Quote:
Originally Posted by Rowglide
I have read the article and gone over the Edson chart... what am I missing?
It appears to me that the loads referred to in the article are at the rudder post, a twisting action. When you move out from the center of the rudderpost by however many inches of radius on the quadrant, you are reducing the load.
Also, if you look at an 11 tooth Edson sprocket, the radius of the chain center is a hair over 2".
No?
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per the article, 3751 lbs on the
rudder acts 2 inches away from the centerline of the rudder post, resulting in a moment of 3751 x 2 = 7502 inch-pounds on the rudder post. The article's quadrant has a radius of 16 inches, so the force on the edge of the quadrant (the wire rope) is 7502/16 = 469 pounds, that is the wire
rope and chain tension. The chain is driven by a 3 inch sprocket, so the moment on the sprocket is 469 x 1.5 = 703.3 inch-pounds. The
helm has a radius of 15 inches, so the force on the edge of the
wheel is 703.3/15 = 46.9 pounds.
More simply, the quadrant provides a mechanical advantage of 8 and the
wheel adds a mechanical advantage of 10 in this example, for a total mechanical advantage of 80. 3751 pounds at the rudder divided by 80 = 46.9 pounds at the wheel. The wire
rope sees 469 pounds. The breaking strength of 1/4 inch wire rope is about 5000 pounds when a straight length of new rope is pulled apart in a lab. Applying an appropriate
safety factor and providing for the loss of strength in bends (thimble) and wire rope clamps gets you to 1/4 inch or maybe 5/16 inch pretty quickly. As mentioned above, the length of the rope may be a factor as well. Edson has a lot of expertise in this area, ignore their guidance at your peril.