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.


  1. Can you share any pictures of this? I am having a bit of trouble visualizing the failure.

  2. For some great pictures of the failure take a look at Carl Miller's last blog post:

  3. Thanks Carl,
    I just replaced my bobstay because it was a little bent and wavy probably from collisions into a dock by a previous owner. I probably would have just had it straightened but I chose to replace it due to the concerns you raised. I learned a little bit in the process. The round fitting with 2 flat sides just above the long end fitting is the actual lock nut. The purpose of the welded hex nut above that is to put a wrench on to keep the bobstay from turning while the other fittings are tightened or loosened. The threads on each end of the bobstay are different. Clockwise and counter-clockwise, like a turnbuckle. One of the bends in my bobstay was right at one of the threaded regions. I was able to back off the long end fitting but not the lock nut due to the bend. I purchased a new 12 foot long 3/4" rod (316SS) from a local supplier and polished it with a buffer.
    I had a local machine shop cut it to length and thread the ends, opposite threads on each end. I then purchased the 3/4" hex nuts (316SS) online from McMaster-Carr. The clockwise threaded nut was cheap but the counter-clockwise nut was a little pricey ($2.00 vs. $30.00).
    The critical flaw that I discovered in the original design was really just an oversight on the part of the welder who welded the hex nuts in place. The weld was only on the top face of the nut, not the bottom. This allowed seawater to creep up into the threads of the hex from the lower face, causing eventual crevice corrosion. The simple fix, which I did was, was to TIG weld both sides of the nut to the bobstay, sealing off the interior of the nut from seawater intrusion. Should be a permanent fix.

    Tom Fish
    S/V Voodoo Child

  4. Interesting. Just now seeing this and inspested mine. Thinking of future modification.

    Hugh Smith
    Stuart Fl. Dec/2020
    The Gray Wolf
    LN 35 15