# QPC of a Sail drive

Discussion in 'Inboards' started by Ogrim, Feb 27, 2013.

1. Joined: Jan 2013
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Location: Southampton [UK]

### OgrimJunior Member

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.

2. Joined: Apr 2004
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Location: Finland

### JoakimSenior Member

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.

3. Joined: Jan 2013
Posts: 10
Likes: 1, Points: 3, Legacy Rep: 13
Location: Southampton [UK]

### OgrimJunior Member

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 ?

4. Joined: Apr 2004
Posts: 892
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Location: Finland

### JoakimSenior Member

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.

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