How Does a Boat Speedometer Work?

At sea, there are no speed restrictions like on land. But keeping an eye on the boat’s speedometer not only helps you to better plan the trip but also increases the safety of the trip. The big question in this discussion is how does a boat speedometer work? 

Boat speedometers measure speed in two ways. The first is the classic method, which consists of dividing the boat’s distance traveled by the time taken to make the journey. The second method is with a pitot tube, which calculates velocity by comparing the water’s density to its pressure.

The speed is always calculated using two basic methods: dividing the space traveled by the time spent on the journey or using the water pressure in the so-called “pitot” tube. In this case, the speed is calculated by the square root of the difference between the dynamic (with the boat in motion) and static pressure of the water