Proper boat ventilation practices are required for several important reasons. USCG regulations require potentially explosive fuel fumes be removed prior to engine starting. This is especially critical after refueling. These regulations apply to all gasoline engines including outboards.
Suitable ventilation of passenger areas, particularly enclosed areas, is crucial to prevent carbon monoxide poisoning. Boats with the right cabin vents will have cleaner, fresher smelling cabins. The right configuration minimizes the appearance of mildew and mold. Adequate ventilation of the head area will also go a long way towards preventing unpleasant odors.
Gasoline Engine Ventilation: Gasoline fumes are volatile, heavier than air, and potentially explosive. Gasoline vapors naturally settle in bilge areas if not properly ventilated. The operation of a bilge blower is necessary to evacuate any possible gas fumes from this enclosed space. Marine specific fuel fume detectors should be installed in enclosed areas as a safety precaution.
Passive or Natural Ventilation: A natural ventilating system typically consists of a minimum of two ventilator ducts connected to cowl vents or an equivalent device. Air intake ducting runs from the open atmosphere to the bottom of the bilge compartment. In addition, the system will have exhaust ducting leading midway from the bilge or near the carburetor out to the open air. Larger boats may require multiple vents and ducting in order to provide adequate under-deck ventilation. A passive system will only function when the craft is underway, therefore, a powered ventilation system is needed prior to engine start-up. Natural systems are required for enclosed fuel tank compartments.
Powered or Forced Ventilation: The USCG revised the regulations in 1980 to require powered ventilation systems on gasoline fueled inboard, inboard/outboard boats. This regulation did not apply to boats built prior to that date. However, it's a good idea to ugrade older gas powered craft to modern systems. This regulation also does not apply to diesel installations. Diesel is not a volatile fuel like gasoline. However, leaking diesel fuel can catch fire, so fume detection is a good idea.
Ignition protected bilge blowers should be operated for at least four minutes before starting engines. The bilge area should also be checked for the possibility of liquid gasoline from leaks and any vapors present.
SAFETY NOTE: Only fuel tanks designated 'for below deck use' should be used in enclosed area. Above deck tanks should NEVER be placed or installed in enclosed areas. Above deck tanks are red in color and are not impermeable to gasoline vapors.
Propane gas Ventilation: Propane gas is similar to gasoline fumes in that it is heavier than air and will sink to the lowest available areas. For this reason, propane tanks should be installed in proper lockers that are sealed from boat interior areas and vented outside of the boat's hull. Marine specific propane gas detectors should be installed as a safety precaution.
Engine Air Intake Vents: Many power boats feature a ducted system to moves fresh air from outside the hull through louvered grills, dorade boxes, air duct hoses and then directly to engine air intakes. It is important that these systems are designed and installed to minimize the possibility of water intrusion.
Passenger Area or Cabin Ventilation: Cabin fans, opening portlights. and deck hatches are important elements of a well designed ventilation system. A solar powered deck vent is an excellent way to circulate fresh air into head compartments. Continually flowing fresh air will help prevent mildew and mold. A combination of powered and passive air vents will ensure adequate flow of fresh air is maintained, even when the boat is unattended.
Carbon Monoxide Precautions: Carbon monoxide poisoning is an insidious, odorless, invisible killer. Carbon monoxide gas (CO) is clear and odorless, often present without the appearance of clouds of exhaust smoke. All boats with gasoline engines, gas powered generators, stoves, heaters or even charcoal grills are potentially at risk for CO poisoning. Engine exhaust system leaks, whether gasoline or diesel, are a potential source of carbon monoxide fumes. All exhaust connections should be routinely inspected for cracked manifolds, damaged pipes, deteriorated hoses, loose clamps, etc. These inspection procedures also apply to generators and all propulsion engines. Hoses should be double clamped for safety. Seal all engine bulkhead compartments to prevent CO gas leakage into passenger areas. Consider installing a carbon monoxide detector as a safety precaution.
Cowl Vents are available in a variety of styles, sizes, and materials. Vinyl cowl vents are a popular choice due to their classic appearance and reliable performance. Vinyl vents work well on deck due to their soft rubber construction. Stainless steel vents work well in locations not liable to see continuous foot traffic. A Dorade box is often used to permit air flow while shedding water. Mosquito screens are a popular accessory, to keep fresh air flowing and insects out.
Mushroom Vents are more difficult to install but offer a lot of versatility. Mushroom vents often have a low profile design, making them desirable for deck mounted and hatch mounted applications. Available in passive and solar powered versions.
Clamshell Vents are the simplest in construction and to install. They are not designed to be directly connected to hoses. Clamshell vents need to be selected and installed carefully as they do not shed water effectively.
Louvered Vents are designed specifically for engine air intake applications. Most are rectangular shaped, and intended to be installed on flat surfaces. Available in stainless steel or ABS plastic construction.
Portlights are designed to be opened for ventilation and sealed shut for heavy weather conditions. Mosquito screens are available to keep the bugs out. Also known as portholes, they can be found in a variety of shapes; round, lozenge, rectangular and oval. Typical port lens material is acrylic; some ports use polycarbonate (Lexan) lenses.
Deadlights are also known as portholes. Unlike Portlights, Deadlights are fixed, in that they admit light but are not designed to be opened. They have no ventilating properties. No screens are needed.
Deck Hatches are designed for through deck and cabin top installations. They feature heavy duty aluminum frames and rugged acrylic or polycarbonate lenses. They are all capable of ventilation functions and can be locked securely for offshore use. The larger ones may also serve as emergency escape hatches.
Access or Inspection Hatches are constructed from resin or plastic, making them suitable for mounting on bulkheads, cockpit sides, center consoles or where foot traffic is unlikely. Available in locking and non-locking versions, they provide offer handy access to lockers, storage bins, fishing tackle, etc. Access hatches must be installed on flat surfaces.
The Chapman Book of Piloting and Seamanship is the essential guide for power and sail boaters. Chapman features an extensive chapter describing the requirements of safe ventilation systems for gasoline powered vessels. In addition, there is a detailed write up of how to safeguard against carbon monoxide poisoning.