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Electronic Detectors for Your Boat


BoatUS Marine Insurance claims files can’t tell us how many disasters were avoided by having an electronic detector on board. But our investigators say many of these types of claims were preventable if the boats had been equipped with alarms.

Photo: BoatUS files

Before affordable electronics, boaters relied on their senses to warn of potential onboard problems. But explosive gasoline and propane vapors sink into the lowest parts of the bilge, making even the most sensitive nose in the cockpit ineffective. Worse, carbon monoxide is odorless and can’t be detected by humans at all. Electronic alarms have replaced human senses in these critical areas, and they do a far better job of tirelessly looking for potential problems. Installing them is a smart winter project.

Gasoline And Propane Vapor Detectors

BoatUS Marine Insurance has seen several explosion claims that occurred right after refueling. Gasoline, like other vapors, has an “upper explosive limit,” or UEL. Too high a concentration means it can’t ignite. A leaking fuel-fill hose can allow gas to collect in the bilge, making vapor concentration too high to ignite. This prevents engines from starting because there are too many gas fumes and not enough air to support combustion inside the engine. But as fresh air gets mixed with the vapors, say after opening the engine cover, the fumes become less concentrated than the UEL, and a spark from a faulty starter or bilge-pump switch can ignite the fumes in a spectacular way. If your boat refuses to start right after fueling, check for a gasoline spill.

Who Needs One?

Any boat with a gasoline fuel tank mounted below decks and/or a propane system needs a vapor detector.

How They Work?

Also known as fume sniffers, vapor detectors monitor for flammable gases. Propane and gasoline fumes have to mix with air before they’ll ignite. These detectors analyze the air around them and sound off long before that concentration is high enough to explode. This air/vapor mixture is called the “lower explosive limit,” or LEL, and varies with the type of vapors. For example, gasoline fumes will explode at about 1.5 percent by volume of air; vapor detectors will sound an alarm when they sense the level is about 20 percent of that, giving you time to find the problem before it gets out of hand.

How To Use Them?

Vapor alarms should be mounted in the engine-space bilge, just above the slosh height of bilge water. Mount the sensor away from the hottest parts of the engine, such as manifolds. Vapor alarms are almost always hard-wired to the boat’s 12-volt DC system. Usually, the unit has a control head mounted at the helm that will sound when dangerous fumes are detected in the bilge. The wire that connects the sensor to the head unit typically can’t be cut, because the manufacturer has calibrated its length. Some vapor detectors can turn on the bilge blower when they detect a buildup, a smart option. The blower, of course, must be ignition protected. Look for an alarm that is UL 2034 listed. Vapor alarms should be tested monthly, using the manufacturer’s procedure. Replace them after no more than five years, or right away if they become submerged.

What To Do When It Sounds?

Shut down the engine, turn off the battery switch, open the engine compartment, and look for the source of a gasoline leak. If you have a propane system, turn off the valve at the tank, open hatches, and air the boat. For a small gasoline leak that can be safely repaired, a bilge blower can help rid the engine space of vapors, but it won’t remove liquid gasoline. If you’re on the water, there’s likely nothing you can do about a ruptured fuel tank that’s spewing gas into the bilge except call for help; a cellphone is a better choice in this case than a VHF, which isn’t required to be ignition-protected. A handheld VHF used away from the spill is safer still. Flares are a bad idea. Have all crew don life jackets, and move everyone to the forward part of the boat; as a last resort, the crew can take to the water, but don’t go far — someone who happens along won’t know your boat is full of explosive fumes. At the dock, larger quantities of spilled gas should be dealt with by pros. Get everyone off the boat and call 911. Don’t operate anything electrical, including the blower; it won’t eliminate spilled gas.

Carbon Monoxide Detector

The symptoms of CO poisoning include headaches, drowsiness, and nausea. With most BoatUS claims, one or more of these symptoms were present, but victims didn’t recognize the danger they were in. How much CO is too much? In parts per million (ppm):

  •    200 ppm – Slight headaches within 2-3 hours
  •    400 ppm – Frontal headaches within 1-2 hours
  •    800 ppm – Dizziness, nausea, convulsions within 45 minutes. Insensible within 2 hours
  •  1,600 ppm – Headache, dizziness, nausea within 20 minutes. Death within 30 minutes
  •  3,200 ppm – Headache, dizziness within 5 minutes. Death within 30 minutes
  •  6,400 ppm – Headache, dizziness within 1-2 minutes. Death in less than 15 minutes
  • 12,800 ppm – Death in less than 3 minutes

Who Needs One?

The American Boat & Yacht Council (ABYC) recommends that a carbon-monoxide (CO) detection system be installed on all boats that have designated sleeping accommodations, a galley area with sink, or a head compartment. Carbon monoxide is emitted from propulsion engines, gasoline generators, and/or cooking or heating devices that burn fuel (e.g., propane, alcohol, CNG). Even rafting up with someone with a gas generator can cause CO poisoning.

How They Work?

Carbon-monoxide detectors not only detect small amounts of CO; a microprocessor also runs the level through a time-weighted chart to determine when a person’s carboxyl hemoglobin (COHb) level would begin to be dangerous. COHb is the level of CO saturation in blood. For example, 70 parts per million (ppm) of CO, weighted over four hours, equals a 10-percent level (approximately the point at which an alarm would first sound). But at 400 ppm, COHb reaches 10 percent in only 15 minutes, and the alarm would also sound. In the past, CO detectors tended to give frequent false alarms due to the outgassing of fumes from carpet, adhesives, and solvents. Most false alarms have been eliminated on new models — a good thing, because many people disconnected this critical safety device as an annoyance.

