ccscott49
Active member
wouldn't work, hydraulic boxes, no oil no drive, no drive no sieze, but nice try!! tee hee. Seen any good lobster pots lately?
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in gear or freewheeling
- Thread starter petersto
- Start date
nicho
Well-known member
When I asked Volvo the same question, they said it did not matter either way - if however, it was left in neutral on a longer journey, eg a channel crossing, the engine should be run, with it in gear, every 6 hours to ensure oil is circulated through the gearbox bearings. Seems we get conflicting advice all the time on this one. I leave my 2030/saildrive in reverse hear all the time whilst sailing.
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zefender
Active member
Course I've seen plenty - a decent couple in the middle of the channel last week too. Now they are exactly what you need to stop your prop spinning.
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Avocet
Well-known member
I've never bothered locking my Hurth gearbox when sailing. The prop turns and I assume that's enough to keep its oil churning. That said, when I lay the boat up for winter it spends several months without "circulating the oil" and that doesn't seem to have bothered it over the last 10 years or so!
Also, we seem to have a majority of opinion suggesting that a locked prop imposes less drag than a freewheeling one and yet the only post with any numerical evidence suggested the opposite. Intuitively I could see how turning a screw into a block of wood might impose less resistance than hammering it in so I'm inclined to believe that a freewheeling prop DECREASES the yacht's resistance to motion. Any thoughts anyone?
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Also, we seem to have a majority of opinion suggesting that a locked prop imposes less drag than a freewheeling one and yet the only post with any numerical evidence suggested the opposite. Intuitively I could see how turning a screw into a block of wood might impose less resistance than hammering it in so I'm inclined to believe that a freewheeling prop DECREASES the yacht's resistance to motion. Any thoughts anyone?
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chippie
New member
I agree with you, recieved wisdom says locked is better but I have yet to see a decent explanation as to why. Maybe the difference is turbulence.
My (faulty?) thinking says that being able to give should create less drag. The propellor presents the same area to the water.
Where is somebody with scientific training that can show us how to look at the problem.
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My (faulty?) thinking says that being able to give should create less drag. The propellor presents the same area to the water.
Where is somebody with scientific training that can show us how to look at the problem.
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wpsalm
New member
yes locked is better.... beleive a helicopter can freewheel to a safe landing in the event the main engine is lost in the event that the main rotor is not able to rotate as in gearbox failure am pretty sure a helicopter crashes thus demonstrating to me at least that a rotating prop produces more drag than a fixed prop....
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Avocet
Well-known member
Don't think the 'copter scenario is as simple as that. I think they can control the pitch of the blades so as it falls through the air the pilot can alter the pitch so the blades accelerate. As he gets nearer the ground, he can suddenly change the pitch again and use the inertia of the spinning blades to slow the fall (somewhat). If it experiences blade seizure, it falls every bit as fast but can't store any of the engergy in the spinning blades.
This could all be complete bunkum of course, but again, intuitively, I could see how it might work...
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This could all be complete bunkum of course, but again, intuitively, I could see how it might work...
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snowleopard
Active member
scientific explanation...
as far as a scientific explanation is concerned, first imagine the propellor as a screw boring through a solid. a 15" pitch propellor will advance by 15" for each turn. there is no 'slip' and the prop is achieving 100% efficiency.
now move on to the real world where the prop is turning in water, for every revolution under power you only get an advance of say 10", i.e. 33% slip. conversely if you drag the prop through the water and let it turn freely, it advances more than 15" for every revolution and the slip is negative.
you can plot a graph of drag against prop rpm, starting from the level of the stationary propellor, the drag increases as the prop gets faster then starts to fall again until the point where there is no slip and drag is zero. the problem is that no prop can freewheel without slip so if the prop is turning freely you will always be in the high-drag part of the graph.
ok, so why would a rotating prop have more drag than a locked one? intuitively one would think it would be lower. the prop is a hydrofoil that works just like a sail, the force generated comes from the difference in pressure on the two sides. our freewheeling prop is turned by water flowing faster over the back surface then the front so having lower pressure at the back. the faster the water flow the more force is generated. as the prop spins the water flow over the blades gets faster and the force increases.
with a close-hauled sail the force generated is a right angles to the sail, part of the force pushes the boat forward but most of it is pushing us sideways against the resistance of the keel. in the case of the prop blade, part of the force is making the prop rotate but the rest is trying to pull it backwards, and that's the force that slows your boat down.
lecture over.
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as far as a scientific explanation is concerned, first imagine the propellor as a screw boring through a solid. a 15" pitch propellor will advance by 15" for each turn. there is no 'slip' and the prop is achieving 100% efficiency.
now move on to the real world where the prop is turning in water, for every revolution under power you only get an advance of say 10", i.e. 33% slip. conversely if you drag the prop through the water and let it turn freely, it advances more than 15" for every revolution and the slip is negative.
you can plot a graph of drag against prop rpm, starting from the level of the stationary propellor, the drag increases as the prop gets faster then starts to fall again until the point where there is no slip and drag is zero. the problem is that no prop can freewheel without slip so if the prop is turning freely you will always be in the high-drag part of the graph.
ok, so why would a rotating prop have more drag than a locked one? intuitively one would think it would be lower. the prop is a hydrofoil that works just like a sail, the force generated comes from the difference in pressure on the two sides. our freewheeling prop is turned by water flowing faster over the back surface then the front so having lower pressure at the back. the faster the water flow the more force is generated. as the prop spins the water flow over the blades gets faster and the force increases.
with a close-hauled sail the force generated is a right angles to the sail, part of the force pushes the boat forward but most of it is pushing us sideways against the resistance of the keel. in the case of the prop blade, part of the force is making the prop rotate but the rest is trying to pull it backwards, and that's the force that slows your boat down.
lecture over.
