Folding propellor lock or not to lock

[geem]

I thought the Bruntons was a variable pitch feathering prop? Vic says his is folding.

Richard

Brunton make folders and feathering props. Two seperate props. The feathering prop just feathers. End of. Its drag is about 15% of a normal three blader when sailing.
The folding prop needs a blast of reverse from the engine before you switch it off to park the blades shut. When you do this they stay shut. I know cos I have one. Both are excellent props.

 

[sailaboutvic]

Brunton make folders and feathering props. Two seperate props. The feathering prop just feathers. End of. Its drag is about 15% of a normal three blader when sailing.
The folding prop needs a blast of reverse from the engine before you switch it off to park the blades shut. When you do this they stay shut. I know cos I have one. Both are excellent props.

No problem with the prop , very happy with it , at less Rev we getting much more speed plus under sail the speed as also increased, my concern is , am I doing any damage to the gearbox locking it in forward gear , I am guessing locking it in forward or Astern wouldn't made any different , correct me if I am wrong guys .

 

[vyv_cox]

No problem with the prop , very happy with it , at less Rev we getting much more speed plus under sail the speed as also increased, my concern is , am I doing any damage to the gearbox locking it in forward gear , I am guessing locking it in forward or Astern wouldn't made any different , correct me if I am wrong guys .

No difference that I can see. The mechanism in both cases is identical other than introducing one more cog to reverse the direction. I have been using whichever works at the time (see post#40) for years with no apparent problem.

 

[RichardS]

The folding prop needs a blast of reverse from the engine before you switch it off to park the blades shut. When you do this they stay shut. I know cos I have one.

Do you have a technical explanation for that behaviour?

Having studied my Flex-O-Folds it appears that the motive force for opening the blades is the moment provided by the drive shaft. If the engine is stopped underway the prop will continue to rotate due to its momentum but since the motive force has now been removed the only force which might be keeping the prop open is the water flowing past the blades due to the forward motion of the boat. However, surely the greater component of that force acting on the blades to rotate them will be the in the direction needed to close them, otherwise it would be a sort of perpetual motion machine. Surely the blades must therefore close after a few seconds?

Richard

 

[sailaboutvic]

No difference that I can see. The mechanism in both cases is identical other than introducing one more cog to reverse the direction. I have been using whichever works at the time (see post#40) for years with no apparent problem.

Thanks , put my mind at rest

 

[lw395]

Do you have a technical explanation for that behaviour?

Having studied my Flex-O-Folds it appears that the motive force for opening the blades is the moment provided by the drive shaft. If the engine is stopped underway the prop will continue to rotate due to its momentum but since the motive force has now been removed the only force which might be keeping the prop open is the water flowing past the blades due to the forward motion of the boat. However, surely the greater component of that force acting on the blades to rotate them will be the in the direction needed to close them, otherwise it would be a sort of perpetual motion machine. Surely the blades must therefore close after a few seconds?

Richard

Many folding props will be held open by centrifugal force if you put the engine in neutral.
Putting the gearbox in astern stops it and the water flow folds it.

 

[RichardS]

Many folding props will be held open by centrifugal force if you put the engine in neutral.
Putting the gearbox in astern stops it and the water flow folds it.

Centrifugal force generated by what?

Your suggesting that the flow of water against the blades causes a force pushing against the front face of the blades which is then deflected by the shape of the blades and resolved into a rotational motion which spins the shaft and thereby the prop which keeps the blades opened out against the force of the stream acting on the front face of the blades ..... etc etc.

I'd be a very rich man if I could make that work.

Richard

 

[alahol2]

Centrifugal force generated by what?

Your suggesting that the flow of water against the blades causes a force pushing against the front face of the blades which is then deflected by the shape of the blades and resolved into a rotational motion which spins the shaft and thereby the prop which keeps the blades opened out against the force of the stream acting on the front face of the blades ..... etc etc.

I'd be a very rich man if I could make that work.

Richard

I claim my millions... My Flex-o-fold continues to spin if I switch off the engine whilst it is in forward gear or even just going to neutral before switching off the engine. I have to go into reverse gear with the engine switched off to fold the prop and stop it spinning.
Last weekend I went into neutral, switched off the engine and for some unknown reason put it in forward gear. It took me about five minutes to realise the noise I was hearing was my prop still spinning. Putting it in reverse soon had it silenced.

 

[RichardS]

I claim my millions... My Flex-o-fold continues to spin if I switch off the engine whilst it is in forward gear or even just going to neutral before switching off the engine. I have to go into reverse gear with the engine switched off to fold the prop and stop it spinning.
Last weekend I went into neutral, switched off the engine and for some unknown reason put it in forward gear. It took me about five minutes to realise the noise I was hearing was my prop still spinning. Putting it in reverse soon had it silenced.

But that's not what we're discussing.

