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Old 02-07-2015, 14:16   #61
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Re: Do you sail with your transmission in gear?

Drag characteristics change dramatically between different Reynolds number regimes. Hydrodynamics of a boat moving 8 knots is a totally different drag regime than prop aircraft moving 300 knots.

Essentially the viscosity of the fluid being moved through is dramatically different and therefore the induced drag is different. The effect is also non-linear and impossible to guess about.

That's why you test. Simple tests prove that a freewheeling sail prop had lower drag. There's also plenty of literature you can google about it.


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Old 02-07-2015, 15:51   #62
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Re: Do you sail with your transmission in gear?

Our Volvo manual says to put it in reverse. We have a folding prop on a Volvo,sail drive.
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Old 02-07-2015, 23:21   #63
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Re: Do you sail with your transmission in gear?

We sail in neutral, but that is due to the the freewheeling prop providing recharge (electric drive).
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Old 03-07-2015, 00:39   #64
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Re: Do you sail with your transmission in gear?

I let the fixed 3 blade prop on my Volvo freewheel when sailing.... downside is wear on the seal.... but they last for years anyway.

Logic ... in the day job ( twin screw) if slow steaming on one engine the plumbers would let the idle shaft and prop freewheel... if they stopped it rotating we had to apply a bucket load more helm to counteract the drag.

And also... if I put the gearbox into 'astern' ( sorry...sailorising expression) while sailing I can't get it out of gear again unless I start the engine....
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Old 03-07-2015, 03:46   #65
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Re: Do you sail with your transmission in gear?

Please stop comparing airplane props with sailboat props. They are completely different!
An airplane prop has far less drag when not spinning.
A sailboat prop has less drag when freewheeling.

While there are several reasons for this appartent inconsistency, the main reason is very simple. The airplane prop is turning the engine over when it is spinning, the boat is turning nothing over when it is spinning in neutral.

The prop on a pisron engine aircraft is bolted to the crankshaft. There is no way to disconnect it, so when it spins, it is turning the engine though compression etc.That is the simple reason why a airplane props absorbs so much power (thus drag) when it is spinning with the engine not operating.
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Old 03-07-2015, 05:14   #66
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Re: Do you sail with your transmission in gear?

Can airplane props not be feathered to reduce drag?
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Old 03-07-2015, 05:41   #67
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Re: Do you sail with your transmission in gear?

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Originally Posted by Catcruiser View Post
Says who Gus? Do you have scientific proof for this? or only hearsay?

A free rotating shaft causes less damage to your gearbox than one that is being held (braked) by it's gear.
Where there is more wear is on the shaftseal and cutless bearing. But thay are easily replaced.
Lock your prop, or let it spin?
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Old 03-07-2015, 06:26   #68
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Re: Do you sail with your transmission in gear?

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Can airplane props not be feathered to reduce drag?
Some can, some not - just like boats

Even those that can be, they are still bolted to the crank (in piston engines). The constant speed unit (CSU) will adjust the angle of attack of the blades but they remain fixed to the crankshaft. By feathering the prop (those with a CSU), the prop will not try to turn the engine over and the prop will stop spinning and thus the stationary prop has less drag.

This is not analogous to a boat prop where a stationary prop is held in position usually by the compression of the engine via the gearbox.
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Old 03-07-2015, 06:29   #69
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Re: Do you sail with your transmission in gear?

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Originally Posted by Sailmonkey View Post
....otherwise the clutch pack for the same direction of travel . . .
Sounds like finally someone who understands how this works. Can you please explain? Why won't the clutch slip in reverse, but it will in forward? A car clutch wouldn't know the difference, but I guess marine clutches work on different principles?
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Old 03-07-2015, 07:24   #70
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Re: Do you sail with your transmission in gear?

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Sounds like finally someone who understands how this works. Can you please explain? Why won't the clutch slip in reverse, but it will in forward? A car clutch wouldn't know the difference, but I guess marine clutches work on different principles?

