Air circulation is very important on a boat, it keeps the air fresh, fights mildew growth, and helps control condensation. There are many ways to achieve air flow in a boat and we will look at some of the options.
There are two main types of vents on a yacht, passive and active. Passive means that air can flow through them if it should desire but there is nothing pushing it to do so. Active air flow is achieved when a fan pushes the air through the vent. By using a combination of active and passive vents, proper circulation can be easily achieved.
Active air flow utilizes fans to push the air through. This can range from your engine room blower to solar vent fans. These systems actively push the air in our out of an area.
Passive air flow typically utilizes dorade vents, grates, hatches, and port holes. These openings can be directed to draw air in without the use of a fan.
Active air flow alone is not as effective as the combination of active and passive. When used alone, they will either create a change in pressure that will work against them. Imagine a sealed engine room with a blower trying to suck all the air out as quickly as possible. It won't be able to! As much as it draws, there needs to be new air to replace the extracted air. The same with a boat riddled with solar fans all drawing air into the cabin: if there is no place for the current air to go, no new air can come in.
When used in conjunction with one another, the blower can draw the engine room air out while vents feed fresh air in; turning over the engine room air and ensuring an explosion free engine start! The same goes for a vessel with a bunch of solar fans pushing air into the cabin, a few vents to let the current air out will greatly increase air flow.
While vents and fans are useful when there is little breeze, hatches and dorades shine when the wind pipes up! Hatches can be angled into or away from the wind, the same with dorades. When angled into the wind, they will draw in great volumes of fresh air. When they are angled away from the wind, they will create a low pressure behind them that will draw out the cabin air. This is why if all the vents aim into the wind and you close the companionway, the draft will stop; the air needs someplace to go.
When it's cold out, no one wants to be opening hatches to keep down moisture levels, but you still need that air turnover to combat condensation. This is where passive vents shine. They will let in and out just enough fresh air to keep the boat much drier.
I typically keep my two dorade vents aimed in opposite directions. This way one is always drawing in air while the other is expelling air.
Vent position is also very important. If all the vents are in the same area, the rest of the boat will suffer from still air. Vents should be laid out in such a way that air will flow through the whole boat. Placing dorade vents near midship will draw air in, and having vents for it to escape forward and aft will draw the air through the whole vessel.
Another place that is often overlooked but greatly appreciates the airflow is the bilge. Airflow helps fight wood rot. If the wood is sealed up with still air and high moisture, spores will begin to activate and eat away at your timbers. Having a grate in the floor is a minimally invasive addition that can help promote bilge airflow. Feeding a large air hose down into the bilge connected to a dorade vent will further promote airflow into the bilge.
If you find that your boat is stuffy inside, think about adding a combination of active and passive vents to help circulate the air inside to make it just as fresh as the air outside!