I have a C 30, not a C 27 but I replaced the forward fixed ports in the head & across at the area above the hanging locker with Lewmar Ocean Series operable ports. Beckson (original equipment for Catalina) plastic ports are slightly cheaper, but the Lewmar are better quality. You may have to cut & enlarge the port opening slightly to fit the #2 size ports, but is easy to do with a jig saw & well worth the trouble. You can't have too much ventilation, especially in the head. I also installed a Nicro solar powered exhaust fan in the head in the cabin top, which I feel is a must on all of our boats, as it really helps to provide positive airflow in the boat when she's all shut up at the dock. I feel that it really helps reduce mold forming & musty smells from lack of ventilation that most boats suffer from.
But beware that the Lewmar ports must be thoroughly sealed when installing as they can & will leak around the inner frame. Also seal the core that you cut through to prevent core rot, especially in a deck mounted Nicro vent. The core that I drilled out for the Nicro vent was 1.5" thick on the C 30 with 2, 1/2 inch thick layers of balsa & marine ply sandwiched in between a quarter inch of fiberglass on the top & bottom of the cabin top.
No wonder the C 30 is over 10,000 lb. empty! They were solidly built back then. Cheers.
