Subject: Which way does a boat turn?
From: "Scott Vernon"

I would guess yes. But I don't know. And I was picturing a side wheeler.

Sv

Ok, continue to picture a sidewheeler.
Now, in hopes of further clarifying what I'm trying to describe, the
paddlewheel is 6' in diameter and you totally submerge it to a point, say, 6"
beneath the water.
Your boat is floating in 200' of water with no land in sight (i.e., you've just
got water with the earth beneath it and water with air, 6" above it).
Looking at the "wheel" from the side and starting at a rotation angle of 090,
as the blade of the wheel rotates clockwise to 180 deg, it is pushing down and
to the left, against a solid non compressible column of water, 200ish feet deep
(it's working as efficiently as it can) and pulling the boat to your right.
Now look at the blades in the opposite quadrant (270 - 360). Starting at 270
and rotating towards 360, the blades are pushing up and to the right, against
........ a 3' 6" column of water with nothing but good old compressible air
above. What happens?
Basically the blades lift that column of water and throw it away into the air,
losing a high percentage of their efficiency to pull down and to the left,
compared to their opposite quadrant.
If you look at 000 - 090, compared to 180 -270, you will see a different set of
comparisons, but I'd bet the net result would still be a greater pull/push to
the right.
Finally, go to a boatyard and look at a power boat on the "hard". Stand in
front of the propellor and pick one blade. Assuming RH, rotate the blade
counter clockwise and as you do, sight 90 deg to the blade pitch and visualize
the direction that the blade will "push" the water. I think you notice that the
blade will be pushing water up, on it's upward swing (into the air), thus
losing efficiency and the net effect will be a pull to the right (your left
standing in front of the prop) .......propwalk.
I don't doubt there are some possible flaws in this description, but it's how I
visualize the overall effect and cause of propwalk.
BTW, propwalk is just as important with twin screw. It's why a twin screw with
inboard turning props handles so much differently as one with outboard turning
props.

Shen