Natural Ventilation

Natural ventilation is a term applied to the provisions of air flow inside a compartment that is induced by non-powered means; i.e. ducts, louvers, clamshells, etc. The requirements for the natural ventilation system are described in detail in 183.630. Any compartment that has a permanently installed gasoline fueled engine must have natural ventilation. Engines for propulsion and auxiliary equipment are included.

EXCEPTION: Compartments qualified as open to the atmosphere do not require additional ventilation. See 183.605 for the definition of open to the atmosphere.

Permanently installed with regard to an engine, means that it is securely fastened to the boat’s structure and the necessary wiring, piping and controls are connected and secured to the boat in accordance with Title 33 CFR Part 183, Subpart I, Electrical Systems, and Title 33 CFR Part 183, Subpart J Fuel Systems. The use of "permanently installed" is to highlight its difference from the use of "portable equipment".

OPEN TO THE ATMOSPHERE

Compartments that are open to the atmosphere do not require additional ventilation. Long narrow spaces formed by side panels or accommodation floors should have openings at each end or along the sides if they are to be considered open to the atmosphere.

ADJOINING COMPARTMENTS - The 2% Rule

Natural ventilation for a compartment adjacent to a compartment requiring ventilation is needed if the total area of all openings between these compartments is more than 2% of the separating structure. If this area is equal to or less than 2%, natural ventilation is not needed for that adjacent compartment.

EXCEPTION: Compartments used for accommodations do not require ventilation if:

• the accommodation is above the compartment requiring ventilation; and
• the accommodation compartment is separated from the compartment requiring ventilation by a deck or other structure.

Accommodation compartments are designed for storage spaces and for living spaces for persons aboard the boat. Examples of specific uses of accommodation compartments include: staterooms, heads (bathrooms), galley, pilot house, navigation, work area and other similar people oriented uses. The uses contrast with engine and fuel tank compartments.

OPEN TO THE ATMOSPHERE

Compartments that are open to the atmosphere do not require additional ventilation.

FUEL TANK COMPARTMENTS

A compartment containing a fuel tank that is permanently installed, as opposed to a portable tank or container, does not require natural ventilation unless the compartment contains an electrical component that is not ignition protected.

Ignition protection is defined in Title 33 CFR Subpart I - Electrical Systems,
183.410(a) as follows:

"(a) Each electrical component must not ignite a propane gas and air mixture that is 4.25 to 5.25 percent propane gas by volume surrounding the electrical component when it is operated at each of its manufacturer rated voltages and current loadings, unless it is isolated from gasoline fuel sources, such as engines, valves, connections, or other fittings in vent lines, fill lines, distribution lines or on fuel tanks, in accordance with paragraph (b) of this section."

Another definition is:

"The design and construction of a device such that under design operating conditions:

• it will not ignite a flammable hydrocarbon mixture surrounding the device when an ignition source causes an internal explosion, or
• it is incapable of releasing sufficient electrical or thermal energy to ignite a hydrocarbon mixture, or
• the source of ignition is hermetically sealed."

"Ignition protected" devices are not necessarily "explosion proof "or "intrinsically safe" as those terms are defined by the National Electrical Code and Title 46 CFR Part 111, sections 105-9 and 105-11. If, however, a device does meet the definition of either "explosion proof" or "intrinsically safe", as referenced above, then it is also "ignition protected" as defined in Title 33 CFR Subpart I - Electrical Systems.

Test standards to determine ignition protection may be found in SAE J1171, External Ignition Protection of Marine Electrical Devices, and in UL 1500, Ignition Protection Test for Marine Products.

Usually fuel level senders and the associated wiring are not sources of ignition and therefore would not normally require natural ventilation.

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