How to Properly Ventilate Your Boat’s Engine Room

Let Them Breathe

It takes more than fuel to make motors happy. They need air, too—and lots of it.

We work a few months ahead of the cover date here at Power & Motoryacht, so I’m writing this in early September. It’s still hot and humid outside, but because I dislike air conditioning, I keep the windows open even if it makes my office a bit warm. That’s okay for most things—typing isn’t very strenuous—but cranking my stationary bike at the end of the workday becomes something of an ordeal: Warm, moist air takes up more volume than cool air, so I can’t cram as much oxygen into my lungs with each inhalation, and my respiration isn’t as efficient. It’s the same with engines: Without ample fresh air flowing into the engine room, even the most perfectly tuned motor won’t perform up to its potential. If the air’s really hot, it’s even worse. Is your engine breathing easy?

How much air does an engine need? Like me after half an hour of pedaling, engines—gas or diesel—are heavy breathers. Determining the precise air requirements is tricky, since some engines are more efficient than others. Turbochargers make a difference, as do electronic ignition-controlled systems for gas engines. Engine manufacturers calculate how much air each of their models need, and the vent sizing necessary to provide it, and usually hide the figures somewhere in the documentation. When you can’t find them, do what most engineers do, and estimate both based on horsepower. The math is simple.

Truckload of Air

According to yacht designer Dave Gerr, whose book, Boat Mechanical Systems Handbook is my bible for this kind of thing, the minimum airflow necessary for an engine equals, in cubic feet per minute (CFM) 2.75 times the horsepower minus 90. (I don’t know where the “minus 90” comes from, but Gerr always has a reason.) Some experts, including the folks at Caterpillar, simply multiply horsepower by 2.5. Take your pick, but more air is always better, with one exception I’ll address in a moment. Using Gerr’s formula, a 315-hp, 4.164L Yanmar 6LPA-STP2 turbodiesel, for example, requires a minimum of about 776 CFM of airflow when running wide-open. That’s enough air to fill a 15-foot U-Haul truck every minute. Okay, most skippers don’t run wide-open very often, but even at 70 percent power, it’s still a heck of a lot of air. And that’s just for one medium-sized engine; to estimate the total required airflow in any engine room, add the horsepower of all main engines and gensets. (Figure genset horsepower as twice the kW rating, e.g., go with 15 horsepower for a 7.5-kW generator.)

There should be additional airflow to cool the engine room, too. Excess heat not only impairs engine performance, but reduces alternator output and battery power, and can damage hoses, belts and other components.

Gerr calculates the minimum total vent area in square inches for a passive ventilation system, i.e, no intake or exhaust blowers, by dividing total horsepower in the engine room by 3.3. He recommends adding 20 percent for most systems to make up for resistance from the ducting system. Screens, louvers, clamshells, water traps, ducting with sharp bends or twists, etc., all reduce airflow, just like resistance in a wire reduces current flow. That’s a vent area of at least 115 square inches for the single Yanmar in the example above, twice that for twins, plus something for the genset. This should provide enough air for the engines, plus extra intake to flush out hot air and keep the engine room cool—no more than 30 degrees higher than the outside, ambient air temperature.

Boatbuilders have engineering departments which figure this stuff out and provide the necessary ventilation based on ABYC and USCG rules and standard practice. But some builders provide just the minimum, and then insulate the engine room so well that the only air that gets in is through the vents. (In the old days, before folks got so paranoid about noise and so forth, engines picked up air from the bilge, from loose-fitting hatches or engine covers and from all kinds of leaky seams as well as through the vents.) If the ventilation system is even slightly damaged or partially blocked, the airflow can fall below requirements. It’s easy to test your boat and see if there’s enough air getting to your motors.

Is My Engine Breathing?

There are two parts to this test. First, check that your engines are getting enough air by running wide-open for a few minutes, keeping an eye on the tach. If you have a genset, start it, too. Give the engines time to inhale the air that’s already in the engine room; sometimes, the revs start to fall as the air is used up. Then open the engine-room hatch or door; if the revs go up, you need more airflow. If the suction from the gasping engines makes it hard to open the door, you really need more airflow. That would be an extreme case: Usually there’s enough air to feed the engines, but maybe not enough to cool the engine room. In a passive ventilation system, the engines, which are powerful air pumps, create enough suction to pull air in from both the intake and exhaust vents. If yours can’t, and there’s nothing obviously wrong—no damaged ducting, clogged louvers or screens, etc.—your ventilation system needs help.

