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Author Topic: Motor power calculation.  (Read 3776 times)

Hagar

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Motor power calculation.
« on: August 07, 2008, 06:24:29 PM »

Is there a simple (not good at maths :embarrassed:) way to calculate the size of motor(s) needed to push a boat at a given speed. Variables such as weight, beam and draft probably come into the equation some place, other than that I have no idea at all.
Oh! if we are talking waterjet propullsion how can you work out the speed from the throughput of the pump?

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boatmadman

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Re: Motor power calculation.
« Reply #1 on: August 07, 2008, 07:34:55 PM »

Hagar,

Yes there is a way to work out your motor,prop,battery esc requirements. I havent found it yet though, so, just ask here and you will get all the help/advice - useful or otherwise you could wish for.

Ian
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SteamboatPhil

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Re: Motor power calculation.
« Reply #2 on: August 07, 2008, 08:29:05 PM »

Its a bl**dy mystry to me, so I'll be watching for the answer too
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Colin Bishop

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Re: Motor power calculation.
« Reply #3 on: August 07, 2008, 08:47:31 PM »

It's basically empirical. You find a setup which works for a similar size and type of boat and go on from there. So if you can post the details of your boat you will probably get quite a bit of advice to choose from.

Colin
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FullLeatherJacket

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Re: Motor power calculation.
« Reply #4 on: August 08, 2008, 07:31:43 AM »

Hagar
Not good at Maths? Then forget it. We did fluids at Uni and, even where the flow rate is slug-like, the calculations involved second-order differential calculus. Introduce any sort of speed in there and you've got a few squintillion more variables to allow for e.g. wetted area, viscosity, skin friction coefficients, temperature, something called Reynold's Number which I never grasped etc etc ad nauseam. I prefer to leave that sort of stuff to the clever b*ggers.

"Suck it and see" always seems to produce something about right after you've built a few models.

Phil
Until you can straight-run going full astern then don't worry your pretty head with this mumbo-jumbo  ;)

FLJ
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SteamboatPhil

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Re: Motor power calculation.
« Reply #5 on: August 08, 2008, 11:05:26 AM »

Your humble servant !!!!!
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malcolmfrary

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Re: Motor power calculation.
« Reply #6 on: August 08, 2008, 12:26:02 PM »

If you know the horse power of the original, and the scale of the model, multiply the original HP by 745 (to get it into Watts), Then dived that by the cube of the scale (e.g. if its 1/100, divide by 100*100*100=lots).  This gives a lowest ball-park figure to give your model the right scale performance IF the hull and all the bits poking out of it are perfectly to scale AND the motor and drive train is as efficient as the real thing.  Generally, if you multiply by something between 1.5 and 2 it will work both without disappointment but without looking silly.
If you set your calculator that is lurking in Windows to scientific, it can be done easily.
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sweeper

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Re: Motor power calculation.
« Reply #7 on: August 08, 2008, 12:26:51 PM »

The attached data may be of some help to someone. As stated, it was copied from a textbook many years ago. The original had some errors in the formula which certainly made life a little difficult at the time.

FORMULAE

This was taken from a modelmaking textbook many years ago, I applied it to a model that I was building at the time. It appeared to be fairly accurate. The origin of some of the terms has vanished in the passage of time.   

ORIGINAL SHIP

Installed power per shaft   (No) Horsepower
Top speed   (Vo) metres/sec    ( 1knot = 0.5m/s )
Screw diameter  (Do) metres
Number of blades (u)
Number of screws  (v)

Non- essential

Screw pitch (H) metres
Screw revs  (no) RPM

CALCULATION

Model scale :  1
Shaft diameter (d)
Scale speed (Vs) = (1/ )  x Vo
Scale power / screw Ns = 1000 x (No/ 3.5 ) Watts

