I first tried to test props with a spring scale on a trolling motor mounted on the dock. Jeremy is right (as usual). I got much different results when I reran the tests in a canoe using GPS speed as the "output" and voltage and current as the "input".
Denny Wolfe
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>On Sun 28/08/11 1:50 PM , "Jeremy" wrote:
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>To test a prop you need conditions where the flow through the prop disc will be close to that seen in service, so putting it in a static tank won't give good answers, I'm afraid. The water in the tank won't circulate in a way that's even close to being representative, so all the tank will really do is provide a drag load, which isn't much help when trying to assess performance.
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>Similarly, testing a prop statically in open water by doing a bollard pull test will give misleading results, as the prop inflow conditions will be way out. A prop has to be designed for the expected in-service hull speed, as the pitch, twist and blade loading are all significantly affected by the true angle at which flow hits the leading edge of the blades. This is one reason why the static thrust figures given by the electric outboard manufacturers are fairly pointless indicators of performance.
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>Short of using a water tunnel, the best way to compare different props is to do controlled tests on a hull at the design speed. The hull doesn't have to be the same size and displacement as the boat the prop is intended for, as long as the speed through the water is the same and the total design hull drag can be simulated (maybe by using a sea anchor). Measuring the motor power is a pretty good way of determining which prop gives best efficiency and this is pretty straightforward to do by recording the voltage and current for a given speed.
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>--- In electricboats@yahoogroups.com, "John Green" wrote:
>>
>> Just a question.....or two.
>> If I wanted to test, say, one prop compared to another, without
>> involving a boat, but maybe in a home bathtub, could I just take the
>> thrust alone as a measure?
>> I am thinking of using a motor suspended over the tub, and set it up so
>> that everything is the same except the prop, (or duct around/behind it).
>> The wife will be away at the time.
>> Do I need to measure the amps. or just use, say, solely a horizontally
>> set up weighing scale to measure the pull/push?
>> I realise that there will be added drag once fitted to a boat.
>>
>> Thanks
>> John
>> PS note to Myles - I hope that you are as proud of the brevity and
>> single topic of this post as I am!
>>
>>
>> --
>> http://www.fastmail.fm - Send your email first class
>>
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