USAMARINE Web Group - Captains' Corner Boatbuilding Tips

Basic Dorade Box

Basic Dorade Box

Called either a Dorade or Collector Box, this item will help to, very simply, extract water from air intakes leading into your engine compartment.  The size and shape will be dictated by your boat, but the principle will still apply.  Most manufactured boats have a form of this system that is factory installed.  Many older boats and home made boats may not, and if your's fits into this later category, here's some tips on getting dryer air to your engine(s).

The Dorade Box is a single chamber system with a baffle between the intake air supply and the engine space air feed.  The baffle helps keep smaller water droplets from being carried through with the velocity of the incoming air.  As a result, most of the water ingested through your air intake drops to the bottom of the box, and is drained away, either into the bilge, overboard through a dedicated above the waterline through hull, or to a sump to be automatically pumped overboard.

A dorade box of around 231 cu in, (1 gallon), should be fine for the average gasoline engine compartment.  Diesel engines will require more air volume and should be computed in size accordingly, (see foot note below).  The tube into the engine compartment should have a hose that leads down within a few inches of the bilge. This allows for further draining and keeps engine fumes vented away from the bilge.

A more complex Dorade system, that I designed for a Bertram 31, can be seen on the "Fortuna" section of our site.

"Fortuna" Hull Inducted Air - Bertram 31

Have fun! Feel free to download and print these drawings, but please don't use it on a website without linking it to

Capt Patrick McCrary

Foot Note - Air Supply to Diesel Engines:
Diesel engines require a far greater volume of air than do gasoline engines.

As a "Rule of Thumb", optimum air volume, to be supplied by the boat's intake, can be calculated in square inches of intake opening.

0.5 sq in of opening will supply 1 rated diesel horse power. ie: A 300 hp diesel engine, at full RPM needs 150 sq in of intake opening to have the optimum volume of air needed to operate at peak efficiency.

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834 Scott Dr., LLANO, TX  78643
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