RPM

[pfmarrack]

Fellow VEGAterians,
I would like to determine if I have the best prop for "Overture" when
I pull her out in a couple of weeks. At the moment I have an unknown
3 blade prop, an unknown gearbox (i.e. - reduction) coupled to a
MD6B. The resulting power is less than generous. Could anyone tell me
the maximum rpm for the MD6B. What are the pros and cons of a 2, 3
and 4 blade prop.

If I understand correctly, the right prop diameter/pitch will allow
the engine to reach it's maximum RPM without cavitation.

Thanks as Always
Paul
V2926

 

[mocap1]

Cavitation is not an issue. The correct prop will consume all the horsepower
available from the engine at max shaft rpm. Three bladed props are more
efficient that two bladed, and four bladed still more efficient. However,
the more blades, the more drag under sail. Four blades on a sailboat are
completely unneccessary. Unless you plan to travel long distances under
power, I would reccomend you stick with a two bladed prop.

Put a mark on the top of your shaft where it comes out of the transmission.
Turn the engine over by hand, and see how many revolutions of the flywheel
are required for one revolution of the shaft. This gives you the reduction.
Call a prop manufacturer or prop shop and give him the engine speed, engine
horsepower, and reduction, and he will give you the optimum prop. Larger
diameter props are more efficient than small ones, but I think the maximum
size you can fit on a Vega without hitting the hull is 13"; check it out.

Mort

 

[pfmarrack]

Mort,
Thanks for the response. I found in the MD6A shop manual that the max
power (10 hp) is at 2400 rpm. Does that sound good to you?

Paul
V2926

 

[mocap1]

Sounds about right. However, what the prop shop needs is the shaft rpm,
which is why you have to measure the reduction.

Mort

 

[pfmarrack]

Got it!!
Thanks