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"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
: When will my boat run fastest: 'Free' running propeller or fixed/locked propeller? ... Simple question, complicated answers, I understand ... When you replace the prop with a folding prop that has nearly zero drag. It does make a difference. I'm not crazy about a prop with so much underwater mechanical parts, myself. Those little critters with the little shells must be of some nuisance to a folding prop with cracks in them. Freewheeling would seem to produce less drag than it being locked up dragging, with no load on the prop. Ever notice how quick the dingy stops when the outboard motor just locks up as opposed to drops to idle speed still turning the prop slowly? I'm also told some transmissions will not tolerate freewheeling. Case in point is when we got this "new-used" Perkins 4-108 from another boater, the transmission on it was much smaller than the original one in Lionheart on her old Perkins 4-108. The mechanics said to keep this "fluid drive" larger transmission because it was made to freewheel for the alternator. The small transmission was not. I've never seen why, though, and don't know why. Oh oh...this is getting too long, again....sorry. |
Thank you Roger - your stamina is impressive - thanks! ... I
start to get your point - Sorry it took so long. Coming from Denmark, where the water is around 13 degrees Celcius these days, this is a very important message to all sailers: Get out and sail, and lock your propeller, so we soon can come out swimming in warmer waters! ... ;-)) ... No Roger you don't have to give me a lecture in how many sailors we need in order to increase the temperature in the waters around Denmark by ten degrees ... you have been very helpful so far .... thank you! -- Flemming Torp "Roger Long" skrev i en meddelelse ... Your intuition is wrong. You never loosen the grip, you just change to a different kind of grip. Ever notice how hot brakes get? That's because they are turning all that energy into heat. When you let the prop go, it is still restrained by the friction of the bearings and oil in the transmission. They get warmer because they are turning the energy being produced into heat. That gets reflected in drag on the boat. When the prop is locked, the shaft break, clutch, or whatever is holding it doesn't get hotter. What gets hotter is the water flowing past the blades. That energy production gets reflected in drag on the boat as well but there is less of it at normal sailing speeds and freewheeling shaft rpm. -- Roger Long "Flemming Torp" fletopkanelbolle2rp.danmark wrote in message . .. My intuition tells me that when you ' loosen the grip' on the propeller shaft and let the propeller turn freely, it reduces the drag on the boat. Right or wrong? -- Flemming Torp |
"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
: My intuition tells me that when you ' loosen the grip' on the propeller shaft and let the propeller turn freely, it reduces the drag on the boat. Right or wrong? It'd be real easy to test. Get one of those plastic props for a small outboard, put it on a shaft. Pull it loosely behind the boat, FREEWHEELING, with no load (which produces no work) using a simple fish weighing scale to measure its drag. Then, lock it to the shaft so it can't turn and drag it at the same speed, yet again. Wanna bet the locked prop draggin' through the water creates lots more drag than the one locked that can't turn? Take the dingy out for a spin. Run it wide open then drop the engine into neutral, freewheeling the prop, and see how long it takes to stop dead. Crank it back up and run it wide open again. This time, leave it in gear and just pull the safety lanyard making it lock the prop dead still. Retime how long it takes it to stop. More the drag, the quicker the stop. You'll find it stops MUCH quicker pulling that STALLED prop through the water than one FREEWHEELING. As Lionheart's freewheeling shaft alternator is TURNED ON to produce power (producing torque on the alternator shaft), it SLOWS the boat as the alternator SLOWS THE PROP. The heavier it pulls (more torque produced), the more it slows the boat and prop. Freewheeling props are producing a minimal amount of torque...and work with no or little load...as load increases, to the point of being stalled at maximum torque, the drag- converted-into-torque INCREASES, not decreases! If you were to spin the prop faster than its drag is capable of, you will pass through a point of ZERO DRAG just at the point where increasing the speed of the prop PRODUCES THRUST. So, as prop speed DECREASES, drag INCREASES! As prop speed INCREASES, it passes through zero drag at the point where it starts to produce thrust by turning it even faster! I'm not a physicist, but I play one on Usenet....(c; Your assignment for tomorrow's class is pages 287 through 312. Do the workbook exercies on workbook pages 42-43 for turn in. The test will be on Friday at 2PM. It might also be a good time to point out that the HELICOPTER rotor CHANGES DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction from freewheeling to provide DOWNDRAFT to hold the craft aloft by the engine. As it decends freewheeling, unless you can reverse the pitch of the rotor, the RISING air through it will make it spin in the opposite direction.....making it not relevant to any of our discussion here as props are fixed pitched and already going in the direction of flow caused by thrust. Geez.... |
