Hi Andrew,
Chris is trying to move more than 2.5 times the displacement, he's 13.5 ton and I'm 5.1. He's only looking for one extra knot, usually one knot faster costs roughly twice the power (and one knot slower takes about half the power). That rule seems to quit if one tries to push past 90% of hull speed. The power graph gets much steeper there. But along with Chris' greater displacement, he's got a longer waterline, so it takes less power to cruise at electric boat speeds.
But what I've noticed for similar types of boats (roughly the same hull form, for auxilary sailboats they are generally pointed in front, fat in the middle and narrow in back, with the widest part 30-40% of the length) our rule of 1kW per ton delivers about 90% of hull speed. That rule seems to work for sailboats from 25 to 40 feet, and may go farther, but we haven't had many bigger conversions report their performance.
Anyway, even with a much larger boat I would guess that his boat will pull around 6500W - 7000W at 6kts compared to my 5100W, just 30% more power to drive almost 3 times the mass. And these are conservative estimates, he might do better... The extra waterline really helps. That extra waterline also makes 7kts possible with reasonable power levels. I don't think that 20kW would push my boat to 6.5kts, that's too close to my hull speed.
All this comes from a variety of boats with different props and motors, as long as they've got the proper reduction to match the motor to the prop. Lower ratios to spin flatter, smaller props faster; higher ratios for bigger, more aggressive props. But they all seem to perform within 10-15% of each other. Of course, 10% is worth a little extra planning, at least to me.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Andrew Gilchrist <andrew@...> wrote:
>
> Chris
>
> Thanks for that reference â€" ME were using it too.
>
> at 2.57 ratio and 856 prop speed the diesel was turning 2200rpm. Perkins
> spec sheets indicate at those rpm the motor could put out up to 55 shaft
> hp â€" and estimated torque of 131*2.57 = 337ft pounds. At part throttle
> who knows...
>
> Your boat displacement how does that compare to Erics? and the prop what
> are its diam pitch and number of blades?
>
> I think you may need way more power - you are (if memory serves) trying
> to propel twice the displacement (needs twice the torque) at twice
> the velocity (usually needs 8 times the power and commensurate torque)
>
> My thought is you need a larger motor with much higher torque per/ amp
> yield and a larger diameter at least and keep the g/box you have that
> ratio will come in handy.
>
> Need to know a bit more about throttle setting at 7 knots - if possible
> how fast at 100% throttle and prop dimensions, blade shape helps
>
> Andrew
>
>
>
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