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post #51 of Old 01-05-2019
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Re: Electric Conversion log for Kahleesea

I haven't had my coffee yet, so f'give me if I don't make any sense.

Originally Posted by Captain Canuck View Post
Hi Sean,
If I'm going to run at 48V, there might be a better choice of motor out there.
You're looking at one of the best compromises of speed/torque/efficiency at your boat's requirements. Short of a switched reluctance motor and controller suddenly becoming available in the 5-10kw @ 48v range. And at <50v ABYC wiring standards. e-10 I think, you really are pushing a lot of current at 10KW. ~200 amps. 2/0 awg is expensive. 10kw is roughly the upper practical limit unless you want do deal with the isolated grounds and relative danger of higher voltages.

Hull speed is more of a personal design challenge. I'm not looking to skip along the water on my wing keel, or maintain it for any period of time, I just want to be able to touch it and say "yeah, the boat can do that". I realize that I would suck the battery dry in no time. I'm going to start with 8kWh and see how far I can get with that. If I need more, I can always keep adding until I run out of space.
You can still get to practical hull speed with the above system by going above the continuous rating of the controller & motor. Thunderstruck says 600 amps for 1 minute into the motor. I imagine that means 300 for 2 or more minutes, then. Their recommended Sevcon 72/80 550 controller is rated for 2minutes @550 amps. So you have plenty headroom to overprop/overpitch the system for brief periods of hull speed, so long as you size your wiring safely. Set it up, and play with the props. Again something with variable pitch will be cheaper long run, props are expensive. You never know, maybe you have a slippery lightweight hull and can get the hull speed on flat water without over currenting... stranger things have happened. test test test!

I have a basic knowledge of electricity, but I'm not a professional, and my knowledge of hydrodynamics is about as complete as someone who's two weeks into their first college course. What I don't understand is why the speed of the prop is an issue. If I get X thrust from a prop spinning at 1200 RPM, and the same thrust from a prop spinning at 800 RPM, what's the difference, regardless of weather conditions, other than the gearing necessary? Won't both props push the boat equally well? You can even argue that fewer RPMs means less wear on all moving components.
Specifically to address your 1200 vs. 800/same thrust question. Thrust at what speed? The efficiency curve of the 1200 rpm prop will not peak as high as the 800 rpm prop will at the particular speed you've optimized for but it will be broader and take less of a hit in different wind/wave conditions it's not optimized for, since the slip will vary less over that range of conditions (Simply because of the higher RPM/lower speed of advance/ratio against actual speed of vessel in prevailing conditions)

At another level you seem conflicted on what you want. You want hull speed -- by definition, inefficient. And you want biggest 2 blade prop for efficiency. wahh? Hey I get it I want fast cheap quality stuff too. But down here in reality we can only do one at a time. Experimenting is fun, and I'm all for it, knock yourself out if you've got the $$ to do so. Report your findings, we all want to know!

For me it's easy, my e29 has enough room for MAYBE a 14.5" prop. May have to modify the rudder to get enough aperture. So I have to go 3 blades. I'll do a kiwi prop, and optimize for headway against crappy conditions, and get my speed fix from my sails.

What more risks are there at 96v vs 48v? I don't know much about DC, but surely the risks can't be as high as the risks for the 120VAC that the boat also runs? Are you talking about undersizing your cables, or is it something else?
Vs AC/shore power/inverter? You can't shut batteries off at the breaker when you're working on the interconnects. They're always on. Always a hazard. They never forgive you when you forget to take your watch off, or drop that wrench you were tightening that bolt with. Add to the 'always on' bit with you're quite a bit more likely to achieve a lethal current through your body's internal+skin resistance than at 48v. And you're in an environment (salt wet) that lends itself to testing the theory quite handily.

The arc gap is twice as long as it is at 48v, and you put twice the current into a given resistance, generating twice the heat into any 'accident' that occurs.

Oh and, by the way, you're welcome. Hope it's useful.

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