Prop rotation when sailing

lw395 said:
No.

The prop always wants to turn.
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Yes, I understand that, but whether it "wants" to or not, is immaterial. It'll still end up doing whatever is easiest. At low speed through the water, I'm sure that will be staying still. As the speed gets high enough to overcome the initial friction, plus whatever other resistive forces are trying to prevent the prop from turning, it will start to turn.
 
lw395 said:
...I would also strongly suspect that increasing resistance from the gearbox at higher rpm is probably quite significant, and probably bring the drag up towards the locked case, to a greater or lesser degree depending on the boats, speed etc.
In some cases it may exceed the locked drag...
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But, of course, it would never get there - otherwise it would lock again! I think it would be a bit like regenerative braking on an electric car. you'd never be able to stop the car with it because as the car got slower, the braking effect would be smaller. I think it's the same (the other way round) with the prop. I agree that the resistance from the gearbox, and maybe other components, would increase with rotational speed, but then the rotational speed would just go down until equilibrium was reached.
 
Avocet said:
Yes, I understand that, but whether it "wants" to or not, is immaterial. It'll still end up doing whatever is easiest. At low speed through the water, I'm sure that will be staying still. As the speed gets high enough to overcome the initial friction, plus whatever other resistive forces are trying to prevent the prop from turning, it will start to turn.
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Easiest for the prop is not the same as easiest for the boat though.
 
Avocet said:
But, of course, it would never get there - otherwise it would lock again! I think it would be a bit like regenerative braking on an electric car. you'd never be able to stop the car with it because as the car got slower, the braking effect would be smaller. I think it's the same (the other way round) with the prop. I agree that the resistance from the gearbox, and maybe other components, would increase with rotational speed, but then the rotational speed would just go down until equilibrium was reached.
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If locking the prop is ever a good idea, it might well be in a regime where the flow around the blades is very much stalled, i.e. detached from the aft face of the blades due to a high angle of attack. In this mode, the torque generated by the prop might be quite low.
Some yacht rudders can be stalled like this, on my Impala, you could heave the tiller beyond 45 degrees, the rudder would cease to do any steering. You had to return the rudder to about 30 degrees to attach the flow again, then the boat would suddenly respond.
That's wandered off topic a bit, but it does illustrate that at yachty speeds it is possible to get out of the linear/normal response of a foil. It also illustrated that re-designing the rudder was a good idea. Could it be that prop design has moved on since 'conventional wisdom' got established?
 
I haven't been able to read all the pages of this saga as I keep losing internet connection, so may be this has been covered.....
A sailor I met, who was also an amateur pilot, told me that a twin engine small plane could take off with only one engine, but the other prop must be lashed down.
There are many similarities between props in water and air, although this doesn't tie in with the recent article about leaving them free to rotate for minimum drag.
 
steve48 said:
There are many similarities between props in water and air, although this doesn't tie in with the recent article about leaving them free to rotate for minimum drag.
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Maybe because the recent article what about yacht props in water, and not aeroplane props in air....There is little similarity between the blade area and diameter of an aeroplane prop and a yacht prop....:rolleyes:
 
lw395 said:
Easiest for the prop is not the same as easiest for the boat though.
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That's the point. The propeller will just keep on absorbing whatever power you throw at it (from boat speed) regardless of what is best for the system as a whole.

To illustrate with an aircraft where it is best to stop the prop for drag and where shaft load is very large:

You are flying along in your Cessna, 2 miles out from the airport and suddenly your engine sputters and stops. The prop continues to free wheel, because of it's inertia but will slow to the point where the airstream is driving it. You put the aircraft at the speed for it's best lift/drag ratio because that's best for maximum range. Unfortunately the drag of the prop (happily spinning away) means that your gliding range is reduced to 1.5 miles. Not good! So you slow the aircraft to close to it's stalling speed (bad for the L/D ratio so bad for the system as a whole) and because of reduced airspeed, the frictional loads are enough to stop the prop. Then you return to the speed for best L/D. Because you've stopped the prop the drag has improved and your range is now 2.5 miles :) The airflow doesn't have the power to start the prop spinning again in this case.

