They are a positive air flow for venting gas fumes out of the hull. If you look closely you will see that the vent fins on one side usually face forward and the fins on the other side face aft. As you run, air is sort of forced into and out of the inner hull. I believe that at some boat length they are Coast Guard required for ventilating possible gas fumes from the bilge. I could be wrong, but that is what I was brought up believing. I think the earlier 20's that had fuel tanks in the console did not require them. Newer models obviously all have them. Somebody else shoot me down on the law thing if I'm wrong.