"Roger Long" skrev i en meddelelse ... OK, here is the answer you are looking for. Unless you have a highly unusual powertrain set up and strangely pitched prop, determine the position in which the most blade area is shadowed by keel and hull. Mark the shaft inside. Stop the shaft in that position. Sail the boat. It's very unlikely you'll go faster doing anything else. -- Roger Long Now we are getting close to 'basics' Roger ... but, but ... as I wrote in the introduction, it has so far been very difficult to get hard evidence from the log when trying to let the propeller run and have it locked, as the speed of the boat is a function of so many things, and I'm convinced that there is not a big difference - so may be my question is of a more theoretical type, as reliable data are hard to get in the real world ... In a bassin, it might be easier .... I have seen som reports, where different kinds of propellers - folding with two blades, folding with three blades, fixed with three baldes etc. were compared ... and the result indicated differences in 'thrust' and in speed up to between ½ - 1 in worst case ... But I have not seen any reports on the comparison between a locked and a free wheeling propeller ... but I have certainly got a lot of input ... also from the aviation world, that I know nothing about ... thank you. But your final proposal is very logical, operational and easy to implement ... when the water gets warmer, we might do what you have recommended ... or do as Larry - use the energy coming from the rotating propeller - og invest in a folding propeller ... time will show, and thank you so much for your keen interest in learning me some physics ... |
"Larry W4CSC" skrev i en meddelelse ... "Flemming Torp" fletopkanelbolle2rp.danmark wrote in : My intuition tells me that when you ' loosen the grip' on the propeller shaft and let the propeller turn freely, it reduces the drag on the boat. Right or wrong? It'd be real easy to test. Get one of those plastic props for a small outboard, put it on a shaft. Pull it loosely behind the boat, FREEWHEELING, with no load (which produces no work) using a simple fish weighing scale to measure its drag. Then, lock it to the shaft so it can't turn and drag it at the same speed, yet again. Wanna bet the locked prop draggin' through the water creates lots more drag than the one locked that can't turn? Take the dingy out for a spin. Run it wide open then drop the engine into neutral, freewheeling the prop, and see how long it takes to stop dead. Crank it back up and run it wide open again. This time, leave it in gear and just pull the safety lanyard making it lock the prop dead still. Retime how long it takes it to stop. More the drag, the quicker the stop. You'll find it stops MUCH quicker pulling that STALLED prop through the water than one FREEWHEELING. As Lionheart's freewheeling shaft alternator is TURNED ON to produce power (producing torque on the alternator shaft), it SLOWS the boat as the alternator SLOWS THE PROP. The heavier it pulls (more torque produced), the more it slows the boat and prop. Freewheeling props are producing a minimal amount of torque...and work with no or little load...as load increases, to the point of being stalled at maximum torque, the drag- converted-into-torque INCREASES, not decreases! If you were to spin the prop faster than its drag is capable of, you will pass through a point of ZERO DRAG just at the point where increasing the speed of the prop PRODUCES THRUST. So, as prop speed DECREASES, drag INCREASES! As prop speed INCREASES, it passes through zero drag at the point where it starts to produce thrust by turning it even faster! I'm not a physicist, but I play one on Usenet....(c; Your assignment for tomorrow's class is pages 287 through 312. Do the workbook exercies on workbook pages 42-43 for turn in. The test will be on Friday at 2PM. It might also be a good time to point out that the HELICOPTER rotor CHANGES DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction from freewheeling to provide DOWNDRAFT to hold the craft aloft by the engine. As it decends freewheeling, unless you can reverse the pitch of the rotor, the RISING air through it will make it spin in the opposite direction.....making it not relevant to any of our discussion here as props are fixed pitched and already going in the direction of flow caused by thrust. Geez.... Thank you for proposing an interesting experiment ... I will try to set up this experiment during the summer holidays ... In the meantime, I must admit, I'm getting pretty much confused ... I have just appreciated and accepted Roger Longs 'lecture' on drag and rotation speed etc., where the conclusion was: The locked propeller gives the maxium speed compared to the freewheeling propeller ... And now, you testify just to the opposite ... My intuition was exactly as you write - I quote: Wanna bet the locked prop draggin' through the water creates lots more drag than the one locked that can't turn? Unquote. I think I understand what you mean, but being a Dane, and having read it quite a few times makes me a little uncomfortable - is the sentence correctly phrased? The beauty of your proposed experiment is, that it is very operational with a little plastic propellerthing and the weight from my wifes kitchen, a string and a piece of wood. And you can repeat the experiment several time under different conditions ... I will have to find out have to keep the propeller from turning and at the same time measure the drag on the weight, without affecting the weight measurement ... I don't