Water Jet propulsion for Yachts

[MarkG]

Does anyone know why we don't see ( well I don't ) any yachts in the 30 to 50 foot range using water jet propulsion?
Seems it would be ideal for the following reasons:
1 - Nothing to snag any warps or lobster pot lines.
2 - the mechanism is relatively simple.
3 - Can redirect output - so can make manouvering simpler.

I was looking at the Schottel system which someone mentioned on another post and other than weight (360 kg !!!) it looks like a winner.

Any comments

 

[Stingo]

Water jet propulsion systems still suck lines in and are incredibly difficult to clear.

 

[Bajansailor]

Here are a few more possible reasons :

Water jet propulsion systems are a lot more expensive than a shaft drive (or even a saildrive) system using the same power.

Water jets at 'slow' (ie less than 20+ knots!) speeds are much less efficient than shaft drives. Water jets become very competitive with shaft drives once speeds get above about 25 - 30 knots, as they then have much less frictional drag than a propeller with its shaft and P or A bracket.

Water jets are usually designed for an almost vertical transom, and they need to have clean undisturbed water flow to the intake grill - it is possible that having a keel in front of the intake could upset the flow of water into the jet.

Re the Schottel systems, the one that kicks up like an outboard engine is rather neat - but as you say they are rather heavy, and no doubt expensive as well!

PS - Just remembered that some of the 40' or 41' Apache catamarans built by Sailcraft in the late '70's (I think) were fitted with twin diesel inboard engines and water jet propulsion units, but I dont know how popular they were.

 

[sarabande]

Well ! Coincidence !
I was talking with Hamilton Jets UK this morning, and am awaiting details of their systems.

Their site has details of a system which provides propulsion and steering, and which looks very inviting.

http://www.hamjet.co.nz/index.cfm/blue_arrow/blue_arrow.html

 

[oldharry]

Also Castoldi jet systems. They do mostly seem to be in the high speed category - their largest production boat being 9.75m powered by twin 160hp diesels..... slightly different league to us Raggies?

 

[Jonny_H]

There was an article in YM a few months back about a company who was designing water jets to be used as bow / stern thrusters. This seems like a good idea, although must admit the complexity would put me off using them for a main engine.

 

[MarkG]

Thanks for all your responses, one learns something new every day...

 

[Woodlouse]

I only have experience with Castoldi and PP jets but from what I can see the part of the jet drive that sticks out the boat would cause fantastic drag. Far more than even a fixed propeller. This doesn't matter on a planing hull because when up to speed the jets are actually out of the water. Add that to whats already been said, about weight, complexity, and size it's not difficult to see why their not on yachts.

 

[TheBoatman]

For yachts I don't think this is a runner for various reasons.

Jet drives are used almost exclusively used on planning hulls.
They require an uninterrupted supply of water to work.
They require more HP to make them work efficiently.
On small boats the engine is always mounted directly to the unit in the transom.
The main steering is by changing the direction of the jet ~ means an outboard leg.

The RNLI tried them out on the Thames ILB's (E class I believe) ~ the crews hated them. Always getting something in the intake, and at speed if the boat jumped a wave the power disappeared.

I would have to ask the question ~ why would you want one?

Jet units are made to run on fast planning hulls - yer average yacht hull speed is nowhere near that of a planning hull.

However, for a bow thruster I could see an advantage if you can solve the problem of where you are going to get the intake fitted right up front in the pointy bit.

Peter.

 

[Major Catastrophe]

[ QUOTE ]

However, for a bow thruster I could see an advantage if you can solve the problem of where you are going to get the intake fitted right up front in the pointy bit.

[/ QUOTE ]

I was talking to a marine engineer about waterjet bowthrusters and he said that the force of the 'suck' took away some of the effect of the 'blow' so you would need opposing inlets for the port and starboard jet. In his opinion an over engineered solution to a simple problem.

As to water jets and ropes. With a prop you can usually clear a fouled rope, but a water jet will ingest it and knacker it.

I discovered this very expensive craft on its first day out after delivery. They discovered that the impeller can 'eat' an entire ski rope.