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Old 01-07-2015, 20:19   #46
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Re: Do you sail with your transmission in gear?

We are outside the OP question. Our transmission is a hydraulic clutch so the engine stopped situation creates a free-wheel situation. We have a Hundested variable pitch blade mechanism between the transmission & prop so we feather the prop to in-line. No rotation and very little drag.
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Old 01-07-2015, 21:17   #47
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Re: Do you sail with your transmission in gear?

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Originally Posted by Nicholson58 View Post
I beg to differ courteously.

A spinning prop will cause a higher parasitic drag than a locked prop. It takes more energy to spin the prop than is generated in the turbulence around a stalled prop. Similarly, a stalled fan exhaust reduces the power consumed by the fan. Also, a stalled discharge of a water pump will draw less power than the same pump delivering flow.
Where have you been? This has been argued ad nauseum around here. You sould have piped up earlier. I know all about airplanes and spinning vs stopped propellers and they say it does not apply here, they say. After a while I guess I believe them but I still stop the prop.
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Old 01-07-2015, 21:56   #48
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Re: Do you sail with your transmission in gear?

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Originally Posted by Nicholson58 View Post
I beg to differ courteously.

A spinning prop will cause a higher parasitic drag than a locked prop. It takes more energy to spin the prop than is generated in the turbulence around a stalled prop.
Not according to the practical tests that have been reported and discussed here on numerous occasions.

Check out Prop Shaft Spinning when Sailing
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Old 01-07-2015, 22:04   #49
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Re: Do you sail with your transmission in gear?

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Did Yachting Monthly give the drag friction when prop is fixed? How does it compare to the 0.5K in neutral?
Yachting Monthly calculated the increased drag for a locked prop at 6kts as 0.5kts. Their test was performed with 2 outboard motors, 1 to provide thrust and 1 to act as a miniature sail drive. They also compared a fixed 3 blade prop against a folding or feathering prop.

At a GPS measured range of speeds the drag was measured with the other prop either fixed or in neutral and plotted graphically. Even at low speeds there was quite a marked increase in drag from the locked prop over the free rotating one. By scaling up the results they calculated that locking the prop at 6kts is the equivalent of dragging a bucket behind the boat (average cruiser type) and by allowing it to rotate you gain an extra 0.5kts speed. The effect of having a folding or feathering prop were even greater.

Obviously you can only have a fixed blade prop in one of 2 states - rotating or locked. YM showed that allowing it to rotate produced less drag than locking it.

At the end of the day it comes down to what your gearbox can take and where there is a choice over locking or rotating the choice is down to the individual.
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Old 01-07-2015, 22:56   #50
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Re: Do you sail with your transmission in gear?

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I leave mine in neutral, but I have a folding prop 👌


Sent from my iPhone using Cruisers Sailing Forum
In case you have any folding propeller as flexofold props opened by centrifugal force Engage reverse, it will stop rotating and the flow of water will make it fold.
Otherwise it will remain open and will rotate.
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Old 02-07-2015, 02:07   #51
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Re: Do you sail with your transmission in gear?

A wind-milling solid (non-folding) propeller, on a boat or an airplane, creates the same amount of drag as a solid disk the same diameter as the propeller. If you have a 14"non-folding or non-feathering propeller wind-milling (or water-milling) while under sail you might as well have a 14" solid plywood disk bolted to the end of your prop shaft. Locking the shaft, either by engaging the transmission or with a shaft lock will reduce the parasite drag to that produced by the frontal area of the individual prop blades exposed to the slip-stream, which depending on the number of blades, could be considerable. With the shaft locked a two-blade prop will therefor produced much less drag than a four-blade prop of the same diameter, whereas water-milling they will produce exactly the same amount of drag. Water-milling props may also cause transmission damage in some installations.
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Old 02-07-2015, 03:30   #52
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Re: Do you sail with your transmission in gear?

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Originally Posted by DMCantor View Post
Yanmar has been very confusing in their recommendations on this, but after considerable research, here is my understanding. This applies only to Yanmar, no data on other engines.



2) If you have a folding or feathering prop, it is best to put the transmission in gear while sailing. This allows the prop to go into low-drag mode. We always use reverse, but I am not certain whether it matters, or which gear is better.

David
s/v Callisto
This is not correct. If you have a Maxprop (feathering) and put the gearbox in reverse the blades will extend fully in revere mode. Maxprop notes in their literature that tis can be used to drive a generator f.eks.

Maxprops manual says take the tranny our of gear while sailing 3 knts - the prop will feather. If it doesn't put it in forward gear for a few seconds then neutral - now it will be feathered.

Under no circumstances should you drop it into reverse
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Old 02-07-2015, 05:47   #53
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Re: Do you sail with your transmission in gear?

