Prop walk

[JordieS]

Hello everyone,
I was just going through my powerboat handling book and I came against prop walk again but I started thinking more deeply about it.

I understand an inboard boat produces more thrust on one side of the propeller as the side of the propeller that is going down has a greater angle of attack. But what I don't understand is how prop walk occurs in outboards and sterndrives since the propellers are parallel to the relative waterflow (or angled straight if that makes more sense).

I would understand if the propeller of the outboard or sterndrive was half in and half out of the water but when it's completely submerged it makes no sense, wouldn't that mean the boat just rolls in the opposite direction of the propeller not cause it to walk sideways?

Think of it this way imagine holding a helicopter by it's rotor and when the engine is running the helicopter would spin around, not walk to one side although of course in real life the helicopters blades are spinning in air which has this effect to a certain degree, hence the tailrotor.

Sorry about the long post, just trying to make myself clear somewhat.

Jordie

[philSweet]

I understand an inboard boat produces more thrust on one side of the propeller as the side of the propeller that is going down has a greater angle of attack.

Are you sure you understand this? The side going down can produce more or less thrust than the side going up depending on the specifics of pitch and drop angle.(you can work it out for two cases, one has the pitch at radius r > drop angle and two has pitch < drop angle) It tends to be more of a vibration issue than a walk issue. Walk has more to do with the way the prop interacts with the hull. Meaning the rotation imparted by the prop can push on the hull. Top/bottom prop force inequality can also be a problem if the incoming stream is disturbed by the hull affecting the upper area of the prop more than the lower area. Basically the inlet condition is slower at the top (and might be angled laterally in a dual installation) comparded to the bottom giving the top a higher angle of attack.

A bit more about the first bit- the side going down is also travelling forwards through the water, the side going up is traveling back. The first order terms affecting angle of attack cancel, but there is a bit of nonlinearity left over if the pitch and drop angles are quite different. Solve the geometry and gin up a speadsheet to play around with it.

[gonzo]

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

[philSweet]

Gonzo, C'mon, water is damn near incompressible. You couldn't measure the difference. I've seen this written all over the place. The static pressure is different, but the density is nearly identical.

from wiki on water:

The bulk modulus of water is 2.2 GPa.[28] The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[28]

Every now and then wiki hands you just exactly what you want. It's then that I start to worry.

[JordieS]

I thought it could interact on the hull but how does that explain prop walk in sterndrives/outboards the water from the prop has nothing to hit in forward except for the ventilation/cavitation plate?

[daiquiri]

JordieS, the prop walk is imho most easily understood if you realize that there is not only a spiral (or swirling) wake behind a prop, but also a spiral inflow to the prop (though of smaller intensity), both induced by the prop rotation. Both of these flows contain a velocity component perpendicular to boats centreline, and interact with the hull by dragging the stern to the same side.

For example, a right-hand prop, when looked at from behind (stern) will create a flow velocity component towards starboard side (to the right), which will tend to drag the stern to the same side. The effect is then observable as if the bottom part of the prop "rolls" or "walks" on an imaginary ground beneath it.

All props have the spiral-like inflow, be it an inboard, an outboard engine or a sterdrive.

Cheers

[JordieS]

This is so confusing, I understand the spiral flow but I just don't understand really how these forces push the stern sideways. Is the walk caused by water coming up from the prop or inflow hitting the left side of the hull (for a RH prop) and pushing it to the right?

Thanks

[daiquiri]

A water flowing over a surface tends to drag the surface in the direction of the flow, because of the friction.
For the same reason your boat has a drag (or a resistance) when moving through the water.

[powerabout]

you will never use prop walk on a sterndrive but useful on a shaft drive
also when the prop drive is angled one side has more effective pitch than the other making un even force
also the lower part of the prop usually has clear water on either side of it left and right and the top doesnt or so close to the surface it does less effective work
and yes daiquiris post 6 makes a single screw put water down one side of the hull when in reverse

I would be interested to know if a duo prop still goes through the water on an angle like outboard/sterndrive or whether they go straight?

[powerabout]

Originally Posted by gonzo

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

so when I run a surfacing prop there is no prop walk?
did you just get home from the pub?

[Submarine Tom]

This could be debated all day and into the night but what really matters here is that the stern will tend to swing the same direction every time reverse is engaged.
This can be used to your advantage when maneuvering. Many choose to see this as an inconvenience when in fact it can be very useful. Who couldn't use a free stern thruster? As mentioned, it affects shaft drives much more than I/O and outboards, and even then, some more than others.

Indulge in your academics as you like, but where the rubber-hits-the-road, this is all that really matters.

-Tom

[JordieS]

Thanks everyone I'm starting to understand it now.

Jordie

[gonzo]

Powerabout: the density of the water is much higher than that of air. I don't understand your comment.

[powerabout]

what i'm pointing out is if a surfacing prop is run at propshaft level then I only have the lower blade and plenty of propwalk

[gonzo]

OK, so where is the disagreement?