|
|
|
|
|||||
|
||||||
From: (no name) (no email)
Date: Sun May 04 2008 - 00:35:56 EDT
In a message dated 5/4/08 12:00:37 AM, JHWard writes:
> BTW - I have never heard a definitive treatise on the drag from a
> windmilling vs locked/stopped marine prop. In the aviation world, a
> windmilling prop
> has orders of magnitude more drag than a stopped one (even if not
> feathered).
> My belief is that water props have less drag if spinning (can someone that
> really knows weigh in on this?). Obviously the best solution would be to
> feather it like some sailboats do. Or take it off.
>
Competitive sailors, of which I was one, differ on this point. The bulk of
the research seems to indicate that a very freely spinning prop has the lowest
drag. The catch is that the spin rate has to be almost the same as the prop
would turn if powered by the engine. If the prop is slowed by friction in the
driveline, the drag increases, reaching a maximum as the prop slows down to
1/2
or 1/3 speed. The drag then declines to a fairly low point if the prop is
locked.
The easiest thing to do when running on one engine is simply to lock the
prop. With purely mechanical transmissions this is achieved by leaving the
tranny
in gear after the engine stops. It won't work with hydraulic activated
gearboxes. In that case use a prop lock. Sailboat catalogs list them but we
used a
large vice grip locked around the shaft. Be sure to remove it before starting
the engine or you will hear loud and probably expensive noises. Sailboats with
twin bladed props lock the prop behind the deadwood to obtain minimum drag.
Larry Z
**************
Wondering what's for Dinner Tonight? Get new twists on
family favorites at AOL Food.
(http://food.aol.com/dinner-tonight?NCID=aolfod00030000000001)
_______________________________________________
http://lists.samurai.com/mailman/listinfo/trawlers-and-trawlering
To unsubscribe or modify your subscription options (get password, change email address, etc) go to: http://lists.samurai.com/mailman/options/trawlers-and-trawlering
Trawlers & Trawlering and T&T are trademarks of Water World
Productions. Unauthorized use is prohibited.
|