Tiller/Rudder pressure

[nicknorman]

Surely with an offset propeller steering in a straight line is going to be a problem? Steering my friend's boat in a straight line is no problem.

 

Can you explain what you mean by the highlighted text? How do you 'ventilate' a propeller? Do you mean cavitation?

Ventilation is where air gets drawn in to the water flowing through the propellor, typically by having the stern insufficiently ballasted.

 

Cavitation is where the local pressure (eg at a blade surface or trailing edge) drops to the point that a vacuum is pulled. Well not really a vacuum of course, just water in its vapour state rather than liquid.

 

I don't think I can go into much detail about your friends boat since I have never seen it, but it can have a combination of lateral and angular displacement which roughly cancel out, it can have a trim tab on the rudder or maybe just wheel steering so you don't realise you have slight rudder on. Anyway, slight lateral offset wouldn't produce much turning tendency but might have quite an effect on rudder effect in the 2 directions. But I am just guessing as I know nothing about sea going boats and this is a canal forum!

[Black Country Lee]

Having tried some research into it on the RYA site it seems that it most commonly occurs when you get the boat up on the plane (haven't managed to do that yet with my narrow boat ). I would suggest that if your stern is far enough out of the water to expose your propeller you have far more to worry about than a bit of 'ventilation'!

 

I agree, it was the simplest way of explaining it though lol, the boat in my avatar suffers a little from ventilation as, when the water tank (in the bow) is full the rear ballast isn't heavy enough and with my pal standing to port to steer creates enough plane to let the odd pocket of air slip through. It can also be caused by chips or damage to the leading edges of the prop.

 

Lee.

[nicknorman]

Having tried some research into it on the RYA site it seems that it most commonly occurs when you get the boat up on the plane (haven't managed to do that yet with my narrow boat ). I would suggest that if your stern is far enough out of the water to expose your propeller you have far more to worry about than a bit of 'ventilation'!

The prop doesn't have to be out of the water normally, but ventilation typically occurs on narrowboats when you hit reverse and air gets sucked under the counter along with the water even though the prop remains below water level. You hear it a lot going uphill in locks in a shallow drafted (or insufficiently ballasted) narrowboat when Bernoulli's effect from the incoming water flow lowers the local water level at the front of the lock, which lowers the boat's bow, which raises the stern a bit, which makes the boat slide "downhill" (forwards) and the frantic reversing from the helmsman is ineffective due to all the air getting sucked in. That is ventilation!

Edited October 13, 2014 by nicknorman

[Wanderer Vagabond]

The prop doesn't have to be out of the water normally, but ventilation typically occurs on narrowboats when you hit reverse and air gets sucked under the counter along with the water even though the prop remains below water level. You hear it a lot going uphill in locks in a shallow drafted (or insufficiently ballasted) narrowboat when Bernoulli's effect from the incoming water flow lowers the local water level at the front of the lock, which lowers the boat's bow, which raises the stern a bit, which makes the boat slide "downhill" (forwards) and the frantic reversing from the helmsman is ineffective due to all the air getting sucked in. That is ventilation!

From what I've now read on ventilation, the effect is to lose drive in that the propeller is just thrashing through air so speed drops and engine speed goes up, no mention of any turning effect from anything I've seen so far.

 

I have to say that I find it odd that you consider something that is recognised by experts (prop walk) as a reality, to be a myth. You have disregarded the initial link ,and I will accept that Wikipedia may not be the most reliable source but is did seem to give a reasonable explanation of the process. If that isn't accepted however these next 2 should be:-

 

http://www.rya.org.uk/cruising/handling-power/Pages/Theeffectofrudderspropsandpropwalkonboathandling.aspx

 

http://www.cruisingschool.co.uk/icc/icc.htm

 

If they are also held to be promulgating myths then I suppose that I may as well give up.