EFFECTIVE SPEED

Single screw  Ve = 0.8 Vs
Twin   screw  Ve = 0.9 Vs
Triple  screw  Ve = 0.84 Vs

Screw diameter  Ds  = 1000 Do
                                      

Propshaft loss  (diameter = d mm)   
 

Na   =   Ns x d x 10 (to the power )4  Watts

Required power / screw

Nm = Ns + Na  Watts

Just checked the layout, this was pasted from a Word document. This board will not accept certain symbols  :(
The missing ones are: Model scale :  box should be Alpha , Scale speed : should be Square root sign then Alpha ,Scale power : should show Alpha to the power 3.5, Screw diameter should show Alpha
Sorry if it appears a confusing layout. It has taken ages to get it into this state!  :'(


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Hagar

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Re: Motor power calculation.
« Reply #8 on: August 08, 2008, 10:05:10 PM »

second-order differential calculus  {-) ok point taken!
Let me put it another way. Boat probably about 6kg all in. speed not a factor, but would like it to move around 1/2 - 1 m second. I am thinking twin water pumps, sort off beefed up thrusters,
12v ?Amp (Max 20A). what sort of motor do you think I will need to run an electric drill pump? Something with plenty of torque I guess. I'm open to all suggestions.

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debssnal

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Re: Motor power calculation.
« Reply #9 on: August 15, 2008, 09:31:39 PM »

HI Hagar have a look at this re powerfull water jets    http://uk.youtube.com/watch?v=Qa5wBkClicI
Kind regards
Alan
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FullLeatherJacket

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Re: Motor power calculation.
« Reply #10 on: August 16, 2008, 10:36:04 AM »

I have a couple of customers who use bilge pumps for driving their bait boats. This one shifts 1100gallons per hour, runs on 12v and doesn't need a huge battery to keep it on song.
Just a bit of lateral thinking...........
http://www.force4.co.uk/ProductDetails/mcs/productID/842/groupID/3/categoryID/28/v/15c78e9e-e5ce-441f-8172-f5014d2466af

FLJ
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Hagar

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Re: Motor power calculation.
« Reply #11 on: August 16, 2008, 04:36:41 PM »

Found these in a web shop up the road from me,
Raboesch bow thrusters. have a tube size of 33mm dia, comes with a 400 size motor , that can be changed out for another, (700?)

http://www.modelskibet.dk/product.asp?product=3602&sub=16&page=1

Do you think they would push a boat of around 6 - 7 kb?
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debssnal

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Re: Motor power calculation.
« Reply #12 on: August 16, 2008, 05:17:28 PM »

Hi it will move the boat but not very fast.
have a look at this
http://www.westbourne-models.com/erol.html#2377X0
then have alook at this
http://uk.youtube.com/watch?v=Jn2QxbhaJUs

kind regards
Alan
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malcolmfrary

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Re: Motor power calculation.
« Reply #13 on: August 16, 2008, 05:42:56 PM »

A 400 size motor (depending on the motor/battery) is capable of moving a boat of that weight realistically using a normal prop.  As main propulsion in a thruster - thats a new set of bets.  If a thruster is designed with a particular motor in mind, it is quite possible that the bearings/gears will suffer in the event of an upgrade (hotter 400) and it is quite probable that a physically larger motor simply would not mate with the mouldings.
The geared prop arrangement should be more efficient than the paddle arrangement.
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Hagar

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Re: Motor power calculation.
« Reply #14 on: August 17, 2008, 11:19:52 AM »

@debsnal:
Does look like a good idea, BUT (and thats a big but ::)) The boat in mind is a bait boat, so I think that such a fast boat would be a tiny bit unpopular with the other anglers on the lake. Bait boats are not that well looked upon as it is. If I was building a fun boat fine.
Also Jet drives have their intakes on the bottom of the hull and I can imagine that all kinds of detrius will get sucked into it on both launch and landing... Not a good idea.

Am now looking at a sort of "overgrown korts". This brings the question: Any ideas as to how one gets the power from the motor into a pipe. Z-shaft?
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