This is the strangest thread we have ever read. There are too many assumption needed to even comment, but here goes;
Before you can start comparing 2 blade to 3 blade, diesel to gas to electric, we need to know the the gear reduction of each.
We need to assume one is going to use the same diameter and pitch whether 2 blade or 3 blade. Then we can START looking at theory and finally compare to actual results.
What exactly do you mean by a prop being a little "too powerful?" Too much pitch, too large a diameter or too much cup?
When you speak of a "smaller prop" do you mean diameter, pitch, cup or all 3?
The last time we checked 1hp=746W, not 760.
In our experience of building aux power packs;
10hp gas powered, requires 7.5hp diesel or 5hp electric for the same loads.
Additionally a lot of consideration on a sail boat needs to be give to the drag of the prop when not under power. Again, we must make the assumption that the prop folds?
With your Yanmar, is would be interesting to accurately measure your fuel burn rate and then look up the hp on the manufactures power curve diagrams.
--- In electricboats@yahoogroups.com, "Mike Gunning" <mike@...> wrote:
>
> A question was asked on the Columbia Yahoo site and I thought I would share my answer with this group. The question was about the boat with a 10hp Volvo with a 2 blade prop and changing it to a 3 blade prop and possible issues. The technical issues of propellers are not as well understood by most of us. I would like to see a technical discussion of these issues and a formula that is easy to apply. This is especially important to the DIY as the system units are engineered.
> Mike Electric Yacht of Southern California
>
> Jeff,
> I agree with Jim in his assessment that the Volvo 10hp is a little light for the boat. The two blade prop is likely the only way the motor can get to its power curve.
> We have a 15hp Yanmar 2QM15 on our boat (the factory offered this motor) plus our electric motor. With the factory 2QM15 a two blade prop was recommended. We installed a 3 blade 14X12 prop and the prop is likely a little too powerful but it does get us to 6 kts with hull speed being 6.57. We know the true shaft hp used when we run the electric with the computer display. We artificially limit the electric because of the small diesel thrush power inhibitor.
>
> This is what we see calculated at shaft hp (shp) and which is not brake hp (bhp). Bhp is a bench test without the losses from transmission and alternator and other loads on the motor and at peak power rpm. These numbers are spot on (formula is (amp X voltage)/760 = hp):
>
> 4 knots at 40amps 48v is 2.25shp
> 5 knots at 80amps 48v is 5.05shp
> Our computer projections calculates the requirement for 6 knots as
> 6 knots at 150amps 48v is 9.47shp
>
> Your Volvo is likely in good enough shape will likely put out about 7.5 shp, less than the 11.25 shp that our Yanmar does.
>
> I would think that the two blade prop is right for your boat. If you want to go with a smaller three blade prop, you should work with a good prop guy and follow their recommendations. I can tell you that the prop is a very important part of the propulsion equation. Get an expert as I know from direct experience when we built a system with the wrong prop information it does not perform. We re-engineer and re-program the system once we get the correct data and it works
> as projected.
> Mike
> sv Electric Fluke Columbia 9.6
> Electric Yachts of Southern California
>
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