[Dalslandia]

Prop walk exist,

On Dalslandia I have a big bent steel plate under the prop as protection, in the beginning it was a whole plate, bent up in level to just under the prop shaft, and it had no prop walk at all, but sometimes when stopping hard in shallow water it could suck up stones and other debris, that could get stuck between propeller and the plate, so I hade 2 big holes cut up on each side of the middle under the prop, so the stones can fall out there, seems to work. but now I have a slight prop walk, bow going port when stopping/reversing, and yes it is a left hand propeller.

 

If going into big harbours or canals that require a Pilot or helmsman on board little bigger ships, the first they ask about is if it is left or right hand propeller...

[nicknorman]

From what I've now read on ventilation, the effect is to lose drive in that the propeller is just thrashing through air so speed drops and engine speed goes up, no mention of any turning effect from anything I've seen so far.

 

I have to say that I find it odd that you consider something that is recognised by experts (prop walk) as a reality, to be a myth. You have disregarded the initial link ,and I will accept that Wikipedia may not be the most reliable source but is did seem to give a reasonable explanation of the process. If that isn't accepted however these next 2 should be:-

 

http://www.rya.org.uk/cruising/handling-power/Pages/Theeffectofrudderspropsandpropwalkonboathandling.aspx

 

http://www.cruisingschool.co.uk/icc/icc.htm

 

If they are also held to be promulgating myths then I suppose that I may as well give up.

Yes the primary effect of ventilation is to reduce prop efficiency, but I think a case can be made that, due to the different densities of water and air, there is likely to be more air nearer the top of the prop and therefore a possible slight paddle wheel effect.

 

Well the RYA link says "may make the stern walk". May. And no attempt to explain it.

 

The Cruising school probably read the rya webpage! But actually it has a theory on the clickable link, but surmises that if the prop shaft is horizontal there is no prop walk. Surely for a narrowboat there is no point in having a non-horizontal prop shaft?

 

So if you want me to say "some badly designed narrowboats have prop walk either due to an insufficiently submerged prop or a non-horizontal prop shaft" then I could go along with that. But paddle wheel effect it is not, unless there is a ventilation issue.

[Wanderer Vagabond]

Yes the primary effect of ventilation is to reduce prop efficiency, but I think a case can be made that, due to the different densities of water and air, there is likely to be more air nearer the top of the prop and therefore a possible slight paddle wheel effect.

 

Well the RYA link says "may make the stern walk". May. And no attempt to explain it.

 

The Cruising school probably read the rya webpage! But actually it has a theory on the clickable link, but surmises that if the prop shaft is horizontal there is no prop walk. Surely for a narrowboat there is no point in having a non-horizontal prop shaft?

 

So if you want me to say "some badly designed narrowboats have prop walk either due to an insufficiently submerged prop or a non-horizontal prop shaft" then I could go along with that. But paddle wheel effect it is not, unless there is a ventilation issue.

I would suggest that there may be a very good reason for a narrow boat having a prop shaft that is not perfectly horizontal. As a narrow boat passes over a shallow area of canal the propeller at the rear 'sucks' the water from below the boat to propel it forwards. This gives rise to your favourite Bernouli effect drawing the boat even closer (or dragging) the canal bottom. If the propeller is angled slightly downwards this will lift the rear of the boat to try to counter this. Would that be a '...badly designed narrowboat...'.

 

I would also suggest that since you are now saying of your ventilation theory that '...I think a case can be made....' it may well also be a myth since I have never personally seen it. Not necessarily saying that it is, but it might be unless you can find conclusive proof to the contrary. I don't know where you will get this proof from since there don't appear to be any acceptable expert sources.

[nicknorman]

I would suggest that there may be a very good reason for a narrow boat having a prop shaft that is not perfectly horizontal. As a narrow boat passes over a shallow area of canal the propeller at the rear 'sucks' the water from below the boat to propel it forwards. This gives rise to your favourite Bernouli effect drawing the boat even closer (or dragging) the canal bottom. If the propeller is angled slightly downwards this will lift the rear of the boat to try to counter this. Would that be a '...badly designed narrowboat...'.