Typically a marine clutch relies on a sort of "ramp" to use the torque or resistance to turn to squeeze the clutch together.

So in the case o a zf gear, when under power you engage the clutch ahead. The act of turning the propellor creates a torque load in a specific direction that is countered by ramp in the gear.....the harder it is to turn the prop the tighter the clutch gets.....inversely if the prop over runs the engine the clutch will become looser.
Now enter astern, this uses the clutch pack that will be tightened when the vessel moves in the direction opposite the way it was intended by gear selection.

I could make this clearer with illustrations and such, but I'm typing with my thumbs on a phone while sitting next to an oil well.....if things go well today I'll have time and can elaborate with a computer.


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Old 03-07-2015, 11:35   #71
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Re: Do you sail with your transmission in gear?

Wow, a whole bunch of bad information here. The info regarding the Hurth gear box is correct for most installations. I don't care what MIT says, a free-wheeling prop creates more drag than a stopped one in both boats and airplanes. I have 40 years experience in both. Add a prop shaft electric generator and you have even more drag. Your wind turbine? Same thing, wind-milling and producing electricity equals max drag, wind-milling and producing no electricity equals less drag. Prop tethered produces the least amount of drag. There is no free lunch. You have to make a decision how much energy you want to extract from the wind to produce electricity and drive the boat. What you get form one you extract from the other. If you are fully charged up tether the wind turbine. Does anyone really think 150 amp (at full capacity) alternator creates the same load on the engine as an 80 amp one? If you do you probably think like an MIT guy. Solar, it doesn't slow your boat down, make noise, hit you in the head or wear out your transmission. You do have to clean the bird crap off it though.
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Old 03-07-2015, 11:51   #72
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Re: Do you sail with your transmission in gear?

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Originally Posted by Wotname View Post
Some can, some not - just like boats

Even those that can be, they are still bolted to the crank (in piston engines). The constant speed unit (CSU) will adjust the angle of attack of the blades but they remain fixed to the crankshaft. By feathering the prop (those with a CSU), the prop will not try to turn the engine over and the prop will stop spinning and thus the stationary prop has less drag.
Dude, you don't know what you are talking about. I doubt everybody wants to hear about all the things you are wrong about. This is a boat forum.
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Old 03-07-2015, 11:55   #73
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Re: Do you sail with your transmission in gear?

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Originally Posted by sailorboy1 View Post
I leave mine in neutral, but I have a folding prop 👌



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I have Volvo stern drives with folding props... when sailing I am in reverse... it closes the folding props. Volvo recommended this procedure with the S130 sail drives.
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Old 03-07-2015, 14:49   #74
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Re: Do you sail with your transmission in gear?

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Originally Posted by jmschmidt View Post
I don't care what MIT says, a free-wheeling prop creates more drag than a stopped one in both boats and airplanes.
It all depends on how much energy it takes to turn the prop and how efficient is the prop itself. A really efficient prop with almost no drag (i.e. heat generated) by the drive train will produce less drag than locking the prop. If the prop shaft is doing work extracting energy from the water flowing over the prop then it will produce proportionally more drag. I did not go to MIT but on this issue they are right. If the prop shaft is truly free and not generating heat somewhere it creates less drag to let it free wheel. But if it generates a lot of heat in the transmission then maybe not a good idea anyway regardless of drag.
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Old 03-07-2015, 19:22   #75
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Re: Do you sail with your transmission in gear?

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Originally Posted by jmschmidt View Post
I don't care what MIT says, a free-wheeling prop creates more drag than a stopped one in both boats and airplanes.
You don't have to listen to MIT, me, or anybody else. This is probably the simplest experiment to perform in the history of hydrodynamics.

Take your boat out on the water, get under sail on a good consistent reach, and record your speed for ten minutes with the prop locked. On the same reach, put your prop in neutral, and record your speed again. Do this a few times to take out the effects of variable winds and waves.

You'll see a .25 to .5 knot improvement in average speed when the prop is freewheeling.

That's how drag works. I've done it, so I know I'm right.
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