Hi Kerry,
So the tug with the wrong prop uses 6 times the hp to go the same speed as the tug with the correct prop at some speeds?
If this is true, then if I hadn't changed the 13x10 2-blade propeller on my boat when I installed my electric drive, then my boat would take 3000W to motor at 3 kts. I know that some people here have converted their boats to electric without changing their props and they don't use 6 times the power to go 3kts.
Does any electric boat here use more than 1500W to go 3kts? If so, what kind of boat and what kind of propeller are you running?
While I believe that my 13x15.5 4 blade prop is more efficient than the old prop, I have a hard time believing that is is 6 times more efficient (1/6 the energy to provide the same thrust). With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts. Better, but not 6 times better.
Given that info, I would believe ratios around 2:1 comparing ICE to electric. However 6:1 still seems like too much.
But though all of this, it is easier to see that the Gerr power predictions don't hold true for our boats at our speeds.
Back to the real question, how can we predict how much power our boats will actually need to motor at a given speed?
Fair winds,
Eric
Marina del Rey, CA
PS. I hadn't done the prop slip calcs on my boat yet. I'm glad that I know those numbers now.
--- In electricboats@yahoogroups.com, "Kerry Thomas" <kjthomas@...> wrote:
>
> Hi Eric
>
> Now had time to give some thought, and calculations, to the subject.
>
> The propellers could account for the difference.
>
> Using as an example (As that is what I have real world numbers for) A tug
> propped with a propeller designed for a high speed, low torque engine
> compared with the same tug with its proper propeller for the powerplant and
> speed does account for most of your observed efficiency difference.
>
> An ICE engine and prop combination designed for high revolutions can be just
> churning up lots of water and wasting power at slow speeds. While a suitably
> propped and slow revving engine can get the same speed with much less
> horsepower.
>
> Of course, like the tug, you could get much the same effect by gearing the
> ICE down and using a bigger prop.
>
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