The problem with polyurethane sealant (I used Sika Flex 291i, but am pretty sure the others are the same), is that it does not elongate as much as advertised before failing. The Sika technical sheet says it will take 300% elongation (tension) before failure. I planned a joint thickness of 3 mm, in order to allow for up to 9 mm of possible movement, an important consideration since the rig pulls upward on the chain plates. I found that the sealant failed with less than 2 mm of stretch. I did clean well with Sika cleaner and I used the recommended primer, so am pretty sure it was not a failure in the preparation. Besides, all 6 chain plate joints failed and leaked.
The product that I switched to is "Bed-It Butyl Tape" (www.pbase.com/mainecruising/butyl_tape), and it works very well. Problem solved, no leaks. By the way, I am now using "Bed-It" for many other leak sealing issues. The original sealant used on the Lord Nelson's is a black, gooey substance that remains pliable even after 30 years. It smells like it is a type of poly sulfide, which would probably also work. The best poly sulfide I have used is 3M 101, but I haven't seen it sold for many years. I think it can still be ordered from Jamestown Distributors.
Donald Dorn-Lopez, firstname.lastname@example.org