(This should follow-on to another thread discussing the safety of sugar scoops.)
Many transom extensions--specifically multihulls but the thought should be extended to monohulls--have extensions that extend only a little above the water line. They add just as much to the water line, weight less, and drain more quickly when a wave fills them. Sides are often added, but they are really cosmetic-only, extending the line of the hull but serving no purpose and actually impeding dock-side loading. Some will argue that the sides add strength, but with composites this can easily be accomplished other ways.
It should be silly to modify a well designed boat but...
1. Many designs are either built overweight, or the designer made insufficient allowance for cruising loads.
2. There seems to be great a pressure to cram too much boat into too little hull length, since every "36" is compared to every other "36." Some of this has to do with slip rental, I'm sure (when I extended mine from 32' to 34' I neglected to change the hull sticker.... heck, with the dingy on davits, the docking length did not actually change).
So do we need the sides at all, or just the part that is in/near the water? In fact, many boats are coming from the factory this way now. Perhaps this is just a catamaran thing. But other than aesthetics and small sides to allow for heel, how would it perform on a monohull?