There should be six deck vents on a 28 Express. Two face forward, and four face aft. Ducts should be attached to each running to a low point in the bilge. If no where else, a duct should be attached to the bilge blower, and run to a low point to ensure that any gas fumes are drawn out.
While underway, the bilge blower need not be run because the four aft-facing vents will cause suction exhaust while the two forward-facing vents supply intake pressure.