1. So, if less resistance (locked prop) makes the boat sail slower, will a larger locked prop make it go faster?
2. How does an auto prop get the energy to fold? Could we build one that acts like a helicopter rotor and flips pitch from postive to negative?
3. Would a prop require a motive force to maintain zero slip with the fluid flow? If so, would that be more or less force than it would take to keep the same prop from spinning?
4. If a prop is locked such as to heel the boat onto a longer water line, will the boat go faster? Or might it go faster if the prop is locked to keep the boat more upright and the sails doing more work?
Etc., etc., and so forth.
There is at least a Winter's worth of study here.
How sailboats sail
Moderator: Jim Walsh
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Parfait's Provider
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berthed Whortonsville, NC
Larger Prop =? Faster Sail
Keep on sailing,
Ken Coit, ND7N
CD/36 #84
Parfait
Raleigh, NC
Ken Coit, ND7N
CD/36 #84
Parfait
Raleigh, NC
Everything is INDEED Relative
Matt
That is certainly a mouthful. You seem to believe that whatever forces are acting on the propeller to either increase or reduce the drag on our boats, they somehow dont have any impact on the spinning of the shaft even though it is fixed to the propeller. The truth is that the shaft spins faster or slower when allowed to freewheel, as a result of the combination of all the forces applicable.
None of the forces that are applicable, fail to contribute to the impact on the velocity of the shafts spinning at any RPM. Whatever the impact of the disturbed flow on the hull and rudder, you must admit it is there at all RPMs, and you would expect it to be greater at higher RPMs than at lower RPMs.
All of that being true, which it is, I dont need to measure the shafts rotation with any fancy instruments, as you suggest. I simply need to determine what does it take to slow a freewheeling propeller at any boat speed, and whether it holds true at all Rpms from "fully freewheeling" to stopped. After all, the only thing Vigor did was watch a toy helicopter fall to the ground to start all this.
We all seem to agree that as one slows the propeller from "fully freewheeling" to any slower RPM, the boat will slow down. What it takes to accomplish this is increased pressure applied to the spinning shaft. The fact that we must apply increased pressure to the spinning shaft to slow the propeller, and thus slow the boat, without a doubt means that we have increased drag on the boat, as the boat wants to speed up and return to its previous sailing speed if it weren't for our pressure applied to the shaft.
Those who believe in the locked prop theory, must believe that while it takes increased pressure to reduce the RPMs of a freewheeling propeller all the way down to some small RPM, that
somehow miraculously, to go from that small RPM to the stopped (locked) propeller mode, no additional pressure is needed. That simply cant be true.
Even the locked propeller disturbs the flow and impacts on the hull and rudder.
We disagree on whether the drag (max to min) is linear, but surely you dont deny that torque on the shaft is RELEVENT to drag on the boat?
So, you may too lock your prop or dont. It wont change the above facts. Do you really think you can stop your propeller (shaft) from any (even very slow) RPM without increasing the pressure on said shaft? Can you or anyone at least answer that?
To all those who cant see any change of speed when locking or unlocking your propeller, that may be. You may have enough friction built into your drivetrain that your propeller and shaft doesn't spin much or at all when freewheeling, so there is no difference to you. Other variables such as wind variation, waves, current, etc. make it difficult to determine true changes in speed.
That is why measuring the RELEVENT pressures needed on the shaft to slow the boat, (slow the propeller) are far more accurate and definitive.
Everything is Relevent.
Darrell
That is certainly a mouthful. You seem to believe that whatever forces are acting on the propeller to either increase or reduce the drag on our boats, they somehow dont have any impact on the spinning of the shaft even though it is fixed to the propeller. The truth is that the shaft spins faster or slower when allowed to freewheel, as a result of the combination of all the forces applicable.
None of the forces that are applicable, fail to contribute to the impact on the velocity of the shafts spinning at any RPM. Whatever the impact of the disturbed flow on the hull and rudder, you must admit it is there at all RPMs, and you would expect it to be greater at higher RPMs than at lower RPMs.
