Prop walk explained

[frygood]

Please could somebody explain prop walk to me in layman terms.

 

Thanks

[Cheshire cat]

Just imagine the boat being on dry land and the propeller being large enough to actually touch the ground. If you started the engine and put it in gear. the propeller would try to walk across the ground and the back end of the boat would move sideways.

 

That's what happens in water but not as dramatically because water, being a fluid, has got give in it where as the ground wouldn't have any. Typically, the bigger the pitch the greater the degree of propwalk.

[frygood]

Thank you Mr Cat i understand now and so does the OH which is more important.So the faster the prop is turning the more effect this has.

 

I have often seen people reversing with the tiller hard over.This is to diminish the effect of the prop walk i take it?

[Cheshire cat]

It will certainly help if they put it hard over in the right direction.

[Arthur Marshall]

 

I have often seen people reversing with the tiller hard over.This is to diminish the effect of the prop walk i take it?

You can steer the thing going backwards (a little) but I think it's the reverse of going frontwards. The direction you shove the tiller will be the direction the pointy end goes behind you - useful if you're trying to keep it straight. I presume it's the effect of the water hitting the rudder and pushing the back end round. That's how it works on my boat anyway (or worked, obviously, I've got no reverse until my gearbox gets fixed...). And it doesn't work at all in a crosswind!

[frygood]

so my bows go to port and stern to starboard.So which way should i put the tiller over.(where should the handle be port or starboard] as we are reversing I'm guessing starboard.?

[alan_fincher]

Just imagine the boat being on dry land and the propeller being large enough to actually touch the ground. If you started the engine and put it in gear. the propeller would try to walk across the ground and the back end of the boat would move sideways.

 

That's what happens in water but not as dramatically because water, being a fluid, has got give in it where as the ground wouldn't have any. Typically, the bigger the pitch the greater the degree of propwalk.

 

Not really quite this simple, I would say, because whilst the bottom of the prop is trying to "walk" the boat to one side, the top is actually trying to "walk" it in the opposite direction.

 

If the two effects were equal, then I would argue it wouldn't "walk" at all.

 

However, the prop is more effective deep down in the water than it is near the surface, so the way it is "walking" the boat at the bottom tends to win, as the offsetting "walk" at the top is less.

 

A significant thing also is whether you have a left or right handed prop, because in one case if you use a lot of reverse thrust to try and pull up, your back end will walk to the left, and the front will swing relatively harmlessly to the right, generally away from any approaching boat.

 

However in the other case the back will tend to walk to the right, an unwanted consequence of which is that the front will swing to the left, and often straight into the path of whatever you are trying to hard avoid!

[ditchcrawler]

There is a lot of writing about it here https://en.wikipedia.org/wiki/Propeller_walk

[Cheshire cat]

Listen. I'm a layman. I explained it in layman's terms.

 

You're all adding bells and whistles. I realise the bottom of the prop has the advantage because it is in water where there is far less turbulence and obstruction and I agree that left handed and right handed props will behave in similar and opposite ways. My propeller is a Crowther and at 17 x 10 prop walk is virtually non existent.

[by'eck]

It can be used to great advantage as Captain Ron demonstrates

 

[ditchcrawler]

Listen. I'm a layman. I explained it in layman's terms.

 

You're all adding bells and whistles. I realise the bottom of the prop has the advantage because it is in water where there is far less turbulence and obstruction and I agree that left handed and right handed props will behave in similar and opposite ways. My propeller is a Crowther and at 17 x 10 prop walk is virtually non existent.

I don't seem to get much ether, The biggest effect in going backwards is water being thrust between the swim and the bank or a puff of wind.

[nicknorman]

You only get prop walk if the top of the propellor isn't properly submerged.

 

On a typical canal you can get far more effect from the proximity of a shallow bit on one side of the boat and not on the other.

[Dalslandia]

you get prop walk even on deep water, it is a bit different from cat walk, and moon walk, but it is a side effect

[biggles47]

You only get prop walk if the top of the propellor isn't properly submerged.

 

 

Isn't torque reaction another factor? That's why you have a tail rotor on helicopters. On some narrow boats it's called a bow thruster.

[alan_fincher]

You only get prop walk if the top of the propellor isn't properly submerged.

