charlie, if spiral prop wash is the reason for p-thrust ever wonder why a tail
dragger with the tail down experiences so much more sidesways turning than the
same aircraft at the same speed with the tail horizontal?

The "p-factor," of the upcoming and downgoing propeller blades having
different angles of attack, is the least signifcant reason for asymmetric
propeller thrust, although the most often touted. I came across a little
article in Flying years ago which did the math, and the effect is very
small. There are something like five reasons that a tail-dragger will yaw
one way, the p-factor being only one.

In my previous boat, Stella B, (www.ctlow.ca/StellaB/StellaB.html), I had
about the same asymmetric thrust whether the outdrive was trimmed in or out.
So much for prop angle.

The main effect is from the spiral prop wash. In reverse, where asymmetric
thrust is virtually always more pronounced, the top half of the prop wash
vortex strikes the hull, and pushes it sideways. The effect will vary
depending on the underwater hull shape, the angle of the prop shaft, design
of prop ...

So, a right hand propeller in reverse turns counter-clockwise. The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.

I hate to say "end of discussion," because that sounds arrogant, and I still
have things to learn, but I am very sure that for most practical purposes,
that is it.

Charles

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Charles T. Low
- remove "UN"
www.boatdocking.com/BDPhoto.html - Photo Contest
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

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"trainfan1" wrote in message
...

Using your theory, Steve, the stern would walk the same way whether the
prop was a left hand or right hand wheel... and this is not the case.

I have always been under the impression, on straight inboards, that the
lower half of the propellor, the arc furthest from the boat/hull
surface, is the one that does the most work/thrust (eg. surface piercing
drives, etc.). The prop has better "bite" in the less turbulent water
away from the hull. This has been my experience too...

Correct Craft & Century inboards: RH prop, walks to port in reverse,
favors left turns in forward... (the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

Mastercraft inboard: LH prop, walks to starboard in reverse, favors
right turns in forward. (again, the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

The rotational force of the shaft/prop is transferred primarily to the
attitude of the boat on a single screw craft.

Rob