Folding propellor lock or not to lock

[alahol2]

We all seem to agree that the prop continues spinning even with no input from the engine. Richard (I think) seems to be able to accept that the prop may continue spinning but in the closed (folded) attitude. Geem, LW and I believe the prop is open when it spins.
If the prop is spinning at all there will be a tendency for the blades to open because of the inertia of the blades. That, after all, is how the prop works under power from the engine.
I would suggest that, maybe, when power is removed this causes the prop to reach a sort of part folded equilibrium between the inertia of the spinning blades trying to open and the tendency of the water flow to close the blades.
I wish I had an underwater camera, that would clinch the matter.

 

[sailaboutvic]

Anyone with an Brunson autoprop .


Dear Vic,

We have fitted the Autoprop to thousands of saildrives now with very good results. The shaft does need to be locked to get the benefit of the Autoprop when sailing. To do this the engine does need to be locked into the ahead position to enable the blades to take the least line of resistance when sailing. To get it into gear whilst sailing with the prop turning and the engine switched off, can prove to be difficult at times and therefore we have said that it is easier to just stop the engine in gear, with the sails powered up. Furthermore, attempting to shift the gear back to neutral when sailing can be difficult, so we recommend that you first slow the vessel by heading into wind, this will reduce the moment on the propeller making easier to engage neutral, so that you can start the engine. In an emergency you can also start the engine in gear.

I hope this information has been of assistance. If you have any further questions please don’t hesitate to ask.

Many thanks.


Kind Regards,


Daniel Woolgar BEng

Naval Architect

 

[RichardS]

We all seem to agree that the prop continues spinning even with no input from the engine. Richard (I think) seems to be able to accept that the prop may continue spinning but in the closed (folded) attitude. Geem, LW and I believe the prop is open when it spins.
If the prop is spinning at all there will be a tendency for the blades to open because of the inertia of the blades. That, after all, is how the prop works under power from the engine.
I would suggest that, maybe, when power is removed this causes the prop to reach a sort of part folded equilibrium between the inertia of the spinning blades trying to open and the tendency of the water flow to close the blades.
I wish I had an underwater camera, that would clinch the matter.

Yes .... the physics would bear that out. The prop would be almost fully folded but with a slight opening which would balance the major force of the water acting to close the blades aganist the minor contrifugal force (minor because, by definition, it's a derivative of the major force .... anything else would defy the laws of thermodynamics).

In this situation, which would only apply in a low friction environment and with certain closed-blade profiles and might therefore be quite unusual, the blades may well continue to rotate until a blip in reverse stops the blades which cannot then recover their spinning attitude due to the new enforced profile and/or inertia.

However, the folding of the blades down to the slightly open aspect (and some designs may well spin even when fully closed) will happen once the turning force is removed from the shaft as the forces are no longer in equilibrium once the engine is stopped.

An extremely interesting and civilised dicsussion which I have never seen explored in such detail before.

Richard

 

[lw395]

Anyone with an Brunson autoprop .


Dear Vic,

We have fitted the Autoprop to thousands of saildrives now with very good results. The shaft does need to be locked to get the benefit of the Autoprop when sailing. To do this the engine does need to be locked into the ahead position to enable the blades to take the least line of resistance when sailing. To get it into gear whilst sailing with the prop turning and the engine switched off, can prove to be difficult at times and therefore we have said that it is easier to just stop the engine in gear, with the sails powered up. Furthermore, attempting to shift the gear back to neutral when sailing can be difficult, so we recommend that you first slow the vessel by heading into wind, this will reduce the moment on the propeller making easier to engage neutral, so that you can start the engine. In an emergency you can also start the engine in gear.

I hope this information has been of assistance. If you have any further questions please don’t hesitate to ask.

Many thanks.


Kind Regards,


Daniel Woolgar BEng

Naval Architect

Brunton autoprop is a different animal.

 

[lw395]

Yes .... the physics would bear that out. The prop would be almost fully folded but with a slight opening which would balance the major force of the water acting to close the blades aganist the minor contrifugal force (minor because, by definition, it's a derivative of the major force .... anything else would defy the laws of thermodynamics).

In this situation, which would only apply in a low friction environment and with certain closed-blade profiles and might therefore be quite unusual, the blades may well continue to rotate until a blip in reverse stops the blades which cannot then recover their spinning attitude due to the new enforced profile and/or inertia.

However, the folding of the blades down to the slightly open aspect (and some designs may well spin even when fully closed) will happen once the turning force is removed from the shaft as the forces are no longer in equilibrium once the engine is stopped.

An extremely interesting and civilised dicsussion which I have never seen explored in such detail before.

Richard

I'd guess the amount the prop is open will vary.
whatever point where the forces balance.
On my Impala, the prop would spin fairly fast, you could see the shaft going round.
Given that it was possible to motor astern using fairly low revs, the centrifugal force must have overcome the drive force of the prop fairly easily.

Props which need to be locked to stop them spinning when closed are different from your (my!) bog standard racing boat folder.
Racing boats often want to run the engine without driving the boat, so needing the box to stay in gear would not be ideal.

 

[geem]

We all seem to agree that the prop continues spinning even with no input from the engine. Richard (I think) seems to be able to accept that the prop may continue spinning but in the closed (folded) attitude. Geem, LW and I believe the prop is open when it spins.
If the prop is spinning at all there will be a tendency for the blades to open because of the inertia of the blades. That, after all, is how the prop works under power from the engine.
I would suggest that, maybe, when power is removed this causes the prop to reach a sort of part folded equilibrium between the inertia of the spinning blades trying to open and the tendency of the water flow to close the blades.
I wish I had an underwater camera, that would clinch the matter.

+1
I am certain that the prop runs in a semi open position when sailing unless you fold it with a squirt of reverse. Certainly with our Prop its the sailing forward that keeps it semi open. When ever I dive under the boat when we are on a mooring like here in Bonaire, the prop has folded on its own. In fact, when ever I dive under the boat the prop is folded.