 

On a typical canal you can get far more effect from the proximity of a shallow bit on one side of the boat and not on the other.

 

 

So why does a boat with a left hand propeller typically slew the opposite way to one with a right hand propeller, (if it is only about the profile of the canal where the boat is?)

[nicknorman]

Isn't torque reaction another factor? That's why you have a tail rotor on helicopters. On some narrow boats it's called a bow thruster.

Torque reaction from a boat propellor results in a roll couple for the boat. Never noticed it - I guess the mass and roll stability of a narrowboat is much more significant than the roll couple from a few hp.

So why does a boat with a left hand propeller typically slew the opposite way to one with a right hand propeller, (if it is only about the profile of the canal where the boat is?)

Because many narrowboats don't have properly submerged props especially in reverse.

[Naughty Cal]

you get prop walk even on deep water, it is a bit different from cat walk, and moon walk, but it is a side effect

Indeed you do.

 

And even boats with counter rotating props which are supposed to eliminate prop walk still get prop walk!

[alan_fincher]

Because many narrowboats don't have properly submerged props especially in reverse.

 

How exactly are you defining "properly submerged"? How submerged must they be to be "properly submerged"?

[sirweste]

I like this thread, top flight geekery.

[nicknorman]

How exactly are you defining "properly submerged"? How submerged must they be to be "properly submerged"?

The point is that the density of water at the bottom part of the prop rotation needs to be the same as that at the top. Of course it is in the static case where the top of the prop is just below the water (water being effectively incompressible). But once rotation starts the flow/pull tends to break up the "solid" water and mixes in some air, resulting in less density of fluid near the top and thus prop walk.

 

Our boat has a well submerged counter and there is a fair bit of clearance between the prop and the counter. I couldn't tell you which way the prop walk is because the boat either stops in a straight line or randomly goes left or right according to the profile of the canal bed. There is probably a slight tendency to go one way but it is masked by other effects.

 

Imagine a submarine at 100' down, what would be the mechanism for prop walk?

Edited November 11, 2016 by nicknorman

[alan_fincher]

The size of the prop may well be one of the biggest influences as to how much it occurs on a narrow boat, I suspect.

 

Our old leisure boat with a 17" "egg whisk" it didn't particularly come into play.

 

On each of the current boats (26" and 25 1/4" diameter props), then I would say it is a very significant factor indeed.

 

It is doubtful the tips of the blades on these are any closer to the surface and/or uxter than they were on the leisure boat, (though of course at the lowest point they are far more submerged).

[ditchcrawler]

 

 

Our boat has a well submerged counter and there is a fair bit of clearance between the prop and the counter. I couldn't tell you which way the prop walk is because the boat either stops in a straight line or randomly goes left or right according to the profile of the canal bed. There is probably a slight tendency to go one way but it is masked by other effects.

 

Ours does the same and on deep open water with no breeze goes backward in quite a straight line if you start off straight. Only difference is according to all on here and books of wisdom, is my prop is too big for the space under the boat.

How about this for an answer

 

http://www.ybw.com/forums/showthread.php?341281-Prop-walk-Pysical-reason-why-does-this-happen&p=3920904#post3920904

[Cheshire cat]

Submarines don't have a counter to mess things up.

[Dalslandia]

Ours does the same and on deep open water with no breeze goes backward in quite a straight line if you start off straight. Only difference is according to all on here and books of wisdom, is my prop is too big for the space under the boat.

How about this for an answer

 

http://www.ybw.com/forums/showthread.php?341281-Prop-walk-Pysical-reason-why-does-this-happen&p=3920904#post3920904

 

If your propeller is to big for the space under the boat, it will not turn around, and that is why you don't have any prop walk, how you get any thrust, i can't explain

​(it is friday)

[twbm]

Just imagine the boat being on dry land and the propeller being large enough to actually touch the ground. If you started the engine and put it in gear. the propeller would try to walk across the ground and the back end of the boat would move sideways.

 

That's what happens in water but not as dramatically because water, being a fluid, has got give in it where as the ground wouldn't have any. Typically, the bigger the pitch the greater the degree of propwalk.

 

frygood, this is close enough to give an understanding of the effect. Hands in pockets, walk quietly away and leave 'em with it ....

 

Or ask another question about migratory coconuts.