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Old 09-08-2009, 08:34 PM   #21
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Country: UK
Location: Poshville, Cheshire
Occupation: IT Manager
Interests: Making loud bangs, drinkin
Boat make: Fletcher
Engines: Mercruiser
Cruising area: Anywhere on North Coast

Join Date: Aug 2009
Location: Poshville, Cheshire
Posts: 7
Quote:
Originally Posted by FLYING FISH View Post
Summat for Capt Chaos to mull over when deciding the finer points of waterjet selection.

These are comparsion transmission curves drawn up for a variety of craft.

The vert axis is wt in lbs/bhp.Red are shaft drives,blue are surface drives,green are waterjets,black are outdrives.Waterjets were mainly used where the owner had a large yacht and need to go into shallow bays and stock surface drives were not available in such large sizes,but they were efficient for Patrol Boats and in one case were used by the Spanish Customs to chase after ex-Shead racers on the Morocco run.On the other extreme they were used on Roger Penske`s 50m Gas Turbine 40kt Motor Yacht.Then there was `The Jet` in 1974, a sister to `Unowot` with castoldis,before surface drives made a name in 1980.One can see from the curves that the dots can be scattered a bit,so it`s not all as predictable as you would like.
Hey Flying Fish, love your data. Unfortunately I didnt fit the jet to the engine it 'came out of the shop' like that. I really wanted to love the jets because at the time I did a lot of messing around the River Fal and yes it was great when the tide left sandbanks by the bucket (those faces hahahahaha) but I can't escape the fact that my loverly little Dowty couldn't go much faster. I agree totally that in the right environment that jet drives are amazing, its just my experiences of them aren't as good as I hoped for.

PS I Still have the jet drive unit in the front of my garden if anyone wants it - The seals may need replacing but hey :cheers
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Old 04-10-2015, 11:21 AM   #22
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Hi anyone know how often jetdrives have to be overhauled when used for normal offshore sea use please ?

How efficient are they compared to props at 20 to 25 knots please
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