The easiest way to add music to a boat with a 12-volt electrical system is by installing a car stereo. By following a few steps, you should be able to install a typical stereo head unit and set of speakers on your boat in an afternoon.
Before you start
Consider where to mount the stereo head unit. If possible, head units should be installed inside. If the head unit is installed outside, it should be weatherproof or water-resistant and protected from the elements. Installing the head unit so that the rear of the unit is directly beneath a shelf or counter will make it easier to attach the mounting strap.
Consider where to mount the speakers. Speakers are less vulnerable than head units, but any speakers mounted outside should be waterproof.
Place speakers as far away from the vessel's compass as possible and magnetically shield them if they are in the vicinity of the compass. Speakers can interfere with the compass.
Mounting the head unit
Trace an outline of the rectangular hole to be cut for the head unit mounting sleeve. Drill 1.3 cm (1/2 inch) holes at opposite inside corners (i.e., upper right and lower left) of the outline. If drilling through wood, use a Forstner bit; if drilling through fibreglass, use a regular high-speed drill bit. Starting at these drill holes, cut out the rectangular hole with a jigsaw -- use the appropriate saw blade.
Insert the mounting sleeve in the hole. If possible, bend the tabs on the mounting sleeve (now on the inside of the hole) to secure the sleeve. Most likely, however, the wall in which you are mounting the unit will be too thick to access the tabs. The walls, however, will be thick enough to support the unit and the mounting strap will keep it in place.
Slide the head unit into the mounting sleeve. Secure the unit with a metal mounting strap fastened to the rear of the head unit and to the underside of the shelf above the unit. If there is no shelf above the head unit, improvise a different mounting point for the strap.
Wiring the head unit
Mount a terminal block with three terminals near the stereo head unit.
Consult the stereo manual to identify the speaker output wires in the wiring harness that plugs into the rear of the head unit. Typically, there will be eight wires -- enough to power two sets of speakers ("front" and "back"). Choose the appropriate wires for one set of speakers, strip the insulation from the ends of each wire and install female bullet terminals with a crimping tool. If you are planning to install two sets of speakers, install female terminals on all eight wires.
Consult the stereo manual to identify the power cables in the wiring harness: typically, there are three such wires -- red, yellow, and black. Strip the insulation from the ends of the wire. Install ring terminals on the end of each wire with a crimping tool.
Plug the wire harness into the rear of the head unit. Attach the red and black wires to the terminal block.
Determine the appropriate wire size needed to power the head unit. Run a red cable from the vessel's fuse or distribution panel to the head unit. Remove the fuse or turn the breaker off. Strip the insulation from the wire and install a ring terminal with a crimping tool. Attach the wire at the terminal block to the red wire coming from the head unit. Connect the wire to the fuse or distribution panel using an appropriate terminal, properly installed using a crimping tool.
Run a black cable (same size as the red cable in step 5) between the vessel's ground and the head unit. Strip the insulation from the end of the wire and install a ring terminal.Attach the wire at the terminal block to the black wire coming from the head unit. Connect the wire to the vessel's ground using an appropriate terminal, properly installed using a crimping tool.
Install the third wire (yellow). The yellow wire coming from the head unit provides electricity to preserve the head unit's user settings when the unit is turned off. Yet even though the unit does not draw much power when turned off, it can still drain the battery if the vessel is left for an extended time. The safest way to connect this wire is to attach it to same terminal as the red primary power supply wire. The unit must be reset every time the battery switch or panel switch is turned off. But usually there is no reason these switches can't be left on.
For each speaker, trace an outline of the hole to be cut to mount the speaker. Drill a 1.3 cm (1/2-inch) hole along the inside of the outline. If drilling through wood, used a Forstner bit; if drilling through fibreglass, use a regular high-speed drill bit. Starting at this drill hole, cut out the rest of the outlined hole with a jigsaw -- use the appropriate saw blade.
Run a duplex speaker wire (for appropriate wire sizes, see References) from each speaker to the head unit.
Install the appropriate terminals on your speaker wires. Install male bullet terminals on the wires at the head unit -- strip the insulation from the end of each wire and install the terminals using a crimping tool. If you are using wire that came supplied with the speakers, it will probably already have terminals on the ends that plug into the speaker. Otherwise, add the appropriate terminals yourself.
Attach the speaker wires to the speakers and head unit. Each speaker has a positive (+) and negative (-) terminal. Hook the wires up correctly. Generally, one of the wires in the duplex speaker wire will have a white stripe or other feature to help distinguish the wires.
Install the fuse or turn the breaker on at the fuse or distribution panel and the stereo is ready to go.
The yellow wire can be attached to its own terminal on the terminal block and connected directly to the battery using a separate wire. This wire must be protected by it own inline fuse, installed as close to the battery as possible. Caution: while the unit does not draw much power when turned off, it can still drain the battery if the vessel is left for an extended time. For radio reception, the head unit still requires an antenna. You can either install a separate antenna or use your boat's VHF antenna by installing an antenna splitter.