Natural Ventilation

OPEN TO THE ATMOSPHERE

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

FUEL TANK COMPARTMENT

Permanently Installed Fuel Tanks

A permanently installed fuel tank must have a vent system in accordance with Title 33 CFR Subpart J - Fuel Systems, 183.520. This section on fuel tank vents requires that vents have a flame arrester and not allow a fuel overflow at the rate of up to two gallons per minute to enter the boat. This requires a fuel tank vent opening that is outside of the boat and will not vent into the compartment. For permanently installed fuel tanks, 183.620 (a)(3) and (5) of this guideline apply.

Portable Fuel Tanks

Compartments used to store vented portable fuel tanks or containers are required to be equipped with natural ventilation.

Cockpit seat lockers in auxiliary sailboats are often used as fuel tank compartments for portable outboard motor fuel tanks. If this fuel tank vents into the locker, then natural ventilation of this locker is required.

Since fuel vapors seek the lowest point of any compartment, gasoline vapors should be considered when designing and constructing a compartment that will contain a fuel tank or container that vents into a compartment. Any openings in or near the bottom of the compartment could permit explosive vapors to flow into the bilge of the boat where an ignition source might ignite the vapors.

TO COMPLY WITH THE LAW

• Is there a fuel tank in the compartment that vents into the compartment?

If YES, then one of the following must be answered YES:

• Is the compartment open to the atmosphere, as defined under 183.605, or
• Is there a natural ventilation system which meets the requirements of 183.630?

OPEN TO THE ATMOSPHERE

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

All plastics and fiberglass will permit gasoline and its vapors to pass through them. This capability of the material is termed permeability. The permeability rate is the quantity of gasoline that passes through the material in a specific length of time or per unit of net compartment volume. See 183.605 of this guideline for a discussion of how to determine net compartment volume.

The permeability rate of a material is affected by the thickness and density of the material. The temperature at which the test is conducted may also have an effect on the permeability rate. The conditions and standard procedures of the test must be controlled when conducting the test.

THE TEST

There are two ways that tests may be conducted to obtain the permeability rate of non-metallic materials used for fuel tanks. The two ways are differentiated by how the test sample is prepared.

• A test sample of an actual fuel tank may be subjected to the test procedure, or
• A 6" x 6" test sample from a piece of fuel tank material mounted on a standardized container and then subjected to the test procedures with the results extrapolated mathematically to obtain the performance of a full sized tank. 

Both of these methods have been used experimentally and appear to correlate. Permeability rate data should be obtained from the tank manufacturer.

The fuel used in the test, to represent gasoline, has been standardized as ASTM D 471 Reference Fuel C.

REQUIREMENTS FOR VENTILATION

Permeable, non-metallic materials may be used for fuel tanks in boats; however, depending on the permeability rate of the material and the net volume of the fuel tank compartment, natural ventilation may be required for the fuel tank compartment.

The regulation evaluates the need for natural ventilation of the fuel tank compartment based on whether its net volume is less than one cubic foot or one or more cubic feet.

LESS THAN ONE CUBIC FOOT

If a fuel tank compartment’s net volume is less than one cubic foot, it is required to have natural ventilation if the fuel tank’s permeability rate is more than 1.2 grams total fuel loss in 24 hours.

ONE OR MORE CUBIC FEET

A fuel tank compartment with a net volume of one or more cubic feet is required to have natural ventilation if the fuel tank’s permeability rate is more than 1.2 grams of fuel loss per each cubic foot of net compartment volume in a 24 hour period.

TEST PROCEDURE:

• Obtain the permeability rate of the non-metallic fuel tank for a 24 hour period in terms of grams of fuel loss.
• Determine the net compartment volume for the fuel tank compartment.
• Calculate the grams of fuel loss per cubic foot of net compartment volume.

Example:

Total fuel loss for a non-metallic tank is found to be 10 grams in 24 hours.

The net compartment volume for this example is 12 cu. ft.

The permeability rate is 10 ÷ 12 or .833 grams of fuel loss in 24 hours per cubic foot of net compartment volume.

Conclusion: The fuel tank compartment in the example does not need natural ventilation to meet the requirements of the regulation.

Question: What is the minimum net compartment volume before natural ventilation is required in the above example?

Answer: Per the example, the fuel tank loss was stated to be 10 grams in 24 hours. The permitted fuel loss may not exceed 1.2 grams per cubic foot in 24 hours.

Therefore the net compartment volume must be at least 8.33 cubic feet in order that no ventilation be required. A compartment of less than 8.33 cubic feet of net compartment volume requires natural ventilation to be provided.

NOTE:
Some non-metallic materials in current use for gasoline fuel tanks have a permeability rate such that natural ventilation of the fuel tank compartment will be required in order to comply with this section of the regulation.

The discussion of this section is included in 183.620(a)(2).

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