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


  1. Thanks for sharing, Carl...I am going to have the bobstay on Veneeni (41LN#49) carefully inspected and adjusted...adjustment may be a way to reveal potential failure...keep posting this stuff...Greg

  2. Thanks Carl, I will put that on my list. I had our bowsprit rebuilt as it had rotted away significantly under the stainless fittings and the ends and the middle. The loose nature of the inner forestay gave it away and the more we looked the more rot was found. Good to have that solid again. Regards, Geoff

  3. Mine broke last month in almost identical fashion. We split a SS rigging screw and inserted the solid rod into the middle. Had to do a bit of grinding to get every thing to clear, but it seems as good as new.

  4. 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