Actually, they are not hollows but more like bulges in between the bulkheads. The forward ones are from my attempt at forcing the plywood to conform as close as possible to the as designed shape. While it seemed OK back then, once the boat is flipped and long battens applied, it is the same as pretty much everyone else's that added to the bulkheads. Actually worse, possibly contributed to the thinner plywood or perhaps the adding of cloth on one side of the plywood and changing how the plywood responds to the changes in weather. In any case, it is between 3/4 and 1 inch at the bulkhead. Besides the forward one, there is one located at 169.5 as well, but that one is only 1/4 inch. Also, one appears between 110 and 124, but that one is really at the puzzle joint and really is a hollow of about 1/4 inch.
We elected to fill these in with foam. i know that at least one boat was cut up due to issues like this and so this method can and will save a boat that has any waviness along the hull. It should also be noted that the issues are symetrical about the center line so we know the boat is not twisted or anything. These issues are only in the skin.
First we purchased low density 1 inch thick foam. Three 2 x 4 pieces are about $ 60.00 delivered. That is more than enough to do all the locations. We used long battens and located the depressions to be filled. They should appear as rough ovals and should go to very little at the chine and the shear. We cut the foam down to 1/2 inch for the shallower areas. We glued the foam onto the area using a notched trough and thickened epoxy. You may have to screw trhe foam in place to make it conform. Once set, we long boarded the foam to shape. You want to take the foam to about 1/8 inch below the final surface.
Save the foam sanding dust as it makes a good thickening agent. We then smoothed the area with the foam dust thickened epoxy putty and immediately applied the cloth. I believe one layer of 6 oz bi-axial is good enough to stiffen the foam. Once all the locations are done this way, do your fairing like normal and apply a layer of the cloth of your choice. You end up with 12 oz. of cloth ( or more) over the foam and that will be fine for normal use.
You can use fancier foam at higher costs and densities, but I do not believe there is any real reason to. The low density is entirely outside the "water tight" hull and the plywood ends up sealed so the only time the low density becomes an issue is from impacts. Which would most likely require a repair regardless of the foam being there or not.
It cost about $ 100.00 in materials in the end and about 16 hours of labor. A cheap way to "save" a hull if any of you out there end up with a problem or two.
