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What Is Prop Walk?


NickF

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I know that when you go backwards in a boat two things can happen to the back end ... but which one is "prop walk"?

 

1.If you are in open water and go astern the back end tends to swing one way or the other ... I think (but am not sure) this is due to the propeller acting like a paddle wheel and pushing the back end sideways ... is that right and how can that work when the entire propeller is under water?

 

2. If you are near the bank and go astern the water being pushed between the bank and the boat pushes you away from the bank .... what is this called? Is this prop walk?

 

I await your expert replies with interest!

 

Nick

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What Is Prop Walk?

 

It's like the Harlem Shake but it never went viral.

 

Seriously though Prop walk is your first description though the effect works in forwards and reverse.

 

It only works due to the effect of the immersed prop though, so the more of the prop there is under water the greater the effect.

 

It isn't strictly a "paddlewheel" effect because if the whole of a paddlewheel was immersed the boat would go nowhere.

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As Carlt has said - it's the former. - Prop walk is caused by the vanes of the propeller moving water. -

 

The propeller needs to be completely immersed for this effect to be at it's greatest - but, of course, the bottom arc of the prop is immersed in deeper water than the top, hence there is a horizontal 'skew' effect.

 

All flyists will have also learned that there is a similar 'prop effect' caused by 'plane propellers too.

  • Greenie 1
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As Carlt has said - it's the former. - Prop walk is caused by the vanes of the propeller moving water. -

 

The propeller needs to be completely immersed for this effect to be at it's greatest - but, of course, the bottom arc of the prop is immersed in deeper water than the top, hence there is a horizontal 'skew' effect.

 

All flyists will have also learned that there is a similar 'prop effect' caused by 'plane propellers too.

Not sure that's true - I suspect it would be worse if the top of the blade is out of the water.

 

Tim

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Not sure that's true - I suspect it would be worse if the top of the blade is out of the water.

 

Tim

I think you're right. The effect is caused by the angle of the prop blades, if they were at right angle to the propshaft there would be no appreciable sideways (or forward) affect. I feel a discussion coming on!!!!

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Grace and Favour is right imo

If you look at the water being pushed out of the back of the boat as you progress (esp if going slow so there is no splash) you will see the flow being twisted around by the screw effect of the whole of the blade being in the water as the prop effectively fails to stand still and turn the boat around it. The same thing happens to aircraft as he said.

Edited by jim and pat dalton
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Grace and Favour is right imo

 

If you look at the water being pushed out of the back of the boat as you progress (esp if going slow so there is no splash) you will see the flow being twisted around by the screw effect of the whole of the blade being in the water as the prop effectively fails to stand still and turn the boat around it. The same thing happens to aircraft as he said.

Surely if the bottom of the prop is in the water but the top isn't, then the bottom will be pushing the boat one way while the top is doing nothing. If the whole prop is immersed, top and bottom will both have opposing effects but the effect at the bottom will be greater.

 

Tim

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Surely if the bottom of the prop is in the water but the top isn't, then the bottom will be pushing the boat one way while the top is doing nothing. If the whole prop is immersed, top and bottom will both have opposing effects but the effect at the bottom will be greater.

 

Tim

It would if it was a true "paddlewheel" effect but it is actually an effect due to the "skewness" of the prop's pitch which isn't reversed at the top of the blade.

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Pardon?

Granted.

 

The forces from the prop doesn't just come straight out at 90 degrees but follow a helical pattern producing sideways pressure also.

 

This helical pattern, unlike the paddlewheel, produces those sideways forces in the same direction both at the top and bottom of the prop.

 

I shall seek out a diagram if I'm still not being clear.

- why is the bottom force stronger?

It isn't

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I'm not quite grasping here what disturbs the symmetry of the forces. Blade pushes water one way at the top, pushes water the opposite way at the bottom - why is the bottom force stronger?

Possibly because the deeper water is at a slightly higher pressure/density?

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I quite like this;

 

Prop walk or transverse propeller thrust is caused by the helical discharge from the propeller and its interaction with the rudder and hull.

With an ahead movement of a right handed propeller:



  • The helical discharge from the propeller creates a larger pressure on the port side of the rudder.
  • A slight upward flow from the hull into the propeller area puts slightly more pressure onto the down sweeping propeller blades.
  • The net result is a tendency for a right handed propeller to give a small swing to port when running ahead.


Transverse thrust is of much greater significance when using an astern movement.

The helical discharge from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so it behaves quite differently. On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force capable of swinging the stern to port, giving the classic kick of the bow to starboard.

(With acknowledgements to The Shiphandler's Guide, by Captain RW Rowe, FNI, published by The Nautical Institute)

 

 

There is even a small chance if I read it enough times, I'll understand it.

