Stiffer hulls are faster hulls for several reasons. Oil-canning, i.e. flexing of the hull skin, when the boat moves over a wave is slow. Stiffening the skin helps the boat to stay rigid as it moves through waves. A stiffer hull flexes less under the dynamic loads imposed by the rigging. Less flexure keeps sail trim where you want it and permits better control of headstay tension, a determinant of jib pointing ability.
Soverel 33 designer Mark Soverel has written an excellent article on stiffening the hull: "Heavy-Weather Tourniquet".
I have followed almost all of the advice in Mark's article and I think it has improved boat performance. I differed on one point, however. Mark recommends an aluminum strut across the top of the existing stiffening rib, just forward of the head. This is a good idea, but in my view the strut only roughly doubles the modulus of the boat skin at that point -- by connecting the two sides of the boat. Doubling the modulus should reduce oil-canning by half. While this is undoubtedly an improvement, I think a more effective measure is justified.
We installed a full bulkhead midway between the two existing forward stiffening ribs. I think this new bulkhead helps the boat enough to justify the added weight. In addition to stiffening the sides of the boat, my new bulkhead supports the deck, which had begun to flex under the weight of the crew, and stiffens the forward part of the hull against torsion. The tradeoff, of course, is that the installed bulkhead adds more weight than the recommended strut and, moreover, this weight is in a very undesirable part of the boat. We estimate that we added 15 pounds with our modification. I'll tell you later whether I've convinced myself that the effort is justified.
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For those who might be interested, here is what we did. We very lightly sanded the gel coat on the inside of the hull in the vicinity of where the bulkhead was to be mounted. We did not make any effort to sand through the gel coat to the glass as the inside layer of glass is very thin. We did not want to damage the integrity of the existing structure. We then built up the glass around the inside of the deck and hull with three layers of 4" glass tape, using epoxy resin, in order to provide relief against the newly imposed shear load. We cut and installed the bulkhead in two parts, using ¾" end-grain balsa layered with fiberglass. This is a commercially available product. The horizontal joint is about two feet from the bottom of the boat at the centerline. We filled our "sins", the holidays between the new bulkhead and the hull, with putty made of a mixture of microballoons and epoxy resin. We then taped the joint all the way around on the forward and aft sides, taped the edges of the access hole through the bulkhead, and painted the addition with epoxy paint.
In order to cut the bulkhead, I made a 5-piece pattern out of thin mahogany plywood. It had to be made in pieces in order to get it in and out of the companionway. Now I'd like to get rid of the template and I wonder if any other owners might wish to use it. I'll be glad to send it along for $50, which should cover the cost of my materials, the packing and UPS shipping.