Ready. Aim. Put out the Fire.

Author: Frank Lanier

When it comes to engine compartment fires, the most natural reaction is also the worst: lifting the hatch to see what’s going on. Doing so provides a rush of oxygen that could easily turn a smoldering fire into an abandon ship–type conflagration. The first rule of successfully containing and extinguishing a fire is fighting it in a way that doesn’t make the situation worse. With engine compartment fires, the safest way to accomplish that is with a fixed automatic extinguishing system.


What They Do

A fixed fire extinguishing system mounted in the engine compartment provides quick, automatic discharge of extinguishing agent as soon as trouble starts. Automatic is a key word here, but there should also be a manual discharge control outside the engine compartment, enabling immediate discharge of the system in the event someone spots smoke or fire before the auto-discharge mechanism kicks in. An audible and visual alarm at the helm to alert you in the event the system discharges is also desirable.

How They Work

Automatic extinguishers use mechanical triggers (e.g., glass or metal) that break or melt at a predetermined temperature, typically around 165 degrees Fahrenheit. Clean extinguishing agent is then released, flooding the entire engine compartment and extinguishing the fire. Clean in this case means the agent leaves no residue. The residue from a dry chemical extinguisher not only creates a huge mess, but it is so corrosive that in some cases it can cause more engine damage than the fire itself.

Today’s automatic systems utilize fire-suppressant agents such as FM-200, FE-241 or HFC-227. Halon was the clean agent of choice in the past, but it’s also an ozone-destroying chlorofluorocarbon whose production has been banned since the mid-1990s.

Sizing Your Suppression Unit

Fixed fire-suppression systems can be customized for your particular vessel, but most boaters will simply buy a pre-engineered off-the-shelf system, which is both cheaper and easier to install.

The first step in planning a fixed system installation is figuring out how large a unit is required to service the space. Start by calculating the cubic volume of the engine compartment: multiply its length times its width times its height.

If the space contains large items such as fuel, water or holding tanks, you can calculate their gross volume and subtract from the overall cubic volume of the engine compartment, to arrive at its net volume. I recommend ignoring this reduction in volume, however, as keeping it adds an additional margin of safety by providing more extinguishing agent.

After calculating the volume of your engine compartment, refer to the extinguisher manufacturer’s recommendations to choose the correctly sized unit. If your engine compartment opens to an adjoining bilge or compartment, add its volume to the engine compartment’s volume prior to selecting a system. Choose a size that meets or exceeds your requirements.


Cylinders for larger systems are typically mounted vertically on the forward or aft bulkhead, while the smaller cylinders for pre-engineered systems can be installed either vertically (with the sensor/discharge port up), horizontally on an engine compartment bulkhead or overhead (depending on the unit).

While you’ll want to follow specific manufacturer’s instructions when installing a fixed system, here are some general guidelines:

1. Install cylinders and controls so they are protected from weather and mechanical damage. Cylinders must be securely mounted and accessible for maintenance (e.g., removal for weighing, inspection).

2. If you’re mounting the cylinder on a bulkhead, locate it as high and close to the centerline as possible, with the actuator pointing toward the opposite bulkhead or the engine. If mounted on the engine compartment overhead, the actuator should be in the center of the compartment with the cylinder parallel to the keel, the top facing the bow and the actuator pointing down. Having the unit centered and up high helps the sensor more readily detect a fire from all points of the compartment and quickly discharge, blanketing as much of the space as quickly as possible.

3. Don’t mount the unit on the underside of a hatch or access door, which could be blown off in the event of an explosion, and avoid placing sensors near engine exhaust manifolds, turbochargers or other engine components that could cause the unit to discharge due to radiated heat. The same holds true for mounting near engine compartment ventilation exhaust or intake vents, which could delay discharge by reducing the temperature near the sensor.

4. Ensure the system is installed so that it doesn’t trap or rest in water, which could lead to corrosion.

5. Never combine the volume of two separate systems in attempts to protect a space. The problem is that the two systems could discharge at different times, failing to blanket the compartment with a high enough concentration of extinguishing agent to put out the fire, and both would fail.

When the Time Comes

Shut down the engine as soon as possible in the event of a fire. A running engine can pump fire suppressant out of the engine compartment while continuing to suck in fresh air. Most automatic units have an option for installing an automatic engine shutdown.

Many automatic units also have the ability to shut down additional equipment, such as generators or exhaust fans, to prevent venting of the extinguishing agent from the space. If your engine utilizes a mechanical shutdown (air or fuel starvation), you’ll have to discuss the options for converting it to an electrical shutdown system with the manufacturer.

Since a clean-agent unit kills the fire without damaging the engine and components, boaters can often restart their engine after locating and correcting the initial cause and return to port under their own power. Make sure you have the ability to bypass the shutdown systems in order to bring the engine back up once the source of the fire has been determined and corrected.


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