Stanley,
The No. 4 Funnel had absolutely nothing to do with ventilation of the boiler rooms. The boiler casings through which the funnels rose from the boiler rooms were transversely subdivided into several sections. The larger mid section contained the lower funnel, while the smaller forward and after end sections were further subdivided into two trunks each. The trunks at the very ends of the boiler casings were the stokehold ventilation trunks; each one terminated in a fan room located on F Deck. Each fan room contained two large Sirocco fans which supplied air through ducting which terminated in the firing aisle in front of each boiler. The trunks within the boiler casing sandwiched between the funnel casing and the stokehold ventilation trunks were the fidley trunks. In addition to acting as a conduit for plumbing and low pressure steam lines for the heating apparatus, the primary purpose of the fidley trunks were to act as an outlet for the hot air rising upward from the boiler rooms below. The stokehold fans were adjusted to supply air in somewhat greater volume than that required by the furnaces so that when combined with the natural draft created by the rising hot air in the fidley trunks, good circulation of fresh air was obtained throughout the boiler rooms. The screened inlets for the stokehold ventilation trunks are easily spotted and can be seen around the bases of the first three funnels in many photographs. The spilled gratings which were placed over the tops of the fidley trunks are harder to spot because they were flush with the top of the boiler casing surrounding the base of each funnel. These are most easily spotted in photos such as the one taken from the top of
Olympic's No. 4 Funnel, and in overhead photos of the wreck.
Regards,
Scott Andrews