 

I would also suggest that since you are now saying of your ventilation theory that '...I think a case can be made....' it may well also be a myth since I have never personally seen it. Not necessarily saying that it is, but it might be unless you can find conclusive proof to the contrary. I don't know where you will get this proof from since there don't appear to be any acceptable expert sources.

Well in summary, whilst prop walk may exist on some boats, it is by no means a certainty.

 

You said it was paddlewheel effect. I asked you to explain it but you couldn't.

 

You then cited some rather dodgy internet links - personally I try not to believe everything I read on the internet when it is not fully explained in a convincing way. However the point about non-horizontal prop shaft was interesting if irrelevant*.

*for a sensibly designed boat.

 

I have no problem reviewing "expert sources" but so far you haven't supplied any.

 

The fact that you don't seem to understand the common phenomenon of ventilation means you lose credibility. Sorry about that, just being honest! I am not trying to make a strong case that ventilation causes prop walk, it just seems the only explanation for those boats that do go sideways in reverse, and if you look back that was supported by a forum member whose boat does ventilate and display prop walk with a forward CofG.

 

Regarding your point about the attitude of a narrowboat changing with power /shallow water this is of course true but for say a 60' boat, dipping the stern by say 6" is really going to change the angle very little. Even if it is enough to generate noticeable asymmetry as per your link, which I doubt, I thought we were talking about prop walk at low speed /reverse? When there is minimal stern dip.

 

Anyway, my discussion style is to make a bold statement and await being proven wrong so as to learn something. If you can give me a convincing explanation of your prop walk, and why our boat doesn't exhibit it, I'll be just as happy as you. But so far you haven't. As I mentioned, we've been here before and no convincing explanation was forthcoming so I'm not holding my breath.

[Black Country Lee]

I have no intention adding 'nutty slack' to this but, assuming I'm the other forum member, my explanation of ventilation was merely a simplified version. Prop wash (or walk if you prefer) is factual. Again simplified, dependant on the hand of the prop in question, I.E. a r/hand prop will walk towards the port and vice versa, the effect going forward will be minimal due to the rudder being in the direct flow zone. However, in reverse is a whole other issue! A r/hand prop running in reverse will drag the stern to the right and no amount of tiller swinging will make the slightest difference as the steerable flow is busy rushing under the flat hull. Steerage = a big fat zero, hence most modern 50'+ modern boats having 'girly' buttons (or bow thrusters if you prefer their technical name). A prime example is the 24'er in my avatar, an arse to steer forward with a full water tank and a 15 stone bloke stood on the rear port comer due to the stern design and lack of ballast, in reverse... You're guess is as good as mine, that all depends on a slight breeze, where you stand, what phase the moon is in in Sagittarius! The ballast is one of my first jobs to fix on it.

 

I hope I've not poked the hornets nest too much?!

 

Lee.

[nicknorman]

I have no intention adding 'nutty slack' to this but, assuming I'm the other forum member, my explanation of ventilation was merely a simplified version. Prop wash (or walk if you prefer) is factual. Again simplified, dependant on the hand of the prop in question, I.E. a r/hand prop will walk towards the port and vice versa, the effect going forward will be minimal due to the rudder being in the direct flow zone. However, in reverse is a whole other issue! A r/hand prop running in reverse will drag the stern to the right and no amount of tiller swinging will make the slightest difference as the steerable flow is busy rushing under the flat hull. Steerage = a big fat zero, hence most modern 50'+ modern boats having 'girly' buttons (or bow thrusters if you prefer their technical name). A prime example is the 24'er in my avatar, an arse to steer forward with a full water tank and a 15 stone bloke stood on the rear port comer due to the stern design and lack of ballast, in reverse... You're guess is as good as mine, that all depends on a slight breeze, where you stand, what phase the moon is in in Sagittarius! The ballast is one of my first jobs to fix on it.

I hope I've not poked the hornets nest too much?!

Lee.

So why doesn't our 59' boat (which is not contaminated with a bowthruster) not exhibit the effect you say WILL happen? Ie when I go into reverse either to reverse, or to stop when going forwards.

