Mast nearly went.....lucky escape!

[Aldebaran_III]

It started a couple of weeks back. Noticed a slight loss of rig tension but a check revealed nothing obvious in the rigging or mast.
About a week later we were sailing in the Anguilla channel, fairly light winds but a bit of a chop, a lot of that from power boats.
All of a sudden there was a loud bang, rig still up, still sailing but rigging now very slack. We couldn't see anything obvious but then I noticed the bowsprit flexing up and down a small amount. We got the sail down pronto, followed by lowering the mast onto the boom gallows and securing everything. Then we motored back to base and put the boat on the lift to make a full inspection.
The problem turned out to be with the stainless tube bob-stay. There is a welded 'T' at the top with a 1/4" bolt running through it. The 'T' had almost completely corroded away and the bolt was very badly bent. (photos)
I removed the bowsprit completely to check for hidden problems. The 4 nuts under the deck are very difficult to reach through the chain/rope locker but I eventually got them undone. Of the four 5/16" shoulder bolts, none had a backing plate (only small washers) and one had sheared off at deck level.
I decided to make a substantial backing plate using the holes in the mooring bollard as a template. Mistake! The holes through the bowsprit and deck had been drilled at all kinds of jaunty angles, so the backing plate would not fit until I filed the holes in various directions.

The 1/4" bolt at the top of the bob-stay was fully threaded, the corrugations weakening the bolt and inviting it to bend under load. The bottom bob-stay bolt was also 1/4" and although it was a shoulder bolt, the shoulder was not long enough and the threads were under load at one side.
I replaced both top and bottom bolts by 5/16" with appropriately long shoulders and drilled the fittings larger to match. Should be much stronger now.
The actual bob-stay I replaced with a better grade of 7/8" stainless tube (best quality 316). By milling a small amount of wood under the bowsprit to accommodate the 7/8" tube, I was able to keep the tube centered and remove the need for a welded "T" piece.
I used all new fastenings, again all 316 S/S

I raised the mast yesterday to adjust the rig tension, but found it was spot on, back to normal.

Lessons:- check the hidden things like the "T" piece under the bowsprit.
Don't use threaded bolts under side loads.
Boat builders should use better quality stainless for critical parts.
Just because a hole should be drilled at 90 degrees it doesn't mean it was.

Just waiting for the wind to drop a little and then back out on the water!!
Cheers, Derek

[Greg]

Wow!

Thanks for the heads-up. I assumed all hardware is 316 SS. Having received a new lower bobstay bracket I thought it a bit odd that the bobstay bolt shoulder did not fully extend the span of the bracket. Now I will look at the upper bracket assy as well. Got me thinking boring out the holes and looking into long clevis pins which are designed for shear loads instead. The threads on 304 or 316 SS bolts, tubes etc. are not strong (ok tensile, not good with shear loads). Many years ago I worked R&D for an aerospace firm and did some material testing on a 316 SS threaded adapter. Pressure stressed with dye, sliced it, polished it and through a microscope saw that when threads are cut into these materials cracks/fissures are created (galling) and extend into the material (sometimes all the way through hence leaks/failures) and the spec had to be de-rated. I would also recommend looking very closely (maybe with a magnifying glass) for hairline cracks in the welds. The fillet metal is very brittle. I assume that when beam reaching or anything more windward there would be significant sideways stress put on the sprit through the forestay as well as vertical stress from higher sheet tensions. This boat is a modification of the Herreshoff America which had a one piece mast, single forestay and no sprit. I think it would wise of us to regularly inspect our rigs. Wash it off and let it breath.