So - nothing to do with boats - but you can see that the prop system will not do what is best for the aicraft as a whole. You, the human operator have to intervene, to make sure that the efficiency of the whole system is maximised.

Yes, aircraft props have different parameters but the way they work is exactly the same. Change the parameters enough and the least drag condition will change. That's just the physics. It is theoretically possible that a boat prop will have less drag stopped, even though the vast majority are better free wheeling.
 
IMO, one disappointment of the article and its conclusions was the lack of test for a 2-blade fixed prop - which is a fairly common setup. Well, it is my configuration so I would say that.

Does anyone have an opinion on how the 3-blade results could relate or be interpolated for a 2-blade or are they too different to compare ?

Would results of a test be less conclusive since a 90 degree turn of prop between tests would result in a large differential ?

Graeme
 
Fascinating stuff in response to my original question.

Over the weekend I did a practical test. Close reaching at about 4.7kn (4.4-5kn) by GPS with locked prop (long keel, 3 bladed prop). I unlocked prop and speed increased within c. 10 sec and showed 5.2-5.6kn. I repeated this several times locking and unlocking prop and got consistent results. Am convinced that, for my boat, drag is reduced. However my free rotating prop makes a horrible whining noise - I will have to go slower with locked prop (at moderate wind speed) or buy ear plugs or better a feathering prop.
 
grazbo said:
IMO, one disappointment of the article and its conclusions was the lack of test for a 2-blade fixed prop - which is a fairly common setup. Well, it is my configuration so I would say that.

Graeme
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oldgitofthewest said:
Fascinating stuff in response to my original question.

Over the weekend I did a practical test. Close reaching at about 4.7kn (4.4-5kn) by GPS with locked prop (long keel, 3 bladed prop). I unlocked prop and speed increased within c. 10 sec and showed 5.2-5.6kn.
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I too have a 2 blade fixed 'skinny' prop and have tried on several occasions to see if locking/unlocking has any effect on speed. The tests have been completely inconclusive so I now don't worry if the prop is locked or unlocked. Just occasionally I get to hear the unlocked prop and will lock it to stop the noise but in most circumstances I wouldn't know if the prop is spinning or not.
 
Easy...why does a sycamore seed have wings?

Whilst props will vary and so will keels/rudders, generally there will be less drag with the prop locked. Less wear on bearings too, but I expect this has all been said before?
 
Laurie said:
Easy...why does a sycamore seed have wings?

Whilst props will vary and so will keels/rudders, generally there will be less drag with the prop locked. Less wear on bearings too, but I expect this has all been said before?
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Nope. Less drag with prop freewheeling as shown by a couple of universities, some individuals and now YM.
 
Laurie said:
Wow, don't tell any pilots!!
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You need to read this vast thread to get through all the argument and counter argument - but in the end it's clear.

The only time a fixed prop might provide less drag is when a two blade prop is locked straight up and down behind the keel.
 
Salty John said:
You need to read this vast thread to get through all the argument and counter argument - but in the end it's clear.

The only time a fixed prop might provide less drag is when a two blade prop is locked straight up and down behind the keel.
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or just possibly, a fine pitched prop with a gearbox that gives a lot of resistance at high speed.
 
Surely there can't be any more to say about this. If previous experience is anything to go by, I will now have killed this thread stone dead.
 
WoodyP said:
Surely there can't be any more to say about this. If previous experience is anything to go by, I will now have killed this thread stone dead.
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The thread is more interesting in terms of the way people think about 'proving' 'facts'.
You can see that a frictionless spinning prop is less drag than a locked prop.
You can see that a helicopter with a dead engine and a freewheeling rotor generates more lift than one with a locked rotor.

A yacht with a real gearbox generating real torque lies somewhere between the two models. To my mind, any measurement which doesn't have the torque and speed realistic proves nothing.

It's the difference between accepting something is true and proving it.
The people have made observations on their own yachts have done a lot more proving than the comic article.

It will be interesting to see how many gearboxes and cutless bearings get shagged out this year. Not many if the weather doesn't improve!
 
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