Best gear position when sailing with saildrive prop ?

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Jobs a good un

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Hi All

After sailing for years with prop always in reverse saw a article today that said less drag is created by leaving prop in neutral ?
Panel opinions ?
Regards

Mark
 
Jobs a good un said:
Hi All

After sailing for years with prop always in reverse saw a article today that said less drag is created by leaving prop in neutral ?
Panel opinions ?
Regards

Mark
Click to expand...

what ever the manufacturer recommends for you particular drive
 
Any rotor will produce less drag when stalled than when driven. Among other things, this is why when a helicopter has engine failure the pilot will attempt to autorotate rather than stall the rotor blades. Similarly aerobatic pilots flying propeller driven aircraft will use a slowly rotating propeller as an air brake. This is simple fluid dynamics.
 
Volvo advice is neutral for fixed blade and reverse for folding. However the drive can be very noisy in neutral if sailin reasonably fast so many people put in gear to stop the noise without any harm.

With my Flexofold I just go into reverse when shutting the engine down which folds the blades, then put in neutral. The blades seem to stay closed.
 
Tranona said:
Volvo advice is neutral for fixed blade and reverse for folding. However the drive can be very noisy in neutral if sailin reasonably fast so many people put in gear to stop the noise without any harm.

With my Flexofold I just go into reverse when shutting the engine down which folds the blades, then put in neutral. The blades seem to stay closed.
Click to expand...
 
ChrisRayner said:
Any rotor will produce less drag when stalled than when driven. Among other things, this is why when a helicopter has engine failure the pilot will attempt to autorotate rather than stall the rotor blades. Similarly aerobatic pilots flying propeller driven aircraft will use a slowly rotating propeller as an air brake. This is simple fluid dynamics.
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It is nowhere near as simple as that.
In a directly coupled light aircraft/motor glider with the engine not running, the drag of a windmilling prop is more than that of a stalled prop because of the work it is doing rotating the engine against significant resistance i.e. compression.
Where you have a prop that windmills freely, as in the case of a boat out of gear with little drag in the gearbox, the rotating prop will have less drag than the locked stalled prop.
A helicopter with failed engine is a different case again. You are deriving lift from the autorotating blades because you also have forward speed i.e. airflow perpendicular (roughly) to the axis of the rotor. That is how an autogyro flies. If you don't have forward speed i.e. airflow is inline with the rotating axis of the rotor, as in the boat case, you will drop ******* fast whether the rotor is turning or stationary!
Is any fluid dynamics simple?
 
ChrisRayner said:
Any rotor will produce less drag when stalled than when driven. Among other things, this is why when a helicopter has engine failure the pilot will attempt to autorotate rather than stall the rotor blades. Similarly aerobatic pilots flying propeller driven aircraft will use a slowly rotating propeller as an air brake. This is simple fluid dynamics.
Click to expand...

It has been shown several times that this statement is untrue. There is considerably less drag on a yacht's propeller when it is turning than when it is stalled in gear. Initially it was done here https://pure.strath.ac.uk/portal/en...ag(9c9e48fd-fbff-4fd8-8f27-1d02169d01dd).html reprinted in part in PBO, then later done again by Emrhys Barrell for Yachting Monthly. It has also been investigated in an innovative way by Maine Sail in US, included here http://www.catamaransite.com/propeller_drag_test.html. All found just the same thing - a freewheeling prop causes considerably less drag.

However, it is not so straightforward as that because some gearboxes are not lubricated unless being driven by the engine. Hence the instruction to do what the gearbox manufacturer says. The only possible rider to that is that a feathering or folding prop stalled offers very little drag, as found in the above investigations, which may change the situation.
 
Little Grebe said:
Volvo - neutral or in reverse with a folding prop
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You forgot to mention the fine print ;)

When in neutral, the rotating propeller on a saildrive can cause significant vibrations.
When in reverse (with a fixed prop), the gearbox will be stuck in reverse until boat speed has dropped below ~ 2 knots.
 
We seem to have regular threads about this and one of the oft stated reasons for the concern is because of the noise when sailing in neutral.

This seems very strange to me because I have two engines so, presumably, twice the noise ......... but if the breeze is strong enough to turn off the engines, say above 7 or 8 knots, then the wind noise drowns out any residual noise from the spinning props unless it's an overnight leg and you are sleeping down below in one of the aft cabins when you can hear a gentle hum ..... which is actually both reassuring (I know we are making good speed) and quite soothing (as I drop off to sleep very quickly :) ).

The fact that my benign experience with "neutral noise" seems different to many others is presumably a) because I am sailing a cat, or b) I have saildrives rather than a shaft drive, or c) a fortuitous combination of both.

I hope it's a) as that is another "positive" I can casually throw into the next "catamaran v monohull" thread! :encouragement:

Any opinions?

Richard
 
Yngmar said:
You forgot to mention the fine print ;)

When in neutral, the rotating propeller on a saildrive can cause significant vibrations.
When in reverse (with a fixed prop), the gearbox will be stuck in reverse until boat speed has dropped below ~ 2 knots.
Click to expand...


I can see that being annoying, almost as much as having to replace the anodes every six months on their folding props!
 
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