How to Hook Up a Boat Speedometer

Analog or mechanical speedometers measure water speed passing the hull.

Speedometers are an integral part of both recreational and commercial boats. Most commercial and yacht-classified vessels are equipped with GPS-based speed monitoring systems. However, recreational boats still utilize the analog speedometer, which measures the rate at which water moves around the hull of the boat. Installing a boat speedometer is a simple and straightforward job that you should be able to complete in one to two hours.

Things You'll Need

• Drill
• 1/4-inch drill bit
• Marine-grade silicone caulk
• Small plastic zip ties
• Wire cutters

Instructions

Preparing for Installation

1. Remove the existing exterior speedometer pilot tube at the waterline of the stern.

2. Cut a hole in the boat's dashboard, with a diameter large enough to receive the speedometer unit.

3. Drill a hole in the stern at the waterline where the pilot tube will be mounted. Make sure that the hole's location is free from any obstructions such as a cavitation plate or keel. Also make sure that the hole is 12 inches or more from the propeller.

Installing the Speedometer

1. Feed the plastic speedometer wire through the 1/4-inch hole and into the boat. Avoid kinking or bending the wire unnecessarily. With the wire through, securely mount the pilot tube in a way that allows water to flow around it freely. Ideally, the pilot tube should be approximately 1 to 2 inches away from the hull of the boat, completely unobstructed.

2. Feed the speedometer wire along the inside of the hull, securing it along the way with plastic zip ties. Mount the wire away from heat sources in a progressively upward slope toward the speedometer unit in the dashboard.

3. Disconnect the positive (red) battery terminal and do a mock connection of the plastic speedometer wire to the speedometer unit, allowing for a length of cable that has 1 to 2 inches of slack when connected.

4. Slip the speedometer unit through the hole in the dashboard and secure the faceplate screws.

5. Attach your boat's common power buss to the positive connection on the back of the speedometer readout. Secure the wire in place with a zip tie to avoid disconnection due to prolonged vibration. Locate the common ground for your boat and connect it to the negative terminal on the back of the speedometer readout. With all wires attached and secured, reconnect the positive battery terminal.

Tips & Warnings

• Use a pneumatic air nozzle to clear the pilot tube hole of debris caused by algae growth if your speedometer isn't working.

• Before you begin work, disconnect the positive battery terminal to avoid any risk of shock.

References

• Faria Marine Instruments: Mechanical Speedometer Installation