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Disclaimer:   I am not a spokesperson for the US Coast Guard or ABYC. For an official interpretation of regulations or standards you must contact the US Coast Guard or other organization referenced..   More..... 

VENTILATION SYSTEMS

The following are not verbatim from the regulations. It is my own wording. Get a copy and read the regulation!

First a note about the reasons behind the fuel, electrical and ventilation standards. Why is all this stuff required?

Federal Ventilation Systems Standards: Title 33 CFR 183.601 apply to:

Boats with permanently installed inboard gasoline engines for:

Propulsion

Generators

Auxiliary Equipment

This does not apply to:

Outboards:

Portable Equipment

Exception: If your outboard powered boat has a permanently installed fuel tank, and there is electrical equipment in the same space as the fuel tank, and that equipment is not ignition protected, then the space must be ventilated! See the electrical page for information on ignition protection.

Exception: If you have a portable fuel tank, fastened down in a compartment and that tank vents into the compartment, then the space must be ventilated.

Natural Ventilation: A natural ventilation system is required on all boats with compartments that contain a permanently installed gasoline engine or a fuel tank, unless that space is open to the atmosphere.

Open to the atmosphere: This means, if there is at least 15 square inches of opening to the atmosphere for each cubic foot of net compartment volume, then you don't have to provide ventilation. Any openings will do. However be aware that gasoline vapors are heavier than air. If all the openings are at the top of the space, and there is no air flow, then the vapors can still be trapped at the bottom:

Natural ventilation

Except for compartments open to the atmosphere:
Natural ventilation
must be provided in:

1. A space that contains a permanently installed gasoline engine.

2. A space that contains a permanently installed fuel tank and an electrical component that is not ignition protected.

3. A fuel tank that vents into that compartment (portables vent to the compartment they are in)

4. Contains a non-metallic fuel tank. (Plastic tanks)

The Natural Ventilation System must be made up of, at least:

1. A supply opening or duct from the atmosphere or from a ventilated compartment that is ventilated to the atmosphere, and :

2. An exhaust opening located on the exterior of the boat

What does all that mean? You must have a means to bring in air from the outside and a means to take air out of the compartment, to the outside of the boat. Air in from the outside, air out to the outside.

Each exhaust opening must originate in the lower third of the compartment. This is because the fuel vapors collect in the bottom of the boat. So the duct that takes the air out has to begin in the lower part of the compartment.

Each supply opening or duct, and exhaust opening or duct must be above the normal accumulation of bilge water. Get it low enough to get the vapors out but not so low it sucks up bilge water.

The openings or ducts must be sized to move enough air in and out of the compartment. The absolute minimum size is an opening of 3 (three) square inches

Here's a graph from the regulations that will help you to figure out the correct size. First add up the volume of the compartment. Subtract the volume of the engine, battery, fuel tank and any other large equipment in the space. (Generators, flotation blocks etc). Then reading up from the bottom of the graph (compartment volume) go up to the curve, and the read over to the left side of the graph to get area of the opening. You can have more than one exhaust opening and more than one intake opening to meet the requirement. The important thing is airflow.

Vent Area

To help determine the rated capacity of a blower for your system look at this graph of air flow versus compartment volume.

To see the full size graphs click on the image

Air Flow Graph

Ducts comes in standard sizes.

4.91 Sq. In. 2,5 In Diam.
7.07 Sq. In. 3 In Diam.
9.62 Sq, In. 3.5 In Diam
12.57 Sq In. 4 In Diam
19.53 Sq. In 5 In Diam

Powered Ventilation:

Any boat that has a permanently installed engine with a cranking motor (starter) must have a powered ventilation system: That simply means almost any inboard gasoline powered boat must have a blower built into the exhaust duct.

Warning Labels:  See also Labels

You must have a label, called a Blower Warning Label. It can be anywhere on the control station but must be near the ignition switch. In other words, next to the key, or starter button. It must say words to the effect:

Blower Label

Beyond the Regulation: This is not law but is good engineering practice and recommended by ABYC.

The Need To Breathe: Engines need air to run properly.  Lots of air. The air provided above is strictly to scavenge fumes from the engine compartment and may not be enough for the engine to breath properly. If this is the case, the engine will run poorly, not reach it's maximim horsepower or rpms, and produce more emissions, especially carbon monoxide. You need to make sure that the vent openings are large enough to provide the amount of air needed for the engine. A quick test on an existing boat is to start the engine, rev the engine to max and open the engine hatch.  If the rpm goes up, it needs more air.  On a boat you are building, you need to find out from the engine specs how many cubic feet of air per minute the engine needs at peak rpm, and make sure the vent openings are large enough to allow that much air in.  Increasing the amount of air flow is easier than you may think. You can double the air flow simply by going from a three inch opening to a four inch opening. You do not have to double the size of the opeings, ducts, etc.  You can also improve the efficiency of the system by eliminating turns, especially right angle turns, eliminating constrictions at connections, and using smooth walled tubing rather than corrugated ducting.

Location of vent openings: Vent openings often have cowlings on them and many times one is pointed forward and the other is point aft. Actually, for natural ventilation this is not necessary. Practical tests have shown that when the boat is standing still the wind direction is what determines which opening is the inlet and which is the outlet. When the boat is moving and the blower is on, the duct that has the blower is the outlet. Also cowlings can often create restrictions in the amount of air flow. Use cowlings that do not reduce the size of the openings. 

Preventing Water Intrusion: One thing you do need to know is that placing the cowls on the side of the boat, or back (transom) of the boat, can result in the boat sinking if water enters the opening. This is not a joke. Numerous boats have been sunk because water came in the vents.  If the vent is on the side or transom, route the duct up in a loop, an upside down U shape (in the plumbing world called a P trap) and then down, so the top of the loop is several inches higher than the vent opening, or use a manifold inside the opening and have the duct enter the top of the manifold. Provide a drain from the manifold out the side of the boat to get rid of accumulated water. This doesn't just happen on small boats. At a boat show, I was looking at a 60 foot houseboat. You could see right into the engine room through the ventilation cowls.  This is a disaster waiting to happen. If you use manifolds to prevent water intrusion, make sure they do not reduce air flow.

Blowers:   Be careful when selecting a blower. The inlet and exhaust openings on the blower may be much smaller than the size of the ducting. It may actually be smaller than the required size of ducting and then restrict the airflow to less than what is needed.  So, if this is the case, go to a larger size duct and get the next larger blower. Also make sure the blower is ignition protected.

Sharp bends in the ducting can reduce the airflow by as much as half. This includes the loop. So, when everything is being installed do a test. Don't guess. You can rent airflow meters. Check out what the actual airflow is at the exit vent.

Do not put air vents within 15 inches of a fuel tank vent, a fuel fill, or an engine exhaust.  Vapors will be sucked in, or if near an engine exhaust, carbon monoxide can enter the boat through the ventilation system.  Also, it is best not to have aft facing ventilation openings on the transom. Intakes there can allow engine exhaust to enter the interior of the boat. So it's best not to put vent openings on the stern.

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