Boat MaintenanceDIY ProjectsSubscriber OnlySystems & Propulsion Seacocks for Thin Hulls Fabricating a fiberglass backing plate for vessels with thin hulls. Drew Frye Published: January 21, 2019 Updated: November 6, 2019 0 Facebook Twitter Email Print A proper below-the-water line sea cock consists of three parts: the outside portion or mushroom, which threads into the flanged valve, the flanged valve, and the backing plate. To continue reading this article you must be a paid subscriber Subscribe Subscribe to Practical Sailor Get the next year of Practical Sailor for just $34. And access all of our online content - over 4,000 articles - free of charge. Subscribe today and save 42%. It's like getting 5 months FREE! Already Subscribed?Click Here to Sign In | Forgot your password?