Do you think it’d be worth the cost instead if it just gave a +1 mast repair per shipwright (giving incentive for multiples to be taken)? 16 points and leaving it alone sound pretty good too though!
I don’t think that first idea would work, due to the no stacking rule. Unless you mean that the ship would be able to repair a number of masts equal to the number of shipwright crew assigned to her?
Leaving it at 16 and keeping the rest of the abilities as-is is also an option to consider.