On 25 Mar 2004 15:17:47 GMT, (JAXAshby) wrote:
it is caused by asymetrical thrust of an angled prop shaft. When backing up the
blade coming up to the hull has a much greater "angle of attack" than the blade
going down from the hull. Thus more thrust on one side than the other.

(there is also asym thrust when going forward, but the rudder can compensate
for this as long as the prop is *pushing* water over the rudder, or the boat is
moving forward)

===========================

OK, the implication of your assertion is that a prop shaft parallel to
the water line, and with little or no hull/aperature entrapment, will
exhibit little or no prop walk. Many full keel lobster boat and
trawler types come close to meeting that criteria.

Does anyone know if that holds up to real world experience?

OK, the implication of your assertion is that a prop shaft parallel to
the water line, and with little or no hull/aperature entrapment, will
exhibit little or no prop walk. Many full keel lobster boat and
trawler types come close to meeting that criteria.

Does anyone know if that holds up to real world experience?


A prop is shaped to move water most efficiently when in forward gear. This is
one reason that it takes more rpm to achieve a given speed in reverse. Some
props are worse than others......(folding props under some sailboats are a
total joke in reverse and you need to put the brakes on pretty early- even at
close to zero knots).

Twin screw boaters have experienced the phenomenon where the reverse wheel
requires a little more throttle than the forward to achieve an almost perfect
pivot.

When a single screw is in reverse, the stern will tend to follow the direction
of the top blade of the prop rotation, not the bottom. Example, a rh propeller
turns clockwise. Viewed from astern, the top blade goes to starboard and the
bottom blade to port when the vessel is in forward gear.

When in reverse, the top blade is moving to port and the bottom blade to
starboard. RH prop vessels routinely back to port.

Prop walk is always present, whether in forward gear or reverse. Yes, the
amount of angle on the prop shaft will increase the amount of prop walk. Prop
walk is not so much a problem in forward because the keel and rudder apply
greater directional influence than the side thrust of the prop.

After a while, we single screw boaters (yes, my wife knew all about that
deficiency when she married me)...learn to use a balance between prop walk and
rudder to steer in reverse. For example, my boat tends to back to starboard. To
back to port, we need a full left rudder and even then we won't start moving to
port until we pick up a bit of speed and the pressure of the water flowing over
the rudder is greater than the pressure produced by the prop walk. To move more
quickly to port, (once we have sternway established), we can take the engine
out of gear, momentarily, so the rudder isn't fighting the prop.

tern will tend to follow the direction
of the top blade of the prop rotation, not the bottom.

yes, that is true. I misspoke by trying to remember which direction my prop
blades are twisted rather than remember which way my engine turns. sorry.