Like most noteworthy inventions, the KaMeWa water jet is relatively simple. To understand its basic operation you need only grab a garden hose and turn it on. As flow increases, so does force in the opposite direction. Turn the nozzle and you feel the direction of the force change. That's almost all there is to it, but like all noteworthy inventions, the water-jet pump is a bit more complex as you get into it. 

The principal difference between the KaMeWa and your garden hose (other than scale) is that the former is a pump and the latter is simply an outlet. In both, the force of the jet stream is dependent on how much water passes through the nozzle per unit of time. In a boat with a jet pump, that has two implications: First, the exit nozzle should be above the waterline so that water can get out of the pump as easily as possible. 

Second, since the ease with which water enters the pump is as critical as its ease of exit, the passage from the hull bottom to the impeller - the inlet tube - is critical. It must be a specific shape and size for the pump to operate efficiently. Furthermore, pump efficiency also depends on harnessing the force of the boat through the water - the velocity head. That's why builders like Denison spend so much time sculpting the inlet tube. 

INSIDE THE PUMP

The KaMeWa is an axial-flow pump with two principal pump components. One is the impeller, which is driven by the engine, and the other is the stator, which is a series of stationary vanes on the pump's interior walls. When velocity is split or vectored, force is reduced, so the stator changes the spiral motion of the water back into lateral motion to increase thrust.

For steerage, KaMeWa uses a nozzle that can be pivoted 30° to either side by hydraulic servos. Nozzles can be operated either independently or in unison, by either a wheel, individually pivoting single-lever controls, or joy sticks.

As long as the engine runs, the pump pumps; that is, the transmission is nearly always engaged in forward and exists primarily to effect reduction; it has no reverse gear. In order to change the direction of the boat, the aft end of the pump contains a plate, or "bucket", which intersects the water stream mid-way, directing part of the flow forward and part aft, splitting and thus negating thrust. As a result, a jet boat in neutral produces a great deal of bubbling and sloshing water from her aft sections but, strangely, goes nowhere.

One advantage of this system is that forward or reverse can be engaged at virtually any speed without damage to any power components. That means outstanding maneuverability and an exceptional emergency stop, but probably a lot of bruised heads and broken crystal.

The KaMeWa pump is mounted so that the static waterline is at the level of the impeller shaft, meaning the pump is half full of water at rest, which allows quick priming. In addition, this system allows the pump to be removed easily for service without dry docking.

EFFICIENCY

The jet-pump system offers outstanding maneuverability, thanks to individually steerable nozzles, shallow draft, low vibration and noise, and increased speed. The advantage in speed is a function of efficiency, but efficiency is in the eye of the beholder. The jet drive actually is less efficient at lower speeds, until the pump can take advantage of the velocity head. Yet even at top speed, in overall efficiency - that is, the ability to move a volume of water at a given speed - the propeller is likely more efficient than a jet pump, although only marginally. Joe Langlois, chief designer at Denison Marine, claims the propeller is about 1% more efficient in moving water at top speed than the pump, but when you factor in the effect of reduced underwater drag - no props, shafts, or rudders - the jet drive has the advantage - about 9% on average. Furthermore, the jet drive allows the engines to be placed farther aft than the inboard, which further enhances planing-hull performance.

There is one major implication to be drawn from Langlois's conclusion: Since the propeller ultimately is a more efficient means of moving water, and the pump's main advantage is in elimination of the drag inherent in underwater appendages, the ultimate drive system is theoretically one that combines both - say, the Arneson or Kaama systems. 

This review/article originally appeared in Boating Magazine, August 1986 and is written by Richard Thiel. For more great powerboat reviews, visit their website and subscribe at: http://subscriptions.boatingmag.com/