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Boat Forum / Sailing / July 2008Locking the prop
I usually sail with the prop locked (by putting it in reverse gear - fixed 2 bladed low tech prop on a sail drive). This avoids the annoying noise of the gearbox turning over and is something that a lot of sailors do. However, the handbook for the Volvo MD 2030, says to leave it in neutral when sailing, and several people have told me that I am significantly increasing drag by locking the prop. What do the URS gurus recommend (Please don't recommend a change of prop, as I want to stay on topic.)  SignatureDuncan Heenan (Speaking personally) >I usually sail with the prop locked (by putting it in reverse gear - fixed 2 >bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 3 lines] >locking the prop. What do the URS gurus recommend (Please don't recommend a >change of prop, as I want to stay on topic.) When you are sailing in a steady wind at a constant speed try reading the speed before and after locking/unlocking. I have never managed to note a change of speed. SignatureMartin > I usually sail with the prop locked (by putting it in reverse gear - fixed > 2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] > increasing drag by locking the prop. What do the URS gurus recommend > (Please don't recommend a change of prop, as I want to stay on topic.) There are 3 issues here. One is what is best for the engine. If the manual says to let it spin, it must do so for a reason, though it would be interesting to know what the reason is. Do they say why? Another is the noise. If it bothers you, lock it. Finally there's the speed thing. Indications are that it makes so little difference that folk aren't sure whether locked or unlocked drag is bigger. I expect Ian will know the answer. My intuition tells me that locked drag is bigger, because although some power is being extracted by the prop from the water flowing past, and expended in overcoming the odd bit of friction here and there, and in stirring (and probably slightly heating) the gearbox oil, it's so little as to be negligible compared with the effect that presumably the overall force "seen" by the water rushing past a fixed blade is bigger than with a spinning blade. Things might be different if one were seeking to extract a serious amount of power from the prop. One might want to decouple it from the engine, and rig up some fancy gearing to drive a reverse windmill to help the boat sail faster... > > I usually sail with the prop locked (by putting it in reverse gear - fixed > > 2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 27 lines] > and rig up some fancy gearing to drive a reverse windmill to help the > boat sail faster... Google found this which seems to make some sense and agrees broadly with what others have said http://books.google.co.uk/books?id=VLg6Lx5yRP0C&pg=RA1-PA220&lpg=RA1-PA220&dq=lo cked+propeller+drag&source=web&ots=jwixLiA42g&sig=-9Dn2K_kytoDWD3M7RFJniPeP5c&hl =en&sa=X&oi=book_result&resnum=1&ct=result Practically I would think that differences in performance that can't be measured it isn't important, though I suspect that a competitive, high performance, well sailed, fully instrumented, polished racing yacht would have a better chance of detecting differences (in the unlikely event that the owner could be persuaded to fit a fixed cruising prop for a trial of course). Performance may not be just about boat speed, Performance % and VMG meters might reveal a different perspective to speed alone. Personally I lock my 3 blade on my N26 long keel in the possibly erroneous belief that it will save wear and noise. ChrisR Ronald Raygun wrote: > Things might be different if one were seeking to extract a serious amount > of power from the prop. One might want to decouple it from the engine, > and rig up some fancy gearing to drive a reverse windmill to help the > boat sail faster... Mmmm, violating energy conservation. -is Signatureseal your e-mail: http://www.gnupg.org/ >> I usually sail with the prop locked (by putting it in reverse gear - >> fixed 2 bladed low tech prop on a sail drive). This avoids the annoying [quoted text clipped - 29 lines] > and rig up some fancy gearing to drive a reverse windmill to help the > boat sail faster... Another issue maybe gearbox cooling. On my last engine the gearbox was cooled by water from the engine. So no engine, no cooling. Personally, I always lock it, as I don't like the noise or the possibility of wear on the stern gland. > I expect Ian will know the answer. My intuition tells me that locked drag > is bigger, because although