have a motor on the dinghy ... I had serious problems understanding the helicopter analogy .... and the way you phrase the case is the way, my intuition saw the situation - I quote: It might also be a good time to point out that the HELICOPTER rotor CHANGES DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction from freewheeling to provide DOWNDRAFT to hold the craft aloft by the engine. unquote. Therefore, I do not see, that this analogy explains anything related to my question. May be an old quotation is in place now: I'm still confused, but - hopefully - at a higher level ??? As to the assignment, I'm afraid I forgot my textbook in the office, and I will be working from other places the rest of the week .... sorry! I'm 'afraid' this debate is not over yet, but you have given me inspiration to make some funny experiments, that might give me and my brother in law a clear and convincing answer, so we can settle our little dispute ... thankyou! -- Flemming Torp |
I'm sory, but I can only see one picture in
alt.binaries.pictures.sports.ocean ... Am I missing something? -- Flemming Torp "Larry W4CSC" skrev i en meddelelse ... "Roger Long" wrote in : Gee, isn't this kind of "niggling and perfectionist", to quote a well respected rec.boats contributor? Oh, you'd have a fit if you looked into that bilge. There's dishwater, some oil, some rust, a few tools too deep to reach even with the magnet, how awful! It's a boat....not house beautiful. Everything from the sinks, shower, everything but the head, dumps in her bilge to be pumped overboard WITHOUT those nasty thru-hull fittings rotting away...(c; Look at the next picture. This is the NEW engine Cap bought from a guy in NC I met on this newsgroup. It's a pullout. See? We didn't even paint it! Looks awful...runs fantastic. The original looked worse. It had over 8000 hours on it. This "new" one is only up to 900 hours, now. Cap'n Geoffrey couldn't even destroy it trying to start it full of seawater when that stupid water injection line to the dripless packing he had a shipyard install BACKED UP seawater into the exhaust ports. Try sailing up Ponce Inlet S of Daytona Beach in the rush of an outgoing tide past the lighthouse and seawall so the tow operator can drag you all the way to Daytona Marina. Scary stuff that day. Cutter Doc did a great job pickling the engine. It looked like we pumped the Exxon Valdez oil slick out of the crankcase...(c; These pictures are old. I need to carry my camera down, now that she's been fitted out, and take some new pictures in her. I'm usually too busy when Geoffrey is around working on his to-do list...(c; He finally gave up trying to pay me. He asked what I wanted for all my work. I told him, "That's easy. Just take me with you." It's worked out great for both of us....(c; |
"Larry W4CSC" skrev i en meddelelse ... "Flemming Torp" fletopkanelbolle2rp.danmark wrote in : When will my boat run fastest: 'Free' running propeller or fixed/locked propeller? ... Simple question, complicated answers, I understand ... When you replace the prop with a folding prop that has nearly zero drag. It does make a difference. I'm not crazy about a prop with so much underwater mechanical parts, myself. Those little critters with the little shells must be of some nuisance to a folding prop with cracks in them. Freewheeling would seem to produce less drag than it being locked up dragging, with no load on the prop. Ever notice how quick the dingy stops when the outboard motor just locks up as opposed to drops to idle speed still turning the prop slowly? I'm also told some transmissions will not tolerate freewheeling. Case in point is when we got this "new-used" Perkins 4-108 from another boater, the transmission on it was much smaller than the original one in Lionheart on her old Perkins 4-108. The mechanics said to keep this "fluid drive" larger transmission because it was made to freewheel for the alternator. The small transmission was not. I've never seen why, though, and don't know why. Oh oh...this is getting too long, again....sorry. I'm still learning - and reading with great interest ... I just wonder whether Roger is in agreement ;-)) -- Flemming Torp |
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If there truly was less drag on a locked prop than a freewheeling one
you would not have to put it in gear to lock it, a prop out of gear would not even freewheel it there was more drag. |
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On Mon, 6 Jun 2005 22:17:44 +0200, "Flemming Torp"
fletopkanelbolle2rp.danmark wrote: My intuition tells me, however, that if you 'let the propeller go', the total resistance/drag will be less and if you start braking the turning propeller, the resistance/drag gets bigger... Fluid dynamics is tough on the intuition. But if you cannot decide between two different opinions, you *could* do what fluid dynamicists do when uncertain: run a test. One way: find a spinning lure and tow it on a swivel so it can turn, behind a boat at moderate speed (you could even do this in the bath, maybe?) and check the drag with an ounce spring scale. Then fix the spinner, and rerun the test. Is the drag greater or less? You will be surprized I expect. [Usual disclaimer: fluid dynamics people and naval engineers know there are scale effects as between boat sized props and little fish size spinners. Still, in comparing results at constant speed and constant spinner size, you *can* get useful results.... ] Brian Whatcott Altus, OK. |
here's more various discussion from nother forum.
