Boat Ventilation is one of the most important systems on your boat. Without adequate ventilation Cabins become musty, hot and suffer from condensation and mildew. This not only makes living uncomfortable, eventually electronics and equipment suffer from corrosion as well as turning your bronze lights green.
Proper marine ventilation, requires moving air into the boat and around the boat and then out of the boat, allowing more fresh air in. To achieve adequate air flow its important to have at least one intake vent and another outlet vent. These vents should be separated so air can come in and circulate, then exit.
It is also important to have vents to allow air out of Heads, galley, lockers and the Engine room. The effectiveness of outlet vents can be increased by fans that move the air around the boat.
Boat Ventilation Anchored or at the Dock
When your boat is tied up at the dock it’s possible to open all ports and hatches to increase ventilation. If you have shades and covers over the ports you can leave them open in rain and in bright sunshine, the shade les air in while keeping the heat out.
Boat Hatches and port-lights when open provide plenty of ventilation, on a sunny day. A good breeze can be directed below pushing air through the boat and escaping through the transom or main hatch. The further aft outlet vents are the more air flow will circulate.
This image shows an open forward hatch which allows plenty of air in, especially when the bow is facing the wind.
This clever sailboat ventilation system uses an awning supported by battens which allows the hatch to remain open in rain and keeps harsh sun from warming up below decks.
The awning also keeps the air around the hatch cool, so you are not letting warm air in.
Ronstan boom tent
If you are looking for a awning try Ronstans Inflatable Boom Tent below. This tent uses inflatable tubes instead of poles which saves time and storage.
This tent is intended to be positioned over a sail boats boom. But it can be used on the foredeck as above or even on a powerboat.
A wind scoop could be added to force more air below.
Windscoops are normally positioned over the forward hatch allowing air to enter the boat and move aft.
left is a nicely designed windscoop from French manufacturer Dorcap.
Advantages are that the windscoop is self-tending and does not need a halyard. We all know the benefits of a windscoop that directs air below.
Dorcap is an all-weather windscoop. The hatch can be left open during rain, plus a baffle on the font stops water from running into the hatch.
There are three sizes; S (small rectangle): 45cm wide by 30cm high. M (medium rectangle), the average hatch size: 50 cm wide by 45 cm high. L (large rectangle): 60cm wide by 55 cm high.
Taylors anchor shade is a smaller shade perfect for small cruiser or open cockpit powerboat. Like a big umbrella with straps at each corner it can provide shade anywhere the imagine takes you
Port lights & Hatches
It is best to have ports and hatches that can be opened if the weather is fine. Many boats do not.
If you have a cabin window that does not open you can add an opening port, like the sailing boat right.
Even though this boat has fixed windows, an opening port has been built into the window, to allow additional air flow.
Most ports cannot be left open in rain. Port shields allow portlights to remain open in rain. These are quickly attached with a peel and stick mounting.
This device is like a wind scoop for a side deck portlight. It is easily inserted in the port from the inside.
Moving air below deck
Above we looked at ways of moving air to below decks. Next we look at moving that air once it got below deck. One important principle is that to move in fresh air you need to remove the stale air
To help we can use fans and escape vets at the transom. Stale air leaving the boat leaves room for fresh air to come in.
With air circulating around the cabin, you need to allow it to move from cabin to cabin even with the doors shut. Louvered vents installed in doors allow circulation. Louvers can also be put in the main hatch boards to allow air to escape.
In the stern of the boat, outlet vents let air get to the extreme ends of the boat and escape. If you do not let air out air will not get in. Good circulation needs vents at the ends of the boat. This boat has 3 large vents mounted on the transom. If you provide good outlet vents, more air can get in
You can put a large home fan over a hatch. I have seen this done, but “the Breeze” is specifically designed to fit a standard hatch.
This 12 Volt fan can be mounted under a mushroom style vent or even a Dorade vent for those windless days to force air through the boat.
Ventilation underway or when you leave your boat has to be engineered around the fact that the boat is closed up and hatches and ports are closed. Its still good to have louvers below deck to aid circulation plus outlet vents.
While at sea you may be able to leave a port open in mild conditions but in a heavy seaway to stop water entering the cabin you need to be watertight. When the boat is closed it soon becomes uncomfortable and the air becomes stale, which can lead to or exaggerate seasickness.
The secret to ventilation at sea is letting air in without letting in water.
The best vents for offshore work and in bad weather are Dorade vents. Dorade vents have been around for a while and can be used in all weather conditions, rain or shine.
The Cowls capture large amounts of air and redirect it below through the box. Any water getting into the cowl is drained back onto the deck.
Even if you are not onboard, Dorade vents allow circulation to keep the boat fresh and help prevent mildew. A pair of decent sized Dorade vents can be amazingly effective if one is facing into the wind and one away from the wind as an extractor.
Dorade vents Design
The Dorade Box got its name from Olin Stephens’s famous racing yacht Dorade. The Dorade consists of a cowl vent and a box, both which can be bought, for DIY installation. Positioning the Dorade combines getting the cowl good air and not interfering with cabinetry below deck.
Installation is quite easy as long as you’re OK with cutting a round hole in your deck. Proper scuppers or drains are cut in the lowest point of the box to allow water to drain.
With just rudimentary carpentry skills anyone can install a Dorade box.
These boat vents do not need a box and can be mounted directly on deck. They still need a hole cut in the deck. The vents can be closed or in some cases have baffles and drains to get rid of any water or rain.
These stainless vents look great but cost around $300. They also need to be mounted on a box adding to the cost.
Rubber Cowl vent.
Plastimo cowl vent is an all in one Dorade. These vents have built in baffles and drains to stop water intrusion, so you do not need to build a Dorade box, just over $100.
Mushroom vents can be installed anywhere there is space on deck which does not interfere with cabinetry below deck. You can mount these in hatches and ports, so that when they are closed they still supply air.
Air Only vents are a new Dorade type vent that only requires single hole thru deck. A typical Dorade vent requires a stand up pipe that vents into the cabin and a cowl vent that is offset from the stand up pipe so that water does not do below deck.
Air only vents have balls that cut off the path below deck. Air is not powerful enough to move the ball, but water can. Therefore, when water goes into the vent it pushes the ball which cuts off the path below deck and so has to escape through the drains. When the water escapes, the ball falls away allowing air below deck again.
Nicro day night vents
Nicro day night vents help give your boat the continuous ventilation it needs, rain or shine, whether your on the boat or not. Solar cells power the fans for efficient, active ventilation. They are designed to move air into or out of your boat and at the same time, keep water from entering. Two fan blades come with the vent, so you can designate the fan to be an extractor or intake vent.
Nicro suggest that you should choose a vent that has the capacity to move 70% of the boat interior volume in an hour. A 4 inch vent moves around 40% more air than a 3 inch vent, you cannot have too much air flow.
Streamlined vent from Plastimo
This streamlined vent from Plastimo allows the function of a Dorade with a small profile and only one hole through the deck. The air enters through the front of the vent and is funneled below. The hole and shut off mechanism is covered with a shield so most light water will not go down the hole and escape through the sides of the box. In case of green water the hole can be closed from the inside.
The small profile means the vent can be installed where regular Dorade vents cannot. They could be installed on the side of a cabin house, on a narrow cockpit coaming for example.
Engines need to breathe just like we do. The problem is out at sea the air is humid and this can cause havoc with an engine. If you rely on your engine, corrosion can be your enemy.
Companies like Delta Systems make air intakes that remove humidity.