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 <[email protected]> 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. _______________________________________________ Email address: [email protected] 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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