Installing Electronics

By Don Casey

You have finally sprung for that new bit of electronic wizardry you've been eyeing, and now you're wondering what else, besides what comes in the box, you will need to get it installed. Usually, not much.


Fishfinder mounting on the helm

The first challenge to installing electronics is finding a good mount location. You want displays to be readily visible and dials and buttons convenient to reach. A dry location is always preferable, even if the unit is waterproof. Generally you should mount electronics high, meaning up under the deck in sailboat, on the bridge of a powerboat. Be sure there will be ample room at the rear of the unit for the wiring.

For a bracket-mounted unit, you will need a drill and the appropriate bolts to install the bracket. Flush-mounting the unit through a panel in the nav station or at the helm requires making an accurate cut-out. The installation instructions in the box will provide the cutout dimensions. Depending on panel material and access, you can make the cutout with a saber saw, a rotary tool, or a keyhole saw. Be sure you know what is behind the panel before sawing; it is all too easy to cut the wiring to other equipment.


As a general rule, mount a new antenna well clear of those already on your boat. For maximum range, mount a VHF antenna as high as possible. The range of SSB and ham radios is less dependent on antenna height, so these antennas can be mounted near the deck. GPS antennas can also be mounted near deck level, but they should have a clear view of the sky, unobstructed by a top or rigging. Radar range and reception are improved by mounting the radar antenna as high as possible. Keep in mind that radar transmissions can damage eyes, so be sure this antenna is well above head level.

A number of antenna mounts are readily available that will simplify your installation. Some are screwed to a horizontal or vertical surface; others simply clamp around a railing. A ratchet mount allows you to fold the antenna down to reduce the vessel's vertical clearance.


Transducers are the underwater component of depth sounders and speed logs. They are generally mounted either through the hull or through an underwater bracket attached to the transom. Depth sounder transducers are sometimes mounted inside the hull to avoid fouling. This can be a good arrangement, but it doesn't work well on some hulls.

Your boat will need to be out of the water to install a through-the-hull transducer. Other than the trauma of drilling a hole through the hull, transducer installation is straightforward.

Transducer illustration 1

Speed log transducers are mounted perpendicular to the surface of the hull, and they can be located almost anywhere as long as they will always be in clear (undisturbed) water when the boat is underway. That generally means aft in a powerboat, and near the keel on a sailboat. They should not be located directly behind another through-hull component or any other hull feature that might alter the flow of water over the transducer. Also pay attention to what is inside the hull where you want to install the transducer.

In a solid laminate hull, you need only use a drill or a hole saw to cut the appropriate size hole — designated in the installation instructions — insert the cable through the hole, run a thick bead of polysulfide sealant around the transducer just above the head, insert it into the hole, and tighten the retaining nut inside the hull. Sealant should squeeze out all around the transducer or you will need to remove it and apply a thicker bead.

Transducer illustration 2

If your hull is cored, the process is more complicated. You must never depend on flexible sealant to keep water out of the core. Rather, after you have drilled the mounting hole, use a sharp tool to dig out the core for an inch or so around the hole. Sand the inner surfaces of the skins where the core was removed and wipe them with acetone. Fill the space with a commercial epoxy paste such as Marine-Tex or with regular epoxy thickened to a peanut-butter consistency with colloidal silica. Let the epoxy cure for 24 hours, then run your hole saw through the hole to clean it up. Now install the transducer exactly as described above.

Depth sounder transducers should be installed perpendicular to the surface of the water rather than the surface of the hull. In a sailboat, you must also be concerned about the keel blocking the transducer's view of the bottom when the boat is heeled, although this will not be a problem under power or at moderate angles of heel. Transducers are sometimes mounted forward of the keel. Dual transducers with a gravity switch that activates the lower one are another option.

Mounting the transducer vertically may require wedge-shaped spacers, called fairing blocks, both outside and inside the hull. Depth-sounder are adversely affected by disturbed water, so the external block should be smooth and streamlined. Also make sure the transducer is not located behind a bubble-generating hull feature.

Transom mounts do not work in the disturbed water behind the prop of an inboard boat, but if your boat is outboard or I/O powered, you can limit new holes in the hull to just those required to screw the bracket to the transom. If the transducer is to give accurate readings, it is imperative that the bracket is absolutely flush with the bottom of the boat--in effect an extension of the hull. Even then, turbulence at the stern, especially at speed, can cause both log and depth sounder to give inaccurate readings. Most transoms have a wood core, so take great care to seal any screw holes you drill.

Signal Wires

The wire from the transducer will have a connector that you simply push or thread onto the appropriate connector on the back of the unit. Never shorten the signal wires from transducers; if the wire is too long, coil the excess.

Antenna wire can be RG8X coax if it is less than about 20 feet long. For longer runs use RG8U or RG213U. Connectors (designated PL-259) need to be soldered to coax, so if you don't have soldering experience, you may want to get help with this part. Solderless connectors haven't done well in the marine environment, although some new solderless terminals may prove to be more durable.

Signal wires illustration

Power Wires

Most 12-volt electronics come with either a red positive lead and a black or yellow negative lead, or a pair of screw terminals marked + and -. Hooking up the unit is essentially a matter of connecting the + lead to the + side of the battery and the - lead to ground.

Power wires illustration

But we don't normally connect electronics directly to the battery. Instead, we take the power from the distribution panel. Since electronic equipment can be sensitive to other loads on the circuit, it is a good idea to give your new gear its own circuit. That means installing a new breaker in the panel if you don't have an open one. If that is not practical, then pick a lightly loaded circuit, one without electric motors or other interference generators. Keep in mind that the breaker is protecting the wiring, not the gear, so you cannot increase the size of the breaker unless you know the size of every wire in the circuit.

Speaking of wire, you need #10 AWG, assuming the new unit is no more than about 20 feet from the distribution panel. The exception to this generalization is SSB, which needs bigger wire (#4). For longer wire runs, consult the ABYC wire size table found in most electrical books (including mine). Buy duplex safety wire with red and yellow conductors; red is +.

Route the wire as high as possible — never through the bilge — and support it with cable clamps at least every 18 inches. Where wires pass through holes in panel boxes, bulkheads and other components, the hole must have a grommet to protect the wire from chafe.

Use crimp connectors on the wire ends: step-down butt connectors to join the supply wires to the light leads, ring connectors for screw terminals, and flanged spades if the screws are captive. If you don't already have a proper crimping tool, add one to your kit.

With the breaker off, connect the red wire to the output side of the breaker. At the unit, connect this same wire to the lead or screw marked +. Connect the yellow wire to the ground bus inside the distribution panel — it will have black and/or yellow wires connected to it — and to the lead or screw marked -. Double check the polarity, then flip the breaker and turn on the unit. You should be in business.

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.


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