Hi Kevin.
I hear what you're saying, and I understand that props are the biggest variable. What I was trying to get at is why, as Eric is suggesting, are Gerr's formulas so far off of observed reality with a variety of electric installations. There must be some assumptions built into those formulas that don't apply, or apply differently, to electric as compared to ICE. I understand that if you're talking true, measured, HP at the shaft then it's apples to apples. A horse is a horse, of course. (sorry)
What I am suggesting is that maybe Gerr applied some empirical "fudge factor" that works ok for ICE but paints a pessimistic picture if applied to electric... Or maybe Eric is misapplying the formula? I dunno. I don't have a copy of Gerr's book, and I'm too much of a math moron to really dig into the formulas anyway, which makes it kind of hard to get my head around this.
Cheers,
Jim
--- In electricboats@yahoogroups.com, Kevin Pemberton <pembertonkevin@...> wrote:
>
> Jim,
>
> (ICE or Electric) Shaft HP is only shaft HP. Measurements of power
> drain don't define Shaft HP but do measure the amount of power needed to
> compensate for the losses if we set up a dynamo. Large Props require
> torque not available from ICE without gearing, that in itself gear
> reduction has larger loss than direct drive that could in theory can be
> achived using many polls and large diameters within our electric motors.
>
> With our efforts many of these statements don't mean anything because we
> are not custom building motors for our boats. But what is common is the
> use of small diameter props designed to slip so ICE size can be small
> with smaller reductions, allowing the ability for our ICE to reach it's
> torque curve. If we design around a big prop and low RPM we will obtain
> more for less on the power curve. Likely however we find that draft,
> and cost, get in the way of optimal battery life.
>
> Kevin Pemberton
>
> On 08/18/2011 04:16 PM, luv2bsailin wrote:
> > What about the assumption that Gerr truly is talking about actual
> > shaft HP, and not some derived value that's based on other assumptions
> > that apply to ICE and not electrics? Is that specifically stated in
> > bis work? Maybe I'm being simplistic, but I gotta ask.
> > Jim
>
>
> --
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