When I bought my Down East 45 schooner, Britannia, it didn’t have any engine room ventilation, either through natural aeriation with cowls, or electric blowers.
The term, “engine-room” is a bit of a misnomer on this boat, because the area is 26’ feet long—from the forward saloon bulkhead to the aft cabin. It is also 4’ feet wide at saloon floor level, tapering 4’6” inches down to the bottom of the bilge. That is a very cavernous area and doesn’t just contain the main engine, but all the other machinery that runs the ship, including a diesel generator, a water heater, nine batteries, a large battery charger, five electric pumps and the large Perkins 4-236 main engine. All these go to form what I call the “equipment bay.”
When everything is humming, and especially when both the generator and main engine are running together, the heat permeating though the 3/4” inch plywood cabin sole could be felt on bare feet. I once placed a thermometer in the space and after five minutes it registered 150F, which isn’t good for the machinery, or my wife’s’ and my feet.
Whether you have a large underfloor area, or a small engine compartment, it will always be beneficial to ventilate the space. All mechanical devices create heat, and diesel engines are designed to run hot, but internal combustion engines also run better when drawing cooler air, which has the effect of increasing the swept volume in the cylinders.
It was obvious to me that a single electric fan would be hard-pressed to suck all the hot air out, and draw cooler air in over such a long area. I thought about whether to have two fans sucking air out, but theoretically that would mean a bigger inlet. A fan would be needed to blow fresh air into the front of the compartment, and also one to suck hot air out somewhere near the stern. Since the floorboards aren’t totally air-tight I was also a bit concerned air might be sucked below from the air-conditioned living areas. The ACs work full-time as it is, and I didn’t want to make it harder. I then had to decide how best to route trunking from on deck, to below the floorboards, yet maximize the air flow.
I found a 4” inch diameter, five blad fan on Amazon.com, for $35.00. The Attwood 1749-4 Turbo 4000 is a 12-Volt in-line blower designed to fit 4” Inch interior diameter piping. The specification said it was water resistant and guaranteed for three years.
The main problem was how to route the trunking to achieve maximum flow, and indeed, what type of trunking to use. I have used flexible plastic wire-wound pipe before. It is frequently used on boats for air conditioning, but my experience has been that it doesn’t last very long. Even if the pipe is protected from vibration through bulkheads, which it inevitably needs to pass, it only takes one small tear in the plastic sheathing to cause a leak and reduce the air flow, and which you sometimes can’t even see, and usually gets worse as time goes by. Furthermore, because it is so thin-walled it conducts heat very easily, but my objective was to get the heat out of the boat as quickly as possible, not to warm the various lockers the pipe passed through. I also found 4” inches diameter corrugated aluminum pipe in my local hardware store, which is stronger than flexible plastic, but also conducts heat even more. Flexible pipe can easily be squashed by other items in lockers, sometimes severely limiting air flow. Rigid pipe is also self-supporting between bulkheads.
After much thought and lots of measurements, I finally decided to use 4” inch diameter rigid plastic pipe—the type used in houses, for sewage lines. This has a wall thickness of about 1/8” inch, and a smooth interior bore, offering the least resistance to air flow. 90░degree and 45░degree bends are also available, enabling a pipe to be routed just about anywhere.
I bought two fans, three 10’ lengths of pipe and a pile of different bends. With this type of project, it’s difficult to know what type of bends will actually be needed, so being able to return unused items to the store was a great advantage.
At the forward end of the equipment bay was a bank of six vertical lockers. Ideal to route a long inlet vent pipe straight down from on deck to the forward end of the bay.
What I didn’t count on was the effort needed to cut the large diameter holes, first through the deck, then through seven more thicknesses of 3/4” inch solid teak. It was hard going, using a 4╝” inch diameter hole cutter on a 90░degree drill attachment, boring through each locker base, then finally through the lower bulkhead into the equipment bay. It was difficult to get the holes exactly level, so the extra ╝” inch gave me some wiggle room. I was finally able to slide a 10’ foot long pipe straight through all the holes. Then, using a 90░degree bend on the end, air was directed straight on to the generator motor.. Not many boats will have such a convenient way to install an inlet pipe, but it doesn't need to come in the area in the center. Anywhere at the front of your engine area will do.