How to Use Them?

Placement is easy because many are powered by a 9-volt battery. CO mixes well with air, and there’s usually circulation on a boat, so positioning them isn’t overly critical. Mount them where you can see them, not in a corner or near a low shelf or berth because a blanket or jacket could inadvertently cover them. Be sure that if a CO detector is hard-wired, it goes directly to the battery (with a proper fuse). A few years ago, two people were killed because their CO detector was wired to come on only when the engine’s ignition switch was on; they were overcome by CO from a generator.

CO detectors should be kept at least a foot away from an opening port or hatch and not too close to a propane stove. According to the ABYC, a CO alarm should be located in the main cabin, plus one in each sleeping area. Look for the stringent UL 2034 Marine labeling; household detectors can’t stand up to the rigors of boat life.

What to do When it Sounds?

You can’t see, smell, or taste CO. If an alarm sounds, it’s for a reason. When it alerts, reset it (it will begin sensing again in a few minutes), get outside into fresh air, and make sure everyone on the boat is accounted for. Call emergency services if any crew member complains of headaches or nausea; if someone is disoriented or unconscious, immediate medical attention is necessary, and a mayday call is justified. Don’t go back in until you’re certain the boat has aired sufficiently and your CO alarm no longer sounds. You’ll need to find the source of the CO before using the boat again. Common problems are leaking exhausts from engines and generators, nearby or rafted boats with generators or engines running, and faulty cooking or heating appliances.

Since 2010, CO alarms have a built-in end-of-life alarm that lets you know with an audible and visual signal when it’s time to replace it, usually after five or seven years. The first of these newer detectors are reaching the end of their life, so a lot of boat owners will start hearing the warning chirps that mean it’s time to replace these alarms. If you have a detector that’s older than seven years, replace it. New boats with accommodation spaces built to ABYC standards will come with CO alarms installed. Most boatbuilders who are members of the National Marine Manufacturing Association build to ABYC standards.

Types of Detectors?

1. Bilge high-water alarm with dashboard-mounted buzzer

2. Bell-style bilge alarms can be easily heard above the noise of engines

3. Carbon monoxide detector as usually found in the home

4. Fire ports allow access to the engine compartment for a fire extinguisher hose without opening up the main hatch

5. Combined device carbon monoxide detector and smoke alarm

Smoke Alarms

Who Needs One?

Unlike CO detectors, smoke alarms aren’t required on boats. But you should have one, especially if you regularly sleep aboard or have an enclosed cabin or engine space.

How They Work?

Smoke alarms use one of two types of sensors: ionization or photoelectric. Ionization detectors are better at alerting to fast fires; photoelectric are best for smoky fires. For boat use, an ionization or combination detector is best.

How To Use Them?

Smoke alarms should be placed in the living space of the boat, but not so close to the galley that making toast will set them off. Some detectors have a mute button that silences them for a few minutes during minor cooking accidents but keeps them operational. Detectors can be hard-wired into the boat’s system or have a self-contained battery. Newer detectors have batteries that last 10 years — also the life of the unit. Battery-powered detectors have the advantage that they can operate even with a dead ship’s battery. Use the test button monthly to check its operation.

What To Do When It Sounds?

Most people have probably heard a smoke alarm go off at one time or another and are pretty attuned to looking around for smoke when it happens. But smoke detectors often sound before you can see smoke, so don’t assume it’s a false alarm.

Because our BoatUS Marine Insurance claim files show that the majority of fires are in or near the engine room, it’s the first place to check if you don’t see obvious signs of a fire. But beware; opening the engine cover may allow a smoldering fire to suddenly erupt when fed with air. Crack the cover and see if the compartment contains smoke or even flames. Installing a fire port allows you to insert a fire extinguisher in the space and put out a fire without opening the hatch. Sometimes a smoke alarm indicates an overheating engine (the rubber exhaust hose melts) or a hot electrical wire. Don’t dismiss the alarm until you’re certain there’s no danger.

There are no UL marine-approved smoke alarms currently available, so choose one designed for the RV industry; they’ve been designed to be more rugged than home units. The box should say that the detector is built to UL 217 standards.

High-Bilge-Water Alarms

Who Needs One?

If you have a bilge, you probably need one. For the past few years, the ABYC has recommended high-water alarms on boats, and new boats built to its standards will have one installed. These are simple devices that typically use a switch to activate an alarm when water reaches a predetermined level.

Some boats have multiple bilge areas; it’s best to have a separate bilge alarm for each area, with a light as well as an audible alarm and a label to indicate the area involved. The warning can give you enough time to find a leak before it’s too late. Some owners link the switch to the boat’s horn, assuring that the alarm will be heard when they’re away.

How To Use Them?

The detector switch should be located high enough above the normal level of bilge water to prevent the alarm from sounding when the bilge has a small amount of water easily handled by the bilge pump; it should be located low enough to alert you if there’s a real problem. The same issues that plague bilge pumps can affect high-water alarms: corroded wire ­connections and jammed switches. While the alarm itself may last indefinitely, float switches need to be checked at least annually.

What To Do When It Sounds?

First, immediately check the level of the bilge water, then locate the leak. According to a recent BoatUS Marine Insurance study, at least a third of boats that sank while underway did so because of leaks at thru-hulls, outdrive boots, or the raw-water cooling system. Many thru-hulls are in the same place as the outdrive bellows and raw-water cooling system: the engine room. Look there first.

Charles Fort

Contributing Editor, BoatUS Magazine