venting for LPG, you exhaust from the stor-
age area or compartment bottom, but for
CNG you need to exhaust from the top.
(NOTE: CNG has much less energy per
pound or per unit volume than LPG. Some
boaters prefer CNG because it rises and thus
won’t collect in the bilge, but the lower effi-
ciency of CNG combined with the difficulty
of locating refill stations makes it a poor
choice, in my opinion.)Chapter 16:Ventilation of Machinery Spaces
Fire Dampers
It’s often overlooked that in case of fire there needs to be a way to cut off air to the engine
compartment.This is done using fire dampers.Many boats have automatic engine compartment
fire-suppression systems installed.But for these to work properly, they should also have all
the engine vent ducts equipped with fire dampers, which will automatically close off the duct
when fire suppression is activated.In addition, an automatic engine shutdown should trigger at
the same time.If the vent ducts are not closed off, a fair portion of the fire-suppression foam or
gas (FE-241 or CO 2 ) can blow out of the vents, where it will do no good.Further, air from the
open vents will feed the fire, making it stronger.In fact,just closing off well-made fire dampers
alone can deprive a fire of enough oxygen to greatly reduce it or even put it out.Note that
the engine vent blowers must also be set up to automatically switch off when the fire-
suppression system is activated.
Fire dampers are available in stainless steel, galvanized steel (not ideal for marine use),
and aluminum.Bearings are often heat-resistant silicone, as are seals.Dampers have mechani-
cal, electric, or pneumatic actuators with manual overrides and are available in round or rec-
tangular shapes to fit the boat’s duct system.Delta-T Systems,Inc.;Greenheck Fan Corpora-
tion, and Actionair are three of several sources for marine fire dampers.Round fire damper (Courtesy Delta-T Systems, Inc.)