Hi Denny,
That sounds like a better expalantion. But Gerr predicts that my boat will use more than 500% more energy than my electric drive consumes at 3kts.
In my book that is more than a mere discrepency. But if this is the case, then a qualifier for the formulas that says that these predictions are more accurate at 80% of hull speed or greater would be appropriate.
So, if we recognize theat the Gerr formulas do not apply for electric propulsion (which is not unreasonable), then what can we do to accurately predict performance? I have been able to make accurate predictions by comparing porposed conversions to practical experience, but something that goes beyond experimental trial and error would be helpful.
This thread started when Bill S. said that his boat needs 30hp (22,500W) to motor at 7.2kts. I would like to know if that is true or if this is an ICE only prediction. A simple observation would put the issue to rest.
Thanks for your input,
Eric
--- In electricboats@yahoogroups.com, Dennis Wolfe <dwolfe@...> wrote:
>
> Gerr's formulas are mostly rules of thumb - not actual physical
> "truths". His formulas are mathematical curves fitted to data points.
> As with most rules of thumb they fit best to data points in the middle
> of the range and often not so well to data points out at the ends.
>
> Our applications are very low power and very slow speeds compared to the
> examples in "The Propeller Handbook"
>
> I would not make too much of discrepancies with observed data.
>
>
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