Re: Electric Trolling Motors (was: Good Girl For Sale)


The trouble with "displacement" for comparison of boats is: what do they _mean_ by displacement. <sigh> Ideally, the displacement given by a boat manufacturer should be the weight of the boat with it's normal load --
the figure the designer used for calculation. Alas, what gets published as displacement can vary from the boat's weight when completely empty, to the weight when loaded to the max -- with no indication what. <sigh> But we have to work with what we can to get some _very_ rough idea about how different boats compare.

I think the crude spreadsheet I worked up a few years ago is useful, nonetheless.

Even though it doesn't take into account hull shape, which can have a big effect on wetted surface, which is the big part of resistance at low speeds. I've been noticing that Tuffy, because of her shapely hull and molded in keel, moves along under sail even in a barely perceptible breeze that would have left flat bottom, fin (technically) keeled Pearl dead in the water. But it would be hard to come up with a simple spreadsheet that included hull shape. <g>

You're right on the money about higher voltage having lower line losses, and lower losses from the Peukert effect.

On 8/9/2020 8:07 AM, johnacord wrote:
Displacement can be deceiving when thinking about moving a hull through the water.  It's not so much the "weight" as it is hull shape which determines the size of the bow wake.  The the bow wake is created by pushing the boat through the water and pushing it requires power. Complicating this, it's somewhat exponential so that as you increase speed the power required goes up a lot, especially as you approach hull speed (top end of displacement motion).  The electric boat community has worked this out well, and as I recall the rule of thumb is doubling the power for an increase of a knot approaching hull speed.  So moderate speeds, in the realm of half hull speed, can be fairly efficient, like your "leisurely, rate of progress".
Regarding 12 vs 24 volts.  From your examples, 5 amps x 24 V = 120 watts  &  30 amps x 12 V = 360 watts;  lower watts = less losses to heating in all components.  If you are using lead acid batteries, either flooded or AGM, higher amperage cause losses due to the peukert effect which can be quite significant at the higher amperage.
John <@Jkohnen>
"My country, right or wrong," is a thing that no patriot would think of saying except in a desperate case. It is like saying, "My mother, drunk or sober." (Gilbert K. Chesterton)
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