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 JordanH 09-14-2012 03:20 PM

Propeller Drag - Challenging my intuition

Hi All,

I generally trust my gut feeling / intuition on things and on another forum someone posted a question about sailing in gear or in neutral. The discussion made me second-guess what I thought was correct.

So... have at it. The basic question is, "Should you sail with the engine in reverse gear, forward gear or neutral?" (Side note for flyingwelshman, I mean while the engine is turned off... ;) )

My gut-feeling is "keep it locked in reverse while under sail."

However, Yanmar (and other) manufacturers say that is incorrect due to strain on the engine. And this article goes even further saying a spinning prop has LESS drag.
http://www.catamaransite.com/propeller_drag_test.html
The latter claim is what blows my mind and goes strongly against my intuition about how the physical world works.

Ok.... DISCUSS!

 asdf38 09-14-2012 03:33 PM

Re: Propeller Drag - Challenging my intuition

I'll be following this because I don't the right answer. I always put my yanmar in neutral and it sounds like that's the right thing.

However as for drag I think the intuitive answer is that of course a spinning prop causes less drag? A spinning prop afterall drives the boat forward. So there is a continum here that goes like this:

prop speed > boat speed = propulsion
prop speed = boat speed = no effect (this one rarely happens)
prop speed < boat speed = drag
prop not moving = more drag
prop driving in reverse = even more drag

What would the argument be otherwise?

 JordanH 09-14-2012 03:42 PM

Re: Propeller Drag - Challenging my intuition

If you've ever fished with a spinning lure on the end of your line you'll notice the significant force that it takes to drag that lure through the water. You can feel the reduction in drag immediately should the spinner get hung-up and stop rotating. Your line immediately goes light and you can reel in quite quickly.

My gut feeling is from observation of dragging many a Mepps through the water.

 arf145 09-14-2012 03:43 PM

Re: Propeller Drag - Challenging my intuition

Actually, my intuition (right or wrong) agrees with MaineSail's findings. Stationary blades have a certain amount of drag. If those same blades are allowed to be pushed out of the way (turning), thus not resisting as much, you get less drag.

 JordanH 09-14-2012 03:55 PM

Re: Propeller Drag - Challenging my intuition

arf145, remember, when they are "pushed out of the way", they are actually displacing water horizontally. It's not like they are being moved into a vacuum.

One added wrinkle, is that I have a full keel and keel mounted rudder. So if my 2-blade prop is locked vertically, there should be no additional drag. A free-wheeling prop, on the other hand, is not. Of course the argument is that I have a slim chance of having that blade lock vertically.

 asdf38 09-14-2012 03:58 PM

Re: Propeller Drag - Challenging my intuition

Quote:
 Originally Posted by JordanH (Post 922241) If you've ever fished with a spinning lure on the end of your line you'll notice the significant force that it takes to drag that lure through the water. You can feel the reduction in drag immediately should the spinner get hung-up and stop rotating. Your line immediately goes light and you can reel in quite quickly. My gut feeling is from observation of dragging many a Mepps through the water.
Without knowing more I'd assume there is a secondary effect happening there. Perhaps the blades retract when not moving or something.

With propellers the analogy in my mind is simply like the rear wheels of a car. It's a hell of a lot better to let them spin than to apply the brakes. Locking the prop on a boat is like dragging a stuck wheel behind it. Better to put that wheel in netutral and let it spin. There will still be friction obviously but better than locking it.

 johnnyquest37 09-14-2012 04:02 PM

Re: Propeller Drag - Challenging my intuition

Quote:
 Originally Posted by JordanH (Post 922241) If you've ever fished with a spinning lure on the end of your line you'll notice the significant force that it takes to drag that lure through the water. You can feel the reduction in drag immediately should the spinner get hung-up and stop rotating. Your line immediately goes light and you can reel in quite quickly. My gut feeling is from observation of dragging many a Mepps through the water.
But if you were to lock the spinner so that it was perpendicular to the direction of travel, I suspect the drag would be greater than if the spinner was spinning.

The fixed propeller with transmission in reverse will not spin, causing drag relative to the area of the blades. If the prop is free-spinning, it is still causing drag, but not nearly as much. This web site http://www.catamaransite.com/propeller_drag_test.html documents one fellow's experiment and he determined that a locked prop creates nearly three times as much drag as the free-spinning prop.

Drag is not the only issue. Preventing damage to the transmission left spinning is the primary reason for preventing the prop from spinning when the engine is off.

 asdf38 09-14-2012 04:03 PM

Re: Propeller Drag - Challenging my intuition

Quote:
 Originally Posted by johnnyquest37 (Post 922251) But if you were to lock the spinner so that it was perpendicular to the direction of travel, I suspect the drag would be greater than if the spinner was spinning. The fixed propeller with transmission in reverse will not spin, causing drag relative to the area of the blades. If the prop is free-spinning, it is still causing drag, but not nearly as much. This web site http://www.catamaransite.com/propeller_drag_test.html documents one fellow's experiment and he determined that a locked prop creates nearly three times as much drag as the free-spinning prop. Drag is not the only issue. Preventing damage to the transmission left spinning is the primary reason for preventing the prop from spinning when the engine is off.
Umm, you didn't read the original post too closely..

 JordanH 09-14-2012 04:05 PM

Re: Propeller Drag - Challenging my intuition

I'm not convinced the wheel analogy is apropos since the 'drag' caused by a locked wheel is high because of friction.

Another point I read in a link was that if you have a folding prop (I don't) then free-wheeling the prop means the blades will/may stay open and cause drag instead of folding and getting out of the way. Clearly there are other variables and considerations to take into consideration.

 peoples1234 09-14-2012 04:05 PM

Re: Propeller Drag - Challenging my intuition

The did a test in Yachting Monthly a few months ago on this exact subject.

Leaving it in neutral and letting the prop spin was much less resistance than not allowing the prop to spin freely. A three blade prop that was not allowed to spin caused as much drag as dragging a 5 gal bucket through the water.

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