Prop walk. Pysical reason why does this happen

[Buck Turgidson]

The energy in the upper part of the helical stream is expended as it interferes with the boat's hull whereas the lower part is free to stream sideways producing thrust. The effect reduces with speed as the stream narrows.

 

[nigelmercier]

I posted a thread (?) on "why two tides per day?" and got equally unconvincing explanations!!!!!!!!!

If you post a link to it, I may have a good reply.

I don't buy the prop angle theory. Sail drives have a vertical prop and still exhibit prop walk.

They may be a nominal vertical, but it is never perfect. This is why the prop-walk on a sail drive is usually much less than with a shaft.

 

[Halo]

I posted a thread (?) on "why two tides per day?" and got equally unconvincing explanations!!!!!!!!!

Took me ages to get an answer on this one.
One tide is caused by the gravitational attraction of the moon acting on the ocean when it is closest to the moon durung the earth's daily rotation.
The other tide is caused by the gravitational attraction of the moon acting on the whole earth. The earth therefore gets a pulled towards the moon and away from the ocean on the side of the earth furthest from the moon. (Imagine a sphere coated in a layer of water if you jerked the sphere some of the water on the opposite side to the jerking action would get "left behind" causing a bulge)
This is a continuous process as the earth and moon are in constant orbit
There is a similar effect from the sun which gives rise to springs and neeps by its effects combining with the moon
Hence there are two standing waves at all times - one towards the moon and one away from it and the earth's rotation acts on these to give 2 tides per day.

 

[nigelmercier]

The other tide is caused by the gravitational attraction of the moon acting on the whole earth.

As the thread has drifted, here goes.

The above is one way of looking at it, but it may not satisfy the physicists. I prefer the explanation that the moon does not in fact orbit the earth! What actually happens is that the moon and the earth both orbit a point in space at their combined centre of mass, which is offset from the centre of the earth. It is this wobble that throws out the bulge on the far side.

 

[davidej]

Could be the effect of cavitation nearer the surface.

Seems to me to be the most likely explanation.

 

[davidej]

Took me ages to get an answer on this one.
One tide is caused by the gravitational attraction of the moon acting on the ocean when it is closest to the moon durung the earth's daily rotation.
The other tide is caused by the gravitational attraction of the moon acting on the whole earth. The earth therefore gets a pulled towards the moon and away from the ocean on the side of the earth furthest from the moon. (Imagine a sphere coated in a layer of water if you jerked the sphere some of the water on the opposite side to the jerking action would get "left behind" causing a bulge)
This is a continuous process as the earth and moon are in constant orbit
There is a similar effect from the sun which gives rise to springs and neeps by its effects combining with the moon
Hence there are two standing waves at all times - one towards the moon and one away from it and the earth's rotation acts on these to give 2 tides per day.

Ditto

 

[lw395]

The energy in the upper part of the helical stream is expended as it interferes with the boat's hull whereas the lower part is free to stream sideways producing thrust. The effect reduces with speed as the stream narrows.

My understanding is that propwalk is caused by the prop acting as a cross between a paddle wheel and a centrifuge, throwing some water out radially, as well as forwards or backwards.
This is particularly so when the boat is not moving and the prop is partially stalled.

The water thrown sidewise by the bottom of the prop reacts against the outside world and pushes the stern the other way.
The water pushed sidewise by the top of the prop pushes partly against the boat, so some of its thrust is cancelled by the reaction on the boat.
So the thrust from the bottom of the prop dominates.

I think this is consistent with prop walk being less on fin keel boats where there is very little to resist the flow of water across the bottom of the hull, and more on boats where (e.g) the prop shaft runs in a skeg or fin under the boat.

 

[Halo]

My understanding is that propwalk is caused by the prop acting as a cross between a paddle wheel and a centrifuge, throwing some water out radially, as well as forwards or backwards.
This is particularly so when the boat is not moving and the prop is partially stalled.

