What Is Prop Walk?

[Dalslandia]

as you say the difference in water pressure is in relationship to the dynamic pressure, (bernoulli just explane part of the job don)

 

but if the diameter is one meter, and the top is at one meter down from surface, the pressure at the borttom blade is double that of the top blade.

 

it will have some effect right?

 

Put the prop shaft in the water line ... the static pressure will still be the same on front and back of the blade, but not if you look at the blade up in the air, and the blade down in the water, my wild guess is that the blade in the water give more forces then the blade up in the air, just the same situation as if the whole prop is under water, it still will have more pressure to work with below the shaft then above.

 

If the diameter is one meter and the draft is 10 meter .... small issus.

 

No I am not talking about ventilation of the blade.

Edited August 5, 2013 by Dalslandia

[NickF]

If you time your mooring stops to come alongside dead slowly and gently and leaving your mooring stop the prop walk should be very mild and not really be much of a bother. Most boaters do far to much revving and roaring of their engines these days when maneuvering, as if they're parking and unparking a car.

Absolutely Bizzard i am with you on that one, low revs and less fuss makes for a much more impressive manoeuvre in my opinion! Much better than lots of noise and wash! After all most of us are on the canals to get peace and quiet and relaxation!

[nicknorman]

 

it will have some effect right?

 

 

That's the thing, I don't see how even a big variation of pressure in an "incompressible" fluid would make any difference.

[twbm]

Can any one explain why, when no one is looking, all prop walk disappears and the boat runs backwards as if on rails?

[Dalslandia]

That's the thing, I don't see how even a big variation of pressure in an "incompressible" fluid would make any difference.

No, it is not easy to understand, if we agree water and air is incompressible, (but neither actually is)

we cant explane how lift or thrust is created by just one theory, we need both buernilli and newton to do that,

Any wing, a prp blade is just a wing too, have an airfoil, good or bad, when moving with some speed, and the zero lift line is angled it will have a suction side and a pressure side, this diffrence in pressure make up som of the lift, the air or water that is leaving the wing or blade trailing edge at an angle, down or back or forward. this motion is creating an force in opposit direction.

 

suction in incompressible fluid sounds strange, but talking boat props, cavitating is result of to much suction often close to leading edge of propeller making the water boil from the low pressure, so it is pressent.

Can any one explain why, when no one is looking, all prop walk disappears and the boat runs backwards as if on rails?

 

The opposit of Murphys law?

Wind, currency, closeness to canal bottom?

Edited August 5, 2013 by Dalslandia

[nb Innisfree]

As already mentioned the top of the prop is in slightly less dense water being near the surface, if prop is deeper, as in a submarine for example, the the effect is said to be much reduced. I reckon what canal boaters consider to be prop walk is really the prop helix travelling forward when in reverse and hitting the swim and counter on one side but going under the swim on the other. That's why the effect seems to be reversed when near the bank, the helix is passing under the swim but bouncing off the bank or pilings and hitting the swim and counter and adding to the helix already hitting that side, this would happen if the bank was on the port side.

 

That's my theory anyway, to me paddlewheel effect is a red herring, it's far too weak to have the effect that we experience.

 

Edited to change 'starboard' to 'port'

Edited August 12, 2013 by nb Innisfree

[nicknorman]

As already mentioned the top of the prop is in slightly less dense water being near the surface

 

No its not, unless you mean because air gets entrained into the water with an inadequately submerged prop. The pressure in the water lower down is greater, the density is the same, or might be greater or lesser depending on the temperature gradient which in any case is neutralised by the stirring action of the prop.

 

Well in truth water is a bit compressible - a mile under water gives a little under 1% compression compared to the surface. Canals (and props) just aren't that deep!

[nb Innisfree]

No its not, unless you mean because air gets entrained into the water with an inadequately submerged prop. The pressure in the water lower down is greater, the density is the same, or might be greater or lesser depending on the temperature gradient which in any case is neutralised by the stirring action of the prop.

 

Well in truth water is a bit compressible - a mile under water gives a little under 1% compression compared to the surface. Canals (and props) just aren't that deep!

I agree about the water density that's why I think it's a red herring. My theory is similar to power yaw with an aeroplane where the prop helix travels in a corkscrew around the fuselage and strikes the rudder/tail on one side but goes under on the other. I just can't see the weak effect of paddlewheel moving a heavy boat in the amiunt it appears to.

[The Pipe]

Well bugger, it does not exist on my boats. Now I'm confused to why they kick to one side.

 

http://www.cruisingschool.co.uk/icc/prop%20walk.pdf

[Dalslandia]

I wrote to kockums Submarin company, and asked if drag is greater or not with big deap water, and higher pressure, they should know, in airplanes density have big effect on every thing, but as stated here the density of water hardly change. akademic question, but we better clear this mystery out. :-)

Edited August 12, 2013 by Dalslandia

[The Pipe]

I wrote to kockums Submarin company, and asked if drag is greater or not with big deap water, they should know, in airplanes density have big effect on every thing, but as stated here the density of water hardly change. akademic question, but we better clear this mystery out. :-)

There is no real difference between aerodynamics and hydrodynamics just multiply the effect by 800ish for water over air.

[The Pipe]

This is more logical explanation forn JayBee in the second post.

http://www.ybw.com/forums/showthread.php?341281-Prop-walk-Pysical-reason-why-does-this-happen/page5

[Dalslandia]

The biggest difference if speaking prop design, it is that in the air pressure and density follow each other, more or less, but not in the water, and boat props is very sensitive for cavitating, and that is effected very much of the pressure/dept.

others it work about the same.