All of that being true, which it is, I dont need to measure the shafts rotation with any fancy instruments, as you suggest. I simply need to determine what does it take to slow a freewheeling propeller at any boat speed, and whether it holds true at all Rpms from "fully freewheeling" to stopped. After all, the only thing Vigor did was watch a toy helicopter fall to the ground to start all this.
We all seem to agree that as one slows the propeller from "fully freewheeling" to any slower RPM, the boat will slow down. What it takes to accomplish this is increased pressure applied to the spinning shaft. The fact that we must apply increased pressure to the spinning shaft to slow the propeller, and thus slow the boat, without a doubt means that we have increased drag on the boat, as the boat wants to speed up and return to its previous sailing speed if it weren't for our pressure applied to the shaft.
Those who believe in the locked prop theory, must believe that while it takes increased pressure to reduce the RPMs of a freewheeling propeller all the way down to some small RPM, that
somehow miraculously, to go from that small RPM to the stopped (locked) propeller mode, no additional pressure is needed. That simply cant be true.
Even the locked propeller disturbs the flow and impacts on the hull and rudder.
We disagree on whether the drag (max to min) is linear, but surely you dont deny that torque on the shaft is RELEVENT to drag on the boat?
So, you may too lock your prop or dont. It wont change the above facts. Do you really think you can stop your propeller (shaft) from any (even very slow) RPM without increasing the pressure on said shaft? Can you or anyone at least answer that?
To all those who cant see any change of speed when locking or unlocking your propeller, that may be. You may have enough friction built into your drivetrain that your propeller and shaft doesn't spin much or at all when freewheeling, so there is no difference to you. Other variables such as wind variation, waves, current, etc. make it difficult to determine true changes in speed.
That is why measuring the RELEVENT pressures needed on the shaft to slow the boat, (slow the propeller) are far more accurate and definitive.
Everything is Relevent.
Darrell
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M. R. Bober
- Posts: 1122
- Joined: Feb 6th, '05, 08:59
- Location: CARETAKER CD28 Flybridge Trawler
Thump thump thump thump
OKOKOKOKOKOKOKOKOKOKOK
I wasn't going to get involved in Vigor's Folly Part 2, but I am lacking in self discipline.
In answer to those who refuse to see, allow me to relate a personal experience--remembering that inference based upon small sample size is unreliable--with locked versus freewheeling props. Last season while on a broad reach in 10 knots of steady wind, with a locked prop in flat water, RESPITE was making 4.75 knots. I unlocked the prop (shifted from reverse to neutral) and RESPITE immediately climbed to 300 feet above sea level. I had to ease the collective to get back into the water. That's my story. I am prepared to have it notarized.
Mitchell Bober
Sunny Lancaster (where you can believe me for two reasons: 1) I am always rights, and 2) I never lie), VA
I wasn't going to get involved in Vigor's Folly Part 2, but I am lacking in self discipline.
In answer to those who refuse to see, allow me to relate a personal experience--remembering that inference based upon small sample size is unreliable--with locked versus freewheeling props. Last season while on a broad reach in 10 knots of steady wind, with a locked prop in flat water, RESPITE was making 4.75 knots. I unlocked the prop (shifted from reverse to neutral) and RESPITE immediately climbed to 300 feet above sea level. I had to ease the collective to get back into the water. That's my story. I am prepared to have it notarized.
Mitchell Bober
Sunny Lancaster (where you can believe me for two reasons: 1) I am always rights, and 2) I never lie), VA
Last edited by M. R. Bober on Dec 16th, '08, 20:13, edited 2 times in total.
CDSOA Founding Member
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Neil Gordon
- Posts: 4367
- Joined: Feb 5th, '05, 17:25
- Location: s/v LIQUIDITY, CD28. We sail from Marina Bay on Boston Harbor. Try us on channel 9.
- Contact:
Darrell,
>>That is why measuring the RELEVENT pressures needed on the shaft to slow the boat, (slow the propeller) are far more accurate and definitive.<<
We agree that force is needed to slow the prop. What we're pondering is whether that force translates into more speed or less speed.
If you're applying energy to squeeze the prop into slowing/stopping, what's wrong with the notion that the energy applied goes to making the boat go faster?
