Vent an engine room
Roger Hughes vents his engine room
Using electric fans to get air in and out of an engine room
When I bought my Down East 45 schooner, Britannia, it didn’t have any engine room ventilation, either through natural aeration with cowls, or electric blowers.
The term ‘engine room’ is a bit of a misnomer on this boat because the area is 26ft long from the forward saloon bulkhead to the aft cabin. It is also 3ft 6in wide at saloon floor level, tapering 4ft 6in down to the bottom of the bilge. That is a very cavernous area and doesn’t just contain the large Perkins 4-236 main engine, but all the other machinery that runs the ship including a 6.5kw diesel generator, a five-gallon water heater, nine batteries, a large battery charger and five electric pumps. All these 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 through the 3⁄4in plywood cabin sole could be felt on bare feet. I once placed a thermometer in the space and after five minutes it registered 66°C (150°F), which isn’t good for the machinery or our 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 considered whether to just have two extractor fans sucking air out, but theoretically that would mean an inlet of twice the size. On the other hand, a fan blowing air into the front of the compartment, and one sucking hot air out somewhere near the stern meant that both tubes could be the same size.
Also, since the floorboards aren’t totally air-tight I was a bit concerned that with two extractor fans the air might come from
the air-conditioned living areas and the air conditioning works full time as it is.
Having decided on a fan in the front and near the stern, I next had to work out how best to route trunking from on deck to below the floorboards, yet maximise the air flow. I found a five-blade fan on Amazon for $35.00. The Attwood 1749-4 Turbo 4000 is a 12V in-line blower designed to fit 4in interior diameter piping. The specification says it will move about 150 cubic feet of air per minute. It’s water resistant and guaranteed for three years.
Trunking route
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; something you can’t always see, and which usually gets worse over time.
Flexible pipe can easily be squashed by other items in lockers, sometimes severely limiting air flow. Furthermore, because it is so thin-walled it conducts heat very easily into lockers, whereas my objective was to get heat out of the boat. Another option was corrugated aluminium pipe, which is stronger than flexible plastic, but conducts heat even more.
After some thought and lots of measuring, I finally decided to use 4in diameter rigid plastic pipe – the type used in houses for sewage lines. This has a wall thickness of about 1⁄8in and a smooth interior bore, offering the least resistance to air flow. You can also get a variety of angled bends, enabling a pipe to be routed just about anywhere. Unlike flexible pipe, rigid pipe is also self-supporting between bulkheads.
I bought two fans, three 10ft 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 used, 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
3⁄4in solid teak.
Tough job
It was hard going, using a 41⁄4in diameter hole cutter on a 90° 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 1⁄4in gave me some wiggle room.
I was finally able to slide a 10ft long pipe straight through all the holes. Then, using a 90° bend on the end, air was directed straight onto the generator motor.
Not many boats will have such a convenient way to install an inlet pipe, but it doesn’t need to come into the centre. Anywhere at the front of your engine area will do.
I left the pipe sticking out of the deck an inch, so I needed some type of vent cover to stop rain entering the pipe, yet still with good air flow. I thought a classic dorade style vent – where air has to pass over a baffle – might restrict the maximum air the fan could draw. I therefore built a simpler teak box, with baffles to keep rainwater out, but still allow good air flow.
I mounted the fan half way down the tube, in a locker big enough to be able to get at it in the event it might fail. When the locker doors are closed, nobody can tell there is a large ventilation pipe inside.
Perfect fit
The pipes and bends slotted perfectly into each other and into the fans, with an airtight fit. I therefore didn’t need to actually glue the connectors together. I simply drilled holes through the joints, then screwed in two self-tapping screws to stop them coming apart through vibration. There’s always a chance of something breaking, so if it’s necessary to dismantle any section it’ll be much easier this way.
One disadvantage of using rigid pipe, especially 4in diameter, is that it’s sometimes difficult to pass long lengths through lockers and bulkheads because it can’t be bent. The way to achieve this is to use a coupling and splice two pipe sections together after they are installed from both ends.
I positioned the rear fan alongside the main engine, so it would suck out the maximum amount of hot air. I then routed the tubing to the back of the boat, through the aft cabin starboard side lockers, and up to the after deck. I cut the holes with the hole-cutter and drill, which were beginning to show signs of fatigue – as were my arms and shoulders.
It took a lot of scrambling through lockers and bulkheads to get the rear piping lined up, but eventually, using both 90° and 45° bends, I managed to bring the pipe out through a hole in the deck. I then caulked it and used a 90° bend and grating to act as a cowl. I also bought a cap which fits on the pipe to keep rain out when we’re not using the blowers.
There is no on/off or reverse switch on these fans. To reverse the air flow it’s just a matter of mounting the fan in the opposite direction, so one way blows and the other sucks. Wiring them was also easy, by connecting to spare 12V contact breakers on the main distribution panel, which I marked, ‘extractor fans’. I wasn’t concerned about the 2.5A current draw from each fan either, because I only use them when the engine or generator are running, both of which have alternators.
Jet engine rush
When I first switched both fans on and felt the enormous rush of air shooting out the rear cowl like a jet engine, I had visions it might give Britannia another knot or two!
The real test, however, was how much the system would displace the hot air, and reduce the temperature in the bay, when everything was operating normally under way. For that test I waited until the temperature had risen to 66°C (150°F) then switched the fans on. The heat coming out of the rear vent would have melted varnish! It still took 15 minutes to drop the temperature to 38°C (100°F) and a further 15 to reduce it to 29°C (85°F), which was the outside temperature that day.
When I started the fans with cold equipment, the temperature remained steady at just a bit above the ambient outside temperature. The forward fan blows out directly in front of the generator motor, which runs cooler when the blower is on. Even blowing 35°C (95°F) air in is cooler than normal running temperatures. I could almost hear the equipment breathing a sigh of relief.
Side benefit
There was another unexpected benefit I’d not considered. The air rushing out of the rear vent smelled – a machine-shop smell – so the system was also removing odours that would normally stay in the equipment bay. Whenever I arrive at the boat, I now switch both fans on immediately and they seem to help remove that ‘closed up’ boat smell.
This was a very satisfactory outcome for all the effort, with not much actual cost. It would be a worthwhile addition to any boat, especially one with all the machinery like Britannia.