some power is being extracted by the prop from [quoted text clipped - 3 lines] > presumably the overall force "seen" by the water rushing past a fixed blade > is bigger than with a spinning blade. Hello! Am I the right Ian? The answer is ... it depends. You certainly /can/ get a hell of a lot more drag out of a rotating prop than a locked one. After all, a twin screw boat would move fine with one prop working and one locked. A prop converts torque into thrust, or drag into torque, so the amount of drag on a spinning prop depends on the frictional torque. If teh shaft spins fairly freely there will be very little drag. A locked prop, on the other hand, has a horrible nasty turbulent wake at any significant speed and that takes energy which is supplied by the moving drag force. So whether to lock the prop or not depends to entirely on how freely the shaft runs. My gut response, unrefined by actual knowledge, is that if the prop shaft can be completely disconnected it's probably better to let it spin, but if it would be churning bits of gearbox around it's probably better to lock it. I doubt if it makes much difference, on the whole. Ian (an Ian, not necessarily the right one) PS Apart from the slight problem of reversing, which could be overcome, a freewheeling hub would be an interesting idea. Does anyone make 'em? >I usually sail with the prop locked (by putting it in reverse gear - fixed 2 >bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 3 lines] >locking the prop. What do the URS gurus recommend (Please don't recommend a >change of prop, as I want to stay on topic.) Well another point is a locked prop is less likely to 'wind on' any debris found in the sea. Given the prevalence of bits of discarded net/rope/plastic this has got to be a consideration. Nick >>I usually sail with the prop locked (by putting it in reverse gear - fixed 2 >>bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 7 lines] >debris found in the sea. Given the prevalence of bits of discarded >net/rope/plastic this has got to be a consideration. or bump start the engine if you happen to surf down mountainous seas in a force 10 SignatureMartin >I usually sail with the prop locked (by putting it in reverse gear - fixed 2 >bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 3 lines] >locking the prop. What do the URS gurus recommend (Please don't recommend a >change of prop, as I want to stay on topic.) The maths says that a locked prop produces less drag, but if you try both and can't tell the difference then I would go with the handbook. -- Alastair > The maths says that a locked prop produces less drag, What maths? >> The maths says that a locked prop produces less drag, > >What maths? I thought you might ask :o) SignatureMartin >> The maths says that a locked prop produces less drag, > > What maths? Tank tests say that the locked prop has less drag, the topic comes up frequently and the intuitive answer is not correct. But if the engine mfr says don't lock it I would follow their advice; if I locked my Yanmar I could only start the engine in gear! >> The maths says that a locked prop produces less drag, > >What maths? There is only The maths. -- Alastair >>> The maths says that a locked prop produces less drag, >> >>What maths? >> > There is only The maths. Well, it appears that Larsson & Eliasson's maths says the opposite. >>>> The maths says that a locked prop produces less drag, >>> [quoted text clipped - 3 lines] > >Well, it appears that Larsson & Eliasson's maths says the opposite. With names like that they must know what they are talking about. -- Alastair > >I usually sail with the prop locked (by putting it in reverse gear - fixed 2 > >bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 6 lines] > The maths says that a locked prop produces less drag, but if you try > both and can't tell the difference then I would go with the handbook. I'm with Ronald and Martin - what maths? SignatureDuncan > > >I usually sail with the prop locked (by putting it in reverse gear - fixed 2 > > >bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 8 lines] > > I'm with Ronald and Martin - what maths? And what assumptions are made on which to apply the Maths :-) ChrisR > I usually sail with the prop locked (by putting it in reverse gear - fixed 2 > bladed low tech prop on a sail drive). This avoids the annoying noise of the [quoted text clipped - 7 lines] > Duncan Heenan > (Speaking personally) Hi Duncan, When I bought my Bav 34 ( same as yours ) they told me that putting the gearbox in reverse whilst sailing could jam the gearbox. The engineer indicated that you