I know it takes energy for the crew to shift from leeward to windward on a breezy day. That energy makes the boat go faster. What's the difference?
>>That is why measuring the RELEVENT pressures needed on the shaft to slow the boat, (slow the propeller) are far more accurate and definitive.<<
We agree that force is needed to slow the prop. What we're pondering is whether that force translates into more speed or less speed.
If you're applying energy to squeeze the prop into slowing/stopping, what's wrong with the notion that the energy applied goes to making the boat go faster?
I know it takes energy for the crew to shift from leeward to windward on a breezy day. That energy makes the boat go faster. What's the difference?
Fair winds, Neil
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
Neil,Neil Gordon wrote:
We agree that force is needed to slow the prop. What we're pondering is whether that force translates into more speed or less speed.
If you're applying energy to squeeze the prop into slowing/stopping, what's wrong with the notion that the energy applied goes to making the boat go faster?
I am very encouraged that you are thinking about this, and you have asked a very pertinent question, thank you.
Now that we recognize that slowing the freewheeling shaft (propeller) on a boat sailing along at any speed , will affect the speed of the boat, lets try to answer your question.
Will the boat speed up or slow down?
I would answer it this way; do you doubt for a second that if we were to speed up the rotation of the shaft (propeller) while all other conditions remained the same, thereby creating thrust, that the boat would speed up?
Of course it would, and it follows that slowing the propeller will slow the boat down, and the more you slow the propeller, the more it slows the boat down.
The water flowing past the freewheeling propeller at any sailing speed is going to cause the propeller to spin at an RPM that is relevent to that boat speed. If you artifically slow the propeller, the additional force created by the water trying to get the propeller back up to the proper RPM, (a force you can feel if you grab the shaft) will slow the boat.
Please ask anything further that you need that will help. I am happy to do this one convert at a time. I am also willing to be proven wrong, if you can.
Darrell
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Neil Gordon
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Darrell,
First of all, remember that although I did go to college, I studied accounting. That said:
>>... do you doubt for a second that if we were to speed up the rotation of the shaft (propeller) while all other conditions remained the same, thereby creating thrust, that the boat would speed up?<<
I'd suggest that you make thrust when the boat makes the prop turn. If the ocean is making the prop turn, that's drag. (We'd all conclude that no prop would be faster than a prop spinning or locked.)
>>... it follows that slowing the propeller will slow the boat down, and the more you slow the propeller, the more it slows the boat down.<<
Either that, or it reduces drag, so the more you slow the prop, the less drag and the faster the boat goes.
>>The water flowing past the freewheeling propeller at any sailing speed is going to cause the propeller to spin at an RPM that is relevent to that boat speed. If you artifically slow the propeller, the additional force created by the water trying to get the propeller back up to the proper RPM, (a force you can feel if you grab the shaft) will slow the boat.<<
Either that, or since the force can't make the locked prop turn faster, the energy has no place to go but to make the boat go faster.
Whenever I have wind on the beam, the force of the wind on the sails tries to turn the boat over. But it can't because the keel stops the wind from doing that; effectively "locking" the hull at about a 15 degree angle of heel. Are you suggesting that if force of the keel was removed, the boat would go faster? What's the difference?
First of all, remember that although I did go to college, I studied accounting. That said:
>>... do you doubt for a second that if we were to speed up the rotation of the shaft (propeller) while all other conditions remained the same, thereby creating thrust, that the boat would speed up?<<
I'd suggest that you make thrust when the boat makes the prop turn. If the ocean is making the prop turn, that's drag. (We'd all conclude that no prop would be faster than a prop spinning or locked.)
>>... it follows that slowing the propeller will slow the boat down, and the more you slow the propeller, the more it slows the boat down.<<
Either that, or it reduces drag, so the more you slow the prop, the less drag and the faster the boat goes.
>>The water flowing past the freewheeling propeller at any sailing speed is going to cause the propeller to spin at an RPM that is relevent to that boat speed. If you artifically slow the propeller, the additional force created by the water trying to get the propeller back up to the proper RPM, (a force you can feel if you grab the shaft) will slow the boat.<<
Either that, or since the force can't make the locked prop turn faster, the energy has no place to go but to make the boat go faster.
