Keel cable size: lecture #2

Posted By: Greg CM26 Seattle CLM005771173
Date: 1/12/08 3:08p.m.

Report: outside forum guidelines

In Response To: Keel cable (pic) (Jake CM26 Portland, OR)

By far, the most common cause of failure in wire rope is fatigue, almost always caused by a combination of abuse and excessive bending. The problem of pinching and crushing caused by erratic lay on the drum is included in the "abuse" category. Clipper boats came from the factory with an amazingly poor keel cable design, which is why virtually every boat out there (possibly with the exception of the Mexican boats) has had at least one cable incident. It is simply and clearly a bad design from the get-go, so just replacing a failed cable does not solve the problem, it just extends it for a while until the next failure. I suspect that many owners look at the failed cable and decide the thing to do is replace it with a bigger one. Not necessarily a good idea... Here's the deal: a wire rope bending around a properly-sized pulley will have a virtually infinite life. When the pulley is a little smaller than it should be, the flexing life is reduced. When the pulley is drastically smaller than it should be, be sure to wear gloves when you handle that cable. What constitues an adequately sized pulley for infinite fatigue life is a function of both the cable construction and its diameter. But here's the crucial point, possibly a little counterintuitive: bigger diameter wire rope requires bigger diameter pulleys, all else being equal. That means that when a cable fails because its pulley is too small (i.e., almost every Clipper built), putting a bigger cable on that same pulley makes the problem worse, and that new, bigger cable will fail even faster than the old one did. Every commercial winch I've ever seen uses a drum that is too small for infinite cable life. That's on purpose. First of all, nobody needs infinite life from their winch. Secondly, that infinte-life drum is usually about 3 times bigger than what you find in a stock winch. The way my 26 was built, it has a turning pulley INSIDE the keel trunk, just below the cable exit. If you take out that bolt, the pulley will fall out. Now put the bolt back in, and you have just created an excellent cable destroyer. I use 3/16" diameter 7 x 19 stainless steel cable. It's nominal breaking strength is listed as 3,700 lbs, which is enough to lift the whole boat right off its trailer. I'm eventually going to re-do my whole keel winch arrangement and use an electric winch, as a few brave souls on this forum have already done. The picture shows how I'm planning to do it. If i could just get off my @ss and away from this computer!