I ran across a term named "natural speed" for boats in a library book over 10 years ago. The "natural speed", they talked about was not hull-speed, but rather, a speed at which a boat could be initially moved and sustain that movement with the most absolutely minimal effort. My boat(A Venture 25) rated at about 1 to 1-1/2 knots. Of course as the boat got heavier with a bigger squared watered surface, the figure would go down. Such that something like a Hunter 35 might have a natural speed of 1 knot(or less).
I have searched in vain for that book and that formula since.
ANYWAY, I think it would be interesting to see how much power(thrust or what-have-you)
I would really hope that someone else has some insight on this "natural speed" too.
John Francis
James,
I think that you made a very relevant point when you stated that your motor acts differently when tied up and when under way. This is why I find real world performance data to be valuable to me for evaluations. I also don't believe that two systems have to be in identical boats for a valid comparison.
Various attributes have a predictable effect on performance, less drag will perform better under electric power. Increased weight will decrease performance. Longer waterlines will be driven easier to the 3 to 5 knot speeds that we're focusing on. Exposed props are more efficient than props in apertures.
If I look at your published performance figures, I can estimate how a similar drive system might perform in my boat. My 1964 Cheoy Lee ketch is about the same weight, a similar waterline length and a narrower beam. My performance hits will come from the full keel and the prop in an aperture. So I don't expect the same performance, but would believe that a 20-25% hit would be reasonable. Of course, we'll find out later this spring.
Similarly, if another type of drive performed in an Ericson 28 as I described in a previous post, that performance can be compared to your drive system in your boat while making allowances for the known differences.
But what would someone with a 42' boat do with this information? I would no more expect to install the same electric drive in a 28' boat and a 42' boat than I would install the same ICE engine in both boats. Since each drive has a range of boats that it is best suited for, real world published data for boats in that expected range can be used, even if the target boat is not exactly the same as earlier installations.
Fair winds,
Eric
--- In electricboats@
>
> Eric,
>
> Static thrust is be a good measure of how well an electric motor
> performs. It has its drawbacks but it is the only baseline that
> everyone could work from. Everything else has way to many
> variables for an accurate comparison, though the information you
> describe is very relevant to boaters with similar boats.
>
> Static thrust, that is, the boat tied to the dock with a strain
> gage, can also give a good indication of efficiency. Motors are
> rated in terms of kilowatts consumed which really has no real
> indication of how well the system performs. Thrust per kilowatt
> consumed at various kilowatt values would be a great indicator.
>
> I have noticed that on Kapowai's motor, when tied to the dock will
> spin at 1400 rpm, however, when cruising at 5.5 knots it is turning
> at 1800 rpm. So other real life data should be kept. But I don't
> know of any other way that two systems could be compared without
> putting them in identical boats.
>
> Perhaps Practical Sailor might do a comparison test of all the
> systems on the market one day.
>
> James
>
>
> www.propulsionmarin
>
>
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