Flame Arresters
By Officer Joe Alvarez

Most gasoline powered inboard boats have something on top of the carburetor called a backfire flame arrester. Some boaters think it's used to keep loose clothing, bugs or other airborne objects from being sucked into the engine. More knowledgeable boaters may know it's a backfire flame arrester, but don't know how important it really is. Backfire flame arresters serve a very important safety function.

Gasoline engines with carburetors vent fuel vapor after shutdown. A ventilation blower will remove any vapor that is lingering in the engine compartment. So long as there is no source of ignition, the vapor itself does not harm. Sources of ignition can be a backfire in the engine intake system when it is restarted or electrical. Flames coming out of a carburetor on a boat can ignite the fuel vapor in the engine compartment.

The flame arrester cools and quenches the flame front by forcing it to pass between closely spaced metal elements that act as a heat sink. Ignition takes place in one cylinder at a time when the intake valve is open and the resulting flame ignites the fuel and air vapor in the intake manifold. The pressure created goes back out through the carburetor or intake system. Only the pressure is released and none of the flame gets out with a working flame arrester.

If the metal elements in a flame arrester are damaged, it can let the flame pass through. Inspecting and replacing damaged arresters should be part of every commissioning. Careful cleaning with oil and grease dissolvers will keep the engine breathing normally.

Inboard gasoline engines are required by federal law to be equipped with an acceptable means of backfire flame control. Backfire flame arresters have prevented a lot of fires and saved lives. Remember, safe boating is smart boating.

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