The relationship between power and speed for boats operating below the onset of significant wave-making drag (i.e. "displacement mode") is close to following a cube law. Doubling the speed multiplies the power needed by a factor of eight.
These figures look to illustrate that reasonable well, taking account of the fixed power losses that have most impact at very low speeds. If your system has around 15W of fixed power loss (probably pretty close for this system) then that 50W at 2mph is really 35W. The power at double this speed should then be about (8 x 35) + 15 = 295W, pretty close to the 300W measured.
The 3mph and 6mph comparison is an exact cube law one, too. 150 x 8 = 1200, so presumably the fixed loss of around 15W got lost in the measurement accuracy.
Jeremy
--- In electricboats@yahoogroups.com, "HargravR" <robert.hargraves@...> wrote:
>
> Power really goes up with speed. The Lyman 13 is about 400 pounds, plus 130# batteries, plus 40# motor, plus 185# captain, plus 55# dog. Water was calm.
>
> 2 mph 50 watts
> 3 mph 150 watts
> 4 mph 300 watts
> 5 mph 600 watts
> 6 mph 1200 watts
>
> I figure the two Optima batteries have 1800 watt hours capacity.
>
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