Whenever I have wind on the beam, the force of the wind on the sails tries to turn the boat over. But it can't because the keel stops the wind from doing that; effectively "locking" the hull at about a 15 degree angle of heel. Are you suggesting that if force of the keel was removed, the boat would go faster? What's the difference?
Fair winds, Neil
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
[quote="Neil Gordon"
>>I'd suggest that you make thrust when the boat makes the prop turn. If the ocean is making the prop turn, that's drag.<<
Are you suggesting that i cant slow a boat that is "motor sailing" along at say 6kts, by simply backing off the throttle. Thrust is relevent too, and cant be created until the propellers RPMs exceed the Rpms of the freewheeling propeller at any given boat speed.
Thrust or drag is not determined by what is making the propeller spin, but rather by whether the propeller is spinning faster or slower than the relevent freewheeling RPM at any given boat speed.
Those that claim that their boat will sail faster if they just keep the engine "ticking" along can only be right if they have enough built in friction in their drivetrain to restrict their propeller from reaching any appreciable freewheeling RPMs, and thus must overcome that restriction by running the engine. Essentially, they are doing the same thing we describe above, they are releasing the artificial pressure that is slowing their boat because their propeller is either "locked" or artificially slowed if in the nuetral gear by friction.
A propeller that is left to do as the forces on it cause it to do, will spin as the boat moves forward under sail because that is where the least resistance lies. The fact that you have to lock the propeller to keep it from spinning tells you that this is not the position of least resistance and it takes a great deal of force to keep it locked.
If a propeller met the least resistance while sitting still, it would sit still without being locked.
Darrell
[/quote]
>>I'd suggest that you make thrust when the boat makes the prop turn. If the ocean is making the prop turn, that's drag.<<
Are you suggesting that i cant slow a boat that is "motor sailing" along at say 6kts, by simply backing off the throttle. Thrust is relevent too, and cant be created until the propellers RPMs exceed the Rpms of the freewheeling propeller at any given boat speed.
Thrust or drag is not determined by what is making the propeller spin, but rather by whether the propeller is spinning faster or slower than the relevent freewheeling RPM at any given boat speed.
Those that claim that their boat will sail faster if they just keep the engine "ticking" along can only be right if they have enough built in friction in their drivetrain to restrict their propeller from reaching any appreciable freewheeling RPMs, and thus must overcome that restriction by running the engine. Essentially, they are doing the same thing we describe above, they are releasing the artificial pressure that is slowing their boat because their propeller is either "locked" or artificially slowed if in the nuetral gear by friction.
A propeller that is left to do as the forces on it cause it to do, will spin as the boat moves forward under sail because that is where the least resistance lies. The fact that you have to lock the propeller to keep it from spinning tells you that this is not the position of least resistance and it takes a great deal of force to keep it locked.
If a propeller met the least resistance while sitting still, it would sit still without being locked.
Darrell
[/quote]
-
Neil Gordon
- Posts: 4367
- Joined: Feb 5th, '05, 17:25
- Location: s/v LIQUIDITY, CD28. We sail from Marina Bay on Boston Harbor. Try us on channel 9.
- Contact:
That doesn't seem to work for the helicopter. Leave its prop to freewheel, and the helicopter falls slowly. Lock it and it crashes. And I'm pretty sure you'd need to exert some force to stop the blades from turning. Gravity (and all else) is constant. What's the difference?darmoose wrote:If a propeller met the least resistance while sitting still, it would sit still without being locked.
Also: There's a difference between (i) using the motor to drive the prop to the same speed as the water flow, and (ii) using the water flow to try driving the prop to the same end.
Fair winds, Neil
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
Neil,
I like the way you are thinking, and your questions are very good. However, if you stick with me, i think i'm gonna getcha
First of all, i dont think the helicopter analogy has any bearing whatsoever on the question of boats and freewheeling propellers (yeah, i can hear Vigor smirking in the background). i do believe and understand (thanks to Matt) the whole autorotation thing. I can only surmize that the differences that Matt explained regarding air foils, lift forces, and all that other stuff make the difference between helicopters and boats. I have always maintained that if the only argument for the "locked prop" theory is helicopters, that is very weak. Please dont try to prove i am wrong by mearly falling back on the helicopter idea (and yeah i know you think i want to take away one of your arguments, and that ain't fair), but the truth is i want to take away your "only" argument, and have you look at the far more pertinent evidence of the workings of the boat itself. That can't be bad.
