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.