You’ll need a couple of thermometers for the second part of the test. Mount one in the engine room and one someplace on deck, in the shade, to read the ambient outside temperature. Again, if you have a genset, start it.

Now, after running the boat long enough and fast enough for things to get good and hot in the engine room, compare the thermometers: If the maximum engine-room temp is more than 30 degrees higher than the ambient temp, or if it exceeds 120 degrees, you need more airflow. (Some experts, including the folks at Caterpillar, say 20 degrees difference is preferable. Bottom line is, cooler is better.) The treatment is the same as above: More airflow.

CPR for Your Engines

If it turns out you have air-flow issues—after making sure there’s no obvious, easily repaired damage to the vent system—call a qualified engine tech to suss the situtation out. The two common solutions are, first, install bigger vents, or more of them, to maintain passive ventilation; or add exhaust and/or intake blowers to create a powered ventilation system. Enlarging vents isn’t always as easy as it sounds, and involves either cutting bigger holes, or more holes, in the boat, and either replacing the current ventilation ducts and components or adding more of them. Exhaust blowers are frequently the better choice. Any qualified marine electrician can install them; although, like most things nautical, some installations are easier than others.

Exhaust blowers increase airflow by creating negative pressure that draws in cool air through the passive intake vents, pulling it through the engine room and ejecting it, along with the heat it picked up along the way. For cooling, the exhaust fans should be mounted high in the engine room, where the hot air gathers naturally, and as far as possible from the intake ducting, which should introduce fresh air lower down in the space. The blowers should be wired to start when the engines do, and stay on for 15 minutes or so after shut down—that will cool the engine room much faster than by passive ventilation. (A time-delay relay will do this automatically; one specifically for marine use is Blue Sea’s ATD Automatic Timer Disconnect. It’s a simple hook-up.) Exhaust blowers also draw out engine-room odors before they can migrate into the rest of the vessel. If intake blowers are also installed, they should be slightly less powerful than the exhausts to maintain the negative pressure.

There’s one caveat: In the unfortunate circumstance of an engine-room fire, exhaust blowers can pull out the fire-extinguishing agent before it has a chance to smother the flames. Fresh air pulled in by the blowers will also feed oxygen to the fire. The solution is to wire the blowers to the automatic fire-extinguishing system, so when the system trips and shuts down the engines, the blowers also shut down immediately. For a very complete fire-suppression system, add automatic louvers to the intake and exhaust vents so they close at the same time. Again, this is a project for a qualified electrician. It’s a bit costly, but with luck it’ll be money wasted.

If your engine room doesn’t get as hot as Satan’s forge after spending the day on the water, and even when you splash out with the throttles the engines breathe nice and easy and hold RPM until you get worried about burning too much fuel, chances are none of the above applies to you. But if any of it does, take steps now to make the air flow more freely over your engines, before excessive heat takes its toll. Your machinery will thank you.

It’s Not a Bilge Blower

Don’t confuse exhaust blowers with bilge blowers. Exhaust blowers live up high, where they can blow hot engine-room air overboard. Bilge blowers draw from the bilge, where smelly and, sometimes, explosive fumes reside. Gasoline engines must, by law, have at least one ignition-protected bilge blower to suck fumes out of the lowest parts of the bilge and discharge them overboard before starting the engine. The intake for the bilge blower is placed in the lower third of the engine compartment, above the normal level of bilge water. It’s usually under the engine.

If a boat’s required to have a blower, the system must conform to federal rules as to both blower capacity and actual airflow. The regulations are spelled out in Code of Federal Regulations Title 33, Sections 183.601 to 183.630. It’s exciting reading. These rules apply only to gasoline-powered boats, but all boats will benefit from having a bilge blower.

Diesel fumes don’t explode, so diesel-powered boats don’t fall under as many regulations as gasoline boats. However, diesel fumes smell bad, so why not install a blower and suck them out rather than breathe them in? Boats with propane galley stoves should have a bilge blower too, even though propane leaks are rare: the tanks should always be stowed in sealed lockers that are vented overboard, and smart skippers install automatic shutoffs at the tanks that stay closed except when something’s on the stove. However, like gasoline fumes, propane fumes are also heavier than air so any gas that does leak can accumulate in the bilge—and if it goes off, it goes off with a bang. I wouldn’t own a boat without a bilge blower, no matter what fuel I was burning.

This article originally appeared in the February 2019 issue of Power & Motoryacht magazine.