You said earlier that "when the boat makes the propeller spin, thats thrust, and when the ocean makes the propeller spin thats drag" I think I can disprove that.
Consider this example..... two sailboats are motoring along (no sails up) at the same speed. The captains decide that the lead boat is going to tow the following boat, and so they fix a towline between them. As long as both captains maintain the same speed, nothing changes, and the towline can be slack.
But, as soon as the captain of the boat being towed reduces RPMs he is no longer in thrust mode (even though his engine is still running). He is in "drag" mode, the towline is taught, and he is controlling the amount of drag on both his hull and on the towing boat by either reducing or increasing his RPMs. As he reduces his RPMs he will slow his boat and the towboat (you cant possibly doubt that, can you?). If he stops his engine and locks his propeller, he will maximize the drag and slow the towboat as much as he can, without turning the engine back on and putting the engine in reverse.
If the boat being towed, puts his tranny back in forward gear, and applies throttle, he will slowly reduce the drag on both his hull and on the towboat, and they both will begin to move faster. However the towline will stay taught, and the boat being towed will continue to be towed until he reaches a speed which takes him from the drag mode to the thrust mode, and the towline will go slack. At this point both boats will be back up to thier original speeds.
The towboat is exactly like our sails and it does not matter if our boat is being sailed a 5kts or being pulled at 5kts. A spinning propeller is in drag mode until the RPMs reach a speed to overcome the greater force of the sails or the towboat, regardless of whether the engine is spinning the propeller, or it is freewheeling. Remember the towline and when it goes slack.
I dont believe the above example can be disputed, and it proves that it does not matter whether the engine or the ocean is causing the propeller to spin, the effect is the same if there is a greater force moving the boat.
It also proves that the drag vs. thrust relationship is LINEAR, because our boat doesnt and cant go from Maximum thrust to Minimum thrust, and then go instantly to Maximum drag (this would be like hitting a wall), or in the reverse our boat cant go from Minimum drag to Maximum drag, and then instantly to Minimum thrust (this would cause a 'big bang' type lurch forward which doesnt happen), and besides we have already proved that more RPMs= less drag, less RPMs= more drag (remember the towline)
These changes from drag mode to thrust mode are subtle and smooth in either direction. The progression is linear, and it does not matter what is causing the propeller to spin. Therefore, a freewheeling propeller causes less drag the faster it spins. A locked propeller causes the maximum drag.
Please dont give me the helicopter thingy, show me that anything I've said in the above example is not true, and that any of the conclusions drawn from that example aren't right.
Darrell
I like the way you are thinking, and your questions are very good. However, if you stick with me, i think i'm gonna getcha
First of all, i dont think the helicopter analogy has any bearing whatsoever on the question of boats and freewheeling propellers (yeah, i can hear Vigor smirking in the background). i do believe and understand (thanks to Matt) the whole autorotation thing. I can only surmize that the differences that Matt explained regarding air foils, lift forces, and all that other stuff make the difference between helicopters and boats. I have always maintained that if the only argument for the "locked prop" theory is helicopters, that is very weak. Please dont try to prove i am wrong by mearly falling back on the helicopter idea (and yeah i know you think i want to take away one of your arguments, and that ain't fair), but the truth is i want to take away your "only" argument, and have you look at the far more pertinent evidence of the workings of the boat itself. That can't be bad.
You said earlier that "when the boat makes the propeller spin, thats thrust, and when the ocean makes the propeller spin thats drag" I think I can disprove that.
Consider this example..... two sailboats are motoring along (no sails up) at the same speed. The captains decide that the lead boat is going to tow the following boat, and so they fix a towline between them. As long as both captains maintain the same speed, nothing changes, and the towline can be slack.
