Jock Strap Failure
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The tang under Kestrel’s mast step failed yesterday. We did three 4.5 mile races in 17-21 knots [recorded nearby] with gusts. We only had five souls aboard, so were not seriously stressing the boat. It failed near the end of the last race. I thought we had hit something. When no debris emerged astern I was puzzled. We found the broken part when we got back to the dock.
It appears to me that the metal fractured and I wonder if the part was accidentally hardened when it was manufactured. It could be a combination of age embrittlement and slow growth of cracks through corrosion at the crack tip. I’ll try to upload pictures so our resident metallurgist can give me an opinion.
Al Holt
Kestrel #114I uploaded a couple of pictures into Kestrel’s album.
AlHere are the pics Al put into his album
Did you inspect the other side? My guess: wear that was enhanced by electrolysis. Two different metals will corrode each other. You have aluminum and steel and stainless steel? Judging also by the red rust…was the pin stainless? Look at the other side..if the hole is elongated into an egg..it is wear from movement. If the metal is disintegrating..electrolysis.
The channel under the mast step is aluminum. The tang, jock strap, and clevis pin are all stainless steel. We’ll get a good look at both sides once I get everything apart, but I don’t think there is enough motion here to cause wear. I suspect the rust-like stains are evidence the hole was actually made by burning rather than drilling. This would account for embrittlement.
I don’t buy the wear/electrolysis explanation.
I’ll send along some closeups for public forensics as winter wears on. I’m too busy to pull the stick at just this moment.As promised, here are pictures of the old and new jock strap tang for Kestrel. The fabricator says there should be meat around the hole equal to one diameter of the hole as a rule of thumb. As you can see, the old tang doesn’t meet that standard. He also pointed out that the thickness of the old tang had be ground down, perhaps to fit inside the clevis on the jock strap fitted by an earlier owner. He wasn’t buying my theory of embrittlement. He also set aside electrolysis and wear as factors.
Al,
Nice polish. Tell me that is not a small crack at the bend point.
No, Jim. That’s just a scratch. It has a cousin on the other side. I think it is an artifact of the bending operation. Those old boys at KATO Marine showed Tubalcain how to make the fasteners for Noah’s yacht. They would not be so silly as to send along a cracked part.
Yes. They do know how to polish stainless steel. I suspect them of using a tumbler, but I’ve never asked.
Al
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