Propulsion System Fires

Taken, in part, from Seaworthy, the Boat US Marine Insurance and Damage Avoidance Report

Volume 22, No.2…April 2004

On a warm Michigan afternoon, a [Boat U.S.] member took seven passengers, five of whom were kids, to a pleasant anchorage off Gull Island in Lake St. Clair. Later, toward evening, the engine on his 28-foot boat was reluctantly started and the group left for home. Shortly after, one of the kids hollered that he could see smoke coming from the engine compartment and immediately the engine died. The member wisely had all his passengers quickly put on PFD’s and get into the water while he tried to determine the cause. When he (unwisely) opened the engine compartment, he was driven out by black smoke and flames. He called the Coast Guard, gave his position, and tried once again, to put out the fire. After emptying a portable fire extinguisher, he decided that since he wasn’t certain that the fire was under control and since he was an accomplished swimmer, the lake looked particularly inviting. The skipper and all passengers were quickly picked up y the local sheriff’s department, uninjured. A blockage in the engine’s cooling system was later found to be responsible for a severe overheat that could have been a tragedy had it not been for the member’s quick thinking (Claim #0205353).

A recent Seaworthy study found that nearly a quarter of all boat fires are caused by the boat’s propulsion system-the engine, transmission, or turbochargers-overheating. Since they tend to generate billowing clouds of thick, black smoke and stop the prop, these overheat fires quickly get a lot of attention.

Keeping Cool

Burning fuel in an engine produces a lot of heat. Pop Quiz-what percentage of  fuel is actually used to make a boat move through the water? If you said none of it, you’re either a sailor or you spend too much time at the dock. If you said all of it, the answer might surprise you-only about a quarter of the energy from gasoline (a little more for diesel) goes toward moving a boat from point A to point B. Most of the rest is wasted as heat and has to be taken away from the engine, lest the engine overheat and begin an expensive meltdown. About half of this wasted heat is eliminated through the engine’s exhaust and the rest is taken out through the cooling system.

 You’re Blocking !

 The majority of propulsion fires are caused by engines overheating. Most are caused by intakes or manifolds becoming blocked or water pumps failing. When that happens, the exhaust system suffers first, since cooling water is no longer carrying off heat from the exhaust gasses. Within seconds of a blockage, rubber hoses that connect to the boat’s exhaust discharge start to melt and make lots of smoke-not surprising when you realize the exhaust gas temperatures without the benefit of cooling water can be well over 1000ºF. If the engine keeps running without cooling water, the hoses can ignite and burn nearby wires and possibly the engine cover (this is when you’ll be glad your gasoline lines are made to USCG A1 Standards, which means they can withstand a few minutes of flame before spewing gas into the mess). If you hadn’t noticed the smoke before, you certainly will when you look back to see why the boat is losing power. During a high-speed run, one member’s starboard engine began to overheat when the intake clogged and within seconds all flexible boots from the exhaust and shift cables began to melt. The member never noticed his temperature gauge climb and only knew there was a problem when the boat started slowing. He looked back and saw a trail of black smoke behind his boat a quarter mile long. By the time he had shut the engine, it was too late. The block had cracked, the heads were damaged and even the port engine had damaged components due to the nearby heat. During the damage inspection, it was found that the engine had no intake screen, which likely contributed to the cause (Claim #0104130).

Most systems use a rubber impeller-type pump to suck cooling water into the engine. These pumps will suck up water and, if you’re not careful, weeds, mud, and sand. A number of engine fires were caused after a boat ran aground and an impatient skipper kept shoving the throttle between forward and reverse to free the boat. With the props stirring up all kinds of junk , some of it got into the cooling system and, just when it was needed most, the system failed which left the boat aground and on fire.

If you can’t get off immediately with the engine power, it’s time to wait for the tide or help from Tow Boat U.S. Impeller-type pumps can also fail completely when their rubber vanes come off due to age or a previous overheat. Not only will the pump no longer operate (bad), but the vanes can get into cooling passages and create blockages (worse). Replacing your impeller every 100 hours or two years, as recommended by manufacturers, can prevent pump failures. Some boaters replace them every season as cheap insurance.

Stopping crud from getting into the system is equally important Your intake through-hull should be equipped with a screen and a strainer should be installed before the pump. Regular cleaning of each will keep the water flowing.

   Heat Exhaustion

Another engine fire was started when a member was pulling a skier. The skier yelled that he could see smoke coming from the cowling vents, at which point the driver stopped the boat. It was later found that the exhaust manifolds were clogged with corrosion. Cooling-water barely trickled through---a classic case of lack of maintenance. The surveyors report speculated that by the time the boat was stopped, the manifolds were red hot, which would explain why, when the member made the mistake of opening the cover to see where the problem was, the engine cover ignited. Quick action with an extinguisher put out the fire before it could spread. Note that opening a smoking engine is not a good idea; the rush of fresh air into the compartment allows a smoldering fire to grow instantly. The use of a fire port or automatic extinguisher is a safe and more effective control.

An exhaust manifold keeps exhaust gasses and cooling water separated so that the water goes directly overboard through the exhaust without getting into the engine. But, the now-hot cooling water also has one more job to do before it can go home---cool the manifold. Without this water, a large cast iron manifold will quickly get red hot.

Manifold Maintenance

Raw-water-cooled engines run water (often salt water) through manifolds, which must also endure hot gas and corrosive acids. Manifolds are often exposed to salt water and vibration, and left idle for long periods while rust and corrosion eat away at the metal and clog water passages. It’s a wonder that they last as long as they do (which isn’t very long). Boats operating in Florida may get three or four years out of manifolds, while those in the Great Lakes may last twice as long. After awhile, rust can build up and clog the all important cooling passages, resulting in a glowing manifold. Removing manifolds to check for corrosion every couple of years should be considered standard maintenance on boats that operate in salt water. Another option is to convert your raw-water-cooled engine and manifold with a fresh water cooling system conversion kit. This will increase overall engine life and you won’t have to worry about raw-water corrosion in the manifold, which is one of the most common causes of over-heating. Note, however, that the risers on top of the manifolds will still be raw-water cooled.

Alert to the Problem---Over-heat Alarms

Watching your engine’s temperature gauge is one way to make sure the engine doesn’t overheat. However, keeping your eyes glued to a two inch gauge isn’t conducive to fun (or safe) boating. The answer to that problem is an overheat alarm, one that is loud enough to hear over engine noise.

Other Hot Mechanicals

What else in the engine room gets hot enough to start a fire? Smoking-hot transmissions and turbochargers accounted for 4% of all boat fires, and it’s easy to see why these components can overheat. Marine transmissions are frequently called on to work at peak loads for hours at a time It’s a good idea to check transmission fluid levels at a regular intervals and check for leaks. Turbochargers (mostly in diesel engines) deal with extremely hot gasses and need a constant supply of cooling oil. If a seal ruptures and no oil reaches the turbo, spinning at up to 100,000 rpm, it goes into meltdown, shooting out flames that can ignite the insulating blanket and anything else nearby.

 Things to Remember

Your hull-mounted screen and engine room strainer need to be inspected frequently to make sure there’s nothing to obstruct the flow of water. Sterndrives often have small intakes that can get clogged with marine growth.

Manifolds have a limited service life and eventually rust begins to narrow the passages. Remove manifolds every couple of years to check for blockages.

Rubber impellers wear with time and eventually the vanes can break off, causing the pump to fail, and worse, clog the system.

Engines and transmissions depend on oil to lessen friction and heat. Look for leaks and correct them before you’re faced with a meltdown.

Make sure flammable materials are not close enough to ignite.