 

On a good day I can reverse 100s of yards straight, provided I stay on top of it, look over the bow to detect any tendency to turn, and correct with the rudder. But once it starts to turn more than a gnats whisker, the turn becomes unstoppable. But that turn will develop either way.

Edited October 13, 2014 by nicknorman

[Black Country Lee]

So why doesn't our boat (which is not contaminated with a bowthruster) not exhibit the effect you say WILL happen? Ie when I go into reverse either to reverse, or to stop when going forwards.

 

I never said it WILL happen, it may happen but varies wildly on a) the design of said boat the weight c) ballast d) the prop fitted compared to gearing, power etc. e) depth of the water f) a million other variables it is too late to wrap my poor old brain around. Basically every boat is different and prop wash is an unpredictable beast and has so many variables makes it impossible to predict. Using the littlun in my avatar again, from 2.5mph (full speed for it's single pot 8hp) to a full stop in reverse is almost akin to handbrake turning a car due to the ballast not being correct, this effect is a lot less so if the water tank is low. Again all variables, your boat, luckily, is one of many that has everything set up correctly so that prop wash isn't an issue, hopefully the same can be said of 'Tara' once I've sorted her ballast out.

 

I hope that's cleared the air now as I don't want nor wish to offend anyone being a relative newbie here if not to the waterways.

 

Lee.

[nicknorman]

I.E. a r/hand prop will walk towards the port and vice versa... A r/hand prop running in reverse will drag the stern to the right.

 

I never said it WILL happen....

Well actually you did. Twice.

 

But never mind, perhaps we can agree that prop walk might happen for reasons we can't yet explain, but it is not a fundamental characteristic of all narrowboats.

 

There is no shame in putting up a good argument even if it is to disagree! In fact especially if it is to disagree (otherwise things would be really boring on here)! What I tend to rile against is folk spitting out something they have heard someone else say, without any understanding of what they were saying. Not saying you were doing that though.

Edited October 13, 2014 by nicknorman

[Black Country Lee]

Ok I said WILL twice All variables though and completely unpredictable. Now that that's agreed can I go to bed? I've got an interview in the morning and got to be up n showered by 6

 

Lee.

[nicknorman]

Ok I said WILL twice All variables though and completely unpredictable. Now that that's agreed can I go to bed? I've got an interview in the morning and got to be up n showered by 6

 

Lee.

Well then you're a very naughty boy, you should have been in bed hours ago, not glued to the internet!

 

Ed: the last job interview I went to, they asked an off-the-wall question "what is red shift?". Fortunately I knew the answer. So maybe it will be your lucky day and they will ask "what is prop walk?"

Edited October 13, 2014 by nicknorman

[Black Country Lee]

I was watching the TV and the internet distracted me from that lol.

 

Lee.

[Wanderer Vagabond]

Well in summary, whilst prop walk may exist on some boats, it is by no means a certainty.

 

You said it was paddlewheel effect. I asked you to explain it but you couldn't.

 

You then cited some rather dodgy internet links - personally I try not to believe everything I read on the internet when it is not fully explained in a convincing way. However the point about non-horizontal prop shaft was interesting if irrelevant*.

*for a sensibly designed boat.

 

I have no problem reviewing "expert sources" but so far you haven't supplied any.

 

The fact that you don't seem to understand the common phenomenon of ventilation means you lose credibility. Sorry about that, just being honest! I am not trying to make a strong case that ventilation causes prop walk, it just seems the only explanation for those boats that do go sideways in reverse, and if you look back that was supported by a forum member whose boat does ventilate and display prop walk with a forward CofG.

 

Regarding your point about the attitude of a narrowboat changing with power /shallow water this is of course true but for say a 60' boat, dipping the stern by say 6" is really going to change the angle very little. Even if it is enough to generate noticeable asymmetry as per your link, which I doubt, I thought we were talking about prop walk at low speed /reverse? When there is minimal stern dip.

