Opening Boat Speedometers: A Step-By-Step Guide

Boat speedometers are an essential piece of equipment for any vessel, helping boaters to monitor their speed and stay within legal limits. They are especially useful when towing water sports enthusiasts, such as water skiers, who typically have a preferred speed. Speedometers can be mechanical (analog) or digital, with each type having its own pros and cons. For instance, traditional boat speedometers that use a pitot tube are prone to inaccuracies due to the influence of currents, whereas GPS speedometers are generally more accurate but can be affected by satellite coverage. In this article, we will discuss the different types of boat speedometers, how they work, and provide a step-by-step guide on how to install a GPS speedometer in your boat.

Manual vs GPS speedometers

Boat speedometers are an essential feature of any vessel's dashboard, helping boaters keep track of their speed, plan journeys, estimate fuel requirements, and ensure they follow local rules and speed limits. There are two main types of speedometers for boats: manual and GPS.

Manual Speedometers

Manual speedometers are a popular option for boat owners as they display how fast the boat is moving. These speedometers use a pitot tube to estimate the boat's speed. The pitot tube is connected to the dashboard through the underside of the boat and sticks out into the water. As the boat moves, water enters the tube, creating pressurised air inside. The change in pressure gives an estimate of the boat's speed. The pressure in the tube changes as the boat's speed changes, resulting in a corresponding change in the gauge reading.

Manual speedometers are a great way to estimate speed, and some models come with additional features such as voltage and fuel gauges. However, they may not provide completely accurate readings due to the influence of water currents and movements.

GPS Speedometers

GPS speedometers provide 100% accurate speed readings by showing how fast the boat is moving through space from one destination to the next. They utilise satellite technology to determine the boat's speed and can provide instant updates on a digital gauge. GPS speedometers are not affected by currents, tides, or other natural factors, making them highly reliable.

GPS speedometers often offer additional features such as distance tracking, trip logging, and navigational charts, providing a comprehensive dashboard for boaters. They are easy to install, typically requiring only a power source and a clear view of the sky to receive satellite signals.

Both manual and GPS speedometers offer advantages for boat owners. Manual speedometers are a more traditional option, providing a good estimate of speed, while GPS speedometers offer unparalleled accuracy and additional features. When choosing a speedometer, factors to consider include accuracy, ease of installation, additional functionalities, and cost.

Pitot tube speedometers

A pitot tube speedometer is a manual speedometer that relies on a pitometer to get an accurate speed reading. This type of speedometer goes through the hull of the boat and attaches to the bottom of the hull or toons, depending on the type of boat.

The pitot tube is a long, thin tube that is open at one end and has a small hole at the other. It is placed in the water so that the open end faces the direction of the water flow. As the water flows past the pitot tube, it creates a pressure difference between the inside and outside of the tube. This pressure difference is proportional to the speed of the water and is recorded on the tachometer as speed.

The pitot tube speedometer is connected to the boat's dashboard through the underside of the boat via a cable. The tube sticks out into the water and travels along the water surface as the boat moves. As the boat gains speed, more water enters the tube, forcing the air inside it upwards and increasing the pressure. This change in pressure is what gives an estimate of the boat's speed.

Electric paddle wheel speedometers

To open an electric paddle wheel speedometer, you will need to refer to the manufacturer's instructions. However, here is a general guide on how to install and open a boat speedometer:

• Choose a location for the speedometer that is easily visible to the driver and does not obstruct the view of other gauges or instruments.
• Drill a hole in the chosen location large enough to accommodate the speedometer's mounting bracket.
• Secure the speedometer's mounting bracket using the provided screws and bolts.
• Install the speedometer's wiring harness as per the manufacturer's instructions.
• Connect the speedometer to the boat's power supply, typically requiring two 12V connections.
• Connect the speedometer to the boat's chassis for grounding.
• Test the speedometer to ensure it is functioning correctly.

It is important to note that regular maintenance and cleaning of the speedometer are essential to ensure accurate readings and reliable performance. Additionally, factors such as water conditions, boat trim, engine RPM, and GPS signals can affect speedometer readings.

Electromagnetic speed sensors

An electromagnetic speed sensor is one of the four common types of boat speedometers. It is also referred to as an EM speed sensor. This type of speedometer measures speed as the boat passes over water and through an electromagnetic field. The electromagnetic field is the actual speedometer that is on the boat.

The sensor is usually mounted on the side of the hull, with the field perpendicular to the direction of travel. As the boat moves through the water, a conductor (usually a metal plate or wire) is forced through the field, and the resulting voltage is used to determine the speed of the boat. The voltage changes as the speed changes. Therefore, it can report the speed to the boat driver as the voltage changes in the sensor. In simple terms, as your boat increases in speed, the voltage produced also increases. The changes in voltage will be reported to the tachometer on your helm.

The electromagnetic speed sensor delivers boat speed and log as well as water temperature. It is installed in a thru-hull housing and is not affected by weeds and other particles suspended in the water. It is free from moving parts and provides accurate data in all circumstances. Robust and compact, it is ideal for racing because it does not create any drag in the water.

To open a boat speedometer, you will need to refer to the installation manual for the specific type of speedometer you have. The process for installing a boat speedometer varies depending on its type. For example, a GPS speedometer will have a different installation process than a mechanical speedometer.

Speedometer maintenance

Boat speedometers are essential for staying safe and complying with speed limits. They are also useful for towing watersports and tracking fuel efficiency, engine performance, and overall boat handling.

There are two main types of boat speedometers: manual and GPS. Manual speedometers use a pitot tube to estimate the boat's speed, while GPS speedometers use satellite technology to provide a more accurate reading.

Maintenance and Repair

Manual Speedometers

Manual speedometers require more frequent maintenance than GPS speedometers. The pitot tube should be regularly checked for any holes, cuts, kinks, or wire ties that may be pinching the tube. If the tube is damaged, it should be replaced. It is important to ensure that the tube is not sagging, as this can cause water to collect and affect the accuracy of the speedometer.

GPS Speedometers

GPS speedometers require minimal maintenance, as long as they have a good electrical connection and the software is up to date. However, they can be impacted by particularly bad weather as they rely on a GPS connection.

Calibration

Proper calibration of the speedometer is crucial for accurate speed readings. Boat owners can test the accuracy of their speedometers by using a GPS-based speed app on their phones while cruising on a day with minimal wind and calm seas.

Installation

When installing a speedometer, boat owners have the option to do it themselves or hire a professional. DIY installation can be simple, especially for GPS speedometers, which only require a power source and a mounting location. Pitot tube speedometers are more complex to install due to the need to drill holes and route the pressure tube.

Tips for Use

• Ensure the speedometer is properly mounted and secure.
• Regularly check the speedometer to gauge your speed.
• Be aware of your surroundings, even if you are complying with speed limits.

Frequently asked questions