Water Jet Propulsion

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Water jets are good in some cases. If you are planning starting s route some 100nm in 30knots this is a good idea. The efficiency is better at high speeds.
Maintenance cost (Know Kamewa owned by RR now) is low.

Thrust is dependent on speed and there are no slip issues.

That means that engine torque is never higher than propeller curve torque unless accelerating against mass inertia. That means that engine can be full load optimised (like steyr) and have a better fuel consumption.

So fore us users this means engine must run at maximum speed to get to plane.

Disadvantage is less maximum thrust and poor part load performance.
 
jets---love'em!

I skipper a 50ft aluminium fast catamaran workboat with twin jets and we work out in the open sea in close quarters situations with various very hard/heavy bits of kit. (I used to have a proper profession, this is my late life crisis after early retirement- I love it and they pay me to do it-bliss!!)
Amazing manoeuvrability sideways backwards forwards and stopping. Wouldnt want props. The hamilton jets have never had a problem over 6k hours. Takes some getting used to after a prop boat but once "the penny has dropped" it is awesome. We need to dock onto various things and the use of the "buckets" instead of a gearbox is easier on the mechanicals and the skipper. Trouble is when I go back to my prop boat for leisure-it all goes wrong in reverse
 
I once got to test drive a Norwegian patrol boat with twin RR water jets after servicing it, it was a 32ft aluminium hull and did 42 knts with twin Penta 74 motors. The only problem was when you tried to turn the hull heeled and the outboard jetpump stalled due to air ingress and you lost 18knts in the turn, so to turn you didn't touch the wheel but used the interceptor style tabs which caused it to turn without heeling, it was weird turning totally flat with no heel and even more weird as there was no selfcentering action, you had to lift the tab to stop the turn otherwise it would just continue skidding round in a flat turn.
But as others have said the close quarters manouvering was fantastic.
 
Thanks for all your replies. I'am not an engineer, but technically minded and looking to learn more about water jet propulsion. It would be nice not to have to worry propellers, shafts, cutlass bearings and P brackets, it would also solve the ''lobster pot'' problem. What is stopping main boat manufacturers from bringing water jet propulsion to say <40ft boats.
David
 
BlackMagic said:
I skipper a 50ft aluminium fast catamaran workboat with twin jets and we work out in the open sea in close quarters situations with various very hard/heavy bits of kit. (I used to have a proper profession, this is my late life crisis after early retirement- I love it and they pay me to do it-bliss!!)
Amazing manoeuvrability sideways backwards forwards and stopping. Wouldnt want props. The hamilton jets have never had a problem over 6k hours. Takes some getting used to after a prop boat but once "the penny has dropped" it is awesome. We need to dock onto various things and the use of the "buckets" instead of a gearbox is easier on the mechanicals and the skipper. Trouble is when I go back to my prop boat for leisure-it all goes wrong in reverse
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I'm guessing you are probably involved in working on the offshore turbines?

One incident occurred a few years ago when a vessel was up against a pile, probably outside of the ideal working limits, that had a potentially serious incident when a large swell rolled through and the jets lost a lot of their water intake and therefore thrust just as some one was about to step off on to the ladder. I believe he had a very lucky escape as obviously the boat shot forwards again as soon as the thrust came back.

I've only used them in small (9m) ribs and they're OK. Lines, etc. not really a problem - plastic bags are on a boat that size - not sure about bigger ones. Also the problem of always having thrust present can be a pain.

W.
 
David of Essex said:
Thanks for all your replies. I'am not an engineer, but technically minded and looking to learn more about water jet propulsion. It would be nice not to have to worry propellers, shafts, cutlass bearings and P brackets, it would also solve the ''lobster pot'' problem. What is stopping main boat manufacturers from bringing water jet propulsion to say <40ft boats.
David
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vested interests mainly. Jets have been around for more than 40 years but have never really caught on except for operating in shallow waters.

However, nobody makes a range of engine/jet packages. Builders want something they can just drop in from a well established manufacturer, and as you know the smaller engine market in this class of boat is dominated by just two - and neither of them has shown any interest in jets.
 
My title is the name of my boat which is a 9.2m "Surfrider" with Cat 350hp inboard driving a 274 Hamilton water jet.

I do jet boat thrill rides out of my home marina in Spain.

As other have said brilliant to manourve even a single, can do great tricks on the ride like stoppees from 25 knts in a couple of boat lengths, and 360 stop turns using helm at same time as reverse thrust.

The advantages of no drives or props to cause drag is great, plus if I am towing a donut, I can get the folks on and off from low the bathing platform secure in the knowledge theres no danger from a prop. I love jet drives, Oh one more thing theres only one sea cock naturally for the engine, no bellows or stern glands, so minimum risk of problems with water ingress from holes in the hull.
 
I seem to recall thet Storebro did a short run of a flybridge with jet drives. Believe that they found that the windage combined with "lack of grip" in the water (no rudders or what drives gives as control) meant that the side drift when on the plane was very high indeed.
Great performance, but not ideal for traditional flybridges. Should be OK on a cat though with twin hulls gripping the water.

Also heard that on lightly used boats that the jet tunnel could get fouled, and as such affect propulsion performance and at worst damage impellor. Hence taken up by commercial and heavy usage, such as channel crossing ferries...
 
working on the offshore turbines
yep of course.
Never had a problem with losing thrust on turbines but I have seen some major damage to prop vessels lack of manoueverabilty. I wouldnt want anything else but each to their own of course.
 
David of Essex said:
. . . it would also solve the ''lobster pot'' problem.
Click to expand...

Perhaps lessen it, but not completely solve it. We sucked up some loose rope (a couple of feet) and were reduced to 500rpm and 5kts. Spent the night on the slipway waiting for the 2:30am tide to float her back onto the trailer!

That said, I think jet drives are great. Are the UK boat buying public too conservative for them?
 
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