Tropical Considerations

Cruising in the tropics brings with it a special set of requirements because of the heat and humidity.

We have always designed our hatches and vents to work in conjunction with specially engineered awnings, which eliminate the need for air conditioning while we are at anchor a majority of the time.

There are several factors to be taken into account. First, there must be a balance between the air flowing into the boat and that which flows out. In other words, there has to be enough cross-sectional area in a non-pressurized area of the boat to let the breeze coming into the hatches and vents escape.

Next, hatch and awnings need to work together to extend the wind speed range in which they can be used when it is raining. Third, awnings should reduce the heat load from insolation through the hatches, windows, and ports (the windows on the Unsailboat being a substantial source of heat load).

Finally, the awning system should be capable of being set in large amounts of wind – we typically shoot for 35 knots as a minimum. And then, if a major squall comes through unexpectedly, the awnings should be easy to furl and tie off. It goes without saying that the squall in question will hit at 0200!

Ventilation is based on 10 medium-to-large Bomar hatches placed on the foredeck for the owner’s suite, the aft deck for the guest cabins, and on top of the saloon. (Above the saloon we have three hatches that are three-feet/0.9m square).

There are also four large Dorade vents.

Each of these cowls feeds the interior through adjustable “eyeball” vents, with which we can direct the air flow. The knob, which you can see inside of the eyeball, controls the air flow velocity, and in heavy weather this can be closed to make the vent watertight. If the breeze is at 8 knots or more, these vents alone keep the sleeping cabins very pleasant.

To reduce heat load on the saloon windows, we use a combination of awnings around the perimeter, a special 3M window film, and shades on the inside.

The shades are dropped in the early morning and late afternoon hours, when the sun is low on the horizon, and below the edge of the house awnings.

The awnings have a pleasant feel from the inside looking out. Your eye tends to go to the outer edge of the awning, and the visual space is increased when these are set.

There are two awnings which protect the Owner’s suite forward.

The awning shown above covers the head, shower, and dressing room hatches. It overlaps the hatch edges so that rain bouncing on the deck does not bounce into the interior.

The other foredeck awning protects the hatch over the bunk from rain, and helps direct air flow into the hatch when the breeze is light.

All four of the hatches in this area have their openings facing aft. This is required as the air flow hits the front of the saloon windows, and then bounces forward at deck level. This flow is so strong that in three or four knots of wind, conditions on the forward suite are very pleasant without running the air conditioning.

The perimeter awnings zip onto the handrails which surround the house, and then tie to the lifelines. There are 3/4″ (19mm) battens on the outer edge. The battens help keep the edges straight, prevent flutter, and make the awnings easy to roll up when they need to be furled. Rolled around the battens, the awnings are then tied with short lengths of light line to the handrails.

The door into the saloon plays an important part in the ventilation system. This is where the air flow exhausts out. The opening is protected on the inboard side by the door.

Outboard and top are protected with covers so that even in heavy rain or spray, the open door is usually dry.

The final piece in the tropical puzzle is the flying bridge awning. This covers the entire seating area and is supported by a combination of aluminum framework and fiberglass battens.

Under most conditions the entire cockpit is shaded. During periods of low sun angles we sometimes move to the opposite side of the cockpit. The important thing is that we can always find a shady spot.

On occasion, when the breeze drops off and it is a little warm at bed time, we’ve found that of we close the boat up, and then run the air conditioning for 45 minutes to an hour, the interior is cooled while humidity drops substantially. We then shut down the air conditioning and genset when before we get into bed. The boat stays cool until the early morning hours, at which point one of us gets up and opens the hatches and lets the early morning breeze back inside.

Posted by Steve Dashew  (October 17, 2011)

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