Prop walk. Pysical reason why does this happen

U

Uricanejack

Well-Known Member
Joined
22 Oct 2012
Messages
3,750
Visit site
Prop walk. Pysical reason why does this happen. I thought I'd ask just for fun.
In all the books I have ever read on sailing, boating, shipping, flying, engineering.
I have never read a physical reason why.
If a prop was only half in the water it would be obvious but it still happens when the whole prop is submerged.
We all know a conventional right hand screw. will walk the stern to port when going astern. and the Bow will go to starboard. Why?:D
The simple explanations in boating books refer to paddlewheel. so ok its a paddle wheel the bottom of the wheel pushes the water to starboard and the stern goes to port. But shurle the top of the padle wheel if it is in the water counters this?
Prop walk happens going forward as well we just don't notice it as much due to the ability to counter with rudder.
It happens when taking of in air planes you have to apply starboard rudder to go straight.:D
 
NealB said:
The density of the water increases with depth....so same idea as your mention of half-submerged porp.
Click to expand...

No marks for your physics. The pressure increases with depth, the density remains constant. Even if you're a porp-oise :)

Fair point, Uricanejack, I'd wondered about that, too. The 'explanations' commonly given are too superficial to warrant the effect.
 
NealB said:
The density of the water increases with depth....so same idea as your mention of half-submerged porp.
Click to expand...

Water is essentially an incompressible fluid and pressure differential isn't that great. I believe that it is due to interference from the hull. The prop is running in clear water on the bottom half of the turn. However, as it rises on the other side it nears the hull.

I've never seen a detailed explanation but it sounds reasonable that the approaching blade will produce increased water pressure at right angles to the shaft as it swings around (as well as producing forward thrust). The pressure dissipates everywhere apart from when the blade approaches the hull. That sets up an imbalance so that side force is greater when a blade is at the bottom of its swing.

On top of that, the prop is probably running at an angle to the surface and that must have an effect as well.

Badly explained but I hope I'm getting the general idea across. Just my thoughts though but a good point. Hope someone comes up with the correct explanation.
 
Last edited:
Aha.....I knew I hadn't made it up myself!

For decades, I've treated my various copies of Eric Hiscock's 'Cruising under sail' as my first source of information on anything to do with sailing (Ok.....ok.....motoring, in this particular case!).

On page 251 of my 1981 edition, Hiscock writes, "any propeller has a turning effect on a vessel owing to its lower half working in denser water than its upper half".

Oh dear...can I not trust him on anything now.....was he a charletan? Please.....do not answer that one!
 
Prop Walk - offset outboard

I get noticeable prop walk on my Jag 25, which has an offset to port outboard. My thought is that it is to flow of water off the prop impacting on the rudder causes the boat to turn bow to port in reverse. Any thoughts?
 
The reason is something called precession.

Studied this during my engineering degree. Its a phenomena of gyroscopes.

An electric motor and flywheel, were mounted horizontally, and fitted to the top of a pole. When the electric motor was running at full speed, we pushed a ruler against the side of it. Instead of moving away, it moved sideways.
The next part of the experiment was to hang weights under the motor at the outside edge of the arm. Instead of collapsing downwards, the motor began to rotate around the vertical pole. This is called precession.

Also, the speed of rotation was proportional to the amount of weights we hung.

Take the results of this experiment and you can apply it to engines, motors, propellers etc.

For example when you drive your car round a left hand bend, the gyroscopic force of the engine flywheel is trying to push the car down, and on a right hand bend it is trying to push the car up.

On single engined aeroplanes this is much more pronounced. twin, and four engined planes are much more stable, because the left hand and right hand engines rotate in the opposite direction to counteract this effect.

On a power boat, the up - down motion will do the same thing.

I'm afraid I can't answer the reason why gyroscopes do this, but I think its a fascinating phenomena.

(OK I'm a geek)
 
Isnt it because of pressure? More pressure on the lower blade so more force. Prop pushes sideways on water, water pushes back. More push on the lower blade. I am not convinced by my own explanation however.
 
macd said:
No marks for your physics.
Click to expand...
Or should that be pysics? ;)

Uricanejack said:
We all know a conventional right hand screw. will walk the stern to port when going astern....
Click to expand...
No, we don't know that, the only way to be sure is to test it.