But, as soon as the captain of the boat being towed reduces RPMs he is no longer in thrust mode (even though his engine is still running). He is in "drag" mode, the towline is taught, and he is controlling the amount of drag on both his hull and on the towing boat by either reducing or increasing his RPMs. As he reduces his RPMs he will slow his boat and the towboat (you cant possibly doubt that, can you?). If he stops his engine and locks his propeller, he will maximize the drag and slow the towboat as much as he can, without turning the engine back on and putting the engine in reverse.
If the boat being towed, puts his tranny back in forward gear, and applies throttle, he will slowly reduce the drag on both his hull and on the towboat, and they both will begin to move faster. However the towline will stay taught, and the boat being towed will continue to be towed until he reaches a speed which takes him from the drag mode to the thrust mode, and the towline will go slack. At this point both boats will be back up to thier original speeds.
The towboat is exactly like our sails and it does not matter if our boat is being sailed a 5kts or being pulled at 5kts. A spinning propeller is in drag mode until the RPMs reach a speed to overcome the greater force of the sails or the towboat, regardless of whether the engine is spinning the propeller, or it is freewheeling. Remember the towline and when it goes slack.
I dont believe the above example can be disputed, and it proves that it does not matter whether the engine or the ocean is causing the propeller to spin, the effect is the same if there is a greater force moving the boat.
It also proves that the drag vs. thrust relationship is LINEAR, because our boat doesnt and cant go from Maximum thrust to Minimum thrust, and then go instantly to Maximum drag (this would be like hitting a wall), or in the reverse our boat cant go from Minimum drag to Maximum drag, and then instantly to Minimum thrust (this would cause a 'big bang' type lurch forward which doesnt happen), and besides we have already proved that more RPMs= less drag, less RPMs= more drag (remember the towline)
These changes from drag mode to thrust mode are subtle and smooth in either direction. The progression is linear, and it does not matter what is causing the propeller to spin. Therefore, a freewheeling propeller causes less drag the faster it spins. A locked propeller causes the maximum drag.
Please dont give me the helicopter thingy, show me that anything I've said in the above example is not true, and that any of the conclusions drawn from that example aren't right.
Darrell
-
Neil Gordon
- Posts: 4367
- Joined: Feb 5th, '05, 17:25
- Location: s/v LIQUIDITY, CD28. We sail from Marina Bay on Boston Harbor. Try us on channel 9.
- Contact:
Darrell,
>>... i dont think the helicopter analogy has any bearing whatsoever on the question of boats and freewheeling propellers ... I can only surmize that the differences that Matt explained regarding air foils, lift forces, and all that other stuff make the difference between helicopters and boats.<<
You want simpler than a helicopter? How about maple seeds? They're half a propeller but they still illustrate the point. In any case you're suggesting that air and water have different physics. Where's the evidence of that?
>>I have always maintained that if the only argument for the "locked prop" theory is helicopters, that is very weak.<<
I'm saying that if the only observable analogy is helicopters (or maple seeds) that is very strong.
>>But, as soon as the captain of the boat being towed reduces RPMs ... the towline is taught, ...<<
Make the tow boat an aircraft carrier and we can ignore the notion that the towed boat slows the tow boat. (This is the same demo as anchoring in a current, then.)
>>... and he is controlling the amount of drag on both his hull and on the towing boat by either reducing or increasing his RPMs.<<
He's controlling the amount of force he's introducing. Remember that for the prop to be effective (even the aircraft carrier's) it can't actually reach it's theoretical maximum speed. (You can only push a car as fast as you can run behind it.) So the prop continues to produce forward thrust even in the slower boat, for a while.
>>As he reduces his RPMs he will slow his boat and the towboat (you cant possibly doubt that, can you?).<<
I've done this when anchoring, but not with that result. I anchor and the boat drifts back with the wind and stops. I put the engine in reverse and if the anchor is set well, tension on the rode increases but the boat speed (i.e., zero) doesn't change. Satisfied that the anchor is holding, I put the engine in neutral and the boat speed doesn't change then, either. (Note: When at anchor, my boat speed is the same whether I lock the prop or not.)
