Is this your boat? If so, certainly you could do it, though if you followed the existing lines you'd end up with an almost canoe stern. Depending on your stern wave now, it may or may not affect seaworthiness in following seas. With the existing swept stern, you could probably add 6 feet of deck without adding much (if any) at all to where the 'addition' joins the lower point of the hull
, just above the waterline, as long as you were willing to have an overhanging rather than a swept stern.
Strength wise, assuming you have a fiberglass hull
, then the addition could be made with glass and any of the various structural foams in situ, or built off the boat and then attached with bolts and glass, and then faired in and painted. If it had no rigging
stresses involved, the main force to worry about would be the effects of waves, but since there's no real reason to alter the existing hull at all (unless you for some reason want to penetrate the existing transom), there should be no problem making the hull addition just as strong as the original hull. I would be leery of attaching mooring cleats
to the addition unless it was specifically engineered for them though.
To me the main question would be how the value of the boat changed. I'm guessing, depending on finishing and accoutrements, 25-40000 USD or maybe more to make the change, and in the end, even if done to the highest standards, will it wind
up devaluing the boat?
And by the way I've actually done this, though on a much smaller scale, in conjunction with recoring and repowering a 24 foot fishing boat. We changed it into a 27.5 foot fishing/diving boat, the extra 3.5 feet is proportionally about the same as the change you propose, the difference is that on this boat the addition was load bearing, as we built in a slight negative camber to help lift
the (slightly underpowered) boat on plane.