Prop walk

[Frosty]

I always understood it like this. At the top of any prop is a flat plate if its an outboard or the bottom of the hull if its a normal drive. There is some thrust radiating out from the prop and when it squeezed between the vavitation plate or hull it has to go somewhere else so it goes to the side of continuing rotation. Hence prop walk changes with prop rotation.

So we can say from that that the closer is the prop is to the hull the more prop walk you will get.

Similarly the small plate of the ouitboards results in less walk than the conventional.

I agree entirely with Sub tom --dont fight it, use it.

[cyclops2]

The unequal fanning out of the water from the prop is VERY noticeable on my overpowered Chaparrel 186 SSI with a 5 L Mercruiser.

Results.
Foward slams the bow left against the dock.
Reverse shoves the stern against the dock. AND at the same time is pulling the bow away from the dock. Impossible for me to dock alone with the wrong wind direction.
Rudder is almost useless in reverse.

Only way to dock in a breeze is with a cellphone calling someone down to the dock. There is only 12" between our boats.

Have to get everything perfect going to the dock in foward ONLY.

[daiquiri]

Originally Posted by gonzo

The density of water is higher on the lower blades and also affects prop walk.

Gonzo, this is true only in terms of a nearly non-measurable difference. Water is, for all practical uses related to ship resistance and propulsors, nearly incompressible - meaning that it requires a really huge pressure to compress a given volume of water even by a very small amount.

To give you an order of magnitude of the pressures invloved, it requires a pressure differential of 215 atm to cause an increase of water density by just 1%.
By knowing this, you can understand that the pressure difference acting between the top and the bottom of any prop is simply too small to cause any measurable difference in density between the two points.

The only time the density can be accounted for is when dealing with aerated water. If there is a significant difference between the amount of dissolved air between the top and bottom blades, then the densities will be different at the two points and the top blades (where there's more air and hence the mixture is less dense) will experience a loss of both thrust and drag.
But the facts is - a prop walk occurs even when water is not aerated, so it certainly cannot be the main source of the problem.

To your defence, I can tell that even respectable authors sometimes stumble and fall on the same issue. Dave Gerr, for example, in his "Propeller Handbook" says (page 20):

"The reason (for prop walk) - in simple terms - is that the water at the bottom of the propeller is a bit denser and freer to flow (there is no hull above it) than at the top of the propeller."

Cheers.

[Jimboat]

Originally Posted by JordieS

...what I don't understand is how prop walk occurs in outboards and sterndrives since the propellers are parallel to the relative waterflow

Prop Walk is normally realized primarily with surfacing propellers.

Originally Posted by daiquiri

Water is, for all practical uses related to ship resistance and propulsors, nearly incompressible - meaning that it requires a really huge pressure to compress a given volume of water even by a very small amount.

Agreed!

[Frosty]

ALL props have prop walk.

[Submarine Tom]

Ya, Ya, but does anyone ever pick-up after them...?

-Tom

[Jimboat]

more on prop walk here