Thanks for finding the typo. No idea how that number got there. In most places I just refer to it as a 32kWh battery bank. I like powers of two. I confirmed the power to maintain 7.2 kts as 30hp a long time ago with my diesel. There are a couple pretty good calculators for power requirements, shaft size, and prop size at http://boatdiesel.com. I've found them to be pretty accurate. Also, Dave Gerr has published a number of formulas in his books on boat design.
My philosphy is that shaft horsepower is shaft horsepower is shaft horsepower - needed to MAINTAIN a given speed. The more power you have the faster you can accelerate to that speed. The higher torque of electric motors lets you get to a given speed faster with a small motor.
My boat, after its refit is actually at 34,600 lbs. That's even after adding an extra thousand pounds of battery weight. Nutrisystem even works for boats! It's also a pretty fast, light displacement design for its length.
-Bill
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Bill,
>
> Thanks for the link. Lot's of good info in there. The key quote that I found was "If I'm willing to have a maximum continuous motoring speed of 7.2 kts, then I only need 30hp".
>
> Have you been able to confirm that your drive energy demand at 7.2kts is 22,500W (about 75A at 300V)? That's pretty good for a displacement of 35,700 lbs.
>
> I did see a small typo. On your design page, the traction bank is described as "3276 AHr 326VDC". That doesn't sound correct. 3276Ah x 326V works out to more than 1000kWh. Your 24 Odyssey 2250's are rated for 126Ah each, so you've got about 1713Wh (126Ah x 13.6V) per battery. Multiply that by 24 batteries and you got a total traction bank capacity of 41.1kWh (126Ah x 326V). If the batteries were in parallel, you would have the same 41.1kWh (3024Ah x 13.6V) but that is not the case. I just thought that you might want to know.
>
> Thanks again for sharing so much information.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, Bill Southworth <whs@> wrote:
> >
> > My boat was built with a design spec of 72 hp to achieve waterline speed in normal conditions. My experience with a conventional engine confirmed that it was about right. I know that many folks recommend using smaller electric power but I felt than the extra power is good to have when you are in rough conditions, a fast moving river, breaking inlet, etc. Admittedly, an electric motor has much more torque at low power, but that doesn't really help the top end.
> >
> > On the other hand, my generator is only half that power, to get 50kW or 100kW requires either using all electric or syncing the generator voltage with the battery output to maximize the combined power to the motor while minimizing battery drain. My boat has a single battery bank and is charged at 352 volts.
> >
> > We're doing the propulsion for a second boat which is a 110' schooner with twin screws. That system will be using two 480 vdc battery banks with 64 kW-Hrs of capacity in each bank. There's some more stuff about my boat at http://barbara-ann.com and about the design approach at http://hybridpropulsion.com. ....
> >
> >
> > - Bill
> >
>
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