Boat part (4)
I believe the answer is:
keel
'boat part' is the definition.
(I know that keel is part of hull)
This is all the clue.
(Other definitions for keel that I've seen before include "Fore-and-aft base to a ship's hull" , "Longitudinal structure supporting the frame of a vessel" , "Strut along base of ship" , "Support on which ship is built" , "Structure along base of ship" .)