The ram air affect which affords the Flats® Cat to run smoothly across the top of the waves also serves another function when entering shallow water. As air passes under the bow and approaches the inside walls of the Cat Hulls, the water under the boat is compressed slightly and in turn rebounds up into the water channel of the twin hulls. It is here that the water comes in contact with the sub-hull. The sub-hull is a slightly downward curve about one third of the way back from the bow. The purpose of the sub hull is to purge most of the air out of the water that is flowing into the Cat Hulls.