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 ----- Original Message ----- 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
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