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When the prop gets tired of you belting it it undoes its nut and waits smugly for you to go into astern gear. When you do that it quickly nips off the shaft raps on the uxter plate to let you know its going off for walkies and cartwheels across the canal bed in glee never to return. smile.png

Water becomes denser, more solid and less disturbed the deeper you go. The prop blades passing through the lower and deeper sector grip-bite more because of this than blades passing through the upper sector which are sweeping through less dense and more disturbed water nearer the surface, less bite. the boats stern end gets shoved to one side going forward and to tother side.when going astern. Which side is which depends on whether you have a left or right hand propeller. It is less noticeable when going 'forward' unless starting off from a standstill because your subconsiously counteracting the affect.

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Water becomes denser, more solid and less disturbed the deeper you go. The prop blades passing through the lower and deeper sector grip-bite more because of this than blades passing through the upper sector which are sweeping through less dense and more disturbed water nearer the surface, less bite. the boats stern end gets shoved to one side going forward and to tother side.when going astern. Which side is which depends on whether you have a left or right hand propeller. It is less noticeable when going 'forward' unless starting off from a standstill because your subconsiously counteracting the affect.

That's the simple explanation that I've assumed to be the case for years. It's evidently much more complex than that, but it supports my personal observation that you have much more trouble with 'walk' when going astern if the prop is near the surface.

 

Tim

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Presumably, the prop will be nearer the surface in reverse. Instead of pulling water from under the boat, it's drawing from a free surface which will lower, and raising the boat at the same time from the water pushed under the hull

 

Richard

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When kids we would often cycle down to the Woolwich dock area mainly to go backwards and forwards on the ferry.

Often on the way or way back we would be stopped at a docks swing bridge while a ship passed through, often these were single screw cargo ships, unladen and not in ballast moving to another dock to load or whatever. We noticed on these ships that roughly the top 1/3 of the propeller blades came out of the water as they slowly rotated threshing the surface and at the same time a good deal of the top of the rudder could be seen which was held over by a fair amount to port or starboard, obvious to me now it was counteracting prop walk although the ship was moving in a straight line from dock to dock. On a calm day some ships seemed to maneuver like this without tug assistance.

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A big high pitch prop have more prop walk, then a small, who could have guessed that?

If you take a Pilot onboard, the first thing he ask if it is left or right hand prop!

 

My thinking of prop walk is that it is the difference in water presure and the often closeness to the hull/ bottom that make the starn walk when reversing, this happends in forward too, but is counter acted by the rudder.

then if the prop shaft, and the propeller is angled down the P factor come in also, the blade that goes down have a different angle then the one going up. This is very notiseble in an aircraft at slow speed/high power like under a climb, the blade that goes down have higher angle then the one going up. also the spiral effect of the slip stream that hit the fin/rudder harder on the side where the blade go up, this act both in the same direction, and a large rudder angle have to been used to correct this.

 

on a boat with a left hand propeller, the bow goes to left when reversing/stoping, and the opposit with a right hand prop, easy to remember.

 

on my boat that have a 32" prop, I have a big bent plate under the prop as a gard, it is not fully half circle but more open then follow the radii from the shaft, before this plate was closed under the prop, and had no prop walk what so ever, but if getting some stones in there when reversing they got stuck between prop blade and the plate, so I opened one big hole each side of the center, under the prop, and now I have a slight prop walk, but the stones fall out at the hole.

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Dunno Ray. It's one of their many claims that I find hard to believe. As they don't provide any useful data to explain it, I'm as much in the dark as you

 

It does seem to be defying the laws of physics though

 

Richard

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Dunno Ray. It's one of their many claims that I find hard to believe. As they don't provide any useful data to explain it, I'm as much in the dark as you

 

It does seem to be defying the laws of physics though

 

Richard

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ax·i·om
/ˈaksēəm/
Noun
(One of the definitions--------)
  1. A statement or proposition on which an abstractly defined structure is based. (i.e. an Axiom Propeller eliminates prop walk) ;)
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Wow what a mine of information this forum is! Thank you all ... I now know what prop walk is and I think I nearly understand why it happens!.and I know it is not a paddlewheel effect!

 

So back to my original post .... what about scenario 2? Is there a name for the stern swinging away from the bank when you go astern right close to the bank of the canal?

Nick

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Wow what a mine of information this forum is! Thank you all ... I now know what prop walk is and I think I nearly understand why it happens!.and I know it is not a paddlewheel effect!

 

So back to my original post .... what about scenario 2? Is there a name for the stern swinging away from the bank when you go astern right close to the bank of the canal?

Nick

A nuisance. smile.png

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