Unfortunately there isn't one foil section that will give the best result for every prop either, plus the section needs to change from the root to the tip of each blade to allow for the very different local flow characteristics at each location (due to the variation in local velocity with diameter).
Many commercially available propellers just use a plain curve for the foil shape, often with a near-flat under surface. It's crude and gives mediocre performance, but it seems that few propeller manufacturers actually worry too much about it. I've measured the best efficiency of some off-the-shelf boat props and few do better than about 55 to 60%; many are much worse.
There are good economic reasons why off-the-shelf props are so basic, mainly to do with the need to keep the stock of different propeller types down to a manageable level. Once you start really optimising a propeller for a particular application you soon realise that it won't work well in any other application, making it a niche product.
Normally, people don't care too much about getting the last few percent out of the propeller - they can just fit a bigger engine, or open the throttle a bit more, to make up for it. It's only really those of us with very limited power sources that can see that there are really big gains to be had by optimising the propeller to best suit the boat. One of the best practical demonstrations of this has been Denny Wolfe's experiments with a simple electric trolling motor. Just by making some fairly simple modifications he managed to get a massive performance improvement, almost doubling his effective range on a given amount of stored energy as I recall.
The electric boat market is too small at the moment to be a significant driver in commercial propeller design, but I think it will change. I took part in a competition a couple of months ago, run by a boating magazine. The idea was to race a boat round a dog leg course powered only by cordless power tools. Out of the dozen entrants, all the very fast boats were using model aircraft props. All the boats using conventional boat propellers were pretty slow. There were a lot of people there who saw this and went away wondering why conventional boat props were so poor............
Jeremy
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> ah yes, i see what you are saying.... what I was looking for was some indication of how well they have designed the foil shape of the prop... perhaps in these days every manufacturer is onto foil shape and one particular shape is used by nearly all.... one could of course use a perfectly flat blade and it would produce thrust but as one approaches the "perfect" foil shape the thrust would improve for the same power applied no matter what the other conditions were contributing.... isn't this true or am i missing something?Â
>
>
>
> ________________________________
> From: Jeremy <jeremy_harris_uk@...>
> To: electricboats@yahoogroups.com
> Sent: Friday, September 16, 2011 9:00 AM
> Subject: [Electric Boats] Re: 1000w per ton? - massively oversized motors
>
>
> Â
> Good question and not one that's easy to answer, I'm afraid.
>
> The main problem is that there is no such thing as a definitive efficiency curve for a propeller, as propeller efficiency depends on thrust, rpm and inflow velocity (usually boat speed through the water). Because these are highly boat-specific, a prop manufacturer can't provide an efficiency curve that means very much.
>
> Here's an example, using my own 13" home-made prop (which is actually 330mm diameter x 385mm pitch):
>
> At 20N thrust, 330rpm and a boat speed of 4mph it is 86% efficient.
>
> At 40N thrust, 365rpm and a boat speed of 4mph it is 78% efficient.
>
> The thrust required to push the boat along at 4mph is dependent very much on displacement/length ratio and hull design and isn't something the propeller designer can predict.
>
> There are ways to model the most efficient propeller for a given hull though. If you know the hull resistance characteristics and the desired cruise speed then you can use one of the "known to work well" propeller calculators. However, some of these calculators are significantly in error for lower speed and power requirements, as they may have been designed for higher speed and power applications, like outboard motors on planing boats. One model that I know works extremely well is Javaprop, by Martin Hepperle. It was intended for optimising model aircraft propellers, but if the fluid parameters are changed to those for water, rather than air, it gives very accurate results for high efficiency boat propellers. It is a little challenging to use, because it requires a lot of iterations to derive the best results, but I've yet to find anything to beat it, other that lots of practical tests. It models my own propeller to within about 2 - 3% of the true
> figure, which seems pretty good to me.
>
> Jeremy
>
> --- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@> wrote:
> >
> > where does one get the efficiency curve for the propeller?    it seems that I would like the 18x10 that i found based on rpms and hull speed but i've not run across any efficiency graphs for propellers....  it seems that the minute any manufacturer mentions efficiency they immediately start writing of the ICEs power curve when combined with a prop but never strictly the prop
> >
> >
> >
> > ________________________________
> > From: Jeremy <jeremy_harris_uk@>
> > To: electricboats@yahoogroups.com
> > Sent: Friday, September 16, 2011 5:42 AM
> > Subject: [Electric Boats] Re: 1000w per ton? - massively oversized motors
> >
> >
> > ÂÂ
> > Partially true, yes, but not the whole story. Certainly the I²R losses are proportional to the square of the torque - i.e. doubling the torque on a given motor means quadruple the I²R loss - but the other motor losses either have no power/rpm/torque dependence (frictional losses in bearings and seals, for example, tend to be fairly constant) or else are strongly rpm (and hence voltage) dependent (eddy current loss in the cores, switching losses in the controller and windage loss from the rotor and any cooling fan).
> >
> > The key to getting good efficiency is to pick a motor carefully to fit your requirement, which means knowing pretty much how much power you need to drive the hull, plus knowing the rpm of the most efficient prop you can fit, and then run that motor in the best efficiency region most of the time.
> >
> > I guess few people would want to try and optimise things to the extent I have (and I freely admit it has soaked up a lot of time), so maybe using a rough rule of thumb for sizing a motor etc would be fine for the majority.
> >
> > Jeremy
> >
> > --- In electricboats@yahoogroups.com, "Steve" <sstuller@> wrote:
> > >
> > > The efficiency is a function of the torque. Even if you cut the voltage in half and the torque(amp draw) remains the same the efficiency doesn't change. Thanks. Steve S.
> > >
> > >
> >
>
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