If you use a mechanical boat speedometer, rather than the "speed of advance" function of your global positioning satellite (GPS) receiver, your indicated speed may not be entirely accurate. A GPS tells you how fast you're moving toward a place. A mechanical speedometer tells you how fast you are moving in relation to the water around your hull. A 20 knot following current may move you along at 30 knots, according to your GPS, when your mechanical speedometer shows you're moving at a paltry 10 knots, in relation to the water around you. Nevertheless, installing speedometers in boats remains a common practice.
Install the Pitot tube on the transom, 6 inches or more from the propeller. so that the intake hole is at least 1-1/2 inches from the hull and unobstructed by the keel or any stabiliser.
Drill a 1/4-inch-diameter hole above the water line of your hull. Pull the tubing for the Pitot through the hole. Apply a marine silicone caulk around the tubing. Surround the tubing with caulk and set the cap plate over the hole, embedding the plate in the caulk. Insert the screws, supplied with the speedometer, through the flanged edges. Tighten the screws until the cap plate is tight against the hull. Push the tubing onto the male fitting on the top of the Pitot.
Bring the end of the tubing from the Pitot forward, toward the dashboard of the boat, by running it alongside the gunwale--the side of the boat--without kinks, crimps or sharp turns. The speedometer kit includes plastic clips to help keep the tubing out of the way, but use plastic wire ties if extra security is necessary. The tube should slope upward as it moves toward the dashboard.
Disconnect the red (positive) cable from your boat's battery. Cut a hole in your dashboard that's as large as the diameter of the speedometer unit. Fit the readout unit through the hole in the dash and secure the unit in place. The speedometer may be clamped in place with the clamp that comes with the unit or held in place by screws through the flange on the sides of the face of the speedometer.
Cut the tubing to length so it will reach the male fitting on the back of the speedometer with a bit of slack to allow for future modifications. Connect the "+" terminal on the back of the speedometer to your boat's common power buss and the "-" terminal to your boat's common ground. Reconnect the positive battery cable to the positive post of your boat's battery.
Don't coil any excess tubing under the dash; leave only an inch of slack so that you can extend the tubing should you move the speedometer some day.