Looking at the contours of a folded Flex-O-Fold, or presumably any folding prop, I can see that it has a contoured shape as with mine:

I can well imagine that water flowing over that contour will, if the shaft/gearbox are relatively low friction, generate a turning moment even when the blades are folded. You can stop this by engaging gear or doing anything which will sufficiently increase the friction in the drive train. The chances are that once the source of the friction is removed, for example by re-engaging neutral, the prop will resume its spinning .... although this will depend upon how marginal the friction in the drive chain is as once the prop has been stopped there may not be sufficient flow to overcome inertia.

However, the posters I'm in discussion with are claiming that they have to engage a gear to cause the blades to fold at all because otherwise a force being generated by the water flow hitting the blades perpendicularly from the front is stopping the blades from folding. I just want someone to explain the physics behind this phenomenon.

Richard

 

[lw395]

The centrifugal force is there because the prop is already spinning.
On your common or garden racing boat folding prop, once you stop it, it will stay folded.
Dunno if flexo-folds or other up market folders are much different.

Often I tend to get sailing, then fold the prop by going into astern for a moment, leaving the engine running for a little while.
The prop folds as its rotation stops once you come out of astern.

If you just go into neutral from forwards, while sailing at a few knots, it will windmill, slowing the boat a little.


I was always taught that stopping some marine gearboxes in gear is baaad, I've forgotten which TBH.

 

[alahol2]

But that's not what we're discussing.

Looking at the contours of a folded Flex-O-Fold, or presumably any folding prop, I can see that it has a contoured shape as with mine:

I can well imagine that water flowing over that contour will, if the shaft/gearbox are relatively low friction, generate a turning moment even when the blades are folded. You can stop this by engaging gear or doing anything which will sufficiently increase the friction in the drive train. The chances are that once the source of the friction is removed, for example by re-engaging neutral, the prop will resume its spinning .... although this will depend upon how marginal the friction in the drive chain is as once the prop has been stopped there may not be sufficient flow to overcome inertia.

However, the posters I'm in discussion with are claiming that they have to engage a gear to cause the blades to fold at all because otherwise a force being generated by the water flow hitting the blades perpendicularly from the front is stopping the blades from folding. I just want someone to explain the physics behind this phenomenon.

Richard

I can't explain the physics but once folded, by putting the gearbox in reverse, the prop will never start rotating again even when the gearbox is put back into neutral. The prop is definitely open when spinning freely, mine has got no 'buffers' as later versions seem to have. I can hear/feel the clunk whenever the prop opens or closes. Mine is a standard setup on a shaft that exits from the 'deadwood'. Maybe it is because, as lw says, the prop is already spinning.

 

[RichardS]

I can't explain the physics but once folded, by putting the gearbox in reverse, the prop will never start rotating again even when the gearbox is put back into neutral. The prop is definitely open when spinning freely, mine has got no 'buffers' as later versions seem to have. I can hear/feel the clunk whenever the prop opens or closes. Mine is a standard setup on a shaft that exits from the 'deadwood'. Maybe it is because, as lw says, the prop is already spinning.

The first part is understandable and matches my hypothesis .... but do you and LW understand the problem with the second statement. In effect, the flow hitting the prop causes it to rotate ..... and the rotation causes the blades to remain open in direct opposition to the force of the flow trying to fold them back. Surely this defies the laws of physics?

The centrifugal force may well be there because the prop is already spinning but that spinning force originates from the engine. As soon as the engine is put into neutral the centrifugal force is removed.

Richard

 

[alahol2]

The first part is understandable and matches my hypothesis .... but do you and LW understand the problem with the second statement.

I fully understand your disbelief. Before I got this prop and became aware of the phenomenon I would have said that if you remove the drive from the prop then the prop would fold and stop rotating. I previously had what I think was a Radice folding prop on a shaft drive Volvo MD7A, I'm almost certain that prop never self rotated but that may have been down to friction in the gearbox.
The only thing I would say is that there is a surprisingly large amount of 'meat' in the blades of a Flex-o-fold close to the tips. This would help increase the inertia of the blades.

 

[lw395]

The first part is understandable and matches my hypothesis .... but do you and LW understand the problem with the second statement. In effect, the flow hitting the prop causes it to rotate ..... and the rotation causes the blades to remain open in direct opposition to the force of the flow trying to fold them back. Surely this defies the laws of physics?

The centrifugal force may well be there because the prop is already spinning but that spinning force originates from the engine. As soon as the engine is put into neutral the centrifugal force is removed.

Richard

Centrifugal force is big enough to overcome the thrust of the blades.
That's how folding props work in astern. And why they often need a good handful of RPM to do so!

The drag of a windmilling prop turning a typical gearbox like Yanmar or Volvo is not immense.

The centrifugal force is m r omega^2
Say the prop is doing 750 rpm. That's 12.5 r/second, omega= 78 rad/s.
omega^2= 6168.
Say the blade is open to a radius of 10cm? and weighs 1kg?
That gives F = 616N or about 63kg force.
Hard to guess the drag of a fairly freely spinning prop, but at 5knots, 600N would be around 2hp which would be a lot of drag, so it's easy to see that centrifugal force is sometimes enough to keep the prop open and spinning.
Which is what I've observed on my Impala, a few sigma33's, X119 and other racing yachts with ordinary folding props.

