Hi Eric,
I've been an accomplice/victim of my marine engineering and naval architect brethren for probably as many years as almost anyone on this list (I'm so old-school that only navies could afford to design and build ships back when I started - I don't even know any "marine architects" ;) Marine engineers and naval architects have long suffered under false assumptions for centuries, so, why change now? As I found out a long time ago, like military and other government entities, they can be proud, tradition-bound professions, unfettered by progress. There are many of these people who don't even understand how or why multi-hulls work - they can't be bothered to think about anything beyond that with which they grew up. Don't even get me started on how few understand how to use high-performance computing, simulation and, in some cases, even CAD-CAM, to assist in design and implementation.
Seriously (who, me?), I'm guessing that Gerr's work has not been updated to reflect knowledge that's been developed through some of the high-tech tools mentioned above, as well as non-traditional prime-mover technology. I'm not even sure that his work can be used beyond a fairly narrow set of hull forms which, as you can imagine, have a significant effect on such calculations - even fractions of a percentage difference in hull planform can result in drastically different drag numbers. Just talk to a hydrodynamicist who's worked on America's Cup hull designs, or any aerodynamicist who knows their way around the many NACA airfoils that can vary by minuscule amounts, and yet whether they get wet from precipitation can result in fatally-different behavior in two nearly-identical airfoils. I'll see if I can come up with a more straightforward or visceral description of what's behind what you're observing, but, I'm pretty sure it's related to the combination of the different torque/power vs. RPM curves with the drag vs. speed curves.
The issue you've identified reminds me a lot of how Otto Lillienthal's lift/drag numbers were accepted as gospel by the best minds in aerodynamics for decades (even years after he died attempting to fly gliders based on them - sometimes, ya just gotta be more curious), including the government's top (well-funded) scientist, Langley. It wasn't until the Wright Brothers came along and couldn't successfully relate Lillienthal's numbers to their kite and glider flying experiences, that they built a simple wind tunnel and discovered that his numbers were off by up to nearly an order of magnitude. Just before that, when they were invited to attend an international conference on aerodynamics, they were excited to finally be able to talk to the "experts" to find out what they were doing wrong, and they were stunned to discover that they already knew more than anyone else in the world (indeed, Langley had been responsible for ensuring they were invited, as they were already known to be secretive, and would abandon their work when anyone not local to Kitty Hawk was in the area).
I can provide lots more examples of how the "experts" have been incredibly wrong for extended periods of time:
- until very recently, dark matter and energy were comic book terms - now, the former comprises two-thirds of the mass in the universe, and the latter is likely responsible for the accelerating expansion of galaxies
- the number of human genes was "known" to be ~100,000 for decades until the first human genome was sequenced around the year 2000, and it was found to have ~20,000
- the fourth form of life, Archea (found around undersea volcanic vents), was completely unknown before ~1970, yet comprises more biomass on the planet than the other three, combined (animals, plants, and single-celled organisms)
- until 1947, the sound barrier couldn't be broken and, even if it could be, humans couldn't survive it
- the universe was believed by the best theoretical physicists in generations to be in a steady state, neither expanding nor contracting, until it was observed to be expanding ... and accelerating
- atoms were the smallest particles; until protons, neutrons, and electrons were; until neutrinos, positrons, quarks, leptons, mesons, gluons, etc., were ...
- eugenics was universally accepted during the time of the Wrights, that genetics alone determined intelligence, morality, etc.
- the speed of light waves was instantaneous, until they were found to also be particles, and not much faster than molasses in January, on the scale of the universe
- it was believed before ~1820 that no human could continue breathing on trains doing more than 30 mph
- there were only five perfect solids, the existence of the sixth, the dodecahedron, was suppressed upon its discovery, and its discoverer was banished to live out his life alone in a remote cave
- the Earth was flat and at the center of the universe ... disagreement was at risk of death
I'm sorry, what were you saying about the applicability of Gerr's work beyond ICEs on cargo ships? ;)
All the Best,
Jim
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> 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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