do it yourself bow-thruster

[Alexis]

Hello there,

I have seen somewhere a sketch of the following system: an electric pump sucks water from an underwater skin-fitting , and pumps it out of another skin-fitting fitted just above the waterline on one side of the bow. Another pump does just the same thing for the other side. I remember seeing a photo of some sort of power trimaran designed to brake the Atlantic crossing record, using such a system for harbour manoeuvering.

Has anyone installed a similar kit and have some of you got the hydraulic/physics knowledge to calculate the power and tube diameters needed for a given displacement ?
Any information is welcome.

Cheers,
Alexiss

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[qsiv]

Vetus used to market such a system - the pump was an engine driven centrifuge, with valving to divert to either side. Seems a perfectly logical system. As for power - I cant help, but my guess is that you would need some 30% more than an equivalant tunnel prop, and the outlet sizes will have to be reasonably substantial to pass the water volumes.

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[DavidGrieves]

I'm no scientist or boffin but this formulae may give you some help:-
Jet Reaction = 1.57 x NP x d squared/10
Jet reaction will be in Newton’s, one Newton is equal to 9.81 Kg, so the weight of the boat you want to move (or at least its bow) divided by 9.81 will give you the Newton’s required to move it.
1.57 is a constant(will not change)
NP= nozzle pressure
d = Diameter of outlet ( skin fitting ) in mm, remember to square it!
Then divide it all by 10.
You should be able to transpose this round to get your answer.

This equation is one used by Firefighters to calculate the jet reaction of the branch he/ she is holding, i.e. how much force is pushing him/she backwards. Newton’s Third law...for every action there is an equal and opposite reaction. Hey, if a fire fighter can do it... and who said we aren't worth 30K?


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[Evadne]

On big ships there are two types of bow thruster: the tunnel (cheapie) and the omni which works exactly like this, sucking up water from below and squirting it through a nozzle, except that the jet nozzle is steerable through 360 degrees.

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[qsiv]

Thats really useful. Only point I would make is that the weight of the boat is a red herring - all you need is the force you want to apply. 200 LbFt (90 Kgf) seems to be a 'medium' size. Vetus have a calculation for the size of thruster needed <A target="_blank" HREF=http://www.vetus.com/products/bowthrusters/bow_thrusters_select.htm>here</A>

Do you know what NP is measured in in your formula?

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[MainlySteam]

And just in case you get it wrong, 1 kilogram force = 9.81 Newtons, not the other way around.

John

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[DavidGrieves]

The NP would have to me in Bar, one bar being about 15 ish? PSI

A 15mm dia outlet with a NP of 3 Bar, would produce about 106 Newtons.

Remember to take into consideration the frictional loss as water passes along your tubing, for a 45 mm hose we would allow 0.2 Bar reduction for every meter, smaller dia hose would be greater than this. You could realy do with a reliable way of measuring the NP at the outlet.

Sorry about the Newtons and Kg thing, I always get that one mixed up...

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[hisw]

why not just use the wind, tide and occasionally a spring?

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[Alexis]

Good point. Wind, tide and springs are much more reliable than aby sort of mecanical/electrical gizmo. Not mentionning the sense of achievment that goes with manoeuvering the way you suggest.
You struck a chord.

Thank you,

Alexis

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