What Is Prop Walk?

[howardang]

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

If you look over the side when you go astern the wedge of water between the bank and the boat forces the stern out. It' s nothing to do with transverse thrust, paddle wheel effect, prop walk etc.

 

Howard

[ditchcrawler]

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.

I think this is the correct answer

[Chris Pink]

It would take a braver man than me to argue with Captain RW Rowe, FNI

[howardang]

It would take a braver man than me to argue with Captain RW Rowe, FNI

Bob Rowe was a recognised authority on ship handling training (he ran the manned model course at Warsash Maritime Academy until he died a few years ago) The explanation of transverse thrust above is what has been taught for many years to merchant navy officers and masters.

 

Howard

[RLWP]

Bob Rowe was a recognised authority on ship handling training (he ran the manned model course at Warsash Maritime Academy until he died a few years ago) The explanation of transverse thrust above is what has been taught for many years to merchant navy officers and masters.

 

Howard

 

 

When we went on the cruise across the Caledonian Canal, the Captain had served time on many ships, including tankers

 

He was surprised to find that canals are different - 'they've got currents' he said ' and wind'.

 

So, I'm sure Captain RW Rowe is a clever chap, until I've heard an explanation from a Number One who has spent years on the Jam 'Ole run I remain to be convinced..

 

Richard

 

Not really

 

[Dalslandia]

That Bruntons AutoProp will highly reduce prop walk That I can understand,

The axiom prop, I can beleve it is reducing prop walk,

a normal prop in reverse will be more of a "paddle" then a propeller, made to give good thrust forward,

the axiom will be equal good or bad both ways, I can't see it will be very efficient, but I have not stydyed it very careful. but it does look like a good bow thruster prop, something like one I designed for my boat years ago.

I wish it was easier to post a photo on here ...

Edited July 30, 2013 by Dalslandia

[by'eck]

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.

 

There is more than one reason for prop walk which can be neutralised or even reversed by shallow waters. Since its most noticeable at slow speeds in reverse though, I think the good Captain's explanation probably the best.

 

[Iain_S]

No.2 is reverse bank effect. The prop is pushing water under and to the sides of the boat. On the bankside, the water builds up more than on the offside, and the slightly higher level pushes the stern out. Most apparent when the boat is stationary (or almost so). If the boat is moving, bank effect (cf. Bernoulli effect) pulls the boat into the bank. This one is due to the reduction in pressure caused by the higher velocity of the water between the boat and the bank.

 

Iain

[by'eck]

No.2 is reverse bank effect. The prop is pushing water under and to the sides of the boat. On the bankside, the water builds up more than on the offside, and the slightly higher level pushes the stern out. Most apparent when the boat is stationary (or almost so). If the boat is moving, bank effect (cf. Bernoulli effect) pulls the boat into the bank. This one is due to the reduction in pressure caused by the higher velocity of the water between the boat and the bank.

 

Iain

 

Don't single handers know it

[bizzard]

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.

[Chris Pink]

Maybe they've been to Venice.

[blackrose]

Propwalk? I thought that was when someone nicked your propeller.

[enigma]

The theory of Prop Walk is here

 

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

 

Enigma

[Ships Cat]

Propwalk is why propeller aircraft need chocks.

 

And why they are not taken away until after the engine is started so the pilot can gain some control over the pitch.

 

What's the NB equivalent of chocks, lol?

[howardang]

The theory of Prop Walk is here

 

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

 

Enigma

This was proposed as an explanation in a similar discussion here a number of years ago.

This theory is not correct because it makes the point that if the prop shaft is horizontal there is no transverse thrust which is obviously not correct. See post 14 for a clear description of what causes prop walk or transverse thrust.

 

Howard

[RLWP]

Propwalk is why propeller aircraft need chocks.

 

And why they are not taken away until after the engine is started so the pilot can gain some control over the pitch.

 

What's the NB equivalent of chocks, lol?

 

Mooring lines when I'm testing gearboxes

 

Richard

[Timleech]

This was proposed as an explanation in a similar discussion here a number of years ago.

This theory is not correct because it makes the point that if the prop shaft is horizontal there is no transverse thrust which is obviously not correct. See post 14 for a clear description of what causes prop walk or transverse thrust.

 

Howard

 

I reckon there are several causes, including the simple paddlewheel effect which some seem keen to dismiss.

