Hi Chris,
I'm kind of interested to hear how you figured the power needed to reach your target speed. I have found that a number of generally accepted methods don't always predict accurate numbers for some types of boats. Here's an excerpt from an earlier post that I made on the subject back in Nov 2011:
"Ealier this summer, there was considerable discussion about the Dave Gerr calculations and how applicable they are to our boats. Because we are operating at such slow speeds and electric motors have very different power curves compared to ICE, much of the information in Gerr's book that has been validated many times over in the ICE world doesn't always hold true for electric boats.
For example, according to Gerr's calculations, my hull would require 3.98hp (3kW) at the propeller to drive it at 3kts. I know that I only draw 500W from my battery bank to power my boat at 3kts and that hasn't included any losses for the controller, wiring, motor or gearbox yet.
In an email from Gerr, he told me that his work doesn't really cover our specific use cases, his focus has been on hulls close to hull speed and beyond.
So while Gerr's book is treated as gospel for mainstream boaters, and rightly so, we should remain open to different answers for our boats.
We can see that just about any prop will make a boat go, but some props will do the job better."
Just wondering out loud....
If your system can change reduction ratios easily, then I don't see any problem with using the prop from your ICE installation, but there are opportunities to reduce slip with more aggressive props that would be completely unsuited for ICE operations. You could start with what you've got to get back on the water as quickly and cheaply as possible, then re-optimize with another prop later.
I went to Motenergy to check the performance graphs for the ME0913 motor and it looks like they never published performance graphs for their brushless motors. So it's hard to tell what your amp load will be at what RPM so that you can match the motor to your prop. I can say that my ME0913 will run as long as there's power at 1900 RPM pulling 106A @ 48V without overheating. So maybe that's a number that you can use...
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Chris Hudson <clh5_98@...> wrote:
>
> Thanks Eric. I know the power required to reach my target speed and the prop RPM needed to get there. I'll keep my current prop for now. With this data shouldn't I be able to calculate my gear ratio?
>
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