 

Anyway, my discussion style is to make a bold statement and await being proven wrong so as to learn something. If you can give me a convincing explanation of your prop walk, and why our boat doesn't exhibit it, I'll be just as happy as you. But so far you haven't. As I mentioned, we've been here before and no convincing explanation was forthcoming so I'm not holding my breath.

I presume that your cynicism covers the internet, written word, description by people who have experienced it in fact everything! Are you are seriously telling me that the RYA are NOT an 'expert source', you are a braver man than I to tell them that they are wrong. I would suggest that your level of confidence in yourself is such that no-one, despite whatever evidence they might wish to present, would ever alter your opinion.

 

Reading the above post by Dalslandia I found interesting and it might (with an open mind) go towards explaining the principle. If I understand what he is saying, he had a hull-plate below his propeller which seemed to prevent Prop walk but unfortunately in shallow water his boat could pick up stones which would jam between the prop and the plate. To prevent this he had a gap cut into the plate and now he experiences prop walk. What I would suggest is that we accept that when a propeller spins some of the force from it will be at right angles to the main driving force (a wet propeller spun in the open air will throw the water off, not throw it backwards so there IS a sideways force). Now when the propeller is installed in the boat this sideways force can only be exerted for the lower 50% of rotation since the hull is in the way for the upper 50% and the force will just hit the hull, and since it is attached to the hull it can't push it sideways. The lower 50% of rotation in in the water, which it CAN push sideways, which will give the effect. Your earlier submarine analogy becomes irrelevant because the propeller is basically suspended in water without a hull above it so whilst the top of the propeller pushes the water to the right (on a clockwise rotating propeller) the bottom will push it to the left cancelling itself out. This is an attempt to explain what Dalslandia is describing and what I have experienced in seagoing boats. The Prop walk may be difficult to see on a narrow boat since there are VERY few opportunities to turn them full circle Clockwise then Anticlockwise on the Inland waterways, this is when prop walk is clearly seen because in one of the directions the circle WILL be smaller.

 

Dalslandia also states that when picking up a Pilot (you know one those people of little experience who steer the boat for you when you can't be a*sed) they ask if it has a left or right hand propeller, why would they do that??

[nicknorman]

I presume that your cynicism covers the internet, written word, description by people who have experienced it in fact everything! Are you are seriously telling me that the RYA are NOT an 'expert source', you are a braver man than I to tell them that they are wrong. I would suggest that your level of confidence in yourself is such that no-one, despite whatever evidence they might wish to present, would ever alter your opinion.

 

Reading the above post by Dalslandia I found interesting and it might (with an open mind) go towards explaining the principle. If I understand what he is saying, he had a hull-plate below his propeller which seemed to prevent Prop walk but unfortunately in shallow water his boat could pick up stones which would jam between the prop and the plate. To prevent this he had a gap cut into the plate and now he experiences prop walk. What I would suggest is that we accept that when a propeller spins some of the force from it will be at right angles to the main driving force (a wet propeller spun in the open air will throw the water off, not throw it backwards so there IS a sideways force). Now when the propeller is installed in the boat this sideways force can only be exerted for the lower 50% of rotation since the hull is in the way for the upper 50% and the force will just hit the hull, and since it is attached to the hull it can't push it sideways. The lower 50% of rotation in in the water, which it CAN push sideways, which will give the effect. Your earlier submarine analogy becomes irrelevant because the propeller is basically suspended in water without a hull above it so whilst the top of the propeller pushes the water to the right (on a clockwise rotating propeller) the bottom will push it to the left cancelling itself out. This is an attempt to explain what Dalslandia is describing and what I have experienced in seagoing boats. The Prop walk may be difficult to see on a narrow boat since there are VERY few opportunities to turn them full circle Clockwise then Anticlockwise on the Inland waterways, this is when prop walk is clearly seen because in one of the directions the circle WILL be smaller.

 

Dalslandia also states that when picking up a Pilot (you know one those people of little experience who steer the boat for you when you can't be a*sed) they ask if it has a left or right hand propeller, why would they do that??

Resorting to insult never helps you win an argument.

 

RYA are people who sail boats, they are not fluid dynamicists. And anyway, don't yachts have sails not props?

