T-Pain Is On A Boat: [Electric Boats] Re: New Member with Questions on Prop Size, Prop Speed, and Reduction Equipment

Sunday, July 5, 2009

[Electric Boats] Re: New Member with Questions on Prop Size, Prop Speed, and Reduction Equipment

--- In electricboats@yahoogroups.com, Gene Fusch <gfusch@...> wrote:
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> As a new member I have questions that may have been discussed before but are new to me.
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> I am starting a new project to build in wood-epoxy a 42' Bruce Roberts Spray (18 ton displacement with a 14.5' beam) sail boat to cruise through retirement.
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> I plan to use a Chinese freestanding lug rig and an electrical auxiliary motor that will recharge while under sail. As I look at various motor options I find their pricing all over the board and limited data provided. A couple questions that I have for anyone who has converted to an electric boat (particularly a sail boat):
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> • My thinking is that a large prop with a high pitch turning slow may be more efficient while under power and for recharging while under sail. From your experience, how large should the prop be for a 42' boat with 18 ton displacement?
> • What should the speed of the prop be?
> • What experience have you had with reduction gears or reduction belt assemblies?
> • What are some good sources of reduction units (to add torque and reduce speed from the motor to the prop)?
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> Thank you,
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> Gene
> Hood Canal
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See "Building a Javelin" in the "files" section for some background info. You are correct that a large prop, at low RPM, with steep pitch is the way to go.

My software says you can get about 5 knots with 10 shaft horsepower, which is about minimum for such a large boat. Ideally you should be swinging a 24 inch prop at a couple hundred RPM. Having a sailboat, you should probably make some adjustments, like a smaller prop (say 18 inch more or less) to reduce drag when you are cruising. If you are going to charge the batteries while underway, pitch should be 1.5 to 2 times the prop diameter. Again this is a first approximation, say 18 x 30 prop size, 250 RPM.

As to reduction gearing, it will be hard to beat the toothed timing type belts at this power level, they are low maintenance, reliable, and more efficient than most other options if setup correctly. This should be the system for final drive, say 4:1 ratio, 1000 RPM on the intermediate (jack) shaft. I would estimate a final drive with belts similar to what Harley Davidson uses on their current motorcycle lineup. The belt manufacturers all offer software to assist in designing this stuff.

Drive from the motor/generator to the jackshaft can be by another toothed belt, although this can be risky if you hit a log or something similar, unless you have a slip clutch that releases at a predetermined torque before something gets damaged. Assuming you get an 1800 RPM motor/generator, this drive reduction is 1.8:1, more or less.

Conventional V belt drive could be used, I am glad to have this setup on my small electric boat, when I foolishly switch fron full ahead to full astern, (or hit a rock with the prop. I get a belt chirp, and nothing is broken. The conventional V belt drive can use adjustable pitch pulleys, which would allow you to "tune" your system to the ideal ratios.

For an application where the motor is used as a generator, the best way to go might be with a continuously variable ratio belt drive system. This would allow different ratios when motoring or charging, and could be especcially helpful if you want to charge in less than ideal sailing conditions. These units are used on snowmobiles, and these sized units are in the proper vicinity of the design conditions we are talking about here. Another alternate is to do this (variable ratios to achieve good propulsion as well as optimum charging) electronically, but you probably need a rocket scientist to design this setup.

As to motor/generator prices, things are expensive at the retail level, but you can find the right stuff on e-bay at very reasonable prices. 10 horsepower DC motors are available, but there are several other motor characteristics that need to be met for your application.

Sorry to be so long-winded for this reply, hope you find the info helpful, and good luck with your project.

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