free spinning prop

[Paddydog`1]

obviously folding feathering props etc reduce drag when sailing compared with a fixed 3 bladed prop. But if the fixed prop is allowed to rotate when sailing this drag is cosiderably reduced. My question is why do some advocate preventing prop rotation when sailing by putting the engine gear in reverse. is it in some way to protect the gearbox? but how can a mechanical gearbox be damaged by a freewheeling prop?

 

[pandos]

This was dealt with here in the past and the answer revolves around complicated counterintuitive laws of physics..

But in a blatant thread hijack....... I wonder if the energy can be harnessed to create electricity in the same way as hydrogenerators operate, surely some pulleys and an alternator could produce some current.

http://www.wattandsea.com/en/cruising-hydrogenerator

???

 

[awol]

My question is why do some advocate preventing prop rotation when sailing by putting the engine gear in reverse.

Well, one reason is that after a while the noise gets on your tits. I have a 2-bladed fixed blade and I stop it aligned with the skeg to reduce drag.

 

[Poignard]

Why do people engage astern gear and not ahead?

 

[Tranona]

Because it will keep turning in ahead. Reverse locks it if it is a mechanical box.

 

[Poignard]

Mine doesn't. I always engage ahead gear.

 

[Tranona]

Think you will find your prop is still capable of turning in ahead.

 

[VicS]

Because it will keep turning in ahead. Reverse locks it if it is a mechanical box.

why will it keep turning if in ahead ?

 

[Poignard]

Think you will find your prop is still capable of turning in ahead.

It does not. I ought to know, I've been doing it for more than 18 years.

 

[awol]

Depends on the gearbox. My Yanmar design locks the prop shaft when in reverse and allows it to spin when the boat is moving forward - works the other way when the boat is travelling backwards (but why would you?). Yanmar, of course, counsel against locking the shaft but I've never had any problems.

 

[Tranona]

Same with Volvo gearbox. Would be very surprised if it locked in forward, although the shaft may not turn when sailing because of the extra drag from the oil in the box.

Volvo and Yanmar instructions are both very clear that reverse gear locks the shaft.

 

[nigelmercier]

... Volvo and Yanmar instructions are both very clear that reverse gear locks the shaft.

But Volvo say not to, don't they?

My Volvo locks the shaft in forward and reverse, but only prevents it turning the other way.

 

[Concerto]

IIRC Yachting Monthly did a test a couple of years ago and they found less drag with a spinning prop compared to a fixing the shaft. The only true way to reduce drag is to fit a folding or feathering prop.

 

[VicS]

But Volvo say not to, don't they?

My Volvo locks the shaft in forward and reverse, but only prevents it turning the other way.

IIRC ( but I might not) Volvo say reverse gear for shaft drive boxes but free spinning for sail drives.

 

[Tranona]

IIRC ( but I might not) Volvo say reverse gear for shaft drive boxes but free spinning for sail drives.

To quote from my Operators handbook.

"Sailing

When sailing set the control lever in astern if a folding propeller is fitted.

If a fixed propeller is fitted the control lever should be set in neutral or reverse. When using a fixed propeller and sailing with the control lever set to reverse the speed is slowed down though less noise is made"

Manual is for D1 and D2 engines with either conventional or saildrive.

Hence my response to parsifal's earlier question as to why people use reverse. of course it may be different with his Bukh, but maybe not.

Advice supports the established view that a spinning prop generates less drag.

Incidentally I return the lever to neutral once my folding prop has folded by putting into reverse. Seems to stay folded until put in gear when the engine is restarted.

 

[johnalison]

When I acquired my boat 16 yrs ago, the advice (Volvo) was different for saildrives and conventional gear. Saildrives like mine should be locked in reverse but I vaguely remember that non-folding props if left to rotate necessitated running the engine every ten hours for a short period. I wondered at the time what people were expected to do on an ocean passage.

 

[vyv_cox]

The situation with my Yanmar KM2P gearbox is very puzzling. I have a Bruntons Autoprop that needs to be locked to feather. I normally put the gear in reverse as soon as we start sailing, which stops the rotation. However, if I forget to do so and remember after, say, 30 minutes, reverse gear does not stop the rotation but forward does! A strange occurrence that I assume is something to do with the position of the cones.

 

[ex-Gladys]

A point that no one has highlighted yet is that cone based gearboxes appear to only lock up in one rotational direction when engaged... you can turn the prop the other way easily... Also, I think if you let the shaft turn that in some gearboxes lubrication doesn't work as it only comes from rotation of the input shaft?

Au contraire to Vyv, my Brunton only stops rotating when ahead is engaged (Technodrive)

 

[duncan99210]

Our Brunton stops spinning as soon as a gear, any gear, is engaged. However, IIRC, Bruntons instructions, in common with the Volvo instructions, tell you to use reverse gear. That's what we try and do but sometimes someone (who will remains nameless) chooses the 'wrong' gear....

 

[laika]

T
When sailing set the control lever in astern if a folding propeller is fitted.

If a fixed propeller is fitted the control lever should be set in neutral or reverse.

Interesting. Why is it called "astern" if a folding prop is fitted but "reverse" if it has a fixed prop?