Getting Fresh Air Down Below

Boats Need More Than Water!

Published: January 2011

Mention a boat and the first element that springs to mind is water. Boats need water. But boats also need air, which is one of those things people tend to take for granted, since it's always there and it's free. But just because air is abundant doesn't mean your boat is getting all it needs; a lack of fresh air on a boat can cause problems ranging from mold and mildew to sputtering engines. The following suggestions are to make sure your boat will always have as much air as it needs to remain healthy and run smoothly.

Eliminating Fungus

Some boaters seem to think that storing a boat out of the water in the winter will help to keep mold and mildew at bay (the term mildew refers to the fuzzy stuff you see that's produced by mold — which is a fungus). That thinking usually lasts until they open up their boat in the spring and are greeted with (at best) a dank, musty smell or (at worst) an interior full of mildewy cushions, carpets, and headliners. Mold spreads by forming spores and every boat already has them – the key is to deny them what they want so they can't grow.

Keeping Water Out

Two things make the difference between a fresh interior and one that smells like a damp basement: air and moisture. Not enough of the former and too much of the latter causes mold and mildew, and it's a lot easier to prevent mold and mildew than it is to stop its growth. Mold growth is accelerated by high humidity and, once formed, it can survive for years, even if conditions change. The U.S. Center for Disease Control says that mold growth is encouraged by warmth and humidity, but, as anyone who has waited too long to clean out their refrigerator can attest, mold can also thrive in cooler weather. Even in frigid climates, the interior of a boat can reach temperatures that will support mold when the sun shines on the hull and in the early days of spring.

All it takes is a single leak to start spores growing. Unfortunately, it's hard to find a boat that's not in a humid location and vulnerable to leaks. Rain leaking through hatches and portlights will make a boat's interior a mold haven, so the first thing to do is find and seal its leaks. Portlights and windows are probably the number one leak spot on a boat, followed by loose stanchions. On sailboats, chainplates that penetrate the deck are a common problem area as are deck-stepped masts. Leaking deck hardware (cleats, rails, windlasses) is another common problem. More bad news: Water that's leaking through the deck core can cause another kind of mold, the kind that causes deck coring to rot. Re-bedding portlights or deck hardware is the only way to stop them from leaking (see Alert) and needs to be done sooner rather than later. Clogged cockpit scuppers are another source of water ingress; if the drains clog with leaves or ice, water can back up and flow into the cabin. In places with large snow accumulations, portlights, hatches, stanchions, and other fittings that normally seem leak-free can begin to drip as snow slowly melts over several days — check them after a heavy snowfall. Keeping a cover on your boat helps keep the water out, but if it prevents air from getting in, you could still be faced with mold and mildew this spring.

Photo of shrink-wrapped boat out of the water Shrink-wrapping is great for keeping water out of a boat, but it can also trap moisture inside. If your boat is shrink-wrapped, make sure there are adequate vents so air can circulate; a boat this size needs at least 8 or ten.

Keep Air Moving

Mold loves a closed-up boat. Air trapped inside tends to hold moisture, which mold thrives on. Unattended boats generate moisture inside through condensation because water, air, and hull temperatures are always changing and at different rates. This process is accelerated in a humid climate. The solution is to exchange inside air for outside air, which greatly reduces the chance of mold forming. Dorades, louvers, vent plates, and other waterproof ventilation systems help with air circulation, but if there is no air movement around the boat, they won't be effective and powered ventilation might be required. There are 12-volt vents for boats, but unless there is a constant supply of electricity, solar-powered vents are a better choice and can move a surprising amount of air. Some models have batteries and can run for 24 hours a day, using stored power to run at night. A good rule of thumb is to replace the air inside the boat every hour (as an example, a 30-foot boat contains about 800 cubic feet of air), and vents are typically rated by how much air they can move in an hour. Larger boats typically need two powered vents, one intake and one exhaust.

Replacing the cabin air won't do any good for closed-up lockers, so keep them open where possible. Don't forget to open covers over the bilge; more importantly, don't forget that there may be an open cover on the sole — leave yourself a note on the cabin door so you or someone else doesn't accidentally step into the bilge in the dark. Removing at least some of the contents of lockers over winter will help air circulate. Small 120-volt heated wands are available that warm the air in lockers and cause it to circulate, but they won't be effective if the cabin air is not vented, and it's often not possible (or legal) to get shore power to a boat in storage. If you're able to use shore power, these heaters are safer than hanging an electric bulb in a locker since they can't get too hot and aren't prone to breaking if the boat is rocked. One thing you should never do is leave an unattended household heater on board; every year boats (and neighboring boats) catch fire from these heaters or their extension cords.

Another way of lowering humidity in a confined space is with chemical dehumidifiers, which use calcium chloride to absorb moisture and then direct it to holding containers. These tubs and bags are safe and available at most marine stores. Depending on the amount of humidity, a couple of packets might last all winter — use more for a larger boat. They're cheap and easy to put into lockers or other places where removing humidity is a problem.

