Wednesday, December 9, 2009

Those Little Things

In writing yesterday's entry, I was reminded that part of the rudder - tiller connection in Journeyman involves a shear pin. I never noticed it until last spring, when on our first sail I felt some give in the connection. The tiller secures to the head of the rudder post with what is essentially a clamp, tightened with a big Allen bolt. I tightened the bolt tight, but still the tiller gave. Scrutiny showed a stainless fastener passing through the bronze clamp assembly, through the top of the rudder post, and to the other side of the clamp assembly. The fastener was tapped or cut flush on each side, but I could see that the diameter of the fastener on one side was perhaps 1/4 inch, and on the other side perhaps 3/16 inch. I had encountered a tapered shear pin, now sheared.

This was a new one on me, and I made my problem known to the Albin Vega egroup. As is usual, someone had encountered the same problem and I was told where to source a replacement pin (McMaster Carr, "Over 480,000 Products"), and the part number.

Upon receiving the pins  - I bought a spare - I lined up the tiller, drove out the broken pin with a hammer and punch, tapped home the new pin, cut the ends with a hacksaw, and touched up the ends with a bastard file. Every once in a while a job is easy.

The broken pin was apparently stainless, yet it had fractured in two places. I assume it is a special alloy, brittle and with a known failure point, exactly what is wanted in a shear pin. By using the right replacement, I still have a tight, play free connection between tiller and rudder, and a weak link protects the rudder and tiller.

By contrast, another egroup member said he had replaced his broken shear pin with a stainless bolt. The years had ovaled the hole, and the bolt now allowed play, not good. And that bolt likely had a far higher shear strength than did the shear pin for which Per Brohall had designed the system back in 1964. I suspect my shear pin broke in the launching process; perhaps the rudder hung up on the trailer's hydraulic arm. Had the pin been a stainless bolt this accident might have twisted the internal structure of the rudder, or fractured the tiller head, instead of breaking a two dollar shear pin.

In most instances a careful engineer or naval architect has designed the various systems on our yachts, and one deviates from the design at his peril. That is not to say one must slavishly follow what has been done or built before, especially if, as certainly is common, time has suggested a failure mode the engineer may not have predicted. But it is sloppy to deviate from the designed system - it is sloppy to replace a shear pin with a bolt - without understanding the system and without deliberately deciding that the new way is probably an improvement.

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