would then have to start the engine in reverse gear to be able to unlock it. Not sure about this, but as I have had a couple of novice crew members jam the box going from ahead to astern too quickly, it is possibly true. As an aside, if this does happen, get a large spanner on the crank pulley and heave as hard as you can in the direction or rotation. It will unlock it. As I am in the same start as you on Saturday, can you let me know your freewheeling intentions so that if you are flying past me I know why. Regards Graham > When I bought my Bav 34 ( same as yours ) they told me that putting > the gearbox in reverse whilst sailing could jam the gearbox. So why not put it in forward instead? > > When I bought my Bav 34 ( same as yours ) they told me that putting > > the gearbox in reverse whilst sailing could jam the gearbox. > > So why not put it in forward instead? Sounds silly, but after 15 years of believing that if you put it in forward it will rotate the engine ( due to water flow over the prop ) , I have never actually tried it.!!! Is this the secret of the RTI then.? >> > When I bought my Bav 34 ( same as yours ) they told me that putting >> > the gearbox in reverse whilst sailing could jam the gearbox. [quoted text clipped - 5 lines] > prop ) , I have never actually tried it.!!! > Is this the secret of the RTI then.? If I put my Yanmar in forward the prop still rotates (it doesn't turn the engine though). If I put it in reverse it jams, or is at least very difficult to put into neutral (heave to and thump it!), if I'm reaching in a F5+. Alisdair >> I usually sail with the prop locked (by putting it in reverse gear - >> fixed 2 [quoted text clipped - 26 lines] > > Regards Graham Aha! We're "Fellowship" sail no 893. What are you? I too have had the gearbox locked by someone going from ahead to astern in one crash move. I wonder it there is a design issue here? Maybe we ought to do some research & see whether it is a common problem. I had to get a mechanic to sort mine out - very expensive. SignatureDuncan Heenan (Speaking personally) > "GRAHAM M" <GRA...@microprise.co.uk> wrote in message > [quoted text clipped - 8 lines] > > - Show quoted text - Spirit of Bavaro ( Bavaro is in the Dominican Republic, , nothing to do with Bavaria) GBR 7408T 7.20 start. The gearbox is a design problem, but easy to rectify. Make sure you have a spanner on board to fit the crankshaft nut. I got the answer from Seastart over the phone. Simply give a big heave until it frees itself. Worked twice so far. I was impressed with your letter to YM ref Bavarias. I posted on YBW.com asking for any genuine proveable instances of Bavarias foundering or otherwise ( keels falling off etc. ) because of design or quality issues ( apart from the famous 35 Match ) and there wasnt a single referance in reply. I too have had mine since 2001 and am very impressed. Apart from the fridge hinges of course. Graham From Larsson & Eliasson's & 'Principles of Yacht design' (2nd edition ; can't really afford the third :-(( ) p. 181 The coefficient in the propeller resistance formula is 0.3 for a free rotating fixed blade prop and 1.4 for a locked fixed blade one. Numerical example : diameter 0.53 m (I am too lazy to convert to "), pitch 0.33 m. At 6.8 knts, the computed resistance is around 120 N for a freely rotating prop (minus 0.2 knts over a properless boat , and 460 N when locked (minus 0.8 knts) A typical folding prop coeff is 0.06, (minus 0.04 knts) Duncan Heenan a écrit : > I usually sail with the prop locked (by putting it in reverse gear - > fixed 2 bladed low tech prop on a sail drive). This avoids the annoying [quoted text clipped - 4 lines] > recommend (Please don't recommend a change of prop, as I want to stay on > topic.) > From Larsson & Eliasson's & 'Principles of Yacht design' (2nd edition ; > can't really afford the third :-(( ) p. 181 [quoted text clipped - 5 lines] > when locked (minus 0.8 knts) > A typical folding prop coeff is 0.06, (minus 0.04 knts) I'm amazed it's that simple. So blade width doesn't matter, and nor does the friction in the shaft for a spinning prop? Andy (who has no engine at all!) Best I can do is to direct you to the scan I've made of the relevant two pages (hoping it is not too drastic a copyright infringement : will delete in a few days) http://francois.lonchamp.free.fr/Pic1/L&E1.jpg http://francois.lonchamp.free.fr/Pic1/L&E2.jpg Andy Champ a écrit : >> From Larsson & Eliasson's & 'Principles of Yacht design' (2nd edition >> ; can't really afford the third :-(( ) p. 181 [quoted text clipped - 13 lines] > Andy > (who has no engine at all!) > Best