The water thrown sidewise by the bottom of the prop reacts against the outside world and pushes the stern the other way.
The water pushed sidewise by the top of the prop pushes partly against the boat, so some of its thrust is cancelled by the reaction on the boat.
So the thrust from the bottom of the prop dominates.

I think this is consistent with prop walk being less on fin keel boats where there is very little to resist the flow of water across the bottom of the hull, and more on boats where (e.g) the prop shaft runs in a skeg or fin under the boat.

This sound the most plausible to me
Most props seem to cause the seas surface to ripple and that would indicate there is potentially less I resistance on the up side than in the deeper water which cannot lift out of the way.

 

[Ex-SolentBoy]

Do submarines suffer from propwalk?

 

[onesea]

As the thread has drifted, here goes.

The above is one way of looking at it, but it may not satisfy the physicists. I prefer the explanation that the moon does not in fact orbit the earth! What actually happens is that the moon and the earth both orbit a point in space at their combined centre of mass, which is offset from the centre of the earth. It is this wobble that throws out the bulge on the far side.

Do not know if its true but I like that explanation, some how it works in my head. Although that is mainly holow so is not affect much by gravity

 

[John Barry]

Do not know if its true but I like that explanation, some how it works in my head. Although that is mainly holow so is not affect much by gravity

It's not true. It's a commonly quoted theory, but it's wrong.

Whilst the Earth-Moon system orbiting the barycentre, and centrefugal / centrepetal force is something you need to understand if you want to know why the Moon doesn't fall on our heads, it's not what causes tides.

There being two bulges is purely a gravitational effect.

Gravity is proportional to the mass of an objects divided by the square of the distance from it. This means that the Moon's gravity is more at a point directly below it than it is on a point at the centre of the Earth, and more there than at a point on the far side of the Earth. The difference between these forces (the differential force) is the force that causes the tides. It's about 1/10,000,000g when the Moon is directly at zenith or nadir, and it's about half that when the Moon is on the horizon. The same effect happens (of course) with the Sun, but it's about 46% of the size of the Moon's effect.

These differential forces (at an angle to the surface) produce tractive forces along the Earth's surface, which produces the two bulges.

You can check the numbers from the mass of the Moon, the distance to the surface of the Earth, and the distance to the Centre of the Earth.

You can have the rest of my lecture on tidal theory if you really want...

 

[Deleted member 36384]

This means that the Moon's gravity is more at a point directly below it than it is on a point at the centre of the Earth, .....

....... The same effect happens (of course) with the Sun, but it's about 46% of the size of the Moon's effect.


You can have the rest of my lecture on tidal theory if you really want...

If the earth and moon were gravitationally isolated from each other, would gravity of each mass just act toward the centre of each planet? Obviously they are not isolated so does the moons gravity act downward because the earth's gravity pulls it off from acting toward the centre of the moon. I am trying to get my head round this point.

 

[John Barry]

If the earth and moon were gravitationally isolated from each other, would gravity of each mass just act toward the centre of each planet? Obviously they are not isolated so does the moons gravity act downward because the earth's gravity pulls it off from acting toward the centre of the moon. I am trying to get my head round this point.

A body's gravity acts towards it's centre of mass.
The Moon's gravity act's towards the Moon's centre of mass.

Its effect at various points on the Earth is 1: directly towards the Moon's centre of mass, and 2: proportional to the Moon's mass divided by the distance from the Moon's centre of mass squared.

The reason we don't fly off towards the Moon is the scale of that the Moon's gravity is a ten millionth of the Eath's gravity. But is does make you lighter if the Moon is directly overhead - by a factor of 1 in 10,000,000.

 

[Halo]

It's not true. It's a commonly quoted theory, but it's wrong.

Whilst the Earth-Moon system orbiting the barycentre, and centrefugal / centrepetal force is something you need to understand if you want to know why the Moon doesn't fall on our heads, it's not what causes tides.

