Boaters always want to know how fast they're going. Whether it's a 5-knot sailboat or a 100-knot racer, the obsession with speed is pervasive. Accurately measuring the speed of a boat is a complex problem, but most of the hard work has already been done by instrument manufacturers- all that's left for the boat owner to do is to install a handful of parts, get out on the water, and start competing for every extra knot of speed.
Find a suitable spot on the boat's instrument panel. Drill a hole in the panel, insert the GPS speedometer, and secure its mounting bracket on the back of the panel as described in the speedometer's manual.
Locate suitable sources of 12-volt power. Most GPS speedometers need a ground connection and two +12-volt connections: one for the backlight and one for the instrument itself. On most boats, the existing instruments are connected to suitable power buses that the speedometer can share.
Crimp terminal fittings onto the ends of the speedometer's power wires and connect these to the ground and power buses. The GPS speedometer should now be ready to use.
Mount the speedometer display and connect its backlight to a power source, in the same manner as for GPS speedometers.
Find a suitable route for the pressure tube. The display will be connected to the pitot by a thin, stiff tube that transfers the water pressure at the pitot to the sensor in the display. The tube has to slope downward toward the transom at all points; if there are sags in the tube, water can collect inside it and mess up the instrument's readings.
Find a place on the transom to mount the pitot tube. It should be well clear of strakes, trim tabs, rudders and propellers, and the tip of the pitot should extend at least 2 inches below the hull. Screw the pitot bracket to the transom with the pitot pointing straight down.
Drill a hole in the transom, well above the waterline, for the pressure tube to pass through. The hole should be angled downward at about 45 degrees. Pull the pressure tube through the hole, connect it to the pitot, and secure it in place with sealant and cable clips. Connect the other end of the pressure tube to the speedometer display, trimming it if necessary.
Buy a paddle wheel or ultrasonic transducer, a display module, and appropriate cables. These systems are expensive, but offer very accurate speed-through-the-water readings and work as well at very low speeds as they do on faster boats. Most modern systems use a single NMEA 2000 cable for both power and data, and NMEA 2000 displays can work with transducers from other manufacturers.
Choose a spot for the transducer through-hull. It should be in an area of relatively clean water flow, and should not be behind a keel, rudder, propeller or other appendage. With the boat out of the water, drill a hole in the hull for the transducer. If you have a cored fibreglass hull, drill the hole a bit oversized and line it with thickened epoxy to seal the core before drilling the hole again at the proper size.
Spread some bedding compound, such as 3M 4200, on the flange of the transducer through-hull and mount it in the hole. Connect an NMEA 2000 cable from a T-fitting on the backbone cable to the transducer. Turn on the boat's electrical system and set the controls on your display unit to show the speedometer reading in the units and format you prefer.
Choose your speedometer type appropriately. Conventional pitot types work well on planing powerboats, but slower powerboats and most sailboats will be better off with a paddle wheel transducer. Very fast powerboats are often better served by GPS-based speedometers, which are also the fastest and easiest to install.