I can do is to direct you to the scan I've made of the relevant two > pages (hoping it is not too drastic a copyright infringement : will > delete in a few days) > > http://francois.lonchamp.free.fr/Pic1/L&E1.jpg > http://francois.lonchamp.free.fr/Pic1/L&E2.jpg François, Thanks for the document - very interesting. that appears to be data for one specific propellor (YD-40) and I'm not sure that the data can be relied on for other designs. Drag from a fixed propellor will be closely related to the frontal area, and not much else - water flow over the blades will be turbulent, and the effect will not be very different to putting a flat plate of the same area in the water. For a spinning propellor drag will come from various types of drag. Firstly, the shape of the blades will be optimised for forward propulsion. Blades are not flat, but a very complicated section. You're undoubtedly aware that props. don't work as well in reverse. That is because of the section. When spinning freely the blades are being driven, not driving, and the flow will not follow the designed contours - so there will be hydrodynamic losses there. Secondly, the surface of a prop won't be perfect. The more damage and growth there is on it, the more the water flow will break up. This too will affect the amount of drag. Thirdly there's the power lost in the gearbox. For a simple example showing that a spinning blade can give more drag than a fixed one you need look no further than the seed of the sycamore tree (Acer pseudoplatanus, or sycomore to you and why did I bother to look that up?) where the spin greatly increases the drag and stops the rate of fall. The blades of a wind generator are another example of how a small blade area can give very high drag - in that case it's useful. I've only once sailed a boat with a significant sized propellor. When freewheeling the blade gave a significant and annoying vibration to the tiller, which was enough of a reason to lock it! Andy > > Best I can do is to direct you to the scan I've made of the relevant two > > pages (hoping it is not too drastic a copyright infringement : will [quoted text clipped - 35 lines] > look that up?) where the spin greatly increases the drag and stops the > rate of fall. That is a very poor example - the drag is maintained because the shape at which it falls is maintained - so that does *NOT* compare to the sycamore seed falling and *not* spinning but maintaining a side on profile as it falls (which it doesn't). That's why it spins, so it *does*. SignatureDuncan > That is a very poor example - the drag is maintained because the shape > at which it falls is maintained - so that does *NOT* compare to the > sycamore seed falling and *not* spinning but maintaining a side on > profile as it falls (which it doesn't). That's why it spins, so it > *does*. Stick two together, so they don't spin. Result - quick fall. It's late autumn there, you might be able to find some and try the experiment! Andy > > That is a very poor example - the drag is maintained because the shape > > at which it falls is maintained - so that does *NOT* compare to the [quoted text clipped - 3 lines] > > Stick two together, so they don't spin. Result - quick fall. Of course. Please read what I said - let me put it another way - *if* they descended *flat* the whole way (which they wouldn't) they'd take longer to reach the ground than if they spun. SignatureDuncan > Thanks for the document - very interesting. > > that appears to be data for one specific propellor (YD-40) and I'm not > sure that the data can be relied on for other designs. If it's a general purpose prop, I'm sure the same principles will apply to most other general purpose props. > Firstly, the shape of the blades will be optimised for forward > propulsion. On some specialist props that may well be the case, but for "ordinary" ones I don't think so. I'm looking at one just now and it looks pretty symmetric. > Blades are not flat, but a very complicated section. Not really. All that happens is that the pitch flattens the further away you get from the middle, because obviously the tangential speed increases linearly with radius, and so if you want to keep the axial water flow per unit area constant, the blade gradient has to be in inverse proportion to the radius. > You're undoubtedly aware that props. don't work as well in reverse. > That is because of the section. More likely because the gear ratio is different. > When spinning freely the blades are > being driven, not driving, and the flow will not follow the designed [quoted text clipped - 3 lines] > growth there is