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Old 13-05-2011, 15:27   #46
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Re: Prop Shaft Spinning When Sailing

anyone know what the manual says about locking up (or not) my 4JH3E ??
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Old 13-05-2011, 15:57   #47
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Re: Prop Shaft Spinning When Sailing

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anyone know what the manual says about locking up (or not) my 4JH3E
It's not really a question of the engine, but of the transmission the engine is coupled to. Some transmissions are ok with unpowered shaft rotation, and some are not.
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Old 13-05-2011, 17:57   #48
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Re: Prop Shaft Spinning When Sailing

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anyone know what the manual says about locking up (or not) my 4JH3E ??
Yanmar recommends neutral. If you want to lock it they recommend a shaft brake.
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Old 13-05-2011, 18:37   #49
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pirate Re: Prop Shaft Spinning When Sailing

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Originally Posted by Maine Sail View Post
Yanmar recommends neutral. If you want to lock it they recommend a shaft brake.
Thats as maybe but I still used to pop them in reverse... the whine gets on my nerves
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Old 14-05-2011, 04:44   #50
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Re: Prop Shaft Spinning When Sailing

Shaft brakes are expensive; I just use a 10 inch vice grip with curved jaws that are well taped. Works great and cost $20. I use it for extended sailing trips and I have used it for about 7,000 sea miles.

We put a red tag on the ignition key to remind us it is on.
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Old 14-05-2011, 06:34   #51
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Re: Prop Shaft Spinning When Sailing

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Shaft brakes are expensive; I just use a 10 inch vice grip with curved jaws that are well taped. Works great and cost $20. I use it for extended sailing trips and I have used it for about 7,000 sea miles.

We put a red tag on the ignition key to remind us it is on.
I've been reluctant to use a vice-grip approach because in an emergency or when I simply forget it, the consequences of them spinning or jamming could be really bad. Heavily taped or with a foam pad and hung with a pc. of line, the shaft would probably just spin but as Murphy's Law would have it, they might just as well bind somehow and knock the entire engine out of line or punch a hole in the hull. I'm going to experiment with a light wooden yoke or maybe a cheap plastic strap wrench....something that can do no damage.
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Old 14-05-2011, 07:23   #52
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Re: Prop Shaft Spinning When Sailing

I have a Yanmar 3YM30 in my boat. I have twice put the transmission in reverse while sailing to keep the shaft from freewheeling. These two occasions were about two years apart. Both times I did this, the transmission bound up under load, and both times I broke my shifter linkage inside the binnacle at the helm trying to get the transmission into neutral again. I will not be doing that again.
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Old 14-05-2011, 16:17   #53
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Re: Prop Shaft Spinning When Sailing

Pilots know that if your engine stops you want the prop to stop as well to reduce drag. A free wheeling prop produces drag almost equal to the disc area that the prop turns in. In other words flying with a free wheeling 48" prop is like flying with a 48" disc of plywood. I know this is true from personal experience. Both water and air are fluids so you would think it would apply to boats as well. I think Main Sails experiment proves that it is not the same. So what was the difference? Main Sail tested with a three bladed prop. The blades on a 3 bladed boat prop are very wide, infact so wide they overlap. An airplane prop is very narrow. My theory is that a 3 or 4 bladed boat prop would have drag equal to it's disc area when it is stopped, so there might be a reduction in drag if it is allowed to spin. I bet a two bladed prop would have less drag if stopped. I wonder if Main Sail ever got to run his test with a 2 blade prop?
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Old 14-05-2011, 17:51   #54
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Re: Prop Shaft Spinning When Sailing

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Pilots know that if your engine stops you want the prop to stop as well to reduce drag. A free wheeling prop produces drag almost equal to the disc area that the prop turns in. In other words flying with a free wheeling 48" prop is like flying with a 48" disc of plywood. I know this is true from personal experience. Both water and air are fluids so you would think it would apply to boats as well. I think Main Sails experiment proves that it is not the same. So what was the difference? Main Sail tested with a three bladed prop. The blades on a 3 bladed boat prop are very wide, infact so wide they overlap. An airplane prop is very narrow. My theory is that a 3 or 4 bladed boat prop would have drag equal to it's disc area when it is stopped, so there might be a reduction in drag if it is allowed to spin. I bet a two bladed prop would have less drag if stopped. I wonder if Main Sail ever got to run his test with a 2 blade prop?
Yes, and the result was very similar more drag when locked.. Both props were 16X11..
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Old 14-05-2011, 18:02   #55
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Re: Prop Shaft Spinning When Sailing

