Your boat's speedometer works because a tube, called the pitot tube, blows air into it from underwater. While one end of the pitot tube is underwater, the other end is connected directly to the back of your speedometer by a barb connector, a metal tube with ridges that, when inserted into the plastic pitot tube, holds the tube in place on the connector. When you speed up, the water attempts to enter the pitot tube at the rear of your boat and compresses the air in the tube which in turn passes that increased air pressure to your speedometer and makes the needle indicating your boat's speed rise.
Disconnect the pitot tube from the speedometer's barbed connector by grasping the tube firmly near the barbed connector and wiggling the tube while pulling until it slips free of the barbed connector. If it has been secured to the connector with wire ties, cut the ties with wire cutters.
Trace the pitot tube to the stern (rear) of the boat, coiling the tube as you go. If the tube goes through the transom, the opening may be caulked with silicone. Cut the silicone with a sharp knife, without damaging the pitot tube, then step out of the boat and find the intake end (the one under water) either at the transom or under the bottom of the boat. The tube is fastened to the hull with a screw-in bracket. Remove the bracket by unscrewing the bracket's mounting screw with a screwdriver and pull on the tube to determine that it's free.
Butt the end of the new pitot tube to the end of the old pitot tube and wrap the joint formed thereby with two layers of duct tape, beginning your wrap at the center and continuing away from the center, spiraling around the tube for a distance of 2 feet from the butt joint. Continue spiral wrapping the duct back toward the center, continuing past the center for a distance of 2 feet.
Grasp the old pitot tube firmly and pull the old pitot tube back into the boat. Since the ends of the two tubes are butted together and secured with duct tape, as you pull the old tube back into the boat, it will carry the new tube with it. Continue to pull until you have enough new tubing to reach the back of the speedometer without it becoming a cosmetic or tripping hazard.
Push the end of the pitot tube onto the barbed connector on the back of the speedometer. Fasten the end of the pitot tube that's on the bottom in place by placing the end of the pitot tube in the bracket your removed earlier and tightening the bracket's mounting screw. Caulk any openings that the pitot tube passes through with marine silicone caulk.
- Before you connect the pitot tube to the bottom of the vessel, blow into the tube and make sure your speedometer moves.
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