free spinning prop

[Tranona]

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

Verbatim from the manual. Guess the two bits were written at different times by different people and not edited for consistency of terminology.

 

[ex-Gladys]

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....

Brunton's comment to me was "use whatever works"

 

[vyv_cox]

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....

If I select forward immediately I turn off the engine the prop continues to spin but in reverse it stops.

 

[rob2]

I recall the trials reported in the press which were said to indicate that drag was greater with a locked prop. However, it struck me at the time that insufficient data was collected to justify that conclusion apart from the one prop with which it was tried. It must surely make a difference what speed through the water and what the area of the blades is compared with the nominal circle of it (can't remember the correct terminology). I would expect a graph of the drag to be greater when locked above a certain speed for each design of prop, but it may well be less below that speed than for a spinning prop. I believe the prop data used to draw the conclusion was a three bladed motoring prop, things may well be very different for a narrow bladed sailing boat prop at normal, fairl;y low sailing speeds!

As I see it, each prop will have its own drag characteristics spinning or fixed and that one must also consider each type of stern gear and gearbox adding more drag, different for each installation. Mind you, I'd willingly lose a fraction of a knot to stop the cockpit sole from vibrating and spending my sailing time irritably worrying about how the gearbox is faring.

Rob.

P.S. I fitted a folding prop so for me it is academic beyond the second or so it takes to fold...

 

[vyv_cox]

I recall the trials reported in the press which were said to indicate that drag was greater with a locked prop. However, it struck me at the time that insufficient data was collected to justify that conclusion apart from the one prop with which it was tried. It must surely make a difference what speed through the water and what the area of the blades is compared with the nominal circle of it (can't remember the correct terminology). I would expect a graph of the drag to be greater when locked above a certain speed for each design of prop, but it may well be less below that speed than for a spinning prop. I believe the prop data used to draw the conclusion was a three bladed motoring prop, things may well be very different for a narrow bladed sailing boat prop at normal, fairl;y low sailing speeds!

As I see it, each prop will have its own drag characteristics spinning or fixed and that one must also consider each type of stern gear and gearbox adding more drag, different for each installation. Mind you, I'd willingly lose a fraction of a knot to stop the cockpit sole from vibrating and spending my sailing time irritably worrying about how the gearbox is faring.

Rob.

P.S. I fitted a folding prop so for me it is academic beyond the second or so it takes to fold...

I believe the original findings were in a thesis by a man at Strathclyde University https://pure.strath.ac.uk/portal/files/257091/strathprints005670.pdf The investigation was pretty thorough and the findings leave little room for doubt. The later YM investigation was perhaps less thorough in that it didn't use many fixed props but it did look at folding and feathering ones. Again, there can be little argument that locking a three-bladed prop adds as much drag as towing a bucket.

 

[Poignard]

A locked prop is trying to turn the boat onto its side whereas a freewheeling one is just stirring up some oil.

 

[VicS]

FWIW the diagram below showing the innards of a KM series gearbox helps to understand why it should be put in "astern" rather than" forward" when sailing.


If forward gear is selected the tendency will be for the (right handed) prop to try to turn the output shaft and the spiral spline that carries the cone clutch assembly in the direction that tries to pull the clutch out of engagement with forward gear. Slipping of the clutch can then cause wear of the forward cone esp as there may be little or no lubrication ( see below) when the engine is not turning

If reverse gear is selected the effect will be for the torque from the prop to drive the cone assembly firmly into engagement with the reverse gear. No slipping occurring and therefore no wear of the cone..


The reason why it should not be allowed to freewheel is that only the forward and reverse gears dip into the oil. They will be stationary if the engine is not running and will therefore not pick up and distribute any oil .

The output shaft will of course still be rotating in its bearings which over a long period of time may suffer wear/failure due to lack of lubrication. No problem, I guess for a thrash round the cans but maybe on an ocean voyage.


N.B. the above relates specifically to the KM gearboxes. Any other type must be be considered quite separately.

 

[Plum]

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?

There has been studies that demonstrated both fixed and free rotating props have lower drag so there must be other factors at work. Last year while sailing in close company with another boat at the same speed for over half an hour I experimented by stopping my three bladed prop for five minutes then letting it spin for five minutes repeating this sequence three times. The result was no speed difference relative to the other boat so I continue my practice of sailing with the prop locked to avoid wear and noise.

Www.solocoastalsailing.co.uk