jet engine

[BrianJ]

Tell me why we cannot use jet boat engines in our yachts... oops no I don’t mean a Boeing 747...I mean the type they use in tourist destinations “to sightsee and give their passengers a thrill and give them a bath at the same time.”

Now that would do away with shafts and props.

Oh I don’t want the speed that the jet boats travel at, but the manourevability must be good and surely you could use a low revving diesel to give you the economy.

Is there a good reason?
Brian J


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[MainlySteam]

Brian, three main reasons: they do not like heavy boats, they do not like displacement mode and they don't like slow speeds (below say 25 knots). Not to say they cannot be used in such cases, but usually only when some particular advantage of the jets is important eg draft, no appendages, etc.

There are quite a lot of good internet sites around giving explanations - just one of many is <A target="_blank" HREF=http://www.ultradynamics.com/sections/ultrajet/considerations.asp?section=3-b>http://www.ultradynamics.com/sections/ultrajet/considerations.asp?section=3-b</A>. I do not know how static that address is, but the part up to the .com will get you there in any event.

Also, unless one has multiple jets with plenty of power, they are not as manoeuvrable at slow speed as many views imagine. I am project managing the design and construction of two 75 foot water jet powered catamarans at present - they each have 4 engines and waterjets and so will do all the things like back up into strong winds, sidle into docks, etc at low speeds.

I have a fin keeled, heavy displacement cruising yacht and, as you infer, it would be wonderful to be able to do all of those things in the marina. Probably result in a lot of spilt pink gins and eye bulging from the gawkers though.

John

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[ccscott49]

You could, but the jet unit takes up a lot of room inside the boat and the debris thing comes into play, with so much floating crap around, plus the wash. Thats the only reasons I can think of, especially the room issue. By the way, jet engines have been used in boats, big ones anyway, like the brave class patrol boats, anmd a lot of the highspeed catamarans, have gas turbines, driving jets. The patrol boats had props, then when they became inefficient, went over to thrust, they used to be a great sight, overtaking, yes thats what I meant, overtaking, the cowes-torquay boats, when used as guard boats. Obsolete now I think. They would make a great conversion, if you could afford the fuel and repairs!!

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[qsiv]

As a cadet I went to sea several time with the CCF on the Brave's.

They were as you say turbine powered, driving props through V drive reduction boxes - the exhaust was directly through the transom. They never actually used 'jet' thrust as far as I was aware. The outer shafts also had smaller diesels on them mounted forward of the V drive, and were engaged via ordinary gearboxes for harbour manouevring. Special care had to be taken in harbours, particularly when going astern as the exhausts (about 30" diameter), had to have 'dustbin lid' covers fitted, otherwise water flooded back up them causing no end of problems.

Even allowing for the fact I was only a sprog, they were mighty impressive. I remember a quick dash to Alderney, then back to Portland for lunch. Oh, and we had breakfast in the mess ashore before we left, and shot at some targets on the range in between. Of course all that was a lifetime ago, when the RN still had enough money to afford fuel and sea time for it's ships.

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[ccscott49]

Yes, fabulous boats, but they were capable of using the thrust, but only in combat type situations, they even experimented with after burners!! Can you imagine! Fry every boat within 1/2 mile!! I know they didn't go ahead with that! Thank goodness.

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[BrianJ]

Bloody good answers from everyone THANKS..I'll stick to Perkins I think..
Brian J

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[Rob_Webb]

If I remember rightly Larry Ellison's superyacht 'Katana' has a third engine fitted which is basically an F16 jet turbine. When I was spectaing Oracle losing to Alinghi in the final of the LV Cup, Larry was aboard Katana (having been thrown off his own Oracle by stroppy Dickson) and when it became apparent that Oracle had lost he headed back into harbour in a high speed sulk with the F16 engine at full tilt - the speed and wash of the vessel was awesome - much of it seemed to be thurst wash caused by the power of the turbine?

Hate to think what the £/hour running cost is at full tilt, must be an expensive day out however you look at it!



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