 

Your attempt at a technical enhancement of dalaslandia's point doesn't hold water (pun intended) as by extrapolation you are saying that fitting a paddlewheel to a submarine would work, if you fitted a flat plate above it. I don't think so!

 

Seagoing boats have completely different hull shapes, rudder/propellor relationships and design priorities, and to be honest I know little about them. But this is a canal forum, maybe you are on the wrong forum if you can't explain it in terms of a canal boat?

 

However on the point you seem so keen on - the differing turning circle in each direction at speed - I would put money on there being some asymmetry of design that explains it, rather than the phantom prop walk, since to notice the difference in turning circles the effect must be very large.

 

But still, if you can explain the effect in a convincing and scientific way I will gladly believe you...

Edited October 13, 2014 by nicknorman

[Dalslandia]

As I understand propwalk, that I have noticed is evidently there, I have been driving the same boat the last 35 years as captain, doing some 60000 locks, sometimes reversing in and out of locks and canals. I also did drive a other slightly bigger boat a few times beeing, maybe a feet or 2 longer and a few cm wider, but a lot heavier, with a propeller with a lot more pitch then Dalslandias. it was a real dog to drive, when stopping it turned 10-20 degree, and when going down the canal, going into the full locks there was no way to see where the edges of the lock entrance was, so going slow was necessarily, meaning going into reverse gear, so had to put the bow on left side of centre then reverse and hopefully the bow got where I wanted it, it didn't help that the bow thruster didn't work.

 

Ok, propellers largest loss is its rotating losses, the radial flow.

 

when reversing with a propeller made for going forward, with the curved side of the propeller forward (called back side!) and the flat face toward the stern, the camber in the blade is all wrong, so we have more losses and drag in the propeller going in reverse. = less axial flow and more rotational

this rotating mass of water leaving the propeller, hits the hull and is shaved off mostly on the side where the up going blade is, creating a high pressure area on that side of the stern/hull/keel, so if it is a right hand propeller, going left in reverse, when reversing/stopping a high pressure area of water forms on the right hand side of the rear hull, pressing the stern to the port and bow to the starboard. so on right/top of propeller the water is steered away to the right, at bottom this spiralled water goes mostly under around the keel/swim, and is also eventually hitting the right side of the swim and underside of hull side, the counter plate.

Edited October 14, 2014 by Dalslandia

[Wanderer Vagabond]

Resorting to insult never helps you win an argument.

 

RYA are people who sail boats, they are not fluid dynamicists. And anyway, don't yachts have sails not props?

 

Your attempt at a technical enhancement of dalaslandia's point doesn't hold water (pun intended) as by extrapolation you are saying that fitting a paddlewheel to a submarine would work, if you fitted a flat plate above it. I don't think so!

 

Seagoing boats have completely different hull shapes, rudder/propellor relationships and design priorities, and to be honest I know little about them. But this is a canal forum, maybe you are on the wrong forum if you can't explain it in terms of a canal boat?

 

However on the point you seem so keen on - the differing turning circle in each direction at speed - I would put money on there being some asymmetry of design that explains it, rather than the phantom prop walk, since to notice the difference in turning circles the effect must be very large.

 

But still, if you can explain the effect in a convincing and scientific way I will gladly believe you...

Since when did describing someone as confident constitute an insult? And if we are looking for insulting comments I would suggest that your earlier reply to Black Country Lee saying,"....What I tend to rile against is folk spitting out something they have heard someone else say, without any understanding of what they were saying..." comes pretty close. If that was addressed towards me I was in fact 'spitting out' what I had experienced towards someone who was alleging that it was a myth.

 

I see that Dalslandia has tried to offer an explanation to you above but I don't hold a lot of hope for you accepting it. If we are looking at theories and myths, try to explain to me in a convincing way the theory of lunar tides. I have never in my life seen water being attracted to a piece of rock but these radicals are trying to tell me that a lump of rock thousands of miles away attracts all of the water on the planet, utter humbug (perhaps ). No real way of 'proving' that either but the theory does help us draw up tide tables!

