Excess engine room temperatures are hard on the equipment and tough on the people. Engine room checks end up less detailed and, if you need to do emergency service, it can become a safety problem. We are all sufficiently motivated to want lower engine room temperatures but its actually not that easy. Engine room temperature specifications from engine manufacturers are either specified as no more than a specific absolute temperature or they specify a deltaT. This latter specification is referring to the allowable amount warmer the engine room can be over the ambient outside temperature. Our engine, a John Deere 6068AFM75, requires a deltaT of no more than 30F. Essentially, that means that the engine room temperature is never more than 30F higher than the outside temperature.

Cooling an engine room sounds easy but, on a trawler, the hot engine is nestled away just above the bilge surrounded by insulating fuel tanks and living spaces. It can take hours to even days for the engine room temperatures to fully develop and max out. Many times we thought we had cooling design changes that worked only to learn that, if we ran for a few more hours, we would still end up considerably hotter than the 30F deltaT goal. With stubbornness we did find a workable solution even when operating 24×7 for days in the tropics. What we learned is that cooling is a black art, “obviously right” answers sometimes can be counter-productive and, even some very small changes can have a very large impact.

The eventual solution centered around the observation that we had great engine room air intakes but inadequate exhaust outlet area. If the air can’t get out, new air can’t come in, and it’s flow that cools the engine room. It’s also important to avoid negative pressure which is bad for the engine and positive pressure which can force engine room air into the house. Dirona has 3-inch ducts at either side at the front of the engine room, and a 3-inch duct up the engine room stack. These would seem sufficient, but even after upgrading the exhaust fans on all three, we had made close to zero progress towards the goal. It’s very difficult to efficiently get enough flow through these three small air ducts, two of which have a circuitous routing to the outside that further reduces flow. The secondary problem is the system has the intakes at the back, but the heat producer, the engine, is at the front of the engine room. The back of the engine room was always much cooler than the front.

The first change we made was to convert the port side intake (left below) to an exhaust, and keep the starboard side air intake as is (right below). The starboard side pushes air forward and the port side pushes it aft. We powered both with a bit more than 700 CFM of active fans on each side. This brought the back of the engine room to near ambient, and the front was much improved, but still not good enough.

The next step was to set up a rotation of the air in the engine room by putting large axial fans circulating the air forward along the port side and aft along the starboard side. This ensures the cold air doesn’t short-circuit out the exhaust but has to travel forward to the front of the engine room which is the hottest area on Dirona.

The last step we took was to experiment with the exhaust fans for the small 3″ ducts on each side of the front of the engine room. We ran for days at a time with them on and off. They consumed power, but didn’t impact the engine room temperature, so we took them both out. The stack blower still is needed to keep the stack cooler.

For air moving, we are using three types of fans: 2 Dayton 1TDR7 for the stack blower, 4 Dayton 4WT44 for circulation and 4 Dayton 3VU66 (2 on each side) for intake and exhaust. Note you do need to seal off the around the air intake and exhaust fans. We mounted them to black marine board and then attached them to the bulkhead, sealing off the intake and exhaust. The circulation fans also are mounted to black marine board using mounts from Sure Marine in Seattle .

While we did go through seven or eight different design ideas and implementations to get there, overall, we think the engine room temperature issue was worth solving. It’s better for the mechanical equipment and it’s better for us. As an additional level of protection, in the case longer service operations are required in hot climates, we also have a large floor-standing industrial air mover we can deploy wherever it’s needed. We use this when working in a hot engine room or lazerette for extra cooling, to provide extra cooling for the SCUBA compressor in hot climates, and sometimes for moving air to cool the house when we aren’t using the air conditioning. It was also useful in experimenting with different flow patterns in the engine room.

If you have high engine room temperature, the first place to look is where is the heat coming from? Make sure the exhaust cladding is of good quality and not letting too much heat escape into the engine room. Once you are sure the boat is not dumping excess heat, the next solution is air flow. And, when looking at air flow, it’s truly amazing how much air flow is required to provide adequate cooling. In the tropics, even with these changes, the engine room falls short of a popular place for cocktails when underway but it’s actually comparatively comfortable for engine room checks and is much safer if longer duration service work is required.