Jet Boat Review

Author: Barry Tranter

Jet Boats - Boat Review by "Modern Boating" October 2009.

"Jet Boats - The Big Squirt"

For many people, waterjet boats are exemplified by those tourist boats that execute crash stops and high-speed spins in order to make their passengers scream. Or PWCs running in the surf, where their speed and maneuverability and lack of a prop make them ideal for their task. Or on TV where those V8-powered skiffs navigate tight channels at ridiculous speeds, and sometimes dump their passengers on the bank.

This noisy, spray-ridden public face of jet boating is misleading. 

Recreational waterjet boats have number of advantages.

Safety is a huge plus, particularly in the smaller family runabouts, which operate around swimmers. “Women, especially, don’t like props,” says Jeremy Bycroft of Australian Marine Leisure. “They worry about the sharp bladed prop even when the boat is at rest.” Water-skiers, wake boarders and swimmers can breathe easy when they are towed behind a jet.

There are other advantages. Most production boats draw only 300mm* of water. (see*below)

If in-water obstacles are encountered—ropes, pots, logs—the jet nozzle tends to ride up and over, rather than snag.

Drive unit manufacturers claim that servicing the drive unit can be as simple as keeping an eye on bearing oil levels and watching for anode depletion. One claims that the owner can perform basic service on a beach, or anywhere with a bit of tidal range.

How do jets boat engines work?

Water is drawn into the jet unit through an aperture in the hull bottom, where an engine-driven impeller accelerates it and squirts the water out of the jet nozzle. Steering is controlled by directing the nozzle. Reverse and neutral are affected by the deflector, which lowers over the nozzle. For reverse the deflector lowers all the way and directs the water thrust forward; for neutral it directs the thrust downwards. So for some maneuvers’ you can set the throttle and forget it, controlling speed purely by deflector position.

As with a conventional drive system, a jet unit has to be matched to hull and engine. The boat’s operating speed is a critical factor as speed through the water affects the rate at which water enters the jet orifice, which is known as ram effect. Cavitation** is a factor, as is the case with conventional propellers.

Manufacturers make the point that the power-to-weight ratio is a vital statistic. “The ability of a waterjet to absorb engine power throughout the speed range is more important than matching to engine power alone,” Jet unit suppliers would rather work with a new hull than an older one, as old hulls can be weakened by age and may need a lot of work just to bring them up to scratch. Matching engine and drive unit to a hull is a job for a naval architect.

*The "bJet" has a draft of just 150mm.

**the rapid formation and collapse of bubbles in a liquid, caused by the movement of something in the liquid such as a propeller, or by waves of high-frequency sound and the pitting of a solid surface as a result of the forces of repeated Cavitation in a surrounding liquid.