Something you can do when you visit your boat this winter (see Coping With Winter) is to open it up on a sunny day and let fresh air in while you're doing an inspection. Go have lunch and then come back and button things up again. Your boat will appreciate the blast of fresh air. Over winter, many boats are shrink-wrapped, which can present problems for air circulation. Shrink-wrap is great for protecting a boat from snow, dirt, and sun, but it limits how much air can get below. Vents can be installed in the shrink-wrap and depending on the size of the boat, several may be required; there are also solar-powered vents designed specifically for shrink-wrap. If there are no security concerns, consider leaving the cabin doors open so air can better circulate.

Finally, take home bedding, mattresses, clothing, towels, and other items that can attract moisture since they can be ruined by mold.

Photo of two large boat engines Two large engines (like these) inhale a huge amount of air and if they're in a small engine space (like this), they need lots of ventilation. Make sure vents and their hoses are clear of obstructions.

Engines Need Air Too

Getting air to the engine is far more important than preventing mildew. Whether you've got a tiny outboard or a roaring V-8, it needs air — clean air — and lots of it. A lack of air can cause gas engines to run poorly and produce excessive carbon monoxide, reduce power, and, in diesel engines, create smoke and soot.

Ted Parsons, a boat owner who lives on Long Island, was puzzled. The performance of his 22-foot ski boat was suffering. So much so that he could barely pull a skier out of the water. The engine lacked power and felt sluggish. But when he brought the boat to a shop to have it checked, it worked fine. The mechanic, standing next to the open engine cover, showed him how the engine started right up and purred like a kitten. When he revved the engine, it sounded just like it should. But when Ted took the boat back to the lake, it once again felt like it was running at half-power and nearly stalled at idle. As he was bobbing on the lake, he opened the engine cover (which seemed unusually difficult) to see if he could spot anything obvious, which suddenly made the engine run smoother. That puzzled him, but the fact that the hatch was hard to open while the engine was running was a clue. On a hunch, he took the boat for a spin on the lake with the cover opened and it felt transformed, with lots of smooth power. Poking around, he finally found the problem — both air intakes had nests in them that nearly cut off the flow of air, causing the engine to struggle. Once the vents were cleaned out, the boat ran like new again.

The story illustrates how important proper ventilation is to an engine. Air contains about 21 percent oxygen, and it's the oxygen that mixes with gas or diesel and allows the fuel to combust. An engine uses far more air than fuel — about 10,000 times as much. If the amount of air is restricted in some way, the engine burns too rich (too much gas), which causes a lack of power, smoke, and carbon buildup inside. A rich mixture also burns cooler, which creates far more carbon monoxide (CO) than a normal engine. Bob Loeser, a well-respected marine accident investigator, performed a test on a twin-engine cruiser and found that by merely opening the engine hatch, CO was reduced by nearly 25 percent. Imagine the increase in this deadly gas if there is a severe restriction in the engine compartment's intake system. Air restrictions also cause a loss of fuel economy. On boats that are equipped with air filters (many just have spark arrestors), replacing a clogged air filter on a carbureted engine can improve fuel economy around five percent and up to 15 percent if the filter is really clogged. Many older engines are only equipped with backfire spark arrestors and don't have filters — the thinking was that boats have little dirt for an engine to ingest. That assumption isn't necessarily true as anyone who has seen the black dust from a V-belt can attest. Trailering a boat can bring lots of dust and dirt inside, too. Many sailboats and trawlers have their engines located under the sole, and dirt and sand from shoes can find its way into the engine. Abrasive dust that's inhaled by an engine can damage its internals. Since gas engines are required by law to have flame arrestors, only special-made marine (not automotive) air filters with built-in spark arrestors can be used if you choose to install one. Proper marine filters are marked "SAE J1928" or "UL 1111" to indicate USCG approval.

Other places restrictions can occur: Check the vents or louvers that allow air into the engine space for obstructions. The hoses that attach to them can get deformed and restrict airflow to the engine, so inspect them, too. Often, when a larger, more powerful engine gets installed, the intake vents are not modified to increase airflow. In some cases, what seems like fuel starvation is being caused by restricted air to the engine. Black smoke from a diesel engine is a classic sign of air starvation. By opening the hatch, and watching for a change in smoke, you can determine whether or not the engine space is getting the air it needs. One more thing: It's generally not a good idea to leave engine space exhaust fans on while cruising, since it tends to pull out more air than it brings in, making it even harder for the engine to inhale while underway.End of story marker

Curing Mildew

As mentioned, it's easier to prevent mildew than it is to remove it, but what do you do if, despite your best efforts, mildew gets started in your boat's cabin? Bleach is the universal mold killer and works well on hard, non-porous surfaces — it kills the growth and can remove the stains. But it’s less effective on wood and can be a disaster on fabric (never use bleach on a PFD as it can damage the fabric or flotation inside). If you choose to use bleach, make sure you do it with lots of ventilation and don't mix it with other cleaners. Dilute it with three parts of water. Vinegar is less toxic and also kills mold, but before you use it on fabric, try it on a spot to see how it reacts. Mix three parts of white vinegar to two parts of water and sponge it onto affected surfaces; if you need something stronger, vinegar can be used full strength too. A half-cup of borax in a gallon of hot water is an effective cleaner and preventative for hard surfaces, too.