My point is that bolting washers would have been even better, not that fender washers will not work if the material is firm. I'm quite sure your oak backers are more than enough by 4 times and that they look sharp.Perhaps I used misleading info regarding the fender washers. They would not be used to allow reefing the nuts with a meter long bar...
the purpose is to eliminate the possible splitting of the wood block as they spread the load out. I did not suggest pine as a backing material, I suggested oak or teak...both are more than robust enough for this application.
For what it's worth, I have essentially the same boat with halyard winches that are about half the size of the OP's which are through bolted using oak backing blocks. I installed the oak for appearance. When I assumed ownership 25 years ago there was no backing blocks, only fender washers. Between the 2 installation methods, the sails have been raised many, many times over a period of 40 years. The winches work fine and do not leak.
I think that sometimes the owners of much bigger boats apply the same standards to situations where less is enough. Respectfully submitted.
Remember to, that at full load the tension on the bolts is 100% WLL on that winch. I've done the calculations. With a softer substrate, full working load is enough to bend the washers; the photos were taken at the WLL, not BS, as made obvious by the fact that the threads are not showing signs of galling. I've seen them bent on brand new boats in shows. It's really common.