Ted Andresen, Gypsy #75
I noticed that the top of my backstay chainplate had moved forward by about 3/8". It had cut through the gunnel and was advancing forward into the quarter deck.
I explored the problem and discovered that my '60 Triton has a chainplate that was acting as a battery. The battery consisted of the stainless steel chainplate and the bronze, or perhaps brass, bolts that secured it to the knee on the transom. Any moisture acted as the electrolyte.
You might take a look at your bolts. I they are bronze and the lower bolt is corroded, consider replacing all of them with stainless steel bolts (3" x 3/8").
As a result of the electrical corrosion my two lower bolts sheered off with just a little bit of torque. The top bolt was in good condition.
The replacement is easy if you remove the rudder head and lay a piece of plywood on top of the rudder post. Support the plywood at the forward end so you can lay on it on your back. Then lay on the plywood, squeeze your head and one arm into the lazarette. This will enable you to work on the bolts with one hand. My bolts required a 9/16" wrench, the nuts used a 5/8" socket. I was able to wedge a wrench on the bolt end (starboard) while I tuned the nut at the other end with a ratchet.
Good luck. This is a tough job. In the future I will check my chainplate bolts every 40 years, whether they need it or not.