Skip to main content

Ventilating Your Boat's Engine Room

Breathing Space

Air is just as crucial to proper engine performance as fuel.

Engine room fan

One of the most valuable metaphors I learned while attending diesel-technician school came from an instructor who claimed to be Swiss but whom we all suspected was German. The Austrian actor Christoph Waltz could have perfectly portrayed Rudy as he directed us in his clipped Prussian accent to think of the internal combustion engine not as an engine but as an air pump. I remember exchanging stupefied glances with my fellow students as we all wondered if he’d been into the schnapps again.

Rudy may have been a tippler but he was also a brilliant engineer, for what he was trying to impress upon us was the importance of getting the right amount of air into an engine, especially a diesel engine. Only if you manage to do this can you then add the proper amount of fuel and produce the requisite power.

I didn’t think much about this until I actually began working on marine engines, which of course operate in enclosed spaces. More to the point, because they are in a wet environment, the admission of air must be controlled so as not to allow water—especially salt water—to enter the engine room. This is usually effected by the size and shape of the openings that admit air to the engine room and by installing baffles or “demisters” that remove moisture from the air. In a future column, I’ll explain how demisters work.

It’s pretty rare to come across a pleasure boat that doesn’t have properly sized engine air intakes but not so rare to find one where,  after a hard run offshore, a fine layer of salt covers everything in the ER. That is to say, there are boats—especially older ones—that do not have proper baffles or demisters. Installing baffles in an existing engine room can be a difficult and expensive job; installing demisters, not so much. If you’re finding even a fine layer of salt on your engine-room equipment, your engines are at serious risk and you need to look into retrofitting demisters.

There are two other aspects of engine-room ventilation of which you should be cognizant. One is making sure that your engines are getting the proper volume of fresh air for combustion. It’s uncommon (but not unheard of) to find a pleasureboat engine room where this is a problem because proper sizing of the engine-room openings is an important part of the design process. The only instances I’ve come across were on prototype and custom boats and production boats where something had been allowed to restrict the engine-room ventilation openings. (I remember one boat on which the captain had carefully lined up all of his cleaning products right in front of an engine room vent. It looked very neat but basically reduced the opening’s effective size by about half.)

There are signs you can look for if you think your engine-room ventilation might be inadequate: Your ears pop when you enter the space with the engines at full tilt (wearing ear protection, of course) or the engine room door is pulled shut when you try to open it. In any case, you’ll probably notice that your engines are not making full power, and in extreme cases, not making full rpm. In this case, your next step is to consult a technician who will probably measure the ambient air pressure in your engine room with the engines at operating speed. The most likely corrective measure is to install intake and exhaust fans.

A more likely (but still relatively rare) problem for the average boater is an engine room in which the air temperature is too high. Hot air is less dense and so contains less oxygen than cooler air, which is what engines need to combust fuel. Of course, engine-room air temperature can be directly related to air volume: If you don’t get enough outside air into an engine room containing a couple of big diesels, things get hot pretty quickly. But the issue is also related to the temperature of the exterior air. It’s more difficult to keep an engine room at the correct temperature in the tropics than in higher latitudes.

So what constitutes “correct temperature?” Engine manufacturers will happily supply you with ambient air-temperature guidelines but as a general rule, a proper engine-room temperature should be relatively comfortable for you to remain in. (Remember, we’re talking the temperature of the air, not mechanical components.) If your engine room is too hot for you to remain in for more than a couple of minutes, further investigation is warranted.

Fortunately, most boats will never have a problem with their ventilation. Just remember not to put your cleaning products up on that shelf in front of the engine room vents, and your engines should have no problem getting their next breath.

Want to learn even more? The diesel engine is the beating heart of your boat. Arguably, a good running engine is your most important single piece of safety gear. Diesel engines are economical, long-lived and reliable, yet there are a number of ways in which they can fail. This seeming contradiction brings us to the distinction that has shaped the Online Marine Diesel Maintenance & Troubleshooting course.Sign up today and use the coupon code: POWERMOTOR for 15% off.

This article originally appeared in the June 2015 issue of Power & Motoryacht magazine.