on it, the more the water flow will break up. This too > will affect the amount of drag. Yes, but both these two factors also affect a locked prop. > Thirdly there's the power lost in the gearbox. Of course, but I think that's unlikely to make a huge difference, unless you can actually feel it getting hot. I don't think conservation of energy/power is the easiest way to resolve this argument heretically, because we can't easily quantify the power lost in the case of a locked prop, can we? The only thing that would resolve the issue practically is to fit a thrust measuring device in line with the prop shaft, perhaps strain gauges on the flexible coupling would do the trick, and even then it wouldn't be easy to compensate for friction from the stern gland when the prop is locked. <snip> >> Blades are not flat, but a very complicated section. > [quoted text clipped - 3 lines] > water flow per unit area constant, the blade gradient has to be in > inverse proportion to the radius. That's not what I meant. The trailing face is (normally!) much less curved than the leading face. The trailing face may even be convex. Stick a straight-edge on the blade at a tangent to the shaft (IYSWIM) >> You're undoubtedly aware that props. don't work as well in reverse. >> That is because of the section. > > More likely because the gear ratio is different. Why would anyone design a gearbox to be deliberately less efficient in reverse? >> When spinning freely the blades are >> being driven, not driving, and the flow will not follow the designed [quoted text clipped - 5 lines] > > Yes, but both these two factors also affect a locked prop. Not as much. The flow over the locked prop is turbulent anyway, while it's likely to be laminar over at least part of the spinning one. Surface imperfections matter far more in laminar flow. <snip> > I don't think conservation of energy/power is the easiest way to > resolve this argument /heretically/, because we can't easily quantify > the power lost in the case of a locked prop, can we? [my italics] Trying to turn this into a religious argument? :P > The only thing > that would resolve the issue practically is to fit a thrust measuring > device in line with the prop shaft, perhaps strain gauges on the > flexible coupling would do the trick, and even then it wouldn't be > easy to compensate for friction from the stern gland when the prop > is locked. True. And I think the paper was from measurements. Andy >>> Blades are not flat, but a very complicated section. >> [quoted text clipped - 6 lines] > That's not what I meant. The trailing face is (normally!) much less > curved than the leading face. The trailing face may even be convex. I'm sure what you say is true for some props, notably those with long narrow straightish blades. The blades I'm looking at are more round, though, like a clover leaf or the famous ears of Mickey Mouse. > Stick a straight-edge on the blade at a tangent to the shaft (IYSWIM) In the case of my prop this just confirms that leading and trailing edges have the same profile. >>> You're undoubtedly aware that props. don't work as well in reverse. >>> That is because of the section. [quoted text clipped - 3 lines] > Why would anyone design a gearbox to be deliberately less efficient in > reverse? So as to be more efficient at a lower speed (because you are unlikely to want to go as fast in reverse as you normally do in forward). OK, typical marine gearboxes might well not have different ratios in forward and reverse, but then I wouldn't expect the kind of prop I have to *work* less well in reverse, and I don't find that mine does. The boat *handles* completely differently in reverse, of course, but that's for different reasons. >> I don't think conservation of energy/power is the easiest way to >> resolve this argument /heretically/, because we can't easily quantify >> the power lost in the case of a locked prop, can we? > > [my italics] > Trying to turn this into a religious argument? :P Ooh, you are a one! > >> Stick a straight-edge on the blade at a tangent to the shaft (IYSWIM) > > In the case of my prop this just confirms that leading and trailing > edges have the same profile. Uh. <slaps forehead> of course I should have realised that the section will be sharp tailed and blunt at the front. Which would mess up the reversing *without* affecting the freewheeling... [realises he has destroyed his own arguments] But you all might like to read this: http://www.physicsforums.com/showthread.php?t=38142 especially response #4. This is describing the far greater lift (drag for us) obtained from a free-wheeling