We are often asked how waterproof these are. Well, you wouldn't fit one to the pressure hull of a submarine, but when they are closed, they are effectively waterproof. When open however, they are not, so I wouldn't advise their use on a foredeck, because one day you will forget to close it. Also very useful, when struck with a belay pin or similar, for waking up the crew! The nominal size is the diameter of the hole.
These have the advantage over the low dome vents that the odd bit of green water over the deck won't find its way into the boat. The trouble is that the upstand means that errant sheets etc can catch under the lip of the dome, either bending it or breaking it off. So you need to be careful where these are sited, particularly on the foredeck of a sailing yacht. The nominal size is the diameter of the hole.
We had a long and fruitless argument as to where these are used most. One camp reckoned they are deck mounted, usually as extractors from engine spaces and similar, the other that they are mounted on cabin sides over tank vents to stop rain water entering. Use them as you will!
It is recommended that these are either fitted on a dorade box, or that a blanking plate or cap is inserted when you are at sea.