T-Pain Is On A Boat: Re: [Electric Boats] Using original transmission.

Tuesday, October 12, 2010

Re: [Electric Boats] Using original transmission.

That sounds like a really nice set up. I've wondered about whether motors could be efficiently run at reduced voltage and speed. I wouldn't mind a big heavy oversized motor if I could run it at much reduced capacity and direct drive as long as the thrust could be accommodated, maybe some sort of inline bearing.

I think my boat and motor combo is right on the edge of what is reasonable to expect out of an electric drive in a sailboat. It's going to work but I'm going to need all the efficiency I can get. Lithium batteries would change the equation but it's all 6V golf cart batteries for me at first!

David

On Tue, Oct 12, 2010 at 1:18 PM, Matthew Geier <matthew@acfr.usyd.edu.au> wrote:

On 13/10/10 03:58, David Goldsmith wrote:
Direct drive is certainly the most efficient, but is it the most effective? The MARS motor I'm using runs flat out at about 3600 RPMs or something close to that. That's about the same range that the ICE was running so the gear box should be a good fit. That's way too fast for the propeller to be effective at moving a big heavy boat at slow speeds.

With my boat, the power / speed at the prop was well with in the specifications of the motor I chose. It's quite possible that if I ran a slight reduction and a higher voltage (I'm using a perm magnet DC motor), I could get a little more electrical efficiency that comes from a higher voltage, but in my case, I doubt it's worth the extra complexity. I could get a slightly larger prop on, which could also change the equation, but again that's really going to be a minor gain.

Horses for courses though - my boat only weighs in at 715kg and requires approx 3kw to reach near hull speed with it's existing prop (The original one off the two stroke engine it used to have). I normally run at a much more efficient 2.5kw. The motor designed to output 13kw maximum (with twice the voltage I run), but the manufacture spec for 36v I run is still 7kw max, 3.9kw continuous. So my motor spends most of it's time running at half it's rated power. It gets warm, but not enough to need forced cooling.