[The Pipe]

The biggest difference if speaking prop design, it is that in the air pressure and density follow each other, more or less, but not in the water, and boat props is very sensitive for cavitating, and that is effected very much of the pressure/dept.

others it work about the same.

Usually on canal boats they do not cavitate it is more ventilation issues due to having the prop close to the surface. My understanding is that Kort propellers do not walk, I cannot remember where I got that from.

Aircraft propellers altho not my field of aviation the big issue is supersonic flows and in the case of helicopters keeping the blade tip speed below supersonic.

[Dalslandia]

I calculate / design airplane propellers, as a hobby. but have never calculated a boat prop, being a boat captain, and privat pilot, and home builder.

 

under Dalslandias prop is a big bent, u-shaped steel plate as prop guard, with that fitted there is no prop walk, but if sucked in some stones or debrie, it can damage the prop, so we made two big holes on each side, left and right, under the prop, so the stones can fall out, with this holes I have a slight prop walk, the shaft is also angled, so it might "help" some. ( Called p-factor on airplanes)

[Phil Ambrose]

Have been following this thread and have decided that I don't really care what causes Prop Walk, I just know it happens and know how to use it to my advantage.

Whatever it is I can make it work for me.

 

Phil

[Phil Ambrose]

Oh just thought of this, a lot of talk about the action of forces on swims etc etc, well my previous boat was a GRP cruiser and in common with all other boats on the Broads it had NO swim yet still had prop walk so that should give food for thought.

 

Phil

[howardang]

I calculate / design airplane propellers, as a hobby. but have never calculated a boat prop, being a boat captain, and privat pilot, and home builder.

under Dalslandias prop is a big bent, u-shaped steel plate as prop guard, with that fitted there is no prop walk, but if sucked in some stones or debrie, it can damage the prop, so we made two big holes on each side, left and right, under the prop, so the stones can fall out, with this holes I have a slight prop walk, the shaft is also angled, so it might "help" some. ( Called p-factor on airplanes)

Prop walk, transverse thrust, paddlewheel effect etc exists in a greater or lesser degree in all propeller driven vessels and is modified by the surrounding conditions- depth, bank effect, squat etc etc. it has existed since propellers were first invented and no amount of over analysis is going to change it. Just live with it, get to know your own vessel and use it to your advantage if possible, but above all, don't be obsessed by it:-) life's too short!

 

Howard

[Dalslandia]

Have been around and on boats my whole life, borne on a small island, traveled this canal for 40 years, some 60 000 locks, I know how different the boat behave from time to time, maybe 4 inch less water or boat fully loaded one day, very light load next day, and it turn and behave different.

This canal is "built" or more correct designed by the brother to John Ericson, that was one of the guys that made a very efficient propeller in later half of 1800 in England.

 

I know how much better the boat turn with the "schilling" type of rudder I designed then what it did with the simple steel plate rudder

 

Can go perfectly aligned into a lock with 2-2,5 knots of speed, and all of a sudden the boat is sucked into one side, I know that and I know why.

It is good to know both the theory and practical side.

 

I know if I miss the lock by a cm I woke up and don't make that misstake for the reminder of the locks, that day :-) We can do things better if we know how and why.

 

We live as long as we learn.

 

Not obsessed, just solving the world problem in the pond

[The Pipe]

Not a case of being obsessed but interested. Life without interest and risk is no life.

[Keeping Up]

Once as I came into a Thames lock and engaged reverse to stop the boat, the lock-keeper said to me "oh that's unusual on a modern boat, you must have a left-handed prop". This puzzled me because I have a right-handed prop, but sure enough the boat does usually behave as if it were left-handed (the bow turns naturally to starboard when going ahead, and the stern immediately swings to starboard when I engage reverse). It is the same in deep water and in shallow water; I have given up trying to explain it, and given up trying to work out if I have got my thinking back-to-front, I just remember it and make use of it whenever I can.

 

Similarly with the effect that pushes the stern away from the bank when stopping; I just make sure that the stern is heading rapidly towards the bank first so that as the boat stops moving forward it also stops moving sideways. The thing I find most interesting is the way the "cushion" of water moves along to the bow if you keep reversing so that the bow then swings outwards; a nuisance at times but useful if you've just dropped the crew off and now want to move away from the bank again.

[Dalslandia]

For those that wonder, a right hand prop, is turning clockwise (analog) when stand behind the boat or plane and have the prop wash in the face

 

What strange for me as an aero prop maker/designer, is that on a boat prop the surface facing the bow is called the back side on a boat propeller,

 

Just got a question if it is a built in heating system in the cusion on the bench, because it was so warm, I said yes and pointed at the sun, he was from Norway, maybe Bergen where the sun never shine.

 

Knowlege is so easy to carry.

Edited August 13, 2013 by Dalslandia

[nb Innisfree]

There is no real difference between aerodynamics and hydrodynamics just multiply the effect by 800ish for water over air.

One is wetter than the other...

[Dalslandia]

One is wetter than the other...

 

barly, at least not last summer

 

I got contact with a man on Kockums, he was a bit busy this week he said, but was happy to come back with his thoughts next week, the story is continuing

Edited August 13, 2013 by Dalslandia

[Phil Ambrose]

I thought the story has pretty much been done to death. Thread so far has been interesting but enough is enough.

To sum up, Prop walk exists, just learn to use it to your advantage, simples.

Phil