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Originally Posted by jmschmidt View Post
A wind-milling solid (non-folding) propeller, on a boat or an airplane, creates the same amount of drag as a solid disk the same diameter as the propeller. If you have a 14"non-folding or non-feathering propeller wind-milling (or water-milling) while under sail you might as well have a 14" solid plywood disk bolted to the end of your prop shaft. Locking the shaft, either by engaging the transmission or with a shaft lock will reduce the parasite drag to that produced by the frontal area of the individual prop blades exposed to the slip-stream, which depending on the number of blades, could be considerable. With the shaft locked a two-blade prop will therefor produced much less drag than a four-blade prop of the same diameter, whereas water-milling they will produce exactly the same amount of drag. Water-milling props may also cause transmission damage in some installations.

Very well written treatise. To bad none of it is correct when applied to sailboat props....
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Old 02-07-2015, 06:12   #54
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Re: Do you sail with your transmission in gear?

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Originally Posted by jmschmidt View Post
A wind-milling solid (non-folding) propeller, on a boat or an airplane, creates the same amount of drag as a solid disk the same diameter as the propeller. If you have a 14"non-folding or non-feathering propeller wind-milling (or water-milling) while under sail you might as well have a 14" solid plywood disk bolted to the end of your prop shaft. Locking the shaft, either by engaging the transmission or with a shaft lock will reduce the parasite drag to that produced by the frontal area of the individual prop blades exposed to the slip-stream, which depending on the number of blades, could be considerable. With the shaft locked a two-blade prop will therefor produced much less drag than a four-blade prop of the same diameter, whereas water-milling they will produce exactly the same amount of drag. Water-milling props may also cause transmission damage in some installations.
Read this: http://www.catamaransite.com/files/propeller.pdf

It shows that a free wheeling fixed blade prop has less drag than when locked. The folks who performed the testing and wrote the report are pretty smart.
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Old 02-07-2015, 06:14   #55
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Re: Do you sail with your transmission in gear?

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Very well written treatise. To bad none of it is correct when applied to sailboat props....
Not being any kind of aeronautical or hydraulic engineer, I would guess as a layman, that one of the reasons for different rules would be that air is compressible, and water, pretty much isn't (at least with normal pressues likely to be induced by flying or sailing).
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Old 02-07-2015, 08:19   #56
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Re: Do you sail with your transmission in gear?

it is my understanding that the prop should be braked (preferably external from the transmission).
for my vessel, the engine must be running for the transmission cooler to keep the transmission from overheating (which can occur if the transmission is being back-driven under sail or from tow).
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Old 02-07-2015, 11:03   #57
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Re: Do you sail with your transmission in gear?

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Originally Posted by CCR2580 View Post
What is the recommend method of sailing...keep your transmission in gear either forward or reverse to stop the prop from turning or just leave in neutral and let prop turn?
I leave it in reverse
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Old 02-07-2015, 11:52   #58
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Re: Do you sail with your transmission in gear?

For the Yanmar 3YM30 with a SailDrive SD-20, the correct procedure according to the manual is to leave the gear shift in Neutral and let it freewheel. If you have a folding prop, put the shifter in reverse until the prop folds, and then return it to neutral.

This is probably correct for any transmission with a folding prop, but check your manual.
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Old 02-07-2015, 12:00   #59
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Re: Do you sail with your transmission in gear?

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Originally Posted by jmschmidt View Post
A wind-milling solid (non-folding) propeller, on a boat or an airplane, creates the same amount of drag as a solid disk the same diameter as the propeller.
That's not true. Here's a paper showing the drag characteristics of a freewheeling prop: "MacKenzie P M and Forrester M A, ‘Sailboat Propeller Drag’, Ocean Engineering, V35, pp28-40, 2007." Sorry, I don't have a URL.

You can test all this yourself. When pointing with a consistent wind and doing close to hull speed, graph your speed over ground with the prop engaged and then disengaged for a few periods. You'll clearly see a .25 to .5 increase in speed corresponding to a decrease in drag when you're freewheeling.

Folding vs. fixed is worth 0.5 knots on my boat, which I've tested.
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Old 02-07-2015, 12:38   #60
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Re: Do you sail with your transmission in gear?

I'm not an Aeronautical Engineer, but I have one that works for me, who happens to be an avid racer as well. We have talked about this apparent difference in prop drag in airplanes vs sailboats, and it's been proven over and over that an airplane prop freewheeling causes more drag than a stopped prop, where a sailboat and therefore I assume even a powerboat prop is just the opposite. What he and I have theorized is the biggest difference is the blade shape, if you look at a common three blade sailboat prop, you can tell almost the entire disk is blocked by prop blade when not rotating, a airplane prop is very thin blades by comparison.
If I were to go to a fixed prop again, I believe I may look real hard at a two blade and fix a shaft brake where the blades were blocked by my considerable dead wood, being a full keel boat.

Someone else on this site wrote about using a motorcycle brake disk and had a golf tee as the key that kept the shaft locked, the golf tee would of course easily shear without damage to anything if you forgot and left it in.
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