When installing elecronic and electrical equipment, it is often necessary to install components in wet areas, or areas exposed to the weather. In these situations, providing a method of keeping components or wiring dry is essential. Fortunately, we can borrow technology from the electronic industry to acomplish this goal.
An electronics industry standard known as NEMA (National Electrical Manufacturer's Association) specifys generic electronic enclosures with varying degrees of protection from the elements. Note that NEMA should not be confused with NMEA (National Marine Electronics Association), which is a totally different standards body.
The enclosures we are interested in are known as NEMA Type 4x. Since this is an industry standard, several manufacturers make these enclosures, which makes them relatively inexpensive. They are available from standard electronics supply houses, such as Mouser Electronics (www.mouser.com). While other types of enclosures exist, using NEMA Type 4x provides an International Electrotechnical Commission (IEC) Standard 60529 Ingress Protection (IP) rating of IP-56 - which offers a moderately high degree of waterproofing. At this level, the enclosure will protect the contents from high pressure water jets applied at any angle. This should be sufficient for protecting equipment at the helm or in the bilge. If you need further protection, you can use a different NEMA enclosure class.
The enclosures can be had with several different options; including a clear or grey cover, internal mounting chassis, and even circuit boards. MY enclosure of choice is the Hammond 1554 series, and they come with clear polycarbonate covers. I also often order the optional metal chassis insert so I do not have to mount components to the enclosure sidewalls. It does no good to have a waterproof box if you drill holes in it.
While realitively inexpensive, they are not exactly cheap. A 6" x 6" x 2" enclosure with optional metal chassis is around $30. But this is a relatively large enclosure, suitable for mounting some electronic equipment. As expected, price varies with size, and a 3" x 3" x 2" enclosure, suitable for wiring terminal boards and cable splices is around $10.
Typically at least one or two holes will have to be drilled into the enclosure for wiring. To waterproof wiring, I like to use Heyco Inc Liquid-Tight Cordgrips. These are rated for IP-68, which maintains a waterproof seal even in permanent immersion in water. Since they are rated higher than the enclosure itself, they will be sufficient. Several different sizes are available, depending on the cable diameter. You can obtain the cordgrips with a variety of different threads, some come with nuts, and some do not. Cordgrips can be purchased from Mouser Elecrtronics for less than $1 each, or from West Marine for about $10 each (a little editorial comment there).
A typical wiring/splice box is shown below. Purchase the correct diameter cordgrip for the cable you intend to use. The cordgrip has a rubber sleeve, and when you tignten down the outer nut, the rubber will squeeze onto the cable, resulting in a waterproof fit. It goes to reason then, that you should only use ROUND wire and cable (the use of "duplex" cable is not satisfactory).
In the second example, a box housing electronic equipment is shown. The box is larger, and there are more cordgrips, but otherwise, the scheme is the same. Using the optional metal chassis, in addition to not having to drill mounting holes into the box, allows the circuitry to be assembled and tested prior to installation in the box.
Shown below is the practical application of the idea. One issue that should be discussed it heat dissipation. Ensure the box is large enough so that electronic components do not overheat. In addition, here is where the metal chassis can help, as it will act as a heat sink. The result is a secure, waterproof enclosure that will protect sensitive equipment in the boat.
NEMA Enclosure Types (PDF)
IEC60529 Ingress Protection Ratings (PDF)