helicopter rotor when compared to a locked rotor. Andy > > > >> Stick a straight-edge on the blade at a tangent to the shaft (IYSWIM) [quoted text clipped - 16 lines] > This is describing the far greater lift (drag for us) obtained from a > free-wheeling helicopter rotor when compared to a locked rotor. Of course it does. I helicopter blade is a wing, and only generates lift when it's moving forward through the air! If it's not spinning, it's not producing any lift, just some 'dirty' drag (it will be stalled in most positions). SignatureDuncan >> This is describing the far greater lift (drag for us) obtained from a >> free-wheeling helicopter rotor when compared to a locked rotor. [quoted text clipped - 4 lines] > If it's not spinning, it's not producing any lift, just some 'dirty' > drag (it will be stalled in most positions). And the difference to a propellor blade is...? > >> This is describing the far greater lift (drag for us) obtained from a > >> free-wheeling helicopter rotor when compared to a locked rotor. [quoted text clipped - 6 lines] > > And the difference to a propellor blade is...? 1. a propellor blade does not equal a wing 2. the propellor blade is stalled when it is being dragged through the water when it's spinning, and when it's locked. SignatureDuncan > In article <CdOdnYnIItZEs_TVRVny...@eclipse.net.uk>, no....@nospam.com > says... > > And the difference to a propellor blade is...? > > 1. a propellor blade does not equal a wing What do you think are the differences? > 2. the propellor blade is stalled when it is being dragged through the > water when it's spinning, and when it's locked. If it's not a wing, how can it be "stalled"? And what do you think characterises a stall? Ian In article <5ea06506-9c16-40df-a98c-a91853163929 @r66g2000hsg.googlegroups.com>, ian.groups@btinternet.com says... > > In article <CdOdnYnIItZEs_TVRVny...@eclipse.net.uk>, no....@nospam.com > > says... [quoted text clipped - 4 lines] > > What do you think are the differences? One is flying through air (aerodynamics). One is moving through water (fluid dynamics). They are quite different. The difference is due that air is quite compressible - water is not. So a wing, looking at it along the chord line, you'll see a greater distrance required for the air to move along the top, than the bottom, so it has to travel faster, this causes a reduction... oh god.. you finish by researching it yerself :) > > 2. the propellor blade is stalled when it is being dragged through the > > water when it's spinning, and when it's locked. > > If it's not a wing, how can it be "stalled"? And what do you think > characterises a stall? Both can have stalled characteristics - the disturbed flow of the water past the blade, v the disturbed flow of air over the top surface of the wing, but please do not *simply* compare the two as being the same - you can see from reading what I've just said, they are very different. Indeed, do not compare a blade going through water, with a wing going through air - the differences in the mediums they are travelling through mean two quite distinct designs for maximum performance. I'll repeat the important bit - the difference is due that air is quite compressible - water is not. Shall we get back on topic? SignatureDuncan > In article <5ea06506-9c16-40df-a98c-a91853163929 > @r66g2000hsg.googlegroups.com>, ian.gro...@btinternet.com says... [quoted text clipped - 11 lines] > (fluid dynamics). They are quite different. The difference is due that > air is quite compressible - water is not. As long as you get the Reynolds numbers the same, the flow patterns will be the same. My glider. Wing chord L = 1m. Typical velocity v = 20m/s. Air density rho = 1kg/m^3. Air dynamic viscosity mu = 2 x 10^-5 Ns/m^2. Re = v L rho / mu = 10^6 My outboard. Blade chord = 4cm. Typical velocity (10cm radius, 2500rpm) = 25m/s. Water density = 1000 kg/m^3. Water dynamic viscosity = 10^-3 Ns/m^2. Re = 10^6. [*] > So a wing, looking at it along the chord line, you'll see a greater > distrance required for the air to move along the top, than the bottom, > so it has to travel faster, this causes a reduction... oh god.. you > finish by researching it yerself :) Why do you think two air particles which arrive at the leading edge together and take different paths must reach the trailing edge together? Oh, and as far as researching it myself goes, I don't really like academic willy waving, but I've been teaching and examining in fluid dynamics at two major UK universities for twenty years. Your background is ... ? > > If it's not a wing, how can it be "stalled"? And what do you