 

Tim

[Dalslandia]

Propwalk is why propeller aircraft need chocks.

 

And why they are not taken away until after the engine is started so the pilot can gain some control over the pitch.

 

What's the NB equivalent of chocks, lol?

Ropes?

 

Jan Captain/pilot

 

Ps. some use the tow hook at the stearn for mooring the airplane under warm up/run up (checking engine)

Edited August 4, 2013 by Dalslandia

[Dalslandia]

 

I reckon there are several causes, including the simple paddlewheel effect which some seem keen to dismiss.

 

Tim

 

The side effect, or paddle effect is from the drag in the water of the propeller blade, the drag is at least 2, induced drag and profile drag, and the denser water give more thrust and drag then the less dens water closer the surface.

the drag is greater and thrust less in reverse with most normal props

 

the up going blade also build up a pressure between the prop and bottom of the boat, even if the bottom is flat, but more so i think if the bottom is angled, there will be a triangle of higher pressure water on the side the prop blade turnes up toward the bottom, this and the paddle effect will force the stearn in opposit direction of the pressure. this is my picture of it anyway,

 

the if the prop shaft is angled the blade will have different angle of attack going up and down, here the shape of the hull also direct the water stream to the propeller in different angles, but maybe mostly going a head.

 

Jan

www.jcpropellerdesign.com

[nicknorman]

... the less dens water closer the surface.

 

 

Jan

www.jcpropellerdesign.com

But the problem with that is that the water lower down is not denser. Water is effectively incompressible and so the density is independent of pressure. Its true that water density is affected by temperature a bit, but surface water vs water 3 ft down may be colder or warmer, depending on the weather. Once the prop is turning, its all mixed up so there can't be a temperature related density difference between water at the bottom and top of the prop.

 

If you want to say the water is less dense due to entrained air, then maybe but not in a well submerged prop.

Edited August 4, 2013 by nicknorman

[Dalslandia]

You are right, wrong choice of word from my side. but still the water pressure is higher deeper down, the static pressure + the dynamic pressure is what gives the effect of the propeller shape. in form of thrust and drag.

 

To much air-head today, as a a side note air is said to be incompressible at speeds below Mach 0,8 So I should know better.

 

have to demand § 2 :-)

Edited August 4, 2013 by Dalslandia

[nicknorman]

You are right, wrong choice of word from my side. but still the water pressure is higher deeper down, the static pressure + the dynamic pressure is what gives the effect of the propeller shape. in form of thrust and drag.

 

To much air-head today, as a a side note air is said to be incompressible at speeds below Mach 0,8 So I should know better.

 

have to demand § 2 :-)

Yes air is considered incompressible below M0.8 but only when its uncontained. You can still compress it into a tyre going at only 30 mph!

 

But I don't see how the higher pressure, which acts all around the lower blade - including both faces - has any effect on the thrust compared to the upper blade?

[Dalslandia]

I know you would say that air can be compressed ...

 

Who don't know that?

 

as I said static + dynamic pressure is what gives the effect we want and some that we don't want from motion of the boat and propeller, the pressure increase with 1 hg/cm2 per meter water hight. so if the propeller diameter is the same as the distance from water line to to upper tip of the propeller, at the bottom the difference is significant. you figur it out ...

[nicknorman]

I know you would say that air can be compressed ...

 

Who don't know that?

 

as I said static + dynamic pressure is what gives the effect we want and some that we don't want from motion of the boat and propeller, the pressure increase with 1 hg/cm2 per meter water hight. so if the propeller diameter is the same as the distance from water line to to upper tip of the propeller, at the bottom the difference is significant. you figur it out ...

I agree that there is a big increase in pressure for a small increase in depth, but I don't see how this translates into a different amount of thrust (and hence drag) at the top and bottom of the propellor.

 

Aerodynamic lift theories involving Bernoulli, faster air on top of the wing etc are these days in disrepute and the best way to consider how a wing works is "by pushing air down". Similarly a boat propellor works by "pushing water backwards" and if the density is the same all round, the thrust will be the same all round regardless of the pressure differences caused by depth. There may be differences at the top because water can slide back at the top being less affected by viscous drag (because there is no water above it) but I don't think that is the effect you are suggesting.

[Biggles]

My opinion to the OP. It can be your best friend or worst enemy. I always try to work with it and plan ahead to that end. Don't always work though.