Natural Ventilation
|
FEDERAL LAW
183.630 - Standards for natural ventilation
(a) For the purpose of 183.620 "natural ventilation" means an airflow in a compartment in a boat achieved by having:
(1) A supply opening or duct from the atmosphere or from a ventilated compartment or from a compartment that is open to the atmosphere; and
(2) An exhaust opening into another ventilated compartment or an exhaust duct to the atmosphere.
|
NATURAL VENTILATION SYSTEM
A natural ventilation system must have two elements:
- a supply opening or a duct
- and an exhaust opening or duct.
NOTE:
An exhaust duct may serve for both the natural ventilation system and the powered ventilation system.
SUPPLY
The supply opening or duct may take air in from any of the following: See Figure 11.
- The Atmosphere: An opening on the outside surface of the boat. It may be fitted with a cowl, louver, clamshell or other suitable ventilation terminal fitting.
- Ventilated Compartment: A supply opening or duct may be installed to take in air from a compartment that is required to be ventilated, provided the supply for that ventilated compartment is on the exterior surface of the boat. See requirements for determining which compartments need to be ventilated in 183.610(a) and 183.620(a).
- Compartments Open to the Atmosphere: A supply opening or duct may be installed totake air in from a compartment that qualifies as open to the atmosphere as described in 183.605.
EXHAUST
The exhaust opening or duct may expel air into any of the following:
- The Atmosphere: If the exhaust discharge point is directly into the atmosphere, there must be a duct from the air and/or vapors intake point to the exhaust discharge point, which is usually at the deck or hull side near the deck.
- Ventilated Compartment: An exhaust opening may be located in a bulkhead or other structure that separates a ventilated compartment from the compartment in which the natural ventilated system is being considered.
The ventilated compartment into which the exhaust opening discharges may not be the same ventilated compartment that contains a supply opening or duct for the compartment being considered.
Air intake openings inside a compartment should be separated from the exhaust duct openings inside the compartment by 24 inches, compartment dimensions permitting.
FIGURE 11 - Natural Ventilation and Exhaust Options
|
TO COMPLY WITH THE LAW
One of the following must be answered YES:
- Is the supply opening or duct from the atmosphere, a ventilated compartment with intake on the exterior surface of the boat or a compartment that is open to the atmosphere?
And, one of the following must be answered YES:
- Is there an exhaust opening into a ventilated compartment other than that in which the supply is located, or is there an exhaust duct to the atmosphere?
|
|
FEDERAL LAW
183.630 - Standards for natural ventilation
(b) Each exhaust opening or exhaust duct must originate in the lower third of the compartment.
|
The location of an exhaust opening or an exhaust duct intake opening is required to be in the lower third of the compartment. The intent is for the exhaust opening to be in a position to remove any flammable or explosive vapors as air from the supply circulates through the compartment and discharges through the exhaust.
If there is uncertainty as to the upper limit of the lower third, perhaps due to a complexly shaped compartment, keep in mind that the lower the exhaust opening or duct intake is located in a compartment, the more effective it is. Normal bilge water level must also be considered as stated in 183.630(c).
Ideally, the exhaust opening or duct intake should be positioned in the lowest part of the compartment where vapors are likely to accumulate. It would then be reasonable to measure the height of the compartment at the position of the exhaust opening or duct intake to determine the lower third level. See Figure 12.
FIGURE 12 - Lower Third Levels
|
TO COMPLY WITH THE LAW
- Are exhaust openings and duct intake openings located in the lower third of the compartment?
|
|
FEDERAL LAW
183.630 - Standards for natural ventilation
(c) Each supply opening or supply duct and each exhaust opening or exhaust duct in a compartment must be above the normal accumulation of bilge water.
|
Consideration must be given to the possibility of normal bilge water accumulations covering the intake opening. Normal accumulations of bilge water occur from propeller shaft stuffing box seepage, spray while operating the boat and from rain. The water remaining in the boat after a bilge pump has completed its normal running cycle would be considered normal. The openings and duct intakes of both supply and exhaust must be above this normal level of accumulated bilge water.
It is important to evaluate compartment design and to locate the openings and ducts in the best positions for the effective removal of any accumulated vapors.
|
TO COMPLY WITH THE LAW
- Are both supply and exhaust openings and ducts located above the normal accumulation of bilge water?
|
|
FEDERAL LAW
183.630 - Standards for natural ventilation
(d) Except as provided in paragraph (e)of this section, supply openings or supply ducts and exhaust openings or exhaust ducts must each have a minimum aggregate internal cross-sectional area calculated as follows: A= 5 Ln (V / 5 ); where:
(1) A is the minimum aggregate internal cross-sectional area of the openings or ducts in square inches;
(2) V is the net compartment volume in cubic feet, including the net volume of other compartments connected by openings that exceed 2 percent of the area between compartments; and
(3) Ln ( V / 5) is the natural logarithm of the quantity (V / 5).
|
The minimum total cross-section of all supply openings and ducts in a compartment is determined by the use of the formula. The formula is also used to determine the minimum total cross-sectional area of all exhaust openings and ducts in a compartment.
THE FORMULA
A = 5 Ln ( V / 5 )
The formula stated in words is:
"the cross-sectional area of either the supply openings and ducts or the exhaust openings and ducts (in square inches) equals five times the natural logarithm of one fifth the net compartment volume in cubic feet."
To use the formula:
Determine the net compartment volume (V) in cubic feet. The net volume of connecting compartments must be included if the openings in the separation structure are more than 2% of the area of the separation structure between the compartments. The exception stated in 183.620(c) for accommodation compartments above a compartment requiring ventilation and separated by a deck or other structure, may be applied.
Divide the net compartment volume (V) by 5.
Determine the natural logarithm (Ln) of V / 5. Natural logarithms are tabulated in books of mathematical tables, engineering handbooks, trigonometry textbooks, etc. Figure 13 is a Table of Natural Logarithms.
Multiply the natural logarithm by 5.
The result is the minimum required area (A) in square inches.
FIGURE 13 - Natural Logarithms
Figure 14 is a graph of the formula. Entering the graph with the net compartment volume in cubic feet you can read the area of openings and ducts directly along the vertical scale on the left of the graph.
The graph is approximate. The formula provides the accurate area.
EXAMPLE
The net volume of a compartment is 80 ft.3. Go to Figure 14 and enter the graph with 80 ft.3 and read the area of the openings and ducts from the vertical scale at 13.92 square inches.
The total cross-section of all supply openings and ducts into the compartment must be equal to or exceed 13.9 in2. with the minimum area of 3in2 per opening.
The aggregate area of exhaust openings and ducts must equal or exceed 13.9 in2.
|
TO COMPLY WITH THE LAW
- Is the total area of exhaust openings and ducts for the compartment and the included connecting compartments at least 5 Ln ( V/ 5 ) square inches?
|
FIGURE 14 - Area of Openings
Previous | Next