Roland,
That is basically the idea. If you reach full revs but have amps to spare, then the motor can carry more load, you add the load by gearing the prop to turn faster, to do more work. The other side of that is if the motor has too much load to get to your desired RPM, then you can decrease the load by increasing the gear ratio and letting the motor spin more freely. For other boats, where one can't change the gear ratio easily, the same kind of adjustments can be made by increasing or decreasing the pitch of the prop.
This analysis can be done more scientifically with electronic equipment. I think that Arby described how he balances the motor to the load with a scope a while back, but in the absence of the proper equipment and know-how, what I described should get you closer to optimum with your boat.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Roland Kruijer <martine_roland@...> wrote:
>
> Thanks for your advice Eric, I apreciate it! One question though, Do you mean that if I measure the used amperage at full speed, this is a way tot determine if I have the right pulley combination? I mean that if I reach maximum rpm but not yet the maximum amps, there is some room for different size pulley?
>
>
> Thanks again,
>
>
> Roland Kruijer
>
>
> To: electricboats@yahoogroups.com
> From: ewdysar@...
> Date: Tue, 6 Jul 2010 05:00:40 +0000
> Subject: [Electric Boats] Re: calculate best measure prop
>
>
> Roland.
>
> With a waterline length of 3.9m, your hull speed calculates to 4.6kts or 8.5km/hr Since your motor is rated to 60A @ 24V, that's 1440 watts or just about 2hp. Realistically with only 2hp, you will not get past hull speed. If the new motor can hit max RPM, then you can make the gear ratio "taller", closer to 1:1. If the motor hits max amp without max RPM, then you should go to a higher ratio, towards 2:1, by getting a smaller motor pulley or a larger driveshaft pully. Because of all of the additional variables involved, a calculated ratio will be merely a suggestion that would have to be proven by experimentation. It sounds like you're close, so I think that you can try different pulleys and you might be able to return the ones that you do not use.
>
> Best regards,
> Eric
> Marina del Rey, CA
>
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