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This is partly based on the Standard Pattern Cowl Ventilator as made to Classification Society Rules. The trunk (the smaller tube that goes into the deck) is made to connect to Evergreen #231 tubing. The Cowl is .688" dia. The trunk matches the tubing at ..344" dia. and has a small stud .288" dia. that matches the tubing I.D. This way you can make the ventilator any height you desire. These were designed for the Engine Room for an O scale / 1:48 ship. The Boiler Room ventilators were larger than these Engine Room ventilators as they (Boiler Room) had a secondary purpose in that they were also used as elevator shafts to lift ash buckets which were used to raise and dump ashes over the side into the dump scows while in harbor, if the steamer is a coal burner. I have the larger Boiler Room ventilators available.
See line-drawing for exact measurements of all cowl ventilators