# Calculating power of scale model outboard

Discussion in 'Hydrodynamics and Aerodynamics' started by Daniella Driver, Mar 5, 2013.

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### Daniella DriverScale Model

Hi everyone.

I am just about to launch my new 2 metre scale model for sea trials, and the NA has asked me to work out how much power the electric outboard produces.

I can always measure the current the motor draws, but the most useful info would be the power transmitted at the prop.

One concept that occurred to me is to hook the model up to some weights in a tank, and see how much weight it can pull - like a tractor derby. ( see illustration ) - i guess this equates to Bollard pull.

I have no idea how to translate such data into usable measure of propulsion eg H.P. to scale it up to the full size calculations

All thoughts welcome.

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the bollard pull is only useful if you know the current inflow degradation of your propulsion system to transfer that to power at speed

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### rwatsonSenior Member

Yes indeed/ That was what I found out subsequently.

I suppose it may be helpful to determine the maximum force that the motor can supply for other calcs, but thats about all .

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### CrowsnestJunior Member

Dear Daniella.
If what you are looking, is for the engine+propeller performance, your test design is almost good, but it neglects the hull effects. For low speeds, it can be accurate enough, check your Froude number.
As I said, if what is required is only knowing the propulsive arrangement properties, try to design your test without the hull, trying to minimize the flow disturbance.
With respect to the whole (hull+propulsor+ ....) test:
You'd need to provide a water stream in order to evaluate the hull form effect on the propulsion means.
Anyway, it all depends on how accurate you want/need to be, and what your budget/time is.

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