Here aboard NAUSIKAA there is a Vetus type of ventilator. Basically, it consists of a screw with a mushroom dome which, when open, allows air to flow past through a mosquito screen but when closed secures the dome against a collar keeping the water out.
Well, that's the theory. In reality mine does not do that any more as the dome parted company with the flange attached to the spindle this morning. Fortunately the weather was good with no sea running and so there was no water ingress. It could have been a very different story.
Obviously,these ventilators are totally unsuited for the task with the spindle flange attached to the dome by only four spot welds each the size of a pin head. My advice: if you have Vetus ventilators then check them, if you are in the market for new ventilators, for your safety's sake, don't buy Vetus.
Aye // Stephen