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Scurvy_Dog 02-18-2003 01:33 PM

Propeller Question
 
I am new to sailing. I am an ex cooperate jet pilot and flight instructor. I understand the dynamics of a propeller in the air. Just wondering if when sailing does a free wheeling prop (eng in neutral) create more drag than a stopped prop? If so how much difference? In a propeller aircraft a stopped prop can offer considerably less drag. Talking to the mechanic who works on my engine he claims it will do no harm placing the engine in gear after shutdown.

Thx

paulk 02-18-2003 03:33 PM

Propeller Question
 
This question comes up often enough to generate considerable discussion on occasion, with various anecdotes, but the upshot is what you know applies in the air also applies in the water. The trick is to mark your propshaft when the blades (of a two -bladed prop) are vertical and thus not projecting beyond the "shadow" of the keel. Then you engage the clutch when the mark shows, and you''re done until you need the engine.

peterpan2k 02-18-2003 06:47 PM

Propeller Question
 
Hi Scurvy
You bet it works the same. as far as puting it in gear. Depends a whole lot what type of trans you have. Trans that are fluid coupled putting it in gear does nothing. there are several folding props both two and three blade for that purpose. As to effectiveness. What? You going for the world cup. You would never know the differance on a Sunday cruise around the bay.
Jim

loki 02-19-2003 03:19 PM

Propeller Question
 
Free wheeling will create less drag than a stopped prop. One of the problems with this is that this can generate heat in the transmission. If the transmission is water cooled, there is no water circulating with the engine off-ergo a potentially damaged transmission. Best check with the engine people first.

Jeff_H 02-19-2003 04:59 PM

Propeller Question
 
Actually the correct answer depends on the propellor and the transmission. There was a lot of research done on this a few years back.

The pretty widely reported results came up that a two blade prop fixed in the vertical position has less drag than a two blade prop either freewheeling or fixed in any other position.

A three or more blade prop with minimal resistance on the shaft will have less drag free wheeling, BUT if there is much friction on the shaft, the turbulence of the semi-free wheeling propellor is greater than that of a fixed prop in the ''Mickey Mouse'' position, which is the lowest drag position for a fixed three bladed prop.

Then there is the issue of transmission wear. In all cases freewheeling causes more transmission wear than being fixed. On most small marine transmissions there is no problem with lubrication or heat build up. That said some larger transmissions have shaft driven lube and cooling pumps which are not being turned when freewheeling.

Jeff


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