Hi Jim,
So what you're saying is that Gerr's formulae, which have been accepted in the marine engineering world aren't applicable to electic drives in the way that we use them.
Sound fair enough. But when we discuss power requirements with a marine architect, how do we convince them that the calculations that they know to be accurate are, in fact, not accurate.
Saying "trust me, you don't know what you're talking about" isn't going to further many conversations. Being able to explain WHY (I'm not yelling) Gerr's predictions may not be accurate, will go alot farther in acvancing the acceptance of electric drives in the mainstream marine world.
One of the things that Gerr states is that his predictions are for shaft horsepower, not fuel or energy consumed. So I believe that introducing thermal efficiency and driveline losses is a red herring. Shaft horsepower doesn't care whether the power came from ICE, electric, or some guy pedaling like crazy. So you don't get to discuss engine efficiency.
Somehow, I think that the explanation might be simpler than what you proposed.
Fair winds,
Eric
--- In electricboats@yahoogroups.com, "jim_ranger_26" <jim_manley@...> wrote:
>
> Hi guys,
>
> Not to confuse things even more (oh, what the hell, everyone else is having too much fun :) but, please bear with me while I get a bit pedantic/pedestrian/Neanderthal and bring things back to first principles (I'm trying to make this approachable to the lurkers we all know are out there, who are too afraid to ask the experts what are actually not-so-dumb questions).
>
> There are only two ways to measure shaft horsepower directly, and that's by either:
>
> - connecting the prop end of the prop shaft to a dynamometer (basically, a cylinder filled with an oil having precisely-known characteristics, and equipped with a calibrated friction-creating mechanism; or a calibrated generator/alternator connected to a calibrated and adjustable load)
>
> - putting differential rotational strain gauges on the prop shaft, just forward of the prop, and on the output side of the engine/motor shaft (obviously, with some means of transmitting strain data remotely while the shaft is turning). For the non-engineers, rotational strain is the measured number of degrees of twisting of the shaft as torque is applied along the length of the shaft, between the engine/electric motor and the prop. Strain should not be confused with stress, which you can consider as being directly related to the torque, so, we'll bundle stress into the torque, and not worry about it any more here.
>
> These allow direct measurement of the instantaneous torque (and therefore, work, and, over time, power, in horsepower, watts, Newton-meters-per-second, or whatever), being applied along the length of the shaft for any given RPM, taking all of the other variables out of the equation, for now.
>
> Of course, we already know that the engine torque(power)/RPM curve will increase exponentially initially, go somewhat linear for a while, then start decreasing in slope sort of parabolically until the peak of the curve is reached, then pretty rapidly fall off at some high-exponential rate (until certain failure at a high-enough RPM).
>
> We also expect that the electric motor torque(power)/RPM curve will have something like an inverse exponential curve, offset to the left such that the maximum torque is available where the curve touches the positive Y axis (i.e., 0 RPM), and gradually petering out as RPM on the X axis increases until the RPM of certain failure is reached.
>
> I suspect Eric now knows where I'm going with this - the two curves are _completely_ different shapes, and that's why Gerr's equations for engines don't hold well for electric drives _ACROSS_SOME/MUCH_OF_THE_RPM_RANGE_ (sorry, I'm not yelling, I'm just trying to emphasize the important stuff). Shaft horsepower _DELIVERED_ is _NOT_ the same for engines as electric drives _FOR_ANY_PARTICULAR_RPM_ (it's not even identical for different engines, and especially different engine technologies, e.g., piston/Wankel-rotary/etc.). In fact, if it weren't for the roughly fourth-power increase in friction (Friction ~= k*(Speed^4) ) of a displacement hull as you get anywhere above about half of hull speed (it's closer to second-power below that, and the curve goes essentially vertical near/at hull speed - you start planing, not necessarily prettily, above that point, and the curve changes radically up there), Gerr's systemic curves (fuel or electrical power consumed vs. speed achieved) would have absolutely nothing in common with each other for the two kinds of power sources. The only reason you are seeing any concordance at all is because of that severe increase in friction with speed - the good ol' Second Law of Thermodynamics is seeing to that (it states that, not only is there no free lunch, but, you can't even get anywhere close to break-even, especially as energy expended is increased - entropy/disorder/chaos increases much faster across the entire system).
>
> What I'm basically saying is that, unless you do the torque/RPM measurements described above and adjust Gerr's formulae for them, you're going to be comparing pineapples to tennis balls, if not outright McIntosh/Delicious/Fuji/etc. to igneous/sedimentary/metamorphic objects.
>
> Tossing of over-ripe fruit, vegetable, and other metaphors now heartily welcome - please try to avoid actual mineral-based objects, but, if you must, be advised that I can duck and run pretty well for someone of my advanced senility! ;)
>
> All the Best,
> Another Jim
>
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