Chainplates are metal bars that rigidly mount to a structural member of a boat. In the simplest of terms, they are the permanent anchor for the wires that hold up the mast. Without them and their counterparts, the mast would topple over.
Above you can see the three chainplates that we pulled and the two bars that we ordered as samples for our new chainplates. When we got the boat we knew that we would need to pull all of the chainplates and that there was a good chance that we would need to replace them. As you can see, there is no doubt that corrosion has begun its gnawing process, effectively weakening the metal at a steady rate. We decided that there was no question, we were going to replace them but that led us to a multitude of new questions. “Should we replace them with the exact same size?” – this would seem logical. “Should we try to buy pre-made chainplates or can we save money by making them ourselves?” “What type of metal is best for this?”
I will admit, I am a naturally skeptical person but I get realllll skeptical around boats. I don’t trust manufacturers or previous DIYers. After all, when we are dealing with things like chainplates, seacocks, rigging, and build quality we are essentially dealing with issues that could be life and death one day. That said, Whitby Boat Works did an incredible job building the Alberg 37 and they are renown worldwide for their indestructible nature.
When I started looking at our chainplates a couple things worried me. In the picture below you can see that there is crevice corrosion where the plate passes through the deck – these are the lines that run across the plate and look like someone scratched it in little lines back and forth. Whats really cool (but not good at all) is that water made those lines. Water actually etched into the steel over time and if it was allowed to keep going it would cut the chainplate in half. Basically stainless steel requires oxygen to form a passive layer which is its protection and constitutes its resistance to corrosion. Without oxygen, this passive (protective) layer breaks down and crevice corrosion is the result. The second thing that worried me was how little steel was left at the end of the chainplate where the pin attaches. You can clearly see in the hole at the bottom of this image that the amount of steel at the end of the plate is the least amount of anywhere! This is the highest load area!
After I got over that detail I was also struck that the starboard main shroud chainplate had pulled up through the main bulkhead about 1/8 of an inch. This may not seem like a lot, but chainplates should not move at all. Any pulling out of the structural member that they are attached to is a serious issue and demands serious attention. We are replacing the entire bulkhead and the chainplate in this situation (this was only one of many problems with this particular bulkhead).
So what to do? First I wanted to know if the chainplates were even the correct size (I told you I was skeptical right). There are mathematical formulas to solve this but there is also a handy chart from Skene’s Elements of Yacht Design and reproduced on p 150 in The Complete Riggers Apprentice.
I got out my caliper and went to work on our 1X19 rigging wire. Our main shrouds, forestay, and backstay are all 5/16 (8mm) and the forward and aft lowers were all 9/32 (7mm).
This is oversized (a good thing) compared to the original Alberg 37 specs which called for 9/32 for the forestay, shrouds, and backstay and 1/4 for the lowers.
Our chainplates were made to the original rigging specs and were never upgraded when the wire rigging was upgraded. According to the chart above, they are a full two sizes too small! (Thank goodness for skepticism!!) I decided we needed all new chainplates, each one 3/8 inch thick. Ready made chainplates are all made of 304 stainless steel – this grade allows for more crevice corrosion. I ruled them out on that alone, not to mention they are 5 times the cost of bar stock metal. So after lots of research I ordered 316L stainless steel bar stock: two 15-5/16 X 1-1/2 X 3/8 inch for the Shrouds, four 13-1/2 X 1-1/2 X 3/8 inch for the lowers, 12 X 6 X 3/8 inch for the backstay, and a 6 X 4 X 3/8 inch backstay backing plate. All of that 316L stainless steel cut to size for only $150.
Next up comes the real work – polishing and drilling the SS bar stock to form it into real chainplates!