Roger,
I'm sure some strengthening may be necessary depending on the boat's construction and just how the towing is done. Lots of hard jecks on the tow line would certainly require more strength than a slow smooth take up of the strain. Most of the strengthening structures I've seen have been associated with how the tow hook is 'connected' to the hull, internal bracing, etc. How much is enough? Back to the old deal about which boat you want to use and how it's built. I've also seen some supporting structures that I thought were a bit 'much', as in metal bracings, really huge clumps of epoxy/fibreglass/whatever stuff holding things to the hull. Given enough of a run, the thing could pull you off of your feet, so I guess it was worth it (was hidden anyway so...).
- 'Doc