I used the largest size they have.
Also, haven’t mounted these yet, and may only use one of the pair purchased: https://www.amazon.com/Mr-Auction-Brass-Clamshell-Cover/dp/B00OTZOM1A/ref=sr_1_sc_1?ie=UTF8&qid=1497234949&sr=8-1-spell&keywords=clamsehll+vent+brass
They’re not brass, but instead are cast iron. Probably salvaged from a ship. I’ll paint a complimenting (or contrasting) color and most likely mount on transom.
Even with the largest clamshell iboats.com has, the openings can’t keep up with the exhaust fan. I suspect the mushrooms vents don’t either, unless two pairs are installed.
Perhaps I should stress is that these clamshell vents must be mounted on flat surfaces. They will not adapt to curvatures.
