Hi Josh,
Prop wash is the disturbed, somewhat aerated,
water left behind the boat when motoring. It can
be a good place to troll for bluebacks in the spring. :)
Cheers, Russ
Sweet 35 mk-1
At 07:09 PM 18/05/2015, you wrote:
Rick, Is their such a thing as prop-wash? If so what is it?
Josh
On May 18, 2015 7:22 PM, "Rick Brass via
CnC-List" <<mailto:cnc-list@cnc-list.com>cnc-list@cnc-list.com> wrote:
Josh, the proper term for what you are
describing is torque steer, not prop wash.
Torque steer to port in forward is present on
all single screw boats with a RH prop, just as
prop walk to port in reverse. A boat with a LH
prop will torque steer to starboard, and prop walk to starboard.
Torque steer and prop walk are primarily cause
by the differential in water pressure between
the upper and lower blades as the prop rotates.
This generates a side thrust perpendicular to
the prop shaft. The amount of side thrust is
impacted by the diameter of the prop and the
pitch of the blades, and by the speed of
rotation of the prop. The more of each, the greater the thrust.
I recently changed from a 17x10 Martec to a 16
1/2x11 Gori that has noticeably thicker blades
with more pitch. I was surprised at the increase in torque steer and prop walk.
Torque steer is generally more pronounced than
prop walk because of the general greater engine
speed in forward gear though when backing at
low speeds the prop walk can generate a lot more
side thrust than the rudder can.
My friend has a new-to-her 29-2 with a 2gm13F
and the standard 14x9RH prop, and that boat prop
walks like a bitch until you get some aft way on the boat.
On power boats with outboards and out drives (no
rudder) the effect of torque steer is generally
compensated by the presence of a small skeg or
adjustable tab on the on the drive housing that
is adjusted to offset the effect of torque steer
and let the boat run in a straight line. Larger
boats with a shaft drive (like ours) have rudders that can compensate.
Twin screw boats typically have counter rotating
(1 RH and 1 LH) props so there is no torque
steer or prop walk with both engines running at
similar RPMs. Unfortunately this isnt always
true the 81 foot tour boat I work on has 2
300HP Volvo Penta outdrives and both are RH. The
torque steer is colossal, and makes handling the boat a constant struggle.
Rick Brass
Imzadi C&C 38 mk 2
la Belle Aurore C&C 25 mk1
Washington, NC
From: CnC-List
[mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Josh Muckley via CnC-List
Sent: Saturday, May 16, 2015 9:45 AM
To: C&C List
Cc: Josh Muckley
Subject: Re: Stus-List Prop Walk C&C 29-2
Sounds reasonable. To add on, would you agree
that the spinning column of water which gets
split by the rudder when going forward is then
responsible for the phenomenon called
prop-wash? I feel some pretty substantial wash
and a pretty strong pull to port when full
throttle but a reasonable and balanced helm when sailing.
Josh Muckley
S/V Sea Hawk
1989 C&C 37+
Solomons, MD
On May 16, 2015 2:08 AM, "Knowles Rich via
CnC-List" <<mailto:cnc-list@cnc-list.com>cnc-list@cnc-list.com> wrote:
Well, this might be just the right time to float
a theory Ive had for some time about prop walk. Here it is:
When the propellor is spinning, it produces a
rotating horizontal column or spinning cylinder
of water molecules which move away from the
propellor along its axis. When the boat is going
forward, this rotating column is left in the
wake and, other than being split equally by the
rudder as the boat moves forward, the spinning
column has little to no effect on the
directional performance of the boat as it is
left behind in the wake and gradually dissipates.
When the propellor is put in reverse, forcing
water to the front of the boat as it pulls the
hull backwards, the column of spinning water
leaving the prop is no longer free to dissipate
in the wake, but encounters the hull of the boat
immediately in front of the propellor. If you
consider the column of water as a spinning
cylinder made up of molecules of water, the
outer wall of the cylinder striking the hull
will cause it to roll up the side of the boat
away from the keel and toward the surface, and
the spinning molecules in the interior of the
cylinder will be directed away from the centre
line of the hull and off to the side.
To see this in action, put your stationary boat
in reverse and note on which side of the boat
the water is agitated. If you have a right
handed prop that turns left when in reverse, the
column of water will be directed to the
starboard side of the boat and will therefore
push the stern of the boat to port. If you have
a left handed propellor that turns to the right
in reverse, the column of water will be directed
to the port or left side of the boat pushing the
stern to starboard. Thus the much cursed and
very useful affect known as prop walk.
Just my theory, but it seems to work for me. Comments welcomed.
Cheers
Rich
Rich Knowles
Nanaimo, BC
INDIGO LF38
Almost sold (really!) in Halifax, NS.
On May 15, 2015, at 07:51, Jean-Francois J
Rivard via CnC-List
<<mailto:cnc-list@cnc-list.com>cnc-list@cnc-list.com> wrote:
Prop walk is a function of the asymmetrical
thrust produced by the angle of the shaft /
rotational angle of the blades vs the water
surface. The more downward angle on the prop /
the longer the blade has to travel going from
bottom towards the surface compared to the blade
that goes from top to bottom. Longer path =
more thrust per rotation for that blade =
unequal thrust.. Pretty simple concept.
See here:
<http://www.castlemarine.co.uk/propwalk.pdf>http://www.castlemarine.co.uk/propwalk.pdf
Increasing either pitch or diameter affects prop
walk and so does prop design. While efficient
for sailing due to a typically smaller diameter,
the Campbell Sailer is known for pretty bad prop walk..
With a little practice prop walk can be useful
when you need to pivot or crab sideways. I use
a Martec folder which is also known for dismal
reverse / prop walk. For what I do, neither
bothers me at all. When I don't want prop walk
I just give it a smooth burst of reverse thrust
then put it in neutral / glide precisely where I
want. (I always back into my slip)
Good luck,
-Francois Rivard
1990 34+ "Take Five"
Lake Lanier, GA.
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