in her own wake (and her maneuverability in close quarters), is only accentuated by her silence and lack of vibration," says Blonder. Parts VI is powered by three MTU turbo diesels driving KaMeWa waterjets pumping 30,000 gallons per minute. Her fuel consumption is 250 gallons per hour at an amazing 30 knots. Blonder goes on to say that, "this futuristic vessel is the next evolution of the motoryacht. Parts VI is so far beyond conventional vessels we have already begun planning a larger version."

Another waterjet yacht about to be launched is Pacifica's new 70-foot sportfisherman with KaMeWas and 12V331 MTUs. Speed, maneuverability and shallow draft are the reasons Pacifica's Bob Gunderson gives for what is essentially an experiment - no other sportfisherman has ever relied solely on jets. A big question mark is how the waterjets will affect trolling. Will fish be attracted or repelled by the bubbles? Stay tuned.

For smaller boats, two new low-horsepower jet units are available. The I-Jeta waterjet is made of plastic reinforced with glass beads. These jets were designed with fun in mind and for inboard engines in the 40- to 70-hp range (gas or diesel). P.P. waterjets from England can be coupled to larger powerplants - up to 300 horsepower. Perkins, Volvo and Fiat engines have all made happy marriages with P.P. jets. They're made of fiberglass and accommodate a wide variety of shaft angles and engine positions. Whatever your hull's configuration and whatever angle of thrust is best suited to your boat, these little lightweight substitutes for props can make you jet-propelled if the concept turns you on.