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chippie
New member
Re: scientific explanation...
A good explanation , thankyou.
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A good explanation , thankyou.
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Avocet
Well-known member
Re: scientific explanation...
Thanks for that, I can see what you're getting at and I can see the sense in it. All I've got to do now is make myself believe it! For ages I had a big problem with the explanation of why a rocket works - when it's in space, all the stuff coming out the back has nothing to "push" against so how does it make the rocket move forwards? I now accept that it's just the mas of the stuff being chucked out the back that has an equal and opposite effect on the rocket and it doesn't matter whether it's up against a wall or in deep space. This took me ages to "get my head round" (same with jet-drives on boats) and now I'll have to try and do the same for props in water! I might even try some basic tests one day if I can think of something cheap and simple. I guess I need a little load cell between the thrust bearing on the propshaft and the boat.
Hmmmmm.
Thanks again!
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Thanks for that, I can see what you're getting at and I can see the sense in it. All I've got to do now is make myself believe it! For ages I had a big problem with the explanation of why a rocket works - when it's in space, all the stuff coming out the back has nothing to "push" against so how does it make the rocket move forwards? I now accept that it's just the mas of the stuff being chucked out the back that has an equal and opposite effect on the rocket and it doesn't matter whether it's up against a wall or in deep space. This took me ages to "get my head round" (same with jet-drives on boats) and now I'll have to try and do the same for props in water! I might even try some basic tests one day if I can think of something cheap and simple. I guess I need a little load cell between the thrust bearing on the propshaft and the boat.
Hmmmmm.
Thanks again!
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seaesta
New member
Re: scientific explanation...
Try this experiment to get a better understanding
1. Switch on the wifes hairdryer
2. BRIEFLY put your hand over the air outlet
3. Note that the fan goes FASTER with a blocked outlet
This occurs because the fan does WORK when pushing air but much less work when it cannot do any work - thus "resistence" is less
A prop is doing work when converting relative motion of the boat and water to spin the shaft - overcoming the friction resistence in the system.
When the prop is locked it is doing no work and hence there is less energy absorbed by the prop. hence the boat goes faster under sail with a lock prop.
Even if you dont believe it then it is better to lock the prop so that plaggy bags and old rope are less likely to jam up the prop.
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Try this experiment to get a better understanding
1. Switch on the wifes hairdryer
2. BRIEFLY put your hand over the air outlet
3. Note that the fan goes FASTER with a blocked outlet
This occurs because the fan does WORK when pushing air but much less work when it cannot do any work - thus "resistence" is less
A prop is doing work when converting relative motion of the boat and water to spin the shaft - overcoming the friction resistence in the system.
When the prop is locked it is doing no work and hence there is less energy absorbed by the prop. hence the boat goes faster under sail with a lock prop.
Even if you dont believe it then it is better to lock the prop so that plaggy bags and old rope are less likely to jam up the prop.
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Avocet
Well-known member
Re: scientific explanation...
I understand the hairdryer thing. All radial and axial compressors will "surge" when the pressure drop across them gets too big and the fluid they're moving in just "shears" round the blade tips. It just seems a bit odd that a stopped prop being dragged through the water causing turbulence round any of its exposed blades (I have a three-bladed prop with quite a large "deadwood" area ahead of it) imposes less drag than one spinning (once it's up to speed). I think this is what the other poster was saying and I accept that the prop might never actually get "up to speed" which would then be like all the blades alternately getting dragged through the water.
I'll have to try some experiments and will get back to the forum if I get any worthwhile results.
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I understand the hairdryer thing. All radial and axial compressors will "surge" when the pressure drop across them gets too big and the fluid they're moving in just "shears" round the blade tips. It just seems a bit odd that a stopped prop being dragged through the water causing turbulence round any of its exposed blades (I have a three-bladed prop with quite a large "deadwood" area ahead of it) imposes less drag than one spinning (once it's up to speed). I think this is what the other poster was saying and I accept that the prop might never actually get "up to speed" which would then be like all the blades alternately getting dragged through the water.
I'll have to try some experiments and will get back to the forum if I get any worthwhile results.
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milltech
Active member
The company that make the propellers where the blades automatically set themselves to the most efficient angle, (the name won't come to me), had a shaft brake on the boat show stand at London. I guess you'd have a handbrake lever next to the throttle.
<hr width=100% size=1>John
<A target="_blank" HREF=http://www.allgadgets.co.uk>http://www.allgadgets.co.uk</A>
<hr width=100% size=1>John
<A target="_blank" HREF=http://www.allgadgets.co.uk>http://www.allgadgets.co.uk</A>