Wednesday, April 26, 2017

More Detail on Bobstay Failure

This is the welded nut on the lower end. Failure occurred about ½ way inside the nut. Inspection would not have revealed an incipient problem. I suppose turning the nut would have broken the rod. The threads backed right out, no sign of stress near the threads.

Broken ends of the ¾” dia. stainless rod. Color is distorted by flash  by crystalline structure is apparent.

No flash color here. Not much held the rod together! This failure was a long time in coming. The threads into the fitting are ugly but seemingly not affected by the fatigue.

The solid end fitting for the rod ends had no toggle, no way to dissipate sideways motion imparted by an anchor line rubbing on the stay, so there is no doubt fatigue caused the failure and water/salt corroded in the fatigue cracks. The top end fitting had no sign of stress.

The replacement was simple and inexpensive:

I hope this experience can keep somebody from the problem happening when it would lead to serious damage.

Carl Miller

Sunday, April 16, 2017

Exumas Cruising

Mike and Linda Wiens, Fairwind 41LN10, are cruising the Exumas.  Read about their on going adventures.

Monday, April 10, 2017

Check Your Bobstay...

Just in from Carl Miller, Saphira 35LN23

In a fair breeze and a 110%genoa, the bottom end of our LN35 (1987) bobstay fractured. Since my local rigger said he had seen the EXACT failure in Hawaii on a 41 Lord Nelson, it makes me think there could be a design issue.

At one rod end, they drilled out a 3/4" nut, and WELDED it to the rod, so there is a hex to rotate the rod for tension adjustment. But the weld heat changes the metallurgy of the SST and with trapped salt under the nut, and some side flexing from the occasional anchor line rubbing, the fracture occurred inside the welded nut. The failure was clearly metal fatigue, and it was a long time coming. The fracture surfaces are crystalline looking, all dark and rusty across the fracture. Because the failure was occurring concealed behind that welded nut, there was nothing to inspect or a crack to see from the exterior. If you have a LN any size and the bobstay rod is OEM, please think about replacing it before something bad happens at a bad time.