I have observed that several owners install much bigger washers under the keel bolts, a practice I have adopted using 100mm square plates 6mm thick with 25mm holes at the centre. The 60mm round washers were just not big enough to share load on the hull. i have observed some vessels where complete plates are used in the several sections of the underfloor to hold four bolts each!
This is a job best done when you have the boat out for an antifoul, and get the advice of a marine engineer on plate sizes and tensions.
