Electromagnetic Compatibility
More than just Preventing Interference

Your boat has several kinds of cables. Obviously, you have electrical power cables. You have high-level radio frequency (RF) cables between radio and radar transmitters, and the associated antennas. You have cables from each computer to its external screens, disk drives, keyboards, etc., and you have cables between computers. Sometimes, it may not be obvious that the short cable between a GPS and a VHF radio has computer signals on it, following the computer traffic rules or protocols, but you do. Almost any modern piece of marine electronics has a small computer in it, perhaps not running Windows or Linux and thus able to run arbitrary programs, but a small computer dedicated to running the fish finder or HF/MF SSB radio or weather receiver.

This page deals with the compatibility among your electronic devices and your power system, which is a different problem than electrical safety. The latter is the province of qualified marine electricians and inspectors, not computer and electronic engineers.  You need to talk toboth disciplines to avoid a ...shocking... experience.

Data cables

Data cables, both computer to local peripheral (typically USB or serial) and between computers (typically Ethernet) have fairly strict limits on length. It is rare that a single computer will reach those lengths, but it is possible. A basic Ethernet cable can be 328 feet/100 meters long, which, given the rule of thumb that the twists and turns of cables make the required cable lengths three times the point-to-point distance, might push the length limit on a large boat.

Data cables, even of proper length, may be susceptible to electrical interference, unless they are optical fiber rather than copper. You can minimize interference, however, by following a number of rules of thumb. Sometimes, violations of the rules may need an experienced eye to spot.

  1. Try to stay away from power or high-level RF cables. If a data cable must be near one of these cable types, do not let them run parallel to one another, within six feet. If they must cross, cross at right angles.
  2. Keep the cables secured with cable ties, clamps, etc.
  3. If either data or high-level RF cables are longer than the run between endpoints, do not coil the excess tightly. First, cables, especially optical fiber and coaxial, have a minimum bend radius. Coiling them more tightly than this limit can cause their internal conductors to break. Second, a coil can act as an antenna and transmit unwanted signals to other data or RF cables. Lay the excess cable in longer ovals, and tie them down -- if you know the sheepshank knot, the loosely coiled cables should look like a sheepshank, with the cable ties holding the bundle together.


Cables running from the transmitter to the antenna carry high-level RF. You don't want this energy to leak directly into the RF receiver, or into data cables. Generally, one radar set will protect its own receiver from its transmissions, but that isn't true of other radar receivers, radio transmitters, etc.  Without precautions, it also is possible for the energy to leak into power cables, and then indirectly into receivers or data equipment.

Antennas, even directional ones, may have sidelobes that might not broadcast far, but still can leak signals. With radar, sidelobes often are present 12.5 degrees or so outside the projected edge of the reflector.


The ordinary kind of hardware -- nuts and bolts -- surprisingly can act as a source of electronic interference.  

Warning: engineeringspeak follows. When two electrical signals of different frequencies meet, they intermodulate, which produces at least two additional frequencies: the sum and difference of the original signal.  There may also be lower-power harmonics of 2X, 3X, etc., multiples of any of these signals. The sum and difference, or harmonics of the original signals and/or sum and difference, may be generated, causing an apparently unrelated signal to leak into electronic equipment.  Corroded hardware, such as a rusty bolt or guy wire, can be a place where inadvertent intermodulation happens.

When looking for non-obvious sources of electromagnetic interference, power down high-level devices for safety, and replace corroded hardware that plausibly might be inside the electrical field.  If there are dissimilar metals with corrosion between them, break the possible intermodulation by putting a nonmetallic insulator between them.

Space between Equipment and Supplies

Materials that can be magnetized should be not be kept within 3, and preferably 6, feet of a magnetic compass. Since not all metals can be magnetized, but many can, it's good practice just to keep large metal objects awy from the compass. This is one good reason to put the magnetic compass on the overhead, or the top of a window, to keep it away from other electronics and hidden magnets in the wheelhouse.

What sorts of magnets are hidden?  Speakers and microphones both use magnets.  Some power supply transformers throw off strong magnetic fields.

Before picking the mounting location for magnetic compasses, you might want to take a good-quality handheld compass, note the position of north while ashore, away from magnets, and then move it near the intended permanent compass location. If the needle deviates from what you saw as magnetic north, it's probably a bad place to mount the compass.  Some compasses do have metal magnetic adjustments that let you compensate for unavoidable magnetic materials, but, if you use them, be sure they are consistent with the readings from a GPS or other nonmagnetic direction-finding system. Remember that GPS will not give you a reliable compass bearing unless you are moving, and you have a GPS display that shows a compass on a screen.

Power cables

Two major kinds of cables will be on fishing vessels and larger recreational boats: 12 volt DC and 120 volt AC. There may be a few additional kinds, such as high-voltage cables for radar and other transmitters, and short low-voltage cables for electronics such as 5 and 9 volt DC, but the twelve and 120 will be the common ones.

You want to keep high-frequency signals, which can be from obvious sources such as radio and radar transmitters, but also from more obscure ones such as data cables that send individual bits and high speed, from coupling to the power lines. Power lines can inadvertently carry high-frequency signals to equipment anywhere in the boat.  Be aware that RF can travel through power grounds

If the interference can be traced to power lines, it may be useful for a qualified electrician (for cables carry dangerous voltages) to install a radio frequency choke, which is a component that "drains off" the high-frequency signals without disturbing main power.


Among the most important ways to avoid interference is good grounding. Ideal ground systems present, to every grounded device on the vessel, an equal-resistance, low-resistance path to the sea, or to an excellent conductor to the sea such as a metal hull. In engineeringspeak, you want to present an equipotential ground plane, which means that you will have large copper cable, copper tape, or other good conductor connected to one another and then to the sea.

"Bonding" and "shielding" tend to be terms used in RF compatibility engineering rather than safety grounding. Try to use them only in that context.  If you are reading the standards of the National Electrical Code (NEC) or the grounding specification of the American Boat & Yacht Council, those documents focus on safety grounding. In contrast, the National Marine Electronics Association is concerned with RF compatibility.