Boomken stay plates parted

I was out on Adventure last weekend, beam reaching in about 18 gusting 20 kts.
There was a significant bang, and I saw the boomken stays dragging in the water behind the boat. We went immediately head to wind, dropped all canvas, and made it safely back to the berth, where we could see that both the Boomken stay plates had parted at the holes.
I’m ashamed to say that the quality of my inspection has obviously been sadly lacking, otherwise this could have been avoided.

Having said that, what to do now ?

I can easily get new plates made here in 316L Stainless, but now I’m thinking about replacing them with substantial bronze ones…
Any comments ?
Has anyone changed these out for bronze, and if so what provisions should be made for dissimilar metal contact?

I presume Port Townsend foundry could supply Bronze Boomken Stay plates ??


Kudos to you, Mike, for your quick action that saved the mast, the backstay, and the boomkin.

I think you have three choices:

  • remake the plates in 316 stainless steel, knowing that it has a working life of about 20 years. 316 (whether standard or the Low carbon version) is prone to stress crack corrosion, pitting corrosion, and crevice corrosion. Any piece of ss subject to shock loads becomes embrittled (and oddly stronger at the same time it becomes more brittle). For Brion Toss’s slightly humourous (but still correct) take on why SS is one of the worst materials for chainplates, see: The Worst Possible Chainplates - All At Sea - Southeast

  • make the plates in a Nickel-Aluminium Bronze (or a Silicon Bronze - both would work). To Australians of my age, the lesson from the two men known to you as Ron and Stan (and hence the company Ronstan) is unforgettable. Ron and Stan were top dinghy racers. And they started making sailboat parts out of SS because they saw too many bronze fittings crack. Unless cast with good technique and at the right melt temperature, bronze is liable to voids and cold joints. The usual workarounds are: 1. to x-ray the bronze to check for the lack of voids and cold joints; and/or 2. to oversize the bronze.

  • to make the plates in titanium (if you can get the right alloy and a workshop able to fabricate) or in a grade of SS superior to 316.

For Zygote’s 15th year refit, we replaced almost all of her chainplates (only plates not replaced are the mast tangs and the gammon iron - all still on the job list). We chose to use duplex stainless steel (i.e. 2205, with 22% Chromium and 5% Nickel) for the chainplates for the whisker stays, shrouds, and the upper plate of the backstay. We chose super duplex SS (i.e. 2507, with 25% Cr and 7% N) for the lower backstay plate, the bobstay fitting, and the boomkin chainplates. That’s because 2507 super duplex ss is almost completely immune from crevice corrosion and much more resistant to pitting corrosion and stress crack corrosion than 316 ss (and even superior to 2205).

We used Roger Olson’s drawings of the chainplates, as found in the BCC Construction Manual, as our guide. We supplied those drawings (as individual *.pdf files) to the metal supplier who then imported the pdfs into their numeric cutting program. That meant that the plates and the square holes for the necks of the carriage bolts were laser cut (side note: I reckon all but one of the stress crack corrosion in Zygote’s chainplates were associated with those square holes, pointing a finger of blame at either the presence of those square holes or the actions of cutting them by drilling and filing/grinding).

See the raw plate material, before we took them to a fabricator for welding, bending etc, as chainplate raw.jpg, about 320 KB.

We also arranged for carriage bolts to be carved out of 2507 super duplex ss; another way to reduce risk from crevice corrosion. And, as discussed in other threads, we bedded the plates in Simson ISR 70-03 (easy stuff with which to work: lay a 2 mm thick bed, allow to cure for 15 minutes, and then tension the nuts).

Of course, we then paid for the finished products to be polished to mirror No. 8 finish - Brion Toss is right, we’re just magpies that like shiny things.

I have the Port Townsend bronze boomkin fittings. They are very nice and quite stout. The folks at PTF do great work.

I also have the Port Townsend boomkin fittings - I agree with Eric Pomber that they are very nice and stout. The only difficulty is getting the attention of Pete and Cathy Langley (owners of Port Townsend Foundry). If you can accomplish that, I think you will be very pleased with their products.

Ahoy Adventure , if you can think “outside the box” , with bronze flat stock , a drill press , and Bosh jigsaw , you can fabricate your own boomkin chain plates top and bottom .

I did see an example , but can’t find the photo , sorry .

Using your SS plates for a pattern , cut out the top and bottom flat plates .

The 3 tangs can be fabricated as thru plate tangs and doweled or pinned on the under side for the back stay and top side for the boomkin stay tangs .

BTW was BCC Adventure owned by that National Geographic photographer in San Francisco ?

Did I inadvertently contribute to confusion? Apologies if that is the case.

I read Mike of BCC Adventure reporting that her boomkin stay chainplates - the plates through-bolted to the hull parted. Not Adventure’s backstay plates (the pair of plates through-bolted to the distal end of the boomkin).

Douglas is of course correct that most bronze alloys can be fabricated with the sort of tools used for hardwood. A few of the bronze alloys, such as Nickel Aluminium Bronze, are tougher. Cobalt tips and drilling a pilot hole first (see:,8664,8703#msg-8703) get the job done.

Annealing bronze plate, by heating and quenching, before bending is necessary and has been addressed on this forum several times before.

One of Mike’s concerns, so far unaddressed, was the risk of bi-metallic galvanic corrosion if he mounted bronze boomkin stay chainplates with 316 ss bolts. One technique is to match bronze plates with silicon bronze bolts (one user on this forum earlier posted that silicon bronze bolts cost as much as the bronze stock - that was also our experience when we bought 2507 duplex ss carriage bolts!!).