someone can correct me if I'm wrong, but I believe the earlier hulls did not have vents. My #252 does, but you still can't trust them to fill your tanks. I tried like I had on previous boats I owned, waiting till the water came out the vent, but when I went down into the cabin, if was also full of water. This has happened more than a few times when overfilling the tanks by mistake. I always fill them from the deck, but have to keep an eye on them through the tank access, then run out to shut the water off. I've tried sealing the access ports, tightening them, etc., but nothing stops them from leaking into the cabin. I've removed all the covers to check for cracks, but the tanks are sound. So vent or not vent, with a 31-2, you seem to have to be careful filling the water tanks!!