I've only found one plausible source for the cause, see here.

[Later] see post #42, much better.
 
Last edited:
panthablue said:
The reason is something called precession.

Studied this during my engineering degree. Its a phenomena of gyroscopes.

An electric motor and flywheel, were mounted horizontally, and fitted to the top of a pole. When the electric motor was running at full speed, we pushed a ruler against the side of it. Instead of moving away, it moved sideways.
The next part of the experiment was to hang weights under the motor at the outside edge of the arm. Instead of collapsing downwards, the motor began to rotate around the vertical pole. This is called precession.

Also, the speed of rotation was proportional to the amount of weights we hung.

Take the results of this experiment and you can apply it to engines, motors, propellers etc.

For example when you drive your car round a left hand bend, the gyroscopic force of the engine flywheel is trying to push the car down, and on a right hand bend it is trying to push the car up.

On single engined aeroplanes this is much more pronounced. twin, and four engined planes are much more stable, because the left hand and right hand engines rotate in the opposite direction to counteract this effect.

On a power boat, the up - down motion will do the same thing.

I'm afraid I can't answer the reason why gyroscopes do this, but I think its a fascinating phenomena.

(OK I'm a geek)
Click to expand...

I'm sorry I think this is rather misleading.
In aircraft, gyroscopic precession is only significant to tailwheel aircraft, at the moment when you lift the tail during the take off run. Precession is the characteristic whereby if you consider a force applied to the edge of a spinning disk parallel with the axis of rotation, it's effect is seen displaced by 90 degress in the direction of rotation. When you lift the tail it's like pushing forward at the top of the rotating propeller "disk" and the result is a strong yaw force to left or right depending on sense of rotation. It is a very noticeable effect.
(You can get a lesser effect lifting the nose of a tricycle undercarriage aircraft at lift off but the pitch change is less and the airspeed is higher so it's easier to control.)
The tendency to turn other than in these circumstances is nothing to do with precession. It is mainly due to the corkscrewing slipstream from the prop striking one side of the tailfin. It's most noticeable climbing when you have high power and low airspeed. A second factor on the ground is increased drag on one wheel due to increased load as a result of torque reaction to the rotating propeller.
I don't think precession has any relevance to prop walk in boats. I understand that to be due to water flow over the bottom of the hull but I'll leave someone else to explain that in detail.

Edit - just read nigelmercier's link, yes assymetric blade effect can be a factor in aircraft in climb too.
 
Last edited:
I agree it's not precession or similar. If you put a yacht engine in astern, especially with the boat tied up, prop-wash emerges very visibly to one side of the hull.
My theory is that this is due to interference between the hull and the rotating body of water generated by the prop. this bias produces a turning effect.
In forward gear, the hull interferes much less with the prop wash, which escapes more or less in line with the hull, hence less turning effect.
I can accept that prop-shaft down-angle also plays a part.
 
Precession is what allows a motorcycle to turn (by countersteering). Can't see the relevance to prop-walk unless you have a very flexible coupling :)

Jag25: as I remember Wiki explains two tides per day quite well. One high tide when a point is closest to the sun, another when it's furthest away.
 
I don't buy the prop angle theory. Sail drives have a vertical prop and still exhibit prop walk.

Newton - to every action there is an equal and opposite reaction.

The prop accelerates an amount of water through an increase in speed. The opposite reaction sends the boat forward or backward. The stream of water coming out of the prop follows a helical path, horizontal and rotational, due to the shape and motion of the prop. In forward gear it is also accelerating the water over the rudder so you have immediate control of this. In astern there is no rudder control until the rudder itself is moving through the water. So the opposite reaction to the direction of rotation of the column of water coming off the prop rotates the vessel.
 
Remember the shallow water effect also, I have a 27" LH prop and 3-1 reduction, when in less than say 2 ft of water below the prop I cannot get the boat to port. Sometimes called 'wheeling', as if the prop were a wheel on the ground.
 
You must log in or register to reply here.

Other threads that may be of interest

ianc1200
Sureseal shaft seal.
2
Replies
20
Views
532
ianc1200
ianc1200
Y
The Seafarer 700 and NASA clipper sounders saga
Replies
10
Views
442
Dellquay13
Dellquay13

Share this page

Top