# Drag and prop selection

Discussion in 'Hydrodynamics and Aerodynamics' started by Sander Rave, Jan 8, 2013.

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### Sander RaveSenior Member

Dear all,

For coming competition season, we are evaluating a new prop for our Mark Mills 43 ft Landmark 43.
Our “Flex-o-Fold” folding propeller has a drag of 3.1 Newton at a 6 knots boat speed. A 2-blade (Gori) race propeller has a resistance that equals “zero”.

Our question: What would be the advantage in time or boat speed over let's say 6 miles at 6 and 12 knots boat speed?

Displacement is 8 tonnes, we try to figure out wetted surface with Mark Mills right away.
EDIT: Wetted surface is app. 34 m^2

Sander

Last edited: Jan 9, 2013
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### JoakimSenior Member

Why don't you ask Mark Mills to do a VPP test for that. Shouldn't take more than a minute or two.

But anyway you are going to get about 0.1% or 0.01 knot difference in speed. Quite a few boats race even with a VP three bladed propeller although it has 5-10 times more drag due to curved blades.

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### Sander RaveSenior Member

Thanks Joakim,

I don't fully agree with you, as your propeller type reflects your rating. If you want speed and acceleration, it makes a difference. Hence, if you don't mind the speed too much as it's calculated in your rating I agree. If you want to sail in free wind ahead of your competition, it's a different story.
Regards,

Sander

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### JoakimSenior Member

I don't think any rating system depends on the brand of the propeller. If a sail drive is used with any folding propeller (or a 2 bladed feathering), the rating will be the same in ORC. I don't know how it is in IRC, but the application form doesn't ask for the size or the brand of the propeller thus only type (folding etc) and number of blades can have an effect.

The VP propeller I was referring to is a Volvo Penta 3 bladed folding propeller. The only difference to yours is that the curved blades do not fold away as well. There is no rating credit for this.

If you have a propeller shaft installation, even the folding propeller dimensions do affect the ORC rating. All 3 bladed folding and feathering propellers are quite draggy in a shaft installation with high shaft angle. I have seen drag measurements of about 100 N at 6 knots for this situation. No rating system gives that much credit.

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