Nicro vents
I have a pair of old Nicro solar vents on my S2 27. They are mounted in factory vent holes just behind the mast, at either side of the cabin top. One was working when I bought the boat. And simply replacing the rechargeable C size battery has the other one cooking better than the first, so I'll replace it's battery as well soon. They are set up as one blowing in and one blowing out. They are not silent, and the one with the new battery is downright obvious when it runs. Which, is most of the time if there's any light available. I've been awakened a couple times by them... Cloudy days are still plenty to activate it.
I'll do some cleaning of the fan blades to see if it quiets down some. They are pretty grungy. I'd have to say they are pretty durable and move a good amount of air. Handy when the boat's on stands.