think > > characterises a stall? [quoted text clipped - 3 lines] > wing, but please do not *simply* compare the two as being the same - you > can see from reading what I've just said, they are very different. I can see that you think they are different. How do you define "stall" in the two cases? > I'll repeat the important bit - the difference is due that air is quite > compressible - water is not. Did you know that compressibility can be safely ignored when modelling wings as long as the airspeed is well below the speed of sound? > Shall we get back on topic? Which is to do with the fluid dynamics of propellors, is it not? Ian [*] Attentive readers will note that this is quite a high speed for a prop, though by no means an impossible one. In practice, the Reynolds number for the prop is likely to be rather lower. The implications are left as an exercise for the informed reader. > > In article <5ea06506-9c16-40df-a98c-a91853163929 > > @r66g2000hsg.googlegroups.com>, ian.gro...@btinternet.com says... > > > > > > In article <CdOdnYnIItZEs_TVRVny...@eclipse.net.uk>, no....@nospam.com > > > > says... > I can see that you think they are different. How do you define "stall" > in the two cases? drag > lift = stall? ChrisR > "Ian" <ian.gro...@btinternet.com> wrote in message > > I can see that you think they are different. How do you > define "stall" > > in the two cases? > > drag > lift = stall? Not necessarily. It's to do with separation of fluid flow. Ian > Oh, and as far as researching it myself goes, I don't really like > academic willy waving, but I've been teaching and examining in fluid > dynamics at two major UK universities for twenty years. Your > background is ... ? Earlier in the thread you wondered whether you were "the right Ian", and you were (i.e. the one I'd meant and hoped would answer). I was thinking about suggesting that you should henceforth be known as "the right Ian", but now you've inspired a much cooler handle: The one with the academic willy. > At 6.8 knts, the computed resistance is around 120 N for a > freely rotating prop (minus 0.2 knts over a properless boat , and 460 N > when locked (minus 0.8 knts) The killer - innocent, baby faced, smiling killer - in that sentence is "freely rotating". Propellors aren;t freely rotating, and the drag depends entirely on how non-free they are. Ian >> At 6.8 knts, the computed resistance is around 120 N for a >> freely rotating prop (minus 0.2 knts over a properless boat , and 460 N [quoted text clipped - 3 lines] > is "freely rotating". Propellors aren;t freely rotating, and the drag > depends entirely on how non-free they are. Right then, Baby Face! :-) Suppose we were to graph drag against prop speed, given some arbitrary constant boat speed. Let F be the speed corresponding to free rotation. There will be a speed slightly above F at which drag becomes negative (i.e. turns into thrust). We've established that drag has a certain highish value at speed 0 and a certain lowish value at speed F. What happens inbetween? Does it go up right away, peak somewhere, and then drop, or does it go down, then up again, possibly exceeding the initial value, and then down again? Or is it even more complicated? If there is a peak drag value, let's say it occurs at speed P. What determines whether (and if so, by how much) drag at speed P is higher than drag at speed 0? About how far between 0 and F is P likely to lie? If P is likely to be fairly close to F, I guess that would mean the system is pretty sensitive to even quite small friction losses, and might also explain why people who try to estimate the difference in effect on boat speed, between locking the proppeller and letting it spin, either struggle to notice any difference at all, or get different results (in sign as well as magnitude) in different situations. > We've established that drag has a certain highish value at speed 0 and > a certain lowish value at speed F. What happens inbetween? Does it go > up right away, peak somewhere, and then drop, or does it go down, then > up again, possibly exceeding the initial value, and then down again? > Or is it even more complicated? I think it's more complicated. Odd things happen when the blades stall. Andy > > The killer - innocent, baby faced, smiling killer - in that sentence > > is "freely rotating". Propellors aren;t freely rotating, and the drag [quoted text clipped - 4 lines] > Suppose we were to graph drag against prop speed, given some arbitrary > constant boat speed. Let F be the speed corresponding to free rotation. OK. > There will be a speed slightly above F at which drag becomes