it's more geared to mounting alternators, but gives insight to prop drag. http://www.ybw.com/forums/showflat.p...e=18& fpart=1 |
The dinghy outboard thought experiment is not valid because the whole
system is coming to a stop and the prop is slowing as the boat is slowing. Do it while towing the dinghy behind the big boat at a steady speed with a scale on the tow rope and you may get a different result. Under some circumstances, a freewheeling prop may have less drag than a fixed one. Props on normal transmissions are not freewheeling however. There is enough drag in the shaftline, bearings, and transmission to upset things. Outboards, with their clutches right in the lower unit are closer to freewheeling. -- Roger Long |
JR Gilbreath wrote:
If there truly was less drag on a locked prop than a freewheeling one you would not have to put it in gear to lock it, a prop out of gear would not even freewheel it there was more drag. Why? |
Gogarty wrote:
In article , says... My answer would be the same as yours and also that if you have a two blade prop, it should be locked in line with the keel if it is possible to determine where that is etc. Well, next time you haul you mark the shaft inside the boat to indicate when the prop is up and down behid the keel or skeg or whatever. After twenty years, I might yet get around to doing that. In our case, the freewheeling prop made so much noise that one of the first things we did was get a feathering prop. We do have evidence that feathering the prop increases our sailing speed, in addition to being quieter. Ours doesn't freewheel in reverse. So you don't really need it then. And the cost of a feathering prop really put me off. The feathering prop also gave us increased speed. That alone made it worth the money. A folding prop and a feathering prop are different and we have the less expensive one. Plus, we found when we took the prop off that we had a scored shaft which had to be replaced. grandma Rosalie |
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"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
: I'm sory, but I can only see one picture in alt.binaries.pictures.sports.ocean ... Am I missing something? Me, too. It's the one with the whole Perkins engine in it. The server must have lost the one with just the shaft alternator behind it. I'll do it again. It should take it a while to propagate to your servers. |
"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
: My intuition was exactly as you write - I quote: Wanna bet the locked prop draggin' through the water creates lots more drag than the one locked that can't turn? Unquote. I think I understand what you mean, but being a Dane, and having read it quite a few times makes me a little uncomfortable - is the sentence correctly phrased? I owe you an apology. I would bet the freewheeling prop drag is LESS than the locked prop drag. I'm sorry I made a joke of my English. I should have read your header before putting my foot in my mouth with the joke. Beautiful country, Danmark. We can only pray leaking, abandoned Russian nuclear facilities don't pollute Scandanavia. I read bellona.no websites about it. How awful. |
"Flemming Torp" fletopkanelbolle2rp.danmark wrote in
: I'm 'afraid' this debate is not over yet, but you have given me inspiration to make some funny experiments, that might give me and my brother in law a clear and convincing answer, so we can settle our little dispute ... thankyou! The thread is wonderful. So many ideas and theories. Thank you for starting it. |
"Roger Long" wrote in
: Under some circumstances, a freewheeling prop may have less drag than a fixed one. Props on normal transmissions are not freewheeling however. There is enough drag in the shaftline, bearings, and transmission to upset things. Outboards, with their clutches right in the lower unit are closer to freewheeling. But the point is, as I load the shaft alternator, and therefore the freewheeling prop, the boat slows more and more as the load increases, terminating in worst case drag as the prop stalls and stops. As the load decreases, and the prop turns faster, speed increases. If you take this beyond the natural drag of the transmission and cutlass bearing limiting prop speed, by adding slight power to it, there comes a point at which the prop crosses zero drag trying to pull the shaft out and starts pushing harder and harder on the shaft as speed increases, where drag goes through zero and turns into THRUST. I've never seen it plotted. It might not be a linear function but appears not to have any weird peaks or valleys in the curve.... |\ D | \ R | \ It's not linear, I'm sure. A | \ Lots more physics going on. G | \ Somewhere it stalls. | \ |------\--------------"freewheeling drag" 0 PROP SPEED - (prop turns = boat speed = no drag here) | \ T | \ H | \ R | \ U | \ S | \ Down here, somewhere it cavitates. T | \ God I hate text graphing....(c; |
This has been going on for a long time this argument. And I don't mean just
in this forum for the last few days. In the late fifties (of last century), my father was building himself a steel ocean racing yacht. The prop drag controversy was alive and well then There was a handicap allowance at the time for prop based on diameter, and some silly ineffectual props that looked likesomething off a model airplane were being tried. Sometimes two in tandem, and they were of course locked in line with the deadwood. Safety regulations required that the boat be able to demonstrate performance under power. In still air and calm water, of speed in knots equal to the square root of the wateline length in feet. I remember a 58ft 10 Meter Class yacht I was crewing on at the time being unable to make headway into a 25 Knot breeze. She had about 50 SHP, but a small prop that was able to transmit only a small fraction of the engine power into thrust. Dad didn't want any of this nonsense, if his boat needed an engine for safety reasons, then it was going to be able to use the power of the engine and go to windward if necessary despite the weather. But he wanted it to be competitive. He got a piece of steel tube about 3 inches in diameter and 18 inches long and machined inside each end to take the outer ring of a tapered roller bearing. The inners of the bearings were installed on the shaft with shoulders so that the bearings were opposed and transmitted the forward and reverse thrust to the outer rings. Outside of the bearings at each end of the tube was a normal