So I stick it in astern for a moment, then the prop folds as it is slowed by the water when I select neutral.
There will also be friction helping keep the prop open perhaps.

The drag of a spinning prop can be noticeable, in light airs beating in smooth water, I've seen perhaps 0.2 knot increase by forcing it to fold.

 

[geem]

Do you have a technical explanation for that behaviour?

Having studied my Flex-O-Folds it appears that the motive force for opening the blades is the moment provided by the drive shaft. If the engine is stopped underway the prop will continue to rotate due to its momentum but since the motive force has now been removed the only force which might be keeping the prop open is the water flowing past the blades due to the forward motion of the boat. However, surely the greater component of that force acting on the blades to rotate them will be the in the direction needed to close them, otherwise it would be a sort of perpetual motion machine. Surely the blades must therefore close after a few seconds?

Richard

With the Brunton Folder, the blade tips are heavy. If you are going forward with the blade spinning the weight of the blade tips keeps the prop open and spinning. A quick blast in reverse stops the spinning of the blades forward and as the blade becomes stationary relative to the boats forward motion, the prop folds. Due to the wash over the blades.

 

[RichardS]

With the Brunton Folder, the blade tips are heavy. If you are going forward with the blade spinning the weight of the blade tips keeps the prop open and spinning. A quick blast in reverse stops the spinning of the blades forward and as the blade becomes stationary relative to the boats forward motion, the prop folds. Due to the wash over the blades.

So the wash onto the blades from ahead is used preferentially to continuously rotate the propellor against the friction of the drive train rather than fold back the props against the friction of the folding gear.

Something just doesn't add up but let's leave it.

Richard

 

[geem]

So the wash onto the blades from ahead is used preferentially to continuously rotate the propellor against the friction of the drive train rather than fold back the props against the friction of the folding gear.

Something just doesn't add up but let's leave it.

Richard

Maybe a didnt explain so well. In forward gear, the spinning prop opens due to centrifugal force. Stop that spinning by momentarily putting the prop in reverse, then neutral and the prop closes due to the wash over a none spinning prop. The Brunton prop has blades like mickey mouses ears and very heavy towards the tips. It closes easily with this simple technique.
When the boat is out of the water, the prop adopts a closed position on its own. The slight angle of my prop shaft downwards is enough to close the prop on its own in air. It wont take much to close it in water if its not spinning. Hope this makes better sense

 

[geem]

So the wash onto the blades from ahead is used preferentially to continuously rotate the propellor against the friction of the drive train rather than fold back the props against the friction of the folding gear.

Something just doesn't add up but let's leave it.

Richard

Maybe a didnt explain so well. In forward gear, the spinning prop opens due to centrifugal force. Stop that spinning by momentarily putting the prop in reverse, then neutral and the prop closes due to the wash over a none spinning prop. The Brunton prop has blades like mickey mouses ears and very heavy towards the tips. It closes easily with this simple technique.
When the boat is out of the water, the prop adopts a closed position on its own. The slight angle of my prop shaft downwards is enough to close the prop on its own in air. It wont take much to close it in water if its not spinning. Hope this makes better sense

 

[RichardS]

Maybe a didnt explain so well. In forward gear, the spinning prop opens due to centrifugal force. Stop that spinning by momentarily putting the prop in reverse, then neutral and the prop closes due to the wash over a none spinning prop. The Brunton prop has blades like mickey mouses ears and very heavy towards the tips. It closes easily with this simple technique.
When the boat is out of the water, the prop adopts a closed position on its own. The slight angle of my prop shaft downwards is enough to close the prop on its own in air. It wont take much to close it in water if its not spinning. Hope this makes better sense

The additional information actually makes the physics even more unbelieveable, I'm afraid. :ambivalence:

It now transpires that the natural inclination of the prop is to close its blades by default, even when no force other than gravity is applied ..... and yet, in a water stream acting directly on the forward face of the open blades and with no centrifugal force being applied by the shaft and with gravity also acting as usual to close the blades, the blades continue to remain open unless the shaft is mechanically reversed to force the blades to close.

Richard

 

[geem]

The additional information actually makes the physics even more unbelieveable, I'm afraid. :ambivalence:

It now transpires that the natural inclination of the prop is to close its blades by default, even when no force other than gravity is applied ..... and yet, in a water stream acting directly on the forward face of the open blades and with no centrifugal force being applied by the shaft and with gravity also acting as usual to close the blades, the blades continue to remain open unless the shaft is mechanically reversed to force the blades to close.

Richard

Well you better start beliving it cos thats how it works!
Is it not possible that once the spinning blades reach sufficient velocity freewheeling, that the blades stay open? This is what we experience. If I forget to give the blip of reverse before I turn the engine off then the prop spins continually whilst sailing. I have to restart the engine and blip it in reverse to overcome the centrifugal force of the spinning prop. The energy to keep the prop spinning comes from the flow over the blades even though the engine is not turning the propeller.