Hei all
I am no engineer and in no way experienced with electric propulsion, but in think it all comes down to the cumulative resistance of a boat at any speed of interest. Therefore I would find it very helpful that at least the ones producing electric propulsion systems for boats would provide charts for their test boats, which show how much force is needed to keep up a certain speed (in calm no current). This can easily be achieved by towing the (propless) boat behind a powerboat at a few different speeds with a springbalance or something similar to record the force on the rope. Then anybody could do the same with any boat (or at a later stage look up the values for a similar boat). Since this chart would at the same time give information about the work done, the overall efficiency if combined with the power consumed when driven by the electric drive in question will be evident.
Maybe my point of view is too simple but as a future customer of an electric propulsion system I would be most interested in how much of the energy I feed into the engine is really driving my boat forward and how much power I need in the end to achieve my intended speeds. Therefore the information provided by torqeedo is nice, asuming I own one of the few boat types they have published their test data on. What I miss is the data on the boat resistance at the given speeds because that would give me an indication how my boat compares to theirs. However, until someone publishes data including boat resistance it won't help me much to ask a friend with a powerboat to pull my little sailboat at a few different boat speeds on a calm and cold winter evening... (starting to think about water density at different temperatures and salt concentrations which might influence the results...)
Matti
--- In electricboats@
>
> 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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