oil seal, installed the right way round so as to keep oil inside the tube. The oil inside the tube was pressurised by a header tank mounted about 3 ft above the waterline. The tube was mounted at the trailing edge of the keel aperture, with the prop sized to the 40 HP diesel engine. Inside the boat between the shaft and the engine gearbox there was a dog clutch, so that the prop and shaft could be completely disconnected from the gearbox. The unit had so little friction that on the slip on a windy day the prop would revolve in the breeze. Driven off the shaft by a small chain was an aircraft tachometer generator, and in the cockpit the tachometer, which was calibrated to read knots. It was about as accurate as anything I've ever used. I once ran a DR plot based on it that was 10 nm out at the end of a 600 nm Sydney Hobart race. 1.5%? Dad ran into the third or fourth owner of the boat in 1980, and when told that the stern tube assemble had just required repair took delight in being able say what a pity it was, as the guarantee had just run out. Going back to the argument about locking or freewheeling: this subject occupied numerous off watch race hours without as I remember any consensus. Had we had a GPS or paddle wheel log then it would have been easy to set the boat up with the prop locked, and then unlock it to see if the speed increased. Though I doubt we would have done this during a race, and in light winds (when the effect will be greatest) we never wanted to sail if it wasn't a race. We had this beaught engine with a big prop. |
In "Flemming Torp" fletopkanelbolle2rp.danmark writes:
"Dave" skrev i en meddelelse ... On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long" said: When the prop is freewheeling, it is producing energy A fairly basic principle: energy can be neither created nor destroyed. Isn't it right, that when the boat is sailing (just for sail that is) you need some energy to keep the propeller from turning - i.e. to keep it fixed - and that gives a certain drag on the boat .... My intuition tells me that when you ' loosen the grip' on the propeller shaft and let the propeller turn freely, it reduces the drag on the boat. Right or wrong? You do not create energy by letting the propellor run. In one answer earlier it was pointed out that a stalled propellor is very different than a rotating propellor. If there were no friction in your propeller shaft when it is running freely the energy needed to spin the propeller could be less than the energy needed for the water to around the propeller blades. If you have a two blade propeller and it is in vertical position behind your keel, the water flow behind the keel is already somehow turbulent and the drag is fairly small. Then in most cases it will cause less drag if it is not rotating. There are so many facotrs involved; the sahpe of the blades, the pitcc, the aspect ratio the shape of the rear of your boat etc. so it is impossible for someone even very knowledgeable person to give you an answer just by reading your description of the situation. There are (fairly reliable) measurements of several sailors sailing and then stopping and letting the propellor to rotate about the changes of the speed. The problem is that in some cases it is faster to let spin and other cases (boats) it is faster to not let to spin. In a case of a three bladed propellor in a 40 feet Colin Archer the difference has bee almost one knot. So I suggest you keep on recording speed differences, there must be some situation where you wave and weather conditions keep for more or less constant for some 20 minutes, so you can stop and let your propeller to roatate by putting the gear on and putting it on neutral. You keep saying that you do not know physics. There are plenty of good (and interesting and well written books) it would be easer to choose from the answers of this group the relevant ones if you were not ignorant. I can promise you that the speed of your boat is not affected by the "votes" given in this discussion. - Lauri Tarkkonen |
In Larry W4CSC writes:
"Flemming Torp" fletopkanelbolle2rp.danmark wrote in : My intuition tells me that when you ' loosen the grip' on the propeller shaft and let the propeller turn freely, it reduces the drag on the boat. Right or wrong? It'd be real easy to test. Get one of those plastic props for a small outboard, put it on a shaft. Pull it loosely behind the boat, FREEWHEELING, with no load (which produces no work) using a simple fish weighing scale to measure its drag. Then, lock it to the shaft so it can't turn and drag it at the same speed, yet again. Wanna bet the locked prop draggin' through the water creates lots more drag than the one locked that can't turn? Your example has one flaw. The sailboat propeller is not loose, it is on a shaft with some bearings and the transmission wheels and oil etc. Do you know that some boat trhansmissions get very hot if you let the propeller to free wheel? Some energy is needed to heat it up. - Lauri Tarkkonen Take the dingy out for a spin. Run it wide open then drop the engine into neutral, freewheeling the prop, and see how long it takes to stop dead. Crank it back up and run it wide open again. This time, leave it in gear and just pull the safety lanyard making it lock the prop dead still. Retime how long it takes it to stop. More the drag, the quicker the stop. You'll find it stops MUCH quicker pulling that STALLED prop through the water than one FREEWHEELING. As Lionheart's freewheeling shaft alternator is TURNED ON to produce power (producing torque on the alternator shaft), it SLOWS the boat as the alternator SLOWS THE PROP. The heavier it pulls (more torque produced), the more it slows the boat and prop. Freewheeling props are producing a minimal amount of torque...and work with no or little load...as load increases, to the point of being stalled at maximum torque, the drag- converted-into-torque INCREASES, not decreases! If you were to spin the prop faster than its drag is capable of, you will pass through a point of ZERO DRAG just at the point where increasing the speed of the prop PRODUCES THRUST. So, as prop speed DECREASES, drag INCREASES! As prop speed INCREASES, it passes through zero drag at the point where it starts to produce thrust by turning it even faster! I'm not a physicist, but I play one on Usenet....(c; Your assignment for tomorrow's class is pages 287 through 312. Do the workbook exercies on workbook pages 42-43 for turn in. The test will be on Friday at 2PM. It might also be a good time to point out that the HELICOPTER rotor CHANGES DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction from freewheeling to provide DOWNDRAFT to hold the craft aloft by the engine. As it decends freewheeling, unless you can reverse the pitch of the rotor, the RISING air through it will make it spin in the opposite direction.....making it not relevant to any of our discussion here as props are fixed pitched and already going in the direction of flow caused by thrust. Geez.... |