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Originally Posted by HopCar View Post
Pilots know that if your engine stops you want the prop to stop as well to reduce drag. A free wheeling prop produces drag almost equal to the disc area that the prop turns in. In other words flying with a free wheeling 48" prop is like flying with a 48" disc of plywood. I know this is true from personal experience. Both water and air are fluids so you would think it would apply to boats as well. I think Main Sails experiment proves that it is not the same. So what was the difference? Main Sail tested with a three bladed prop. The blades on a 3 bladed boat prop are very wide, infact so wide they overlap. An airplane prop is very narrow. My theory is that a 3 or 4 bladed boat prop would have drag equal to it's disc area when it is stopped, so there might be a reduction in drag if it is allowed to spin. I bet a two bladed prop would have less drag if stopped. I wonder if Main Sail ever got to run his test with a 2 blade prop?
The main difference between an airplane prop and a boat prop is that the boat prop can be disconnected from the engine when it is free wheeling (ie transmission in neutral) whereas the airplane prop remains connected to the engine.

The airplane prop therefore has to rotate the engine through compression of all cylinders for it to spin which is why you want to stop it spinning by feathering the blades.

You can't compare the two systems as they are quite dissimilar in their operation.
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Old 14-05-2011, 21:50   #56
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Re: Prop Shaft Spinning When Sailing

Hi all, I have a beta /kubota diesel, and in the manual iit says put shifter in reverse while sailing. Red
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Old 14-05-2011, 22:30   #57
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Re: Prop Shaft Spinning When Sailing

Wotname, My personal experience with a stopped propeller and a free spinning propeller was in an ultralight airplane The engine connected to the prop by a centrifugal clutch. When I shut off the engine, the prop was disconnected from the engine and very free to spin. The ultralight was equipped with a sensitive variometer that measured how fast I was going up or down in feet per minute. There were several of us that tried this on different ultralights. We would climb to altitude and shut off our engines and glide to the ground. Our sink rates were much higher than we expected. One of the guys developed a prop brake that we could engage after shutting down the engine. As soon as you engaged the brake you could feel the aircraft accelerate. When you slowed to your previous speed your sink rate was dramatically less. Drag was obviously reduced a bunch and the prop was not turning the engine before the brake was applied.

Maine Sail's test blew my theory on two bladed props so now I have no idea why the difference between air and water.
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Old 15-05-2011, 00:28   #58
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Re: Prop Shaft Spinning When Sailing

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Wotname, My personal experience with a stopped propeller and a free spinning propeller was in an ultralight airplane The engine connected to the prop by a centrifugal clutch. When I shut off the engine, the prop was disconnected from the engine and very free to spin. The ultralight was equipped with a sensitive variometer that measured how fast I was going up or down in feet per minute. There were several of us that tried this on different ultralights. We would climb to altitude and shut off our engines and glide to the ground. Our sink rates were much higher than we expected. One of the guys developed a prop brake that we could engage after shutting down the engine. As soon as you engaged the brake you could feel the aircraft accelerate. When you slowed to your previous speed your sink rate was dramatically less. Drag was obviously reduced a bunch and the prop was not turning the engine before the brake was applied.

Maine Sail's test blew my theory on two bladed props so now I have no idea why the difference between air and water.
HopCar, thanks for the explanation, I hadn't considered ultralights (mainly because I know nothing about them).

Your experience is very interesting and does suggest a reduction in drag of the locked prop.

BTW, do ultralights ever have constant speed unit attached to the prop or any other way adjusting the pitch (in flight)?
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Old 15-05-2011, 10:34   #59
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Re: Prop Shaft Spinning When Sailing

Wotname, I only saw one adjustable pitch prop on an ultralight. It was a very strange single blade prop. I don't think the guy that made it ever managed to sell any.
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Old 14-06-2011, 06:41   #60
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Re: Prop Shaft Spinning When Sailing

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.... I bet a two bladed prop would have less drag if stopped. I wonder if Main Sail ever got to run his test with a 2 blade prop?
I have a pair of 2 blade props coupled to a pair of Yanmar 1GM10 diesels. While sailing well under hull speed, the difference between locking the props in reverse and free wheeling them is more than 1 1/2 knots of boat speed. With faster speeds achieved with the props freewheeling. My 2 blade propellers are pretty standard fixed, bronze type.

This confirms Main Sail's results. Free wheeling my props is definately faster, and is spec'd in my engine manual..
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