I agree Kevin.
In my experience, virtually all ICE installations do have gear reduction. In fact, the gear reduction ratio is a prominant part of every propeller sizing calculations. But gear boxes and transmissions rarely introduce more than a 15% loss of efficiency, the gearbox in my electric conversion is 97% efficient, better than many belt drives. Remember that almost all friction losses show up as heat, if your ICE V-drive absorbed 20% of 50hp coming out of an engine, it would be a 7500W heater in your bilge. Now push that up to twin drives at 300hp a piece, and you got 90,000W of heat coming out of your tranmissions alone. So obviously, normal gearboxes are more efficient than that.
My electric motor drives the boat more efficiently at slow speeds than the diesel ICE, and it drives the boat faster too. The old system was already optimized for the engine's torque curve and propped and geared appropriately. I dont think that the torque curves are what is throwing Gerr's formulas off.
Fair winds,
Eric
> 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
>
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