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e31 04-18-2004 08:30 PM

lowest drag for fixed 3 blade?
 
I have an Ericson 31I with a 15" fixed three blade prop. Being that a self-feathering prop is out of the budget, what is the lowest drag situation for this fixed 3-blade gear:

1- freely rotating

or

2- locked in the "mickey mouse" position with two blades up and one inline with the keel?

or

3-quit worrying about the details and just sail.


Any thoughts?

Doug

39512 04-19-2004 04:50 AM

lowest drag for fixed 3 blade?
 
How about a used two blade fixed prop? I suspect that there are some out there, since it is a common refit to change to a folding two blade prop.

Otherwise, I think it should be locked. You might find which position is best by marking the shaft with two blades up and two blades down then testing.

PaulBl 04-19-2004 07:06 AM

lowest drag for fixed 3 blade?
 
A fixed prop has about the same drag no matter who makes it. You can vary the diameter or the picth but thats all there is.

As far as locked or unlocked goes. It is more important to consider the transmission. My Hurth transmission requires the transmission locked in reverse so the prop does not freewheel under sail. Not all transmissions are like that so you need to find out. Some others can freewheel fine.

As far as if you can freewheel if it better I''ve seen it argued both ways. Since I have to lock mine I don''t care<g>.

If I had the money I would go with a Martec 3 blade. For now I lock the prop and just sail the boat.

jkumin 04-19-2004 09:26 AM

lowest drag for fixed 3 blade?
 
A German University ran tests a few years ago on several folding and feathering three bladed props. They used a fixed three blade as a control. At 8 knots the fixed prop drag was 420 Newtons locked and 180 free-wheeling. By contrast the better low drag (folding or feathering) props were around 12 Newtons drag. A Newton is around .22 pounds.

The point above is correct about transmissions - some need the input shaft turning for proper lubrication. There is also the noise and wear issue. Always nice to hear that low rumble stop when the prop
is locked.


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