Jet power is an element of the power driven world that is subjected to a wholly undeserved degree of prejudice. You often see ‘jetskis’ (more accurately known as personal watercraft or ‘PWs’) outlawed from operation on a waterway, a marina, a launch site or even a stretch of coastline - and yet the water jet has a heritage, a modern relevance and a spread of merits for the leisure boater that few other forms of propulsion can match.
What is jet power all about?
Jet power is essentially about sucking large quantities of a substance through an intake and squeezing it back out through a smaller outlet under very high pressure. That is what provides the thrust, and while a jet aircraft uses air, a jet-propelled boat uses water. It is a very simple concept and by no means a new one. In fact, the idea has been around in one form or another since the 17th century, and has been recorded in use on boats as early as the 1770s. It might have taken until the 1950s before Kiwi, Bill Hamilton, really refined the system but the modern jet is now a common phenomenon, not just in marine recreation but also in tour boats, military craft and commercial applications.
So what makes a jet so useful?
One of the main benefits of a jet-powered craft is that it has no substantial working parts beneath the planing surface, making it far better suited to shallow water running than a craft with a fragile and exposed propeller. Not only does this mean a jet is safer for people in the water (which is why it is so commonly used for rescue operations), but it can also bring otherwise restricted boating regions within realistic navigational reach.
Another major beneﬁt of jets is the fact that the engine tends to have a much easier life. On a prop-driven craft, going astern means dropping the revs while the appropriate gear is selected, causing both the gears and the engine to do a great deal of work. But a jet uses a ‘bucket’ (basically a thrust deflector) over the jet outlet (or nozzle), so it has no need of a gearbox. Instead, going astern simply involves lowering the bucket over the nozzle to redirect the ﬂow of water forward and propel the craft in a backward direction. There’s no gearbox, no fuss and no frantic loading and unloading of the engine.
But perhaps the greatest practical benefit of the jet system is the manoeuvrability it brings. Directional control is achieved not by a fin or rudder dipped in the water flow but by the angling of a plate in the end of the jet nozzle itself. It means that in order to maintain steerage, you need to use positive adjustments with far more throttle engagement than with a propeller, but it also means that (with practice) you can make tremendously sensitive adjustments, spinning on the spot or, with twin jets, even crabbing sideways.
A massive fun factor
Practicalities aside, a jet boat is also one of the most enjoyable craft you can drive - not least because you can go from full ahead to full astern at maximum revs without fear of destroying your gearbox. Known as a ‘crash stop’, it enables a jet boat to come to a complete standstill from 30 or 40 knots in no more than its own boat length. It’s faintly unnecessary of course but it’s also fantastic entertainment.
And no less enjoyable is the ferocious handling ability of a jet boat. Not only can you pull a simple 360 on the spot from a standstill, but you can also do pretty much the same thing at pace, spinning the craft 180 and facing your own wake in a great flurry of spray and adrenaline, before hooking up and flying off again in search of your next bit of entertainment.
It is true to say that a jet-driven craft must preserve proper contact with the surface at all times, otherwise both drive and steering will be lost - and you can often hear that on a lumpy sea, when the aft section of the hull is separated from the surface, creating a shrill whine, followed by a chopping rasp as it drops and the jet hooks up once again. But the same is true (though to a lesser extent) with a conventional prop. It protrudes some distance beneath the hull and so limits the period of impotence, but driven well, there is little real world difference between the two forms of propulsion - even on a lumpy sea.
What about the negatives?
No system is perfect and the same is true of jet propulsion. In the best part of 20 years on boats, the bulk of my breakdowns at sea have been on jet driven craft where the intake filter has become clogged with debris or the impeller has sucked up an unforgiving procession of grit and stones. Fixing such issues is rarely a big deal and can often be sorted in seconds, but it does reveal the importance of keeping a clean flow of water for the system to operate properly.
It is also true that a jet (which works indirectly by pushing water against water) is also inherently less efficient than a well-rigged prop, which generates its thrust by virtue of direct contact with its medium. But with lightweight boats and improved four-stroke and diesel engines, a more sensible approach to throttle application on the part of the driver can radically minimise this efficiency difference.
A combination of manoeuvrability, fun, safety and ease of use makes jet power a genuinely exciting means of propulsion. Not for nothing is it such a popular option for the discerning sports tender market, where the likes of Williams and Avon do tremendous business. There are of course those who consider any jet powered craft a little too brash and aggressive for their tastes but the prejudice is entirely unfair. With modern jet driven vessels ranging from tiny one-man personal watercraft to super-stylish tenders and enormously potent 10m+ RIBs, jet power is as varied in its application as it is effective. One thing is for certain. Take the time to drive a good jet boat and you will be keen to come back for more…
Alex Smith is an ex-Naval officer, with extensive experience as a marine journalist, boat tester and magazine editor. Having raced as a Pilot in the National Thundercat Series and as a Navigator in the inaugural Red Sea RIB Rally, he has now settled in the West Country, where he lives and works as a specialist marine writer and photographer from his narrowboat in Bath.