Ships just don’t look right without their ventilators, but in paper it can be a problem. When I first started building ships in paper I cheated—I would find an appropriate ventilator in a plastic kit, make a plaster mold of it, and cast some in low melting temperature alloy. Definitely not paper! Here is how to make reasonably good looking ventilators in the real thing—paper.
I have made one example in a very large scale for a demonstration. This is one of the very tall ventilators on Starke, a model by Cony. His site is at http://hometown.aol.com/conysmodellbau/page4.html Starke was an ice ship, a rail car ferry built in 1931, and used as part of the rail system in Sweden.