|The point of the Dorade Box is to allow the air to pass below while trapping any water and allowing it to drain overboard.|
There is a formula for working out how much ventilation will be provided by a particular size of cowl.
However, the number of vents you have and where they are sited on deck will determine what areas of your boat will be aired.
A pair of cowls will create a flow of air throughout the boat if fitted at either end of the boat with one facing forward and one facing aft.
But in practice where you fit the vents and how many you have will depend on the size of your boat and the space available.
Suffice to say that the more you have and the larger the cowl opening the more air will flow below.Plastimo Low Line Pvc Cowl VentilatorFor a small boat where deck space is limited it is worth considering the plastic dorades such as those made by Vetus.
They come in a variety of sizes and are light, functional, compact, the flexible plastic is less likely to foul lines and much more friendly to bare shins and toes.
One problem with the plastic
ones is that they tend to
discolor in strong sunlight, fortunately this can be remedied by
the paint used for coating vinyl plastic and PVC.
However, if you are reluctant to spoil the looks of your wooden with plastic bits on her deck, a wooden dorade box complete with a brass cowl will be an ornament to be proud of.
Or you could compromise by fitting a toe friendly plastic cowl to a wooden box.
But it will need to be of,
Have sufficient drainage to allow water to out without unduly reducing the air flow,
A method of controlling the air flow from the inside,
A mesh screen to filter the air,
Cowl necks long enough to avoid any deck water,
The scoops should be free to turn through 360 degrees.
With a bit of thought, at the design stage the box could double up as a place to site your navigation lights or with a couple of clips somewhere to store the boat hook or any non load bearing hardware.