QPC of a Sail drive

[Ogrim]

Hello,

I would like to know what would be a typical Quasi propulsive coefficient for a Sail Drive.
I know it is less than a classic shaft, but I have no idea of how much.
If anyone have an idea ?

I guess only the rotative efficiency is going to change.

Thank you for your answers =)

 

[Joakim]

How do you know that a sail drive is less efficient than a classic shaft? I wouldn't think so.

It all depends on the boat and how the installation was made, but generally I would assume for most sail drive installation that QPC is equal to open water efficiency.

 

[Ogrim]

You can generaly assume for a in-line shaft a QPC of 50 - 55%
Basically the QPC = hull eff. x open water eff. x rotative eff.

Let's assume I have a hull given the following values at 10kts speed with a in-line shaft:
hull efficiency = 0.9, Rotative efficiency = 0.95, open water efficiency = 0.65 (it is fully assumption by could be significant)
given a QPC around 55%. If I now replace my shaft line by a sail drive. What coefficients are going to change ?

Thank for your reply and don't hesitate to tell me if I am misleaded.

 

[Joakim]

This depends a lot on the hull form and location of the propeller, but generally a sail drive positions the propeller further away and not "behind" the hull. Thus it is less affected by the hull and all the correction terms to open water efficiency are closer to unity.

65% open water efficiency would be exceptionally high for a typical sail drive propeller as well as the 10 knots speed.