Good ventilation is one of the most important of cruising amenities. Our lessons on this subject came the hard way. We first arrived in the Marquesas Islands aboard Intermezzo in the summer, and we had just two small dorade vents, no fans, no way to open the hatches at sea or when it was raining, and an awning that was awkward to use with blue stripes which increased the temperature substantially. Oh, and the topsides were a dark blue/gray shade, nicely absorbing the morning and afternoon heat. If you want a primer on ventilation, just do the opposite of everything we started out with!
When anchored or moored so the boat can swing head to wind, the foredeck hatch is usually best opened facing aft, so air can exhaust through it. Adding a foredeck awning will enable the hatch to be left open during rain squalls.
There are large sections in our Offshore Cruising Encyclopedia devoted to the details of dorade construction, awnings, and spray shields, so we’ll just touch on the highlights here.
To begin with, the ventilation system needs to be looked at in the context of a wet environment, where it is raining (probably hard) or there is occasional spray on deck. This means that some form of protection needs to be provided. Hatch storm covers which open with built-in side panels are one approach. It helps to combine these with a small awning to keep rain from hitting the deck and bouncing below.
You cannot have too many dorade vents. We typically fit a minimum of two of these for the owner’s cabin, one for each sleeping cabin, and two (preferably four) for the saloon/galley area. Something in the size range of the Vetus “Yogi” cowl is about as big as we’ve been able to find. The down pipes should be as large as the inlet from the cowl. The eyeball vents we use on the inside of these allow the air flow to be throttled back and directed (you can get these from Air Concepts in Tucson, AZ).
The more area the awning covers, the better. This will reduce heat load and give better protection. We like to see at least a foot (300mm) of overhang beyond the outline of the open deck hatch.
For cruising in warm climates, light colors on the hull, deck, and awnings are a must. Dark colors look nice, but are not cool! A light gray is as dark as we like to go. In order for there to be good air flow, the incoming and outgoing air must balance. One thing that surprises most people at anchor is that this process usually starts with air coming in the companionway, especially if it is aft and there is a large dodger. This works in our favor, as the companionway is well protected at anchor. We arrange our forward hatch so that it opens aft, which allows the pressure from the companionway and any dorades to escape. We usually fit a small awning over this hatch, tied to the life lines, to protect the hatch from rain.
When there is no breeze – or when the hatches must remain closed – is when fans come into play. For the tropics, we want one for each bunk and seating position. Often it is not desirable to have the fan running at full speed. In this case, a unit with a speed control is a good idea – or use a selector switch to feed the fan a lower voltage (tapped from the batteries).