Sail drives have both prop walk and torque steer, as do outboards. With an outboard the tendency is to turn the motor off center slightly so the thrust offsets the prop walk. And see my comment about outboards having a tab to compensate for torque steer; on a Mercury engine or Mercruiser outdrive the tab is usually one of the zincs that you adjust to some angle off center.
Try an experiment next time you are in your dinghy. Put the outboard in forward gear at idle speed, loosen the bolt that resists rotation of the motor, and then let go of the steering handle. If you have a RH prop, the handle will move to starboard. In reverse, the handle will drift to port. The 72 foot sail training schooner Jeanie B has a 671 Detroit with an absolutely horizontal shaft driving a 4 blade prop (30x24, IIRC) in an aperture. Even at 80,000 pounds and having a full keel, she prop walks to port. That, and the occasional breast line used as a spring, is how we bring the stern over to the dock when we come back to the pier after a sail. Having a prop shaft that is not along the centerline, but angled slightly to the side – which is pretty common - can either enhance prop walk or oppose it, depending on which side the shaft is angled toward and whether you have a RH or LH prop rotation. As far as having the prop shaft angled downward, I can see two things happening as the angle increases: 1) The pressure differential between the upper and lower blades decreases, as does the vertical distance between the center of effort of the upper blade and the CE of the lower blade. Both would tend to reduce the side thrust. 2) As the angle of the prop shaft increases and approaches vertical, the CE of the lower blade is increasingly in front of the CE of the upper blade and you would generate torque in the horizontal plane that would try to make the boat rotate, much as a helicopter without a functioning tail rotor will rotate. Since the keel will resist the boat swinging, the torque will cause the stern to move sideways. I need to do a bit of doodling, but my first impression is that the thrust would be in the same direction as the thrust from the pressure differential on a vertical propeller. 3) So as the prop shaft is angled more and more downward, the thrust from pressure decreases and the thrust from the horizontal plane increases. The net result is prop walk. But remember, your engine is installed with the CL of the crankshaft at an angle less than 15 degrees from horizontal. So the horizontal distance between the CE of the two blades is pretty small, the moment is pretty small, and the resulting side thrust is relatively minor. Rick Brass Washington, NC From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Chuck S via CnC-List Sent: Tuesday, May 19, 2015 6:50 PM To: CNC boat owners, cnc-list Cc: Chuck S Subject: Re: Stus-List Prop Walk 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
_______________________________________________ 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