Edited October 14, 2014 by Wanderer Vagabond

[nicknorman]

Since when did describing someone as confident constitute an insult? And if we are looking for insulting comments I would suggest that your earlier reply to Black Country Lee saying,"....What I tend to rile against is folk spitting out something they have heard someone else say, without any understanding of what they were saying..." comes pretty close. If that was addressed towards me I was in fact 'spitting out' what I had experienced towards someone who was alleging that it was a myth.

 

I see that Dalslandia has tried to offer an explanation to you above but I don't hold a lot of hope for you accepting it. If we are looking at theories and myths, try to explain to me in a convincing way the theory of lunar tides. I have never in my life seen water being attracted to a piece of rock but these radicals are trying to tell me that a lump of rock thousands of miles away attracts all of the water on the planet, utter humbug (perhaps ). No real way of 'proving' that either but the theory does help us draw up tide tables!

I see you have no new theories of your own, but at least hopefully you now realise that the trite repost "paddle wheel effect" is definitely a myth, even though it is often repeated. But no need to thank me for helping you clear up that misconception. On my comment emboldened by you above, I didn't say it was directed to you but I suppose if the cap fits, you might as well wear it.

[nicknorman]

As I understand propwalk, that I have noticed is evidently there, I have been driving the same boat the last 35 years as captain, doing some 60000 locks, sometimes reversing in and out of locks and canals. I also did drive a other slightly bigger boat a few times beeing, maybe a feet or 2 longer and a few cm wider, but a lot heavier, with a propeller with a lot more pitch then Dalslandias. it was a real dog to drive, when stopping it turned 10-20 degree, and when going down the canal, going into the full locks there was no way to see where the edges of the lock entrance was, so going slow was necessarily, meaning going into reverse gear, so had to put the bow on left side of centre then reverse and hopefully the bow got where I wanted it, it didn't help that the bow thruster didn't work.

 

Ok, propellers largest loss is its rotating losses, the radial flow.

 

when reversing with a propeller made for going forward, with the curved side of the propeller forward (called back side!) and the flat face toward the stern, the camber in the blade is all wrong, so we have more losses and drag in the propeller going in reverse. = less axial flow and more rotational

this rotating mass of water leaving the propeller, hits the hull and is shaved off mostly on the side where the up going blade is, creating a high pressure area on that side of the stern/hull/keel, so if it is a right hand propeller, going left in reverse, when reversing/stopping a high pressure area of water forms on the right hand side of the rear hull, pressing the stern to the port and bow to the starboard. so on right/top of propeller the water is steered away to the right, at bottom this spiralled water goes mostly under around the keel/swim, and is also eventually hitting the right side of the swim and underside of hull side, the counter plate.

Rotational flow - yes of course, and more rotation and less flow in reverse. But I question your point about the flow causing increased pressure. Normally flow causes reduced pressure. But whatever, the rotational flow must be at the root of this and I know you made this point when we discussed it before. I just wondered if WV would get there himself. Depending on the design of the hull / propellor/ rudder the effect will be more or less evident in reverse, but you would think mostly absent in fwd presuming a rudder that equally covers the upper and lower flows, making WV's point about turning circles surely due to something else like a design asymmetry, or an odd-shaped rudder (but if due to the rudder you would think it would want to turn a bit on its own due to asymmetric contact with the spiral flow).

 

Our boat stops well and there is a very pronounced flow coming forward of the swims when hitting reverse. I have never noticed a difference between the L and R side flows, which is perhaps why our boat doesn't exhibit prop walk.

[bizzard]

Prop walk;- ''To wrestle with a clothes prop'' whilst trying to prop up your washing line in a gale of wind to prevent your washing from dragging in the dirt.

[furnessvale]

Since when did describing someone as confident constitute an insult? And if we are looking for insulting comments I would suggest that your earlier reply to Black Country Lee saying,"....What I tend to rile against is folk spitting out something they have heard someone else say, without any understanding of what they were saying..." comes pretty close. If that was addressed towards me I was in fact 'spitting out' what I had experienced towards someone who was alleging that it was a myth.

