Perhaps a bit more food for thought?...

There are several effects that all contribute to prop walk (and I am
assured by an ex-Navy acquaintance that even ships with a horizontal
prop shaft still have prop walk). The one noted below (from Chapman)
may not be the best to use for THIS case.

Chapman's explanation here uses the incline of the prop shaft relative
to the flow of water past the blades. I believe this is meant to be
the angle between the inclined shaft and the HORIZONTAL flow of water
past the prop caused by the (forward) motion of the boat through the
water. (And this is confirmed by his later paragraph that explains why
engines are mounted lower to reduce this angle between the shaft and
(horizontal) flow of water past the prop.)

As the boat is jammed against the dock and not able to move through the
water there will be no horizontal flow of water due to forward motion.
The only flow past the prop will therefore probably be a flow parallel
to the shaft caused by the blades pushing the water in that direction
(possibly modified slightly by hull effects) and therefore the angle of
the ascending blade and the descending blade relative to this flow is
more nearly equal. In this case the difference in thrust from an
"ascending" vs a "descending" blade is probably minor and in fact the
concept of "ascending" and "descending" loses meaning if not referenced
to the horizontal in this case perhaps.

However the rotation of the prop is causing a swirling effect of the
water leaving the prop and the water leaving the ascending blade (on the
port side of the boat) may produce more push on the port side of the
hull than does the water swirling down from the descending blade (on the
starboard side) creating a net push on the port aft end of the boat,
adding to the turning effect to starboard. Note that if a single rudder
is mounted directly behind the prop the water hitting each side is
probably pretty equal - rather it is the water hitting whateve hull is
left in the way that probably has the greater effect and the water
corkscrewing away from the descending blade on the starboard side misses
the hull.

I have even heard the explanation that since the water is more dense at
the bottom of the rotation than at the top, the blade gets more "bight"
at the bottom of its swing and "rolls" the stern to starboard. Surely
this density difference is small so this contribution to the overall
effect from this must also be a minor part.

Dave

Gould 0738 wrote:

"T-boned to a dock"?

Assumptions:

1. You are describing a boat with a rh prop.

2. "T-boned to a dock" means the stem is jammed against the dock to prevent
forward movement.......(until the stern swings far enough that the angle is no
longer sufficient to brake the boat).

With those assumptions...........



Q: same question but rudder centered?

A: the stern will move to port due to prop walk

Incorrect. Here are some excerpts from my old copy of Chapman's. We can
probably assume that propellers still behave the way they did 20 years ago.

"Another factor also effects a boat's readtion to propeller rotation. While
this factgor is sometimes referred to as 'sideways blade pressure' it is more
properly an 'unequal blade thrust', exerted by the ascending and descending
blades of the propeller. Figure 709, top.

Here we are looking at the starboard side of a propeller shaft, inclined, as
most shafts are, at a significant angle to the water's surface and the the flow
of water past the blades. The actual pitch of the blades as manufactured, of
course, is the same, but the water flows diagonally across the plane in which
the blades revolve.

Figure 709 shows clearly how the effectof this is to increase the pitch of the
descending starboard blade, (right hand propeller) as compared with the
ascending port blade, when considered relative to the direction of water flow
past the propeller.

The importance of this factor is reduced as the shaft angle is decreased, and
naval architects sometimes take pains to have the engine installed as low as
possible to keep the shaft nearly parallel to the water's surface and to the
flow of water past the blades. This contributes to greater propeller eficiency,
and is a factor worth considering if it is consistent with other design
requirements. Once a boat is built, shaft angle is difficult, usually
impossible, to modify.

The relatively greater blade pitch on the starboard side creates a stronger
thrust on this side, causing the bow to turn to port.
As far as this single factor is concerned, THE STERN OF A SINGLE-SCREW BOAT
WITH A RIGHT-HAND PROPELLER
THUS NATURALLY TENDS TO GO TO STARBOARD WHEN THE PROPELLER IS GOING AHEAD, AND
TO PORT WHEN IT IS REVERSING."