Water Pump as a Jet Drive

[Fanie]

Hi Guys,

Why can't one use a petrol driven water pump as a jet drive ?

I've had a look at some water pumps, the inlet is 50mm (2") as is the outlet, and on another the inlet and outlet is 80mm (3").

If you put a nozzle on the outlet with a reduced diameter to accelerate the water out, why won't it work ? You may not get the same performance as a jet ski's more efficient designed jet drive unit, but the pump and motor can be obtained a lot cheaper.

[CDK]

Works like a charm Fanie!

There are cheap Chinese pumps on the market, powered by a 10 HP diesel engine. The manufacturer warns that the cage must be firmly attached to a concrete or steel base before the pump can be used because without it, the 80 kg unit will topple over as soon as the throttle is opened.

[Fanie]

Hi CDK,

Now the question is, how big a pump and motor to propel how big a boat, or rather how can one determine the amount of thrust of a unit ?

[michael pierzga]

Thirty years ago before, bow thrusters for yachts were widely available , I sailed a motorsailer which used a big fire pump powered via a coupling from a diesel generator as a bow thruster The system worked but the problem was filtering the huge intake water flow. Always clogged with debris. I cant imagine it being a satisfactory means of propulsion. Also the water flow thru the plumbing made very much noise

[Fanie]

Hi Michael,

I had a look at the fire fighting model also. The nozzle seems extremely small, and my female instinct tells me one may get better thrust just making the normal pump outlet the right diameter, say from 80mm to 40 or 30mm, perhaps even 20mm. The fire fighting unit's nozzle is around 4 or 5mm.

You get the same pumps in a slurry pump model, it's made from polyprop and will be sea water resistant, and can aparently pump solids up to 25mm. If they have the same pumping capability as the other models, then I think this would be the thing to go for. You have to put a grid of sorts in front of the intake in any case or you may chum when you pass through a school of fish.

[michael pierzga]

I cant remember pump output or diameters...this pump was huge. The boat was about 75 tons by 25 meters. The plumbing was at least 80mm with a nozzle at the bow for thrustering . port and starboard was controlled by air operated valves to direct water jets at the bow and a " loop " with an air operated valve was installed at the pump so the water could circulate ..."in Neutral" I do remember that the system worked fine. Only prone to clogging .

The the other water pump jet powered boat I know had problems with cavitation...sucking air...in a seaway.

[CDK]

Originally Posted by Fanie

Hi CDK,

Now the question is, how big a pump and motor to propel how big a boat, or rather how can one determine the amount of thrust of a unit ?

Looking at this particular pump's depth and placement of the exhaust flange, I estimate the minimum force to throw the unit on its side to be 120 kg. That -or more than that- is the thrust you obtain when the pump is placed open-ended at the stern of your boat.

[Fanie]

I'm considering using a water pump in a kayak. It's firstly to annoy some okes locally and secondly because it's a cheap alternative propulsion. The kayak is fairly large and should be ok with the weight etc. I'm not very keen on this rowing idea in any case, and the easiest seems to be a water pump with a petrol motor.

You get some hobby jet drives, but I suspect they may be slightly small and is also a tad more expensive.

It's a pity there isn't a real range of jet drives available. You either get small or big, but nothing in between

[michael pierzga]

Ahh a jet kayak !!!!!! .perhaps just a scuba tank of air...a tank of water then

BLOW BALLAST !!! thru an aft facing jet and " burst" speed 50 meters ahead of your mates as they look on , scratch their heads and wonder...How did he do that ?

[michael pierzga]

Like one of these " sneaky" motor canoes......slow down your padling too fast !!

[Fanie]

" burst" speed 50 meters ahead of your mates as they look on , scratch their heads and wonder...How did he do that ?

By pretending to row in slow motion. And not only 50m, wherever I go...

[michael pierzga]

You might be able to generate substantial kayak propelling burst " POWER' by consuming Bilitong, boiled eggs and beer the week before ? I not aware of the proper formula..be careful when experimenting

[Fanie]

Yes well I want propulsion, not explosions that can be fatal

[srimes]

part 1:
http://www.duckworksmagazine.com/03/...jet/drive1.htm

part 2:
http://www.duckworksmagazine.com/03/...jet/drive2.htm

[Fanie]

I saw that article, thanks Srimes.

Another option is the model one

http://www.cornwallmodelboats.co.uk/...pner_2347.html

It may be a lot easier to install, requires a 2.2hp or bigger motor