H33 passive vent replacement
Jim, I just measured the annular cutout that I saved when I replaced two of the passive ventilators on my 79 H33 with 4" Nicro day and night solar ventilators. The hole that the original passive ventilators fit into is about 4 1/4" diameter. The Nicro 4" deckplate which is the foundation for all of the Nicro 4" ventilators requires a rough cutout of 4 3/4". According to the West Marine catalog, the 3" ventilator requires a 3 3/4" rough opening. So the original hole is 1/2" too big for the 3" deckplate and 1/2" too small for the 4" deckplate. Whether the 3" Nicro deck plateflange will span this gap allowing purchase for the flange mounting screws could be determined by pulling one of the things out of the box at a marine supply store and measuring it I suppose. I am guessing that the screw purchase for a 3" vent is going to be skinny, leaving the 4" option as the only viable option. I didn't measure a 3" ventilator when I installed mine, (for one thing, I didn't ahead of time know the rough opening of the original passive ventilators) The cutout is not difficult. The thickness of the cabin top is less than 1 1/4", cutting through about 1/2" of end grain balsa and the rest laid up fiberglass. Just be sure that your hole is at least the 4 3/4" minimum specified to allow clearance for the deck plate flange and the lower trim ring. I have the feeling based on the condition of my passive ventilator innards that this is going to be needed on all of the H33s within a few years and so is worth treating in some detail. Mounting in the plastic of a hatch isn't really much of an option on a H33 with old ventilators through the deck that have to be replaced anyway.
