See full version: The Holy Place Let It Breathe – Engine Compartment Ventilation
A few years ago, the owner of a new trawler visited the boatyard I manage. He sat in my office and dictated a moderately long list of improvements, modifications, and repairs that required attention. One of his minor complaints was that the engine-compartment door was sticking. He complained that when the engine compartment was hot, the door was difficult to open.
We’ll return to that engine-room problem a little later on. Before doing that, however, a discussion on the importance of proper engine-room ventilation is in order. Beyond the obvious and above example—doors and hatches that are difficult to open when these compartments aren’t properly ventilated—there are a number of other important reasons to ensure that your engine and the compartment in which it lives are supplied with ample quantities of fresh air. here
While the need for these two roles may appear obvious, there are a few subtleties that are worth mentioning. The heat produced by engines that isn’t removed by the seawater cooling process (it doesn’t matter whether your engine uses seawater, a heat exchanger, and a wet exhaust or a keel cooler and a dry stack; heat is still transferred from the engine’s coolant to the seawater) is commonly referred to as “heat rejection to the atmosphere.” Additionally, engine-compartment heat may also be created by auxiliary machinery, such as air conditioning compressors, generators, and various electric motors and pumps. Additionally, the heat load on an engine compartment may be reinforced by the color of a vessel’s hull; black, blue, and green hulls will become considerably warmer than lighter colors. On occasion, I’ve measured hull temperatures as high as 180 degrees F where a dark-colored hull was exposed to several hours of direct sunlight. The degree of hull insulation (cored hulls offer excellent thermal insulation) will determine how much of a factor this becomes on engine-compartment temperature. [links]
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. more
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.
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.
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.
Most of those inline blowers (axial fans) have flow ratings around 125 cfm free air, which means no connections to it, or even measured with a bellmouth inlet adaptor. Connect to corrugated flex with rough interior, make a bend or two, add an exhaust louver and an intake screen, and you're moving half that amount or less. more
Chris: I have never had a problem with a blower running constantly while under power. when running gas engines that was standard procedure for me and I can only think of one time, after many, many hours, one of those fans crapping out.
The amount of heat an inboard diesel, even a small one, will shed to the interior while running is a Goliath to the blower's diminutive Samson-esque air movement. The engine intake will likely remove more air (probably around 70 cfm for a 100 cid 3-cylinder 4-stroke) than the blower.
If you want it to keep the smell down when docked and sleeping on the boat, go for it. For keeping the engine cooler, I suspect it's like spitting on a frying pan.
When a boat is under power, the engine can create an amazing amount of heat – so it’s important to ensure the marine engine room ventilation system is doing its job. Boat engine blowers are a great place to start, since they are made to push a huge amount of air out of the engine room, but it can also help to position boat engine vents in strategic locations to help cool the engine room off after the boat has arrived at its destination. Fisheries Supply stocks a wide variety of all these types of air vents – so shop today and keep your engine room cool! more