Prindle 19MX rudder problems

Did the gudgeons just fail or did they rip out of the transom?

A new set of gudgeons?

Response to failure of gudgeons.
Saturday; bolt heads sheared off lower gudgeon. I replaced gudgeon and used 'Locktight' on new bolts.

Sunday: bolts ripped out with upper gudgeon.

Thanks,
BB.

That helps.... Bolts shearing could have been a result of an impact a long time ago or corrosion. Since they survived this long, and there are many other succesfull implementations of this system, it would be safe to assume that the failure is not from bad design. Once the lower bolts sheared, the uppers were undoubtedly put under a large amount of stress trying to handle the load (and they too likely have the same age and history as the lower bolts). It would also be safe to assume that this bolt failure would be because of their history and the sudden load placed upon them when the lowers failed.

That said, I would recommend replacing the upper and lower bolts together with fresh washers and lock nuts on the inside of the hull. Locktite will not prevent the bolt from shearing - only the hardware from coming loose. It may also be of note that locktite will react badly with the nylon in a nylock nut, making it hard and brittle, if they come in contact.

I have had the same problem The aluminum plate inside the hull becoms corroded over time. To eliinate the problem I replaced the bolts and installed stainlesss nuts on the inside. To access the inside you must remove the deck by prying it up with a stiff putty knife. When you are finished simply bond the hull and deck together again with resh resin and some strips of fiberglass mat.

The stock 1/4" bolts will not take the additional stress of the new rudders over time. Replace with 5/16", usually not a problem to carefully drill and tap. The al backing is pretty thick if it is still anchored to the transom. DO NOT Peel off deck. With patience and ingenuity you can work from the gap at the bottom of the bulkhead. If you want to go faster or through-bolt, holesaw a handhole through the bulkhead.

My guess would be crevice corrosion of the stainless steel. In a closed, corrosive environment such as bolt threads or where the bolt shank passes through the hole, the stainless steel can't develop its protective layer and it eventually pits or corrodes internally almost like tiny worms in wood. 316 SS helps, and so does a liberal dose of Never Seize which helps to keep water out of the crevice, but even with that, routine inspection and replacement of SS fasteners is a good thing.