Anybody know about Waterjets?

Bertramdriver

Bertramdriver

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The concept of Waterjets fascinate me but I know little about them. What I'm interested in is at what stage does a Waterjet become more efficient than props. I've seen them on commercial cats and smaller working boats but I've often wondered why we don't see them on mobo's. any body out there know?
 
Propulsion is always an interesting subject, and I remember doing some writing and research into surface props.....

Anyway, I believe waterjets are equally as facinating, but does have some limitations.... some of which was pointed out to me a couple of weeks ago...

In respect to what stage they become efficient, is probably when overcoming the drag from P-bracket, Rudders etc., become high % of engine load, which I suspect is between the 18 - 25 knot range..

However, they will be designed (pump impellor size) will be maximised for cruising speeds, and apparently below this they are not as efficient ... so if you do a lot of pottering around at low speeds, they will not be for you..

I have also been led to believe that they are sensitive to fouling in the water intake ... Ie they therefore are reliant on relatively frequent use, which in the leisure boat market reduces their attractiveness..

Low draught, no props spinning, thrust in direction of travel (not angled downwards), constant engine load, manouverability etc., are of course some of the attractions...

I recall that Storebro once did some trials with waterjets on one of their range of J series boats (J52 I believe)... and found that whilst efficient, the fact that the boat did not have props etc., to grip the water, meant that they were suffering from more side drift (leeway) when underway... they later did a J34 which got some more following..
 
And I thought it was a Typhoon doing a flyover. Listen for mine 3rd sept when we get afloat again. If you sit still in a quiet place you might just hear the seismic rhythms travel through the earths crust. Or is it the fracking?
 
I know of a fishing boat rigged with a jet drive, it needs 3000 rpm, the Ford Sabre had to be opened up, and is normally connected to the engine with a carden shaft. This one had a PRM box, so the intake could be reverse flushed. This is a substantial FV, (Cygnus Cyclone 26, see google) the draft is less than a foot. Goes like s off the s.
 
Bertramdriver said:
The concept of Waterjets fascinate me but I know little about them. What I'm interested in is at what stage does a Waterjet become more efficient than props. I've seen them on commercial cats and smaller working boats but I've often wondered why we don't see them on mobo's. any body out there know?
Click to expand...

I've driven a triple jet 'pilot boat', on windfarms, very manouverable & easy to go sideways.
Stop on a sixpence.
 
Also note that at most speeds they do feel a little squiggly, you feel that you are not so planted on the water like a shaft drive, small movements can make the back end seem a bit flighty, a bit like a supercar getting to the limits of grip as you push harder into a fast corner. Never feels as if you are going to go side ways or anything just feels a bit spongey maybe
 
They are very manoeuverable, especially in catamarans, and will walk sideways with ease.

As Alf mentioned above, they start to become competitive against shafts once you have speeds of around 25 knots.
And if you are going along at perhaps 25 knots, and decide to drop the buckets, thereby immediately converting all the thrust from ahead to astern, the boat stops within her own length, and the crew have to worry about being catapulted into orbit...... :)

They are very popular with dive boats (especially re not having any propellers to chew up the punters with); there were a couple of dive boats here with water jets, but they both had lots of problems with internal corrosion, despite an apparent abundance of zincs.

Hamilton Jets are made in New Zealand, and have a good reputation - http://www.hamjet.co.nz/
I was involved in the design of a 40' ally cat water taxi for the Thames 25 years ago, and she had a pair of Mermaid Monarch 250 hp engines with Hamilton jets (273's I think), and she could happily cruise at 25 knots. Fling her into a tight turn and she would stay absolutely level, ie no inclination to lean in or out during the turn. More G forces.....

And they can also be designed to have a fair amount of torque - Traktor Jets are the business here http://www.namjet.com/

Here is a neat little catamaran tug (ALN 149) from Alnmaritec with Traktor jets - http://www.alnmaritec.co.uk/boats/all_boats140.shtml
She has 4 tonnes of bollard pull from 1,000 hp - a conventional displacement tug with large propellers might achieve perhaps 8 tonnes bollard pull for the same power, but it wont be doing 25 knots when free running......

And here is a mussel harvesting cat (ALN 152) fitted with Hamilton jets - http://www.alnmaritec.co.uk/boats/all_boats150.shtml

One of their main disadvantages is that they are phenomenally more expensive to buy initially than a similar set up with conventional shafts and propellers.
 
Pretty Cat workboats used on the the Thames by the PLA, built by Alnmaritec......choice of jets or props.
http://www.alnmaritec.co.uk/downloads/Wave Guardian.pdf

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PICT00081.jpgI saw these used on small workboats in Alaska. I thought they would be brilliant for dive clubs and dinghy yacht club rescues where there is the potential to mince people.
 
Re how they are effective when the jet of water emerges above the surface - it is all about momentum.
A simplified (I hope!) way of looking at it is described below.

Unlike conventional propellers which need to have a 'bite', the momentum of the water being expelled doesn't need to 'push' against something dense (like water).
The jet propels a relatively small quantity of water out of the back of the boat into the open air at very high speed.
The momentum of this water is M x V, ie its mass times its velocity.
(or small m, very big V)
This energy is converted into moving the boat forward (with some of the energy being used up along the way in frictional and wavemaking resistance of the hull).
Now we have a heavy lump of a boat moving ahead relatively slowly.
(ie very big M, small v)
It is the same principle (in a way) as the recoil of a gun when you fire it.
 
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