My understanding, also. Gary S/V Expresso ~~~~~~~_/)~~~~~~
On Tue, May 19, 2015 at 6:49 PM, Chuck S via CnC-List <cnc-list@cnc-list.com > wrote: > I read somewhere the *primary cause of prop walk is the shaft angle*. > Sail drives and outboards have zero prop walk because the shaft is > straight. > > Chuck > Resolute > 1990 C&C 34R > Broad Creek, Magothy River, Md > > ------------------------------ > *From: *"Rick Brass via CnC-List" <cnc-list@cnc-list.com> > *To: *cnc-list@cnc-list.com > *Cc: *"Rick Brass" <rickbr...@earthlink.net> > *Sent: *Monday, May 18, 2015 7:22:08 PM > *Subject: *Re: Stus-List Prop Walk C&C 29-2 > > 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 isn’t 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" < > cnc-list@cnc-list.com> wrote: > > Well, this might be just the right time to float a theory I’ve 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 < > 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 > > 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. > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > > > > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > > > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > >
_______________________________________________ Email address: CnC-List@cnc-list.com To change your list preferences, including unsubscribing -- go to the bottom of page at: http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com