In JR Gilbreath writes:
If there truly was less drag on a locked prop than a freewheeling one you would not have to put it in gear to lock it, a prop out of gear would not even freewheel it there was more drag. The rotating propeller after it gets some speed is a very different animal that the locked propeller. - Lauri Tarkkonen |
In "Roger Long" writes:
The dinghy outboard thought experiment is not valid because the whole system is coming to a stop and the prop is slowing as the boat is slowing. Do it while towing the dinghy behind the big boat at a steady speed with a scale on the tow rope and you may get a different result. Under some circumstances, a freewheeling prop may have less drag than a fixed one. Props on normal transmissions are not freewheeling however. There is enough drag in the shaftline, bearings, and transmission to upset things. Outboards, with their clutches right in the lower unit are closer to freewheeling. -- Roger Long I will second Roger on this. The dinghy experiment would be valid on a very naiive level of thinking and the experiment with the fishing lure is invalid because the boat propellors is NOT freewheeling. There is quite a bit of friction involved. In some cases the gearbox get too hot to hold a hand on it because the transmission fluid is not pumped around and while lubricating it is also transmitting the heat away. - Lauri Tarkkonen |
"Roger Long" wrote in message
... I'll try. The answer will be found in conservation of energy. The energy books have to balance in every system. The amount if drag of the water flowing over the prop will exactly equal the energy being produced. With the prop stopped, it is simply pull aft on the shaft. The prop isn't very efficient that way. Try propelling a boat by putting the prop out ahead on a long line and then pulling it in quickly. When the prop is freewheeling, it is producing energy that is absorbed by heating up the bearings and lube oil in the transmission, (or in Larry's case by charging batteries). It's slightly counterintuitive but, whatever energy is produced has to have an exact mirror image in drag on the boat. Since the prop is making energy more efficiently while turning, there has to be more drag. -- Roger Long Your explanation is pure nonsense. When the prop is fixed the energy created is dissipated as heat in the water. When it is free to rotate some is dissipated in the grease and bearings and some as heat in the water. -- Peter Aitken |
Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling however. There is enough drag in the shaftline, bearings, and transmission to upset things. Outboards, with their clutches right in the lower unit are closer to freewheeling. Roger Long I will second Roger on this. The dinghy experiment would be valid on a very naiive level of thinking and the experiment with the fishing lure is invalid because the boat propellors is NOT freewheeling. There is quite a bit of friction involved. In some cases the gearbox get too hot to hold a hand on it because the transmission fluid is not pumped around and while lubricating it is also transmitting the heat away. - Lauri Tarkkonen Actually, the dingy experiment is on the right track. It seems to me that the energy dissipated in the prop/shaft/transmission/oil train is irrelevant. That is energy that *would* had been dissipated in redirecting water flow. Look at it simply; you have *one* energy source, and that's the water 'stream' past the prop (not quite accurate of course, since the overall drag will affect the 'stream' velocity, but it's an easier way to envision it). The amount of energy extracted from that source will be the result of 1) frictional losses, 2) inertial forces, i.e. energy required to redirect the mass of water striking the prop surfaces (not straightforward to calculate), and 3) impact forces (momemtum, if we're thinking in terms of a 'stream'). Now, the only issue is whether the drag at the prop is more or less when fixed. So *if* the freewheeling prop has less drag than the fixed prop, then so will the turning prop attached to the prop/shaft/transmission/oil train. To the extent that energy is dissipated by the drive train, the prop will be rotating slower, and the system will behave somewhere in between fixed and freewheeling. Now, whether the fixed or turning has less drag, under any particular set of parameters, I don't know. I suspect it's a wash. Clearly a totally freewheeling prop would have less, as the impact forces are reduced, as is the angular deflection of the water stream, leaving mostly frictional losses. I suspect though, as appears the case on my Catalina 30, that the prop when turning significantly slower than it's pitch rate (i.e. dragging the shaft and trans with it) creates nearly the same drag as the fixed prop. Keith Hughes |
Flemming Torp wrote:
"Roger Long" skrev i en meddelelse ... OK, here is the answer you are looking for. Unless you have a highly unusual powertrain set up and strangely pitched prop, determine the position in which the most blade area is shadowed by keel and hull. Mark the shaft inside. Stop the shaft in that position. Sail the boat. It's very unlikely you'll go faster doing anything else. Now we are getting close to 'basics' Roger ... but, but ... as I wrote in the introduction, it has so far been very difficult to get hard evidence from the log when trying to let the propeller run and have it locked, as the speed of the boat is a function of so many things, and I'm convinced that there is not a big difference - so may be my question is of a more theoretical type, as reliable data are hard to get in the real world ... In a bassin, it might be easier ... I have seen som reports, where different kinds of propellers - folding with two blades, folding with three blades, fixed with three baldes etc. were compared ... and the result indicated differences in 'thrust' and in speed up to between ½ - 1 in worst case ... But I have not seen any reports on the comparison between a locked and a free wheeling propeller ... but I have certainly got a lot of input ... also from the aviation world, that I know nothing about ... thank you. But your final proposal is very logical, operational and easy to implement ... when the water gets warmer, we might do what you have recommended ... or do as Larry - use the energy coming from the rotating propeller - og invest in a folding propeller ... time will show, and thank you so much for your keen interest in learning me some physics ... Roger's advice is correct, but it does sidestep your original question. Locking a prop in the "shadow of the hull" is generally a winner. Even with a 3-blade prop you should have one blade lined up with the hull. With a two blade prop the gain can easily be enough to win a race, or come in before dark on long passage. I've heard of two studies that addressed this. One, a publicized MIT study showed that free-wheeling had less drag. However, this did not directly model yacht props so it really doesn't apply. (In fact, their prop wasn't actually free, it was powered at a speed that minimized turbulence.) Other study, which I've heard of third hand, "proved" that locked was less drag for yacht props. Dave Gerr, in his "propeller Handbook," says rotating is less drag (unless you can lock it behind the keel) but he doesn't give a reason or cite a reference. Consider two cases: first take a hypothetical "flat prop" with zero pitch. Obvious, it won't spin and it will have a lot of turbulence, indicating a lot of drag. Now give it a bit a pitch. It will start to spin, but the small pitch will mean that it has to spin very fast fast to match the boat speed. Since it can't, there will still be a huge amount of turbulence. Will it be less than the flat blade? Hard to say, because there are a variety of factors - the shape of the turbulence is different and the rotation means that a larger volume of water is disturbed. This approximates the yacht prop where the free rotation is very unlikely to match the boat speed and the turbulence will be high and quite complex. Now consider a feathering prop, where the blades are lined up with the flow and cause no turbulence. Give them a some pitch - as long as the flow is smooth there will be little drag. Increase the pitch (actually a smaller number since feathering the pitch is infinite) to the fastest spin with low turbulence. The drag will still be low at this point. Now lock it - the turbulence (and the drag) will jump up. Here's a case where the free rotating prop will have less drag than the locked one, but it probably does not resemble a typical yacht prop. It does come close to large ship props, so its not surprising that there are some studies and anecdotal evidence from other forms of props, that say free wheeling is less drag. My point is (as I mentioned in my first post) that its impossible to determine intuitively the drag for the case of a yacht prop, where the turbulence is high whether locked or not, and analogies from other type of props simply don't apply. The issue of the heat buildup in the transmission is a "red herring." It is a significant side issue, since yacht transmissions do have a lot of friction, and thus heat problems, when free wheeling, but the dominant factor is the turbulence. This approach tries to measure the energy extracted from the water by the work done by the shaft, but this is only valid if the turbulence is low and the prop is working in a relatively efficient mode. When the turbulence is high, most of the energy goes into heating the water, and this is rather difficult to measure. |
In article , "Flemming Torp" fletopkanelbolle2rp.danmark wrote:
I'm not sure I get your conclusion ... ;-) Sorry, but do you recommend me to let the propeller turn og should I stop it from turning, if I'm only concerned with the speed of the sailboat - when only using the sails? I guess I'm too long winded. It is my guess that a freely turning propellor would produce less drag than a locked propellor. This is assuming that there is no load on thepropellor other than the friction of the bearings that support the propellor shaft. |
Why 65 replies on a simple question. Amazing. ;-)
Now tackle this one...how many sailors does it take to screw in a light bulb? |
In "*JimH*" writes:
Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. Now tackle this one...how many sailors does it take to screw in a light bulb? There are not enough wise men to enlighten the ones that prefers to stay ignorant. Looks like no lightbulbs are needed. - Lauri Tarkkonen |
In HarryKrause writes:
Lauri Tarkkonen wrote: In "*JimH*" writes: Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he last had one, he said he spent most of his time aboard it while it was tied to a marina's dock in southern Lake Erie. Thus, a discussion about feathering or engaging a sailboat prop would not be something to which he could contribute, at least not from a point of knowledge. Perhaps his point of discussion is arrogance. - Lauri Tarkkonen |
"Lauri Tarkkonen" wrote in message ... In HarryKrause writes: Lauri Tarkkonen wrote: In "*JimH*" writes: Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. Jim Hert (JimH) doesn't actually have a boat, and, when he claimed he last had one, he said he spent most of his time aboard it while it was tied to a marina's dock in southern Lake Erie. Thus, a discussion about feathering or engaging a sailboat prop would not be something to which he could contribute, at least not from a point of knowledge. Perhaps his point of discussion is arrogance. - Lauri Tarkkonen Not at all Lauri. First of all, if my original post offended you or anyone in this NG I apologize. It was certainly a brain fart on my part as I certainly respect the nautical and operational knowledge I have personally observed from most of the sailors I have had the opportunity of meeting or observed in action. As to my boating experience that Harry Krause questions: We have owned boats for over 25 years. We had to sell our 32 footer at the end of the 2003 season due to my wife's medical condition, a condition that caused her to take a medical retirement from her teaching career. Krause knows that yet for some reason shows little consideration for that fact. That alone should speak loudly of his moral character. Yes, we are currently boatless but hope to purchase a smaller more manageable (for me) boat in 4 years when