Designing new chainplates
**WARNING: Due to the blizzard in Newport, I have written a technical blog post detailing how I went about designing new chainplates. It is dry as a bone and there is only one, uninteresing picture. You have been warned.
I should also mention that to the right of this post is a link to Amazon and you should see some products scrolling by. These are not random ones chosen by Amazon, but ones I have personally hand-picked because I use them almost daily. I'll go into more detail about them in subsequent blog posts. You'll also see a sprinkling of books, movies and other media I come across that I've found interesting, useful and/or inspirational. Yes, I do get a small kickback from each sale, but in return you get a product that has not only met my standards, but has held up to my daily use in the marine environment.
Chainplates are important. For the uninitiated, they hold your mast up and keep you sailing in all kinds of weather. Not something to skimp on. I discovered this shortly after purchasing Soveraine:
Brown water was dripping down the inside of the v-berth and coming out of the bulkhead where, you guessed it, the chainplates were fastened to. Once I had decided I was not heading south this year and I was on the dock, I tore into the fiberglass to find out what exactly was going on (for some reason someone thought it was a good idea to fiberglass over the chainplates and associated nuts and bolts, that is what you see in the top of the picture). There was indeed surface rust on the stainless chainplates that are now 50 years old. Time to replace them.
I decided on silicon bronze for its comparable strength to stainless steel and superior corrosion resistance, and I also decided to move them outside of and bolted through the hull. This would stop the water from permeating the deck, which caused the surface rust in the first place. Lots of questions arose. How thick should they be? How long? What was the original design calculations for breaking strength? What configuration on the outside of the hull? How many bolts and what size?
I started with what I know. Based on the thickness of the wire rope that went to the top of the mast (shrouds), the "breaking strength" is 9,600 lbs. I use quotes around that term because it is ambiguous. A call to a structural engineer I knew from college confirmed what I had thought; there are two types of "breaking strengths" on a piece of metal. A lower value called the yield strength is the point at which the metal is permanently deformed and will not return to its original geometry. The second, higher value called the point of failure is when it tears apart completely.
For silicon bronze this yield strength is 55,000 psi, stainless is closer to 60,000 psi. How can a strength be a pressure (pounds per square inch)? Because the strength of a metal is based on geometry! If you multiply the pounds per square inch of yield strength by the square inch cross sectional area of the chainplate, you get a value in pounds. So, taking the 55,000 psi for the yield strength this would give an area of 0.18 square inches to meet the 10,000 lbs of load to match the shrouds.
Talking with a naval architect friend, the rule of thumb is a hole diameter all around the hole you drill to put a bolt through. I decided on 3/8" bolts (the originals were 5/16") and the most common plate in stock where I got the metal is 1/4". If I make them 1.5" wide, minus the bolt hole of 3/8", that leaves 1.125" in width, and with a thickness of 1/4", an area of 0.28 square inches, well above the 0.18 calculated! With that width and thickness the yield strength is 15,469 lbs, also well above the 10,000 lbs of load on the shrouds!
As far as configuration, in order to keep them aligned with the mast correctly, I devised a "forked" design that will go on either side of the bulkhead they were previously attached to. This will make more sense when I install it if you can't visualize what I'm talking about.
It is now a waiting game; I can't do a whole lot with the chainplates until the shrink wrap comes off, probably sometime in April. Green and I spent the past two days removing the transmission...but thats another blog post...
I should also mention that to the right of this post is a link to Amazon and you should see some products scrolling by. These are not random ones chosen by Amazon, but ones I have personally hand-picked because I use them almost daily. I'll go into more detail about them in subsequent blog posts. You'll also see a sprinkling of books, movies and other media I come across that I've found interesting, useful and/or inspirational. Yes, I do get a small kickback from each sale, but in return you get a product that has not only met my standards, but has held up to my daily use in the marine environment.
Chainplates are important. For the uninitiated, they hold your mast up and keep you sailing in all kinds of weather. Not something to skimp on. I discovered this shortly after purchasing Soveraine:
Brown water was dripping down the inside of the v-berth and coming out of the bulkhead where, you guessed it, the chainplates were fastened to. Once I had decided I was not heading south this year and I was on the dock, I tore into the fiberglass to find out what exactly was going on (for some reason someone thought it was a good idea to fiberglass over the chainplates and associated nuts and bolts, that is what you see in the top of the picture). There was indeed surface rust on the stainless chainplates that are now 50 years old. Time to replace them.
I decided on silicon bronze for its comparable strength to stainless steel and superior corrosion resistance, and I also decided to move them outside of and bolted through the hull. This would stop the water from permeating the deck, which caused the surface rust in the first place. Lots of questions arose. How thick should they be? How long? What was the original design calculations for breaking strength? What configuration on the outside of the hull? How many bolts and what size?
I started with what I know. Based on the thickness of the wire rope that went to the top of the mast (shrouds), the "breaking strength" is 9,600 lbs. I use quotes around that term because it is ambiguous. A call to a structural engineer I knew from college confirmed what I had thought; there are two types of "breaking strengths" on a piece of metal. A lower value called the yield strength is the point at which the metal is permanently deformed and will not return to its original geometry. The second, higher value called the point of failure is when it tears apart completely.
For silicon bronze this yield strength is 55,000 psi, stainless is closer to 60,000 psi. How can a strength be a pressure (pounds per square inch)? Because the strength of a metal is based on geometry! If you multiply the pounds per square inch of yield strength by the square inch cross sectional area of the chainplate, you get a value in pounds. So, taking the 55,000 psi for the yield strength this would give an area of 0.18 square inches to meet the 10,000 lbs of load to match the shrouds.
Talking with a naval architect friend, the rule of thumb is a hole diameter all around the hole you drill to put a bolt through. I decided on 3/8" bolts (the originals were 5/16") and the most common plate in stock where I got the metal is 1/4". If I make them 1.5" wide, minus the bolt hole of 3/8", that leaves 1.125" in width, and with a thickness of 1/4", an area of 0.28 square inches, well above the 0.18 calculated! With that width and thickness the yield strength is 15,469 lbs, also well above the 10,000 lbs of load on the shrouds!
As far as configuration, in order to keep them aligned with the mast correctly, I devised a "forked" design that will go on either side of the bulkhead they were previously attached to. This will make more sense when I install it if you can't visualize what I'm talking about.
It is now a waiting game; I can't do a whole lot with the chainplates until the shrink wrap comes off, probably sometime in April. Green and I spent the past two days removing the transmission...but thats another blog post...
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ZBH