negative > (i.e. turns into thrust). OK again. We're now in the regime where torque has to be applied to the shaft in the "forward" direction to drive the prop faster than it wants to go naturally. > We've established that drag has a certain highish value at speed 0 and > a certain lowish value at speed F. What happens inbetween? Does it go > up right away, peak somewhere, and then drop, or does it go down, then > up again, possibly exceeding the initial value, and then down again? We're now in the regime where the torque is the other way round - prop turning the engine. Offhand, and without doing any sums or even much thinking, I'd expect the drag force to rise smoothly - and probably parabolically - as torque increased and shaft speed decreased until you got to the stall. When the angle of attack of the blades reached about 20 degree or so, the torque and drag would fall rapidly and stay at a lower and fairly constant value as shaft speed decreased to zero (or less, if you want to drive the shaft in reverse). > If there is a peak drag value, let's say it occurs at speed P. > What determines whether (and if so, by how much) drag at speed P is > higher than drag at speed 0? Propellor pitch, diameter, number of blades, blade area, aspect ratio, immersion, water viscosity. All of which will boil down to a few dimensionless coefficients - Reynolds number and so on. Wild guess? For every 1N drag with the prop freewheeling I'd expect about 10N at maximum drag and 2N with the prop locked. > About how far between 0 and F is P likely to lie? Another wild guess? 20% of F. These are all interesting questions, and worth looking into in detail - I bet it has been done. Those of us who still trail a Walkers log from the taffrail as god intended know, by the way, just how much tauter the string gets, and how much further back the log gets pulled, whan a small piece of seaweed wraps itself around the sinker, giving that little bit of extra torque on the spinner ... Ian No-one appears to have discussed the merits of this aspect on the Brunton-Weill auto reversing prop! Any experience? >I usually sail with the prop locked (by putting it in reverse gear - fixed >2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] >increasing drag by locking the prop. What do the URS gurus recommend >(Please don't recommend a change of prop, as I want to stay on topic.) No-one appears to have discussed this aspect,whilst using the Brunton-Weill Auto Reversing prop!! Any experience? STABU >I usually sail with the prop locked (by putting it in reverse gear - fixed >2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] >increasing drag by locking the prop. What do the URS gurus recommend >(Please don't recommend a change of prop, as I want to stay on topic.) N0-one appears to have discussed this aspect whilst using the Brunton Weill Auto Reversing Prop. Any experience. STABU >I usually sail with the prop locked (by putting it in reverse gear - fixed >2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] >increasing drag by locking the prop. What do the URS gurus recommend >(Please don't recommend a change of prop, as I want to stay on topic.) No-one appears to have discussed this aspect whilst using the Brunton Weill Auto Reversing Prop!! Any experience?? Stabu >I usually sail with the prop locked (by putting it in reverse gear - fixed >2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] >increasing drag by locking the prop. What do the URS gurus recommend >(Please don't recommend a change of prop, as I want to stay on topic.) > No-one appears to have discussed this aspect whilst using the Brunton Weill > Auto Reversing Prop!! Any experience?? Stabu You only had to post once, not 4 times. :P I looked it up, and couldn't find it. However, Brunton's do a range of propellors including folding and feathering ones. They would be far better under sail than any simpler prop. - although I don't like the idea of a complex mechanism below waterline. Andy Thanks Andy! I was perhaps impatient. The company name is BRUNTONS and the prop is a 3 bladed swivelling one via ball bearings.The main advantage is that in the leading edge in Forward..becomes the leading edge when put into Reverse,and they do FEATHER> >I usually sail with the prop locked (by putting it in reverse gear - fixed >2 bladed low tech prop on a sail drive). This avoids the annoying noise of [quoted text clipped - 3 lines] >increasing drag by locking the prop. What do the URS gurus recommend >(Please don't recommend a change of prop, as I want to stay on topic.) |
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