HI Michael,
I'm not sure where you're headed with this. Are you trying to justify installing only 500W of drive power per ton instead of 1kW per ton? If so, yes your boat will still work, and it will be underpowered. Seriously, 1kW/ton is not oversized, though some people might consider it undersized. But most members here consider 1kW/ton to be a pretty good compromise.
For our boats, increasing your speed one knot takes about twice as much power. So you will use about 4 times the power at 8kts compared to 6kts. But with your 32.5' waterline, I doubt you would ever see 8kts, even with 150hp. Your hull speed spec's out around 7.5kts (the Fisher 37 is not known as a fast cruiser). With 16kW, I would guess that full throttle will be somewhere around 7kts. That means 6kts should be around 8kW and 5kts around 4kW. With my boat, which weighs less than 1/3 what your boat does, 6kts is above 5kW. So 8kW for the same speed for your boat seems reasonable. Remember that the numbers that I stated for your boat are pure speculation, and if you do better, then think of how happy you'll be. I'm a firm believer in estimating conservatively while you're still in the planning phases, I think that it sets more realistic expectations. However, I look forward to your initial performance after you complete your conversion to see how close my estimates are.
But to your point, I don't see any reason to put in a 30kW drive system, that would be oversized and the extra power would only provide an extra 1/2kt.
Since you mentioned that you could accept the sacrifice of cruising at only 6kts instead of hull speed, we can translate that into amps. If 6kts takes 8000W, that is 160A at 50V. Moving up to 72V will drop your cruising load to 110A. Even at 144V, you would still need 55A. Given that, how small of a motor do you want to use?
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> there is a sort of cascade effect to over sizing motors.... if you put in a larger motor than you can use and WANT to be able to use them then you will need a larger battery bank and larger gen sets to recharge them in the same amount of time.... it seems to me that running a motor at 60% (30 out of 50 possible amps) isn't that much harder on it than running it at 30% (30 out of 100 possible amps)... boat manufacturers over estimate hull speeds to the point that the sweet place is probably about 70% what they claim... being as electrics have all that torque for maneuvering and such they have a tremendous advantage around the marinas where i would mainly use them anyway... if i have to cruise at 6.0 knots rather than 8 and in the process double my cruising distance it seems a very good trade off... it's a personal choice of course but I am going to have to do some serious thinking/testing to determine against the idea of using smaller
> motors... i think that everybody's knee jerk is to go larger but what i really want is bullet proof... i'd rather carry a spare motor, controller and adjustable dc-dc converter while combining this with two 7.5kw gens and enough batteries for about 2 hours cruising than have to oversize everything for that 2 knots that i don't care that much about.... it would be a very expensive two knots...   the back emf regen problem also rings true a bit as i have now heard more than once that you will actually regen more IF you run the motors at very low amp rather than set them for no power at all....  blathering a bit, sorry about that... thinking with my fingers... input gratefully accepted
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