The Higgins PT boats initially had 10 funnel ventilators as standard equipment plus powered fan ventilation in the engine room. Ventilation was critical not only for the humans inhabiting the below deck space, but also for exhausting carbon monoxide and gasoline vapors that might accumulate in unventilated spaces.
In later boats, after the turrets were relocated aft, two of the tall funnel ventilators were removed and a smaller funnel vent added to the engine room hatch.
Funnel ventilators under construction. The larger vents were built up using half inch plastic water pipe as a base and adding shaped card stock for the upper part of the vent. These will probably be used for actual ventilation on the model, to provide cooling for the electric motors. The smaller vents are made similarly but start with 3/8″ copper pipe as the base. In this photo you can see some crew members being made from Sculpy clay, and several copper wire/aluminum foil armatures for more crew on the bench.