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post #1 of Old 12-18-2000 Thread Starter
Tom Wood
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Building New Dorade Boxes

I am replacing the cowl vents on the deck of my boat and need plans to build new Dorade boxes or water traps. The mounting plates for the vents are four inches in diameter and they are 12-1/2 inches tall.

Do you know of a good design I could us to build the boxes? I know they need to be built right to make sure they don't leak and provide lots of ventilation. And of course, since they are on the deck, aesthetics are also important.


Tom Wood responds:

I have seen some sketches of Dorade boxes and remember scribbling one on the back of an envelope once. They are simple, and yet complicated. Here’s how to design one.

First the simple part—you need a four-sided box with a closed top and an open bottom about twice as long as it is wide, and a baffle in between the two halves. The box can be of wood, fiberglass, metal, or plastic, but must be strong enough to withstand errant feet, waves, and spinnaker poles. The top of the box must be flat and level with the horizon, so the bottom of the box must be shaped to the deck, and an ample mounting flange incorporated in the design.

A hole through the deck must be provided, and this should have a flange sitting slightly above deck level to discourage any drops of water from falling on your head in the cabin. This hole must be the same size as the throat of the cowl vent, in your case a diameter of four inches—larger if it will have an insect screen or other obstruction. If the deckpipe is too small, airflow will be restricted.

Since most decks have camber outboard and many flow downward as they move aft, the cowl vent should be placed in the rear half of the Dorade box with the forward end covering the deckpipe on most boats. This arrangement allows the drain hole to be placed on the aft, outboard corner of the box where the water will normally collect. If the deck is relatively flat, put drain holes in both aft corners so that the Dorade box will drain on both tacks. The drain hole(s) must be flush with the deck to avoid trapping water inside the box and must not be too large or vast quantities of air will escape with the water.

Now for the hard part—the total cubic inches of available airflow through the whole system must be uniform if it is to work properly. A cowl vent with a four-inch throat has a cross sectional area of 12-1/2 inches (Pi x Radius squared is equal to 3.14159 x 2 x 2 = 12.5). We’ve already determined that the deckpipe needs to be at least four inches in diameter—more if it is to have screens or shutters. But what if you wanted a square deckpipe, or oval? For a 12-1/2 square-inch minimum, a square one would have to be 3 1/2 inches in both dimensions to be adequate—an oblong deckpipe could be 4-1/8 inches x 3 inches.

The last calculation is to make sure that we have the same capacity over the baffle. A four-inch cowl vent comes with a deck plate normally about 5-1/2 inches across. So let’s say that your box is six inches on its outside dimension and 5-1/2 inches on the inside dimension in width with double those dimensions in length. To get the 12-1/2 square inches of airflow over the top of the baffle between the halves, the baffle has to be 2-1/2 inches from the top of the box (5 1/2 wide x 2 1/2 high = 12-3/4 square inches). If the baffle is going to be 2-1/2 inches high off the deck to keep water from running into the deckpipe compartment, the resulting inside height of the box is five inches.

There are some nice touches to the design of a great Dorade box if you have the time and patience to build a work of art rather than just a plain old watertrap box. Angling the sides to match the angle of the boat’s cabinsides and other lines makes them look as if the boxes belong there. Putting a section of glass such as a round deadlight or a section of acrylic or Lexan over the front half of the box allows an amazing amount of light to flood down the deckpipe, especially if the deckpipe is chromed or painted white. The best design I’ve ever seen was a second deck plate mounted in the forward half of the Dorade box top over the deckipe so that the cowl vent could be placed directly over the deckpipe for straight through ventilation in fair weather. The plug for the unused deck plate was made of clear plastic that allowed light through. This arrangement had the added advantage that the forward end of the box could be cleaned. Swapping the cowl and the plug took a matter of a minute.

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