B323 Prop Rotation Under Sail

[DJBrookster]

Engine is a Yanmar 3ym20. Not sure about the gearbox, is this better left in neutral or put in reverse?

[Rakuflames]

Originally Posted by DJBrookster

Engine is a Yanmar 3ym20. Not sure about the gearbox, is this better left in neutral or put in reverse?

I *just* had that engine put in my boat by a really good mechanic. He said to put it in reverse.

[svHyLyte]

Lock it in reverse and hang your key on the shifter lever so that you remember to shift back to neutral before you start the engine.

FWIW...

[DJBrookster]

Thanks, I was concerned that locking in reverse would either damage the gearbox or cause lubrication issues.

[Astrid]

The answer would be in the transmission manual for your make and model.

[DJBrookster]

Thanks Astrid but if I had the manual that would be good. It's the problem with 2nd hand boats you don't always get all the literature.

[Astrid]

what make tranny is it?

[Tynesider]

Originally Posted by DJBrookster

Engine is a Yanmar 3ym20. Not sure about the gearbox, is this better left in neutral or put in reverse?

Why do you want to lock it?

When under sail I always leave mine in neutral so the prop turns, why lock the prop and create drag, if the prop is turning then you are sailing with a free bonus by creating propulsion from the prop.

In my ZF maual it doe say 'IF required' lock the prop by putting it in 'reverse', but agree with others never leave it in 'foreward'.

[roverhi]

There has been a recent post by someone who supposdely got the word dorectly from from Yanmar that reccomends leaving the transmission in neutral. You might want to google it and see what the reasoning is. I always put my 3gm30 in reverse because I can't stand the noise of the prop turning. Some transmissions can be ruined by allowing the prop to rotate.

Depending on your boat type and prop, there may be significant reduction in drag by locking the prop in position. At the worst, it's only a limited benefit to let the prop spin. There is no thrust from a prop spinning from hydrodynamic pressure, just a possible limited reduction in drag that mainly diehard racer's would care about.

In my own experience sailing identical full keel boats, saw no difference in speed fixed or spinning. Three identical boats on 3 plus day ocean passages in ideal reaching conditions, one with a 2 bladed prop fixed in the shadow of the deadwood, one with a spinning 3 bladed prop, and one with a fixed three blade. The differential in daily mileages covered was less than one percent. Obviously these weren't side by side tests but the conditions were as identical as it's possible to get. Average daily speeds were slightly better than the theoretical hull speeds of the boats, irregardless of the prop or whether it was rotating.

[svlamorocha]

Originally Posted by DJBrookster

Thanks, I was concerned that locking in reverse would either damage the gearbox ...

There is another significant risk of locking the transmission in reverse.. If the boat goes fast enough the prop may start turning the transmission and engine BACKWARDS (leading to catastrophic damage). Imagine what that can do to your engine.. I understand this is why both Volvo and Yanmar recommend leaving the tranny in neutral while under sail.

If you want to balance both issues you can a) lock it in reverse while going slow when there is no risk of breaking the tranny or turning the engine backwards (I do recall seeing a specific speed posted by the builder of the Sweden 45) and b) let it freewheel when going faster at the expense of wear but without risk of catastrophic damage.

[Astrid]

You would really have to be haulin a** to get the prop to overcome the compression of the diesel engine. Given most boat props, I doubt the normal sail boat could manage enough energy through the water to do that.

[svlamorocha]

Originally Posted by roverhi

Depending on your boat type and prop, there may be significant reduction in drag by locking the prop in position.

Are you referring to fixed-blade props other than 2-fixed-blade props that can "hide" behind a keel? If so I am curious as to the source of your statement, which goes against the fluid mechanics stuff from engineering school and the results from tank tests.. See Propeller Drag under sail, Article for results of a tank test that quantified the drag reduction from letting a fixed blade prop spin..

[svlamorocha]

Originally Posted by Astrid

You would really have to be haulin a** to get the prop to overcome the compression of the diesel engine. Given most boat props, I doubt the normal sail boat could manage enough energy through the water to do that.

I believe you are right; that is exactly what I was wondering when I saw that label in that new Sweden 45... Trying to find a reason to believe on something coming from such a fine builder, I let myself be persuaded that the *force from the prop* that depends on speed does not go down over time, while the compression on a non-running engine will eventually escape through the rings/valves and let it turn once or twice.. Maybe I am taking that label too seriously?

[Pete7]

Originally Posted by roverhi

Depending on your boat type and prop, there may be significant reduction in drag by locking the prop in position. .

Some light reading on the subject, but I think a free wheeling prop produces less drag on a modern fin yacht.

Sailboat propeller drag - Strathprints

Pete

[Pete7]

Originally Posted by Astrid

You would really have to be haulin a** to get the prop to overcome the compression of the diesel engine. Given most boat props, I doubt the normal sail boat could manage enough energy through the water to do that.

Managed it once with a Nicolson 55 surfing down some fair sized waves, sailing along the English South Coast. Frightened the living daylights out of us as we shot down the wave at 11 knots only for the engine to fire up in forward with a wide open throttle setting and suddenly we really did accelerate.

Pete