[samuelcooper]

I'm currently restoring a 69 20 ft. I had the same concerns about the hull near the stern. I think that part of the problem is the way the cap is attached to the hull. I think it is called a shoebox design. After removing the rubrail which was obviously an add on, the "weakness" was more obvious because the screws holding the rubrail on went through the hull also, thus holding the cap to the hull. There were a few rivets left. There is only a small lip 1/2 to 3/4 inch of cap overlapping the hull. After removing the transom, I found that there is a massive amount of glass in the bottom of the hull and in the stringers, but it is fairly thin on the sides of the hull. I really don't think it is a structural issue since the bottom of the hull and stringers are so heavy. But it could explain the water in the bilge (spraying up in the gap between the hull and the cap. This is an issue with many new production boats and they try to "fix" it with a bead of silicone in the gap, which fails. Some new boats have a better way of attaching cap to hull.
I'm thinking on how to make a better bond between the cap and the hull, thus stiffening the sides of the hull. Have several ideas but haven't decided.
Remember, these boats have no foam. Newer boats that are foamed may feel more solid but it just be the foam.
If you are really concerned about it, you could glass the inside of the hull but it would add lots of weight aft and probably change the ride a good bit.
Again, I'm no expert.