This 110mm hatch cover is designed to provide a complete water tight seal for your boat. The external seal is formed by placing the A.338 O ring around the fixed gasket on the hull, under the small lip. Once the hatch cover lid is screwed on tightly, it forms a seal around the external diameter of the hatch. Keep in mind that this hatch cover requires O ring A.338, which is not included. For optimal performance, the A.338 should be installed around the outside of the fixed gasket on the hull before screwing the lid on.