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Hi all on my B28 there are two louvered vents on on each side of the boat they are about where the step is 3 inches below the rub rail and on mine they are blocked off on inside
Any idea what they are /were for ?

Either air induction for the engine box or exhaust vent for bilge fumes, usually attached to the exhaust hose extending from the bilge blowers. What year boat and what engines?

I put my money on blower vents. If the boat was converted to diesel then blowers are not really needed so the were probably blocked off.

If the vent is anything like mine, a 8 by 8 cm (approx) small SS thin louvered sheet, then it's for the blower.
PS: I wanted to get rid of my blower, but I heard that in the case of a mishap the insurance will jump on you...I don't necessarily want to irritate my insurance broker!

Not my blowers they are connected to the big clam shell vents on the gunnel. one bringing fresh air into engine compartment and one removing it one each side

it is a 1977 with 350 mercruisers.

I don't think they are original. My 79 28 has a single SS vent for the blower on the port side, just below the gunwale about even with the step-up in the cockpit at the aft end of the engines. A friend has a 74 set up just like mine.

I think you are correct my small vent was replaced and blocked off for large vents which let in lots of air and water.