we retire riverside (Huron River off Lake Erie). We live in Northern Ohio and would normally log 55-80 hours on the boat annually....not many hours according to hard core fishermen or year round boaters, but we did indeed spend every weekend on the boat as well as 2 full vacation weeks during our summer vacation. We would use our boats mainly for cruising to swimming spots or (as in the case of our 27 and 32 footers) cruise to various ports in Canada and Michigan. Overnight trips on the hook in protected areas were magnificent. And to the surprise of Harry Krause, we would actually spend weekends on our boat (along with our 2 children) even when the weather/Lake conditions were bad, spending time at the marina. Our fishing would be limited to early season (me and my buddies) walleye fishing and late season perch fishing. Our children fondly recall our time on the boats (they are now 17 and 20 years old respectively) and I would not give up our experience on our boats as a family (even though they do not meet Harry Krause's requirements). Our children were with us on the boat since the age of 6 months. Do not pay attention to Harry Krause. He is a bitter old man who for some reason has to spend his time posting personal attacks on folks who disagree with him politically. I actually feel sorry for the guy. |
"*JimH*" wrote in message ... "Lauri Tarkkonen" wrote in message ... In HarryKrause writes: Lauri Tarkkonen wrote: In "*JimH*" writes: Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he last had one, he said he spent most of his time aboard it while it was tied to a marina's dock in southern Lake Erie. Thus, a discussion about feathering or engaging a sailboat prop would not be something to which he could contribute, at least not from a point of knowledge. Perhaps his point of discussion is arrogance. - Lauri Tarkkonen Not at all Lauri. First of all, if my original post offended you or anyone in this NG I apologize. It was certainly a brain fart on my part as I certainly respect the nautical and operational knowledge I have personally observed from most of the sailors I have had the opportunity of meeting or observed in action. As to my boating experience that Harry Krause questions: We have owned boats for over 25 years. We had to sell our 32 footer at the end of the 2003 season due to my wife's medical condition, a condition that caused her to take a medical retirement from her teaching career. Krause knows that yet for some reason shows little consideration for that fact. That alone should speak loudly of his moral character. Yes, we are currently boatless but hope to purchase a smaller more manageable (for me) boat in 4 years when we retire riverside (Huron River off Lake Erie). We live in Northern Ohio and would normally log 55-80 hours on the boat annually....not many hours according to hard core fishermen or year round boaters, but we did indeed spend every weekend on the boat as well as 2 full vacation weeks during our summer vacation. We would use our boats mainly for cruising to swimming spots or (as in the case of our 27 and 32 footers) cruise to various ports in Canada and Michigan. Overnight trips on the hook in protected areas were magnificent. And to the surprise of Harry Krause, we would actually spend weekends on our boat (along with our 2 children) even when the weather/Lake conditions were bad, spending time at the marina. Our fishing would be limited to early season (me and my buddies) walleye fishing and late season perch fishing. Our children fondly recall our time on the boats (they are now 17 and 20 years old respectively) and I would not give up our experience on our boats as a family (even though they do not meet Harry Krause's requirements). Our children were with us on the boat since the age of 6 months. Do not pay attention to Harry Krause. He is a bitter old man who for some reason has to spend his time posting personal attacks on folks who disagree with him politically. I actually feel sorry for the guy. BTW: For your information about this person posting as Harry Krause: 1. He has stalked me and my family on the internet because my political views did not agree with is. 2. He subsequently posted my real name (I had posted under an alias for obvious reasons), address and wife's name to rec.boats. 3. He obtained a picture or our house via the internet. 4. He made personal threats to me and my wife. As a result we have tracked Harry Krause's posts and continued threats to us and forwarded them to our attorney. He has admitted to all these things in rec.boats. There are many active member of rec.boats.cruising who can attest to this type of behavior by Harry Krause as similar things happened to them. He is a sick man and I suggest you treat him with caution. |
On Tue, 07 Jun 2005 15:43:23 -0400, HarryKrause wrote:
Lauri Tarkkonen wrote: In "*JimH*" writes: Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. - Lauri Tarkkonen Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he last had one, he said he spent most of his time aboard it while it was tied to a marina's dock in southern Lake Erie. Thus, a discussion about feathering or engaging a sailboat prop would not be something to which he could contribute, at least not from a point of knowledge. Are you not able to make enough personal attacks in rec.boats? -- John H "All decisions are the result of binary thinking." |
"John H" wrote in message ... On Tue, 07 Jun 2005 15:43:23 -0400, HarryKrause wrote: Lauri Tarkkonen wrote: In "*JimH*" writes: Why 65 replies on a simple question. Amazing. ;-) Because it is not as simple as you and the majority of posters seem to think. By the way this discussion is repeated about once in three years and same unfounded arguments and simplifications are represented over and over again. - Lauri Tarkkonen Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he last had one, he said he spent most of his time aboard it while it was tied to a marina's dock in southern Lake Erie. Thus, a discussion about feathering or engaging a sailboat prop would not be something to which he could contribute, at least not from a point of knowledge. Are you not able to make enough personal attacks in rec.boats? -- John H The simple answer is..........................................NO. Quite sad. I dare anyone to show a NG where Harry Karuse is both respected and shown as credible and believable. Not much to ask. I dare you. |
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