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Weld line separation like what appears in the photographs you have posted is very common with neglected Montauk 17 boats that have not had their brass drain tubes replaced and re-sealed properly. The condition is cosmetic in nature right now. You have relatively minor swelling of the wood causing the weld to separate in cold weather. Wait for a warm day above 65 degrees to seal it; I always use West System Six10 epoxy for a minor separated weld-line. No need to worry about the structural integrity of your transom.