There being two bulges is purely a gravitational effect.

Gravity is proportional to the mass of an objects divided by the square of the distance from it. This means that the Moon's gravity is more at a point directly below it than it is on a point at the centre of the Earth, and more there than at a point on the far side of the Earth. The difference between these forces (the differential force) is the force that causes the tides. It's about 1/10,000,000g when the Moon is directly at zenith or nadir, and it's about half that when the Moon is on the horizon. The same effect happens (of course) with the Sun, but it's about 46% of the size of the Moon's effect.

These differential forces (at an angle to the surface) produce tractive forces along the Earth's surface, which produces the two bulges.

You can check the numbers from the mass of the Moon, the distance to the surface of the Earth, and the distance to the Centre of the Earth.

You can have the rest of my lecture on tidal theory if you really want...

You sound very well read on this but from your description I still cannot understand how you think gravitational forces put a bulge of water on the far side of the earth (as seen from the moon).

"never argue with old idiots they drag you down to thier level then beat you with experience"

 

[Sailfree]

Prop Walk is caused by the paddle wheel effect as someone has mentioned thats why it is more noticeable when boat is slow and engine revs high with prop effectively thashing the water.

When prop is effectively "scewing" itself forwards through the water there is no prop walk.

Bottom of the prop is whisking more dense water and freely . Top of the prop is whisking less dense water and only a smaller amount gets past the obstruction of the hull above to cause a side thrust.

Remember a Feeling 39 I tried one weekend by thrashing the prop you could turn through almost 90 deg - worked as good as a bow thruster!

 

[Polly1]

Do outboards get prop walk? There is no hull for them to interfere with

 

[ProMariner]

One problem with the hull as an end plate causing asymmetric water flow off the bottom of the prop theory: Long keel boats, with symmetrical prop aperture, would mean no prop wash. Right?

 

[JonJon]

Right I think I'm clear now ; propwalk is to do with the pull of the moon on the boat.

Thanks

 

[Plevier]

Actually very relevant in helicopters too.

W.

Yes, sorry, I was only talking about the fixed wing case.

 

[John Barry]

Thread drift

You sound very well read on this but from your description I still cannot understand how you think gravitational forces put a bulge of water on the far side of the earth (as seen from the moon).

"never argue with old idiots they drag you down to thier level then beat you with experience"

Gravitational attraction is Gm/d^2
From the Moon at the centre of the Earth this is 6.67E-11 * 7.35E22 / 384400000^2/9.8066
= 3.38E-6g
To the near side of the Earth this is 6.67E-11 * 7.35E22 / (384400000-6378137)^2/9.8066 = 3.498E-6g
To the far side it's 6.67E-11 * 7.35E22 / (384400000+6378137)^2/9.8066 = 3.274E-6g

The differential forces are therefore:
Moon side of Earth 3.498E-6 - 3.38E-6 = 1.15E-7g
Far side of Earth 3.274E-6 - 3.38E-6 = -1.10E-7g

The positive sign indicates that the force is towards the Moon (upwards as the Moon it at zenith)
The negative sign indicates that the force is away from the Moon (away from the Moon is still upwards as the Moon is at nadir)

With the Moon on the horizon the force is the same as the average, but it's at a slight angle (due to the Moon's horizontal parallax, about 1°), so the differential force is 3.38E-6 sin(1°) = 5.90E-8, and this force acts towards the centre of the Earth (a function of the direction of the parallax).

Between all these points the differential force is at an angle to the surface, and these forces divide into vertical and horizontal components, the horizontal (tractive) parts vectored towards the zenith and nadir poisitions. It's these tractive forces that move the water and create the two bulges.

However, this is all about the equilibrium tide, and the associated forces (the equilibrium tide is about the position with the Earth covered in water and the forces at balance). Getting from this to details of tidal prediction is quite a long haul, and I'm running out of electrons...

Nerd mode off. Or perhaps we need a new thread that's not about propwalk.