Building Rudder/Sealing Marine Plywood
Rudders are usually built with a core of wood or other material, encased in fiberglass. The shape that you are suggesting, with a rounded leading edge and a squared-off trailing edge, doesn''t sound like an efficient foil.
What company manufactured your boat? For example, if your boat is a Catalina, replacement rudders can be purchased from the company, or, if you log onto the Catalina 25 owners website, you can get plans for building your own rudder. Plans for building rudders are probably available for boats manufactured by other companies as well. Sometimes you can find "classified ads" on the owner''s websites, listing used rudders and other parts for sale. I just bought a used rudder for my boat by logging onto the owner''s website. Tell us the manufacturer of your boat, and perhaps someone on this forum can tell you where to find the owner''s website for your boat.