Makers of marine engines schedule regular maintenance for their motors based on the number of hours they are used. Mercruiser inboards should have their oil changed every 100 hours. Several models of Evinrude outboards suggest an engine tune-up every 500 hours. However, many boat manufacturers don't include an engine hour meter in the instrument gauge package on the boats they produce. If this is the case with your boat, adding an hour meter will ensure that you are able to maintain your boat's power-plant on schedule.
Locate a position on the dashboard to install the hour meter. You will want it to look good, but more important is that the location has to be large enough to accommodate the face of the meter on the front side. Also, looking behind the instrument panel, you need to make sure there are no hidden components directly behind that location that will impede the installation.
Access the rear of the dashboard so you can see the wires running to the instrument panel and the backs of the equipment and gauges mounted on the dash.
Locate the rear of the boat's ignition switch. The meter can be positioned anywhere, but it will eventually be wired into the ignition switch. Mounting the hour meter near the ignition switch will facilitate the wiring job later.
Measure the barrel of the hour meter. Select a hole saw slightly larger so the barrel will slide into the hole but the flange on the face of the meter will obscure the cut-out. Many hour meters come with a template or directions indicating the proper size hole saw to use.
Paste a layer of sticky tape, such as duct tape or masking tape, on the dashboard where the meter will be located. This will help prevent the fibreglass or other material to be cut from chipping excessively while the hole is being made.
Cut the hole. Remove the sticky tape. Insert the meter into position on the dashboard.
Insert the wires on the rear of the meter through the retaining ring. Screw the retaining ring onto the threads of the meter's barrel until the unit is locked firmly into place.
Determine which of the wires on the rear of the ignition switch is powered when the key is in the run position, using a 12-volt test light.
Turn the ignition key to "on" or "run." Attach the grounding clip of the test light to a metal stud or framework connected to the boat's electrical grounding system. Touch each of the wire connections on the rear of the ignition switch to see which one is powered.
Touch the same wire with the ignition switch in the off position and then again with the ignition switch in the accessory position, if so equipped. The wire should only be powered when the key is in the run position; it should not powered when the key is in any other position.
Connect the hot wire (usually red) on the back of the hour meter to the run wire on the back of the ignition switch, using a Scotch lock wire connector.
Attach the ground wire (usually black) on the back of the meter to the boat's grounding system. Do this by connecting the wire to another ground wire with a Scotch lock, or connecting it to a grounding stud using an appropriate solderless wire connector.