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This is based in part on the Standard Pattern Cowl Ventilator as made to Classification Society Rules. The Cowl is .250" dia. The trunk (the smaller tube that goes into the deck) is made to connect to Evergreen #224 tubing. The trunk matches the tubing at .125" dia. and has a small stud .069" dia. that matches the tubing I.D. This way you can make the ventilator any height you desire.
see line-drawing for exact measurements for all cowl ventilators