 

I see that Dalslandia has tried to offer an explanation to you above but I don't hold a lot of hope for you accepting it. If we are looking at theories and myths, try to explain to me in a convincing way the theory of lunar tides. I have never in my life seen water being attracted to a piece of rock but these radicals are trying to tell me that a lump of rock thousands of miles away attracts all of the water on the planet, utter humbug (perhaps ). No real way of 'proving' that either but the theory does help us draw up tide tables!

I could understand that bit. What gets me is that it only attracts HALF the water on the planet.

 

AIUI the other half goes the other way, away from the moon, forming the (approx.) 12 hour pattern of tides.

 

How does that work then?

 

Confused of Tunbridge Wells

[Dalslandia]

Rotational flow - yes of course, and more rotation and less flow in reverse. But I question your point about the flow causing increased pressure. Normally flow causes reduced pressure. But whatever, the rotational flow must be at the root of this and I know you made this point when we discussed it before. I just wondered if WV would get there himself. Depending on the design of the hull / propellor/ rudder the effect will be more or less evident in reverse, but you would think mostly absent in fwd presuming a rudder that equally covers the upper and lower flows, making WV's point about turning circles surely due to something else like a design asymmetry, or an odd-shaped rudder (but if due to the rudder you would think it would want to turn a bit on its own due to asymmetric contact with the spiral flow).

 

Our boat stops well and there is a very pronounced flow coming forward of the swims when hitting reverse. I have never noticed a difference between the L and R side flows, which is perhaps why our boat doesn't exhibit prop walk.

 

Speed make a dynamic pressure, and then as you say steady flow over an profile make a reduction in pressure if it is speeded up and increased pressure if slowed down. the propeller wash is a higher speed then the surrounding, specially if going forward and propeller work in reverse, you will have more flow going forward on one side then the other, that might have non,

The other passenger boat I drove had a horizontal prop shaft, my boat have a tilted shaft. so the so called P-factor (aeronautical) can not explain this difference, but why are some seeing almost non and other a lot of prop walk? maybe it is the shape of the hull, the pitch make a difference we know that.

[Wanderer Vagabond]

I see you have no new theories of your own, but at least hopefully you now realise that the trite repost "paddle wheel effect" is definitely a myth, even though it is often repeated. But no need to thank me for helping you clear up that misconception. On my comment emboldened by you above, I didn't say it was directed to you but I suppose if the cap fits, you might as well wear it.

This didn't start as a 'theory' of my own, this started as an attempt to explain an effect that I had experienced, but you had dismissed as a myth. It is described as a 'paddle wheel effect' to try to simplify the explanation as to why the stern of a boat can be (slightly) pushed sideways by the rotation of the propeller. The simplified description of 'paddle wheel effect' is irrelevant to the discussion, you had declared that Prop walk was a myth, not paddle wheel effect, so is seems that you have now changed your 'argument' in an effort to claim some sort of 'victory'.

 

My ironic rubbishing of the theory of lunar tides was intended to display that most people will repeat '..something that they have heard someone else say without any understanding of what they were saying..' when it gives a reasonable explanation of their experiences. I haven't seen any practical visual demonstration of lumps of rock attracting water and have no real understanding of how it happens, but accept that it is a reality (unless someone can come up with a better explanation!). So your ridicule of this acceptance is a bit misplaced, I don't understand atomic theory but I'm sure that those who were at Hiroshima wouldn't doubt it's reality.

 

And finally your approach to discussion of,"... my discussion style is to make a bold statement and await being proven wrong....." is the total opposite of both legal and scientific method. In science you propose a theorem that fits with observations and then try to find out where and why it doesn't fit, in legal circles you have to prove something happened since it is impossible to prove something didn't. If I were to make the 'bold statement' that your ancestor was Jack the Ripper it would be impossible for you to disprove it. It is from this line of reasoning that we get the crop of various idiotic conspiracy theories that the Internet is plagued with.