>>If he stops his engine and locks his propeller, he will maximize the drag and slow the towboat as much as he can, without turning the engine back on and putting the engine in reverse.<<
There's a variable you introduce, which is the force of the engine. Our discussion only has the force of the flow over the prop, spinning or not. You need to make your case without helping the prop to spin.
>>A spinning propeller is in drag mode until the RPMs reach a speed to overcome the greater force of the sails or the towboat, regardless of whether the engine is spinning the propeller, or it is freewheeling.<<
Yes, a spinning prop can be in drag mode but that's when the energy needed to spin it is diverted from its intended purpose. We're using wind to push the water out of the way. But then we use some of that energy to spin the prop. Prove that the prop spins by itself (i.e., doesn't use any of the water flow energy) and you might win a Nobel prize.
>>Therefore, a freewheeling propeller causes less drag the faster it spins.<<
A freewheeling prop uses more energy the faster it spins. (Try cranking anything you like... the faster you crank, the more your arm will hurt.)
>>... i dont think the helicopter analogy has any bearing whatsoever on the question of boats and freewheeling propellers ... I can only surmize that the differences that Matt explained regarding air foils, lift forces, and all that other stuff make the difference between helicopters and boats.<<
You want simpler than a helicopter? How about maple seeds? They're half a propeller but they still illustrate the point. In any case you're suggesting that air and water have different physics. Where's the evidence of that?
>>I have always maintained that if the only argument for the "locked prop" theory is helicopters, that is very weak.<<
I'm saying that if the only observable analogy is helicopters (or maple seeds) that is very strong.
>>But, as soon as the captain of the boat being towed reduces RPMs ... the towline is taught, ...<<
Make the tow boat an aircraft carrier and we can ignore the notion that the towed boat slows the tow boat. (This is the same demo as anchoring in a current, then.)
>>... and he is controlling the amount of drag on both his hull and on the towing boat by either reducing or increasing his RPMs.<<
He's controlling the amount of force he's introducing. Remember that for the prop to be effective (even the aircraft carrier's) it can't actually reach it's theoretical maximum speed. (You can only push a car as fast as you can run behind it.) So the prop continues to produce forward thrust even in the slower boat, for a while.
>>As he reduces his RPMs he will slow his boat and the towboat (you cant possibly doubt that, can you?).<<
I've done this when anchoring, but not with that result. I anchor and the boat drifts back with the wind and stops. I put the engine in reverse and if the anchor is set well, tension on the rode increases but the boat speed (i.e., zero) doesn't change. Satisfied that the anchor is holding, I put the engine in neutral and the boat speed doesn't change then, either. (Note: When at anchor, my boat speed is the same whether I lock the prop or not.)
>>If he stops his engine and locks his propeller, he will maximize the drag and slow the towboat as much as he can, without turning the engine back on and putting the engine in reverse.<<
There's a variable you introduce, which is the force of the engine. Our discussion only has the force of the flow over the prop, spinning or not. You need to make your case without helping the prop to spin.
>>A spinning propeller is in drag mode until the RPMs reach a speed to overcome the greater force of the sails or the towboat, regardless of whether the engine is spinning the propeller, or it is freewheeling.<<
Yes, a spinning prop can be in drag mode but that's when the energy needed to spin it is diverted from its intended purpose. We're using wind to push the water out of the way. But then we use some of that energy to spin the prop. Prove that the prop spins by itself (i.e., doesn't use any of the water flow energy) and you might win a Nobel prize.
>>Therefore, a freewheeling propeller causes less drag the faster it spins.<<
A freewheeling prop uses more energy the faster it spins. (Try cranking anything you like... the faster you crank, the more your arm will hurt.)
Fair winds, Neil
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
s/v LIQUIDITY
Cape Dory 28 #167
Boston, MA
CDSOA member #698
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Marc Theriault
- Posts: 85
- Joined: Feb 6th, '05, 18:05
- Location: Contessa 26 s/v Sun Wave Lake Champlain NY/VT
Ty owners you are the winners
The beauty of the TY,
Locking or not locking the prop....Why?
Just rease the motor and the debate is close.
Happy holidays to all
Marc
Locking or not locking the prop....Why?
Just rease the motor and the debate is close.
Happy holidays to all
Marc
Proof requested and provided...
Neil,
I must thank you for providing the thought that lead to the proof,
but why wont you, or anyone, set aside your analogies for just a moment (you can always come back to them, i promise), and simply address any single sentance in my example above and tell me if you disagree and why?
My initial point was to address your ascertion that a propeller being spun by the boat can't be in drag mode. I have proven that it certainly can. The towline don't lie.
Once you can understand that, you cant avoid accepting that a propeller going from maximum thrust to a stopped position is a progression from minimum drag to maximum drag, and it is linear.
Or you can stick with toy helicopters, maple seeds, or appleseeds and refuse to learn anything, no skin off my nose. I have offered to let you convince me otherwise, and told you how you can do it.
In the meantime, i will stick to what i know to be fact.
Sooner or later , this will hit home for anyone who has an open mind.
Merry Christmas to you
Darrell
I must thank you for providing the thought that lead to the proof,
but why wont you, or anyone, set aside your analogies for just a moment (you can always come back to them, i promise), and simply address any single sentance in my example above and tell me if you disagree and why?
My initial point was to address your ascertion that a propeller being spun by the boat can't be in drag mode. I have proven that it certainly can. The towline don't lie.
Once you can understand that, you cant avoid accepting that a propeller going from maximum thrust to a stopped position is a progression from minimum drag to maximum drag, and it is linear.
Or you can stick with toy helicopters, maple seeds, or appleseeds and refuse to learn anything, no skin off my nose. I have offered to let you convince me otherwise, and told you how you can do it.
In the meantime, i will stick to what i know to be fact.
Sooner or later , this will hit home for anyone who has an open mind.
Merry Christmas to you
Darrell
Questions for Matt (or anyone else who knows the answers).
My understanding of the freewheeling vs. locked schism is frustrated by not knowing the answers to the following two questions:
1. Is a sailboat prop more like a woodscrew (i.e., the thrust results from Newton's third law) or more like an airplane propellor (i.e., it operates as an airfoil and the thrust comes from horizontal lift)? Why?
2. In which direction does a freewheeling prop spin; i.e., does a lefthand prop continue spinning to the left when it freewheels or does the force of the water on the blades cause it to reverse direction? Why?
I know some of you will think these are stupid questions but please go easy on me.
1. Is a sailboat prop more like a woodscrew (i.e., the thrust results from Newton's third law) or more like an airplane propellor (i.e., it operates as an airfoil and the thrust comes from horizontal lift)? Why?
2. In which direction does a freewheeling prop spin; i.e., does a lefthand prop continue spinning to the left when it freewheels or does the force of the water on the blades cause it to reverse direction? Why?
I know some of you will think these are stupid questions but please go easy on me.
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SurryMark
- Posts: 302
- Joined: Nov 18th, '08, 10:04
- Location: Formerly CD27Y, Tula. Now Luders Sea Sprite 34
- Contact:
offsetting the upsetting disturbance
Here the proven and approved Lake Mt. Tamalpais Racing Society method for neutralizing prop drag. Drill 3/16" holes centered and 1/2" from the tip of each blade. Insert a balloon through each hole, with the mouthpiece part of the balloon facing forward. Blow each balloon to between 11" and 12" diameter, taking care to inflate each balloon the same, give or take 10%. Hold shut the mouthpiece of each balloon until you are ready to go to the surface. Release balloons at the same time. This will effectively neutralize any prop drag to the level of a shaft without prop, if the shaft is ground to a 20º x 3" bevel with a 1.5" radius at the end. Note: Take care to use a cordless drill, not a cord drill, if you are making the holes in place.
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Matt Cawthorne
- Posts: 355
- Joined: Mar 2nd, '05, 17:33
- Location: CD 36, 1982
Hull # 79
Gas
Perhaps that explains my results. I have noted here that when the shaft is locked the boat went the same speed. That test was done the day after I taped a bag of beans around the prop overnight. In the morning I removed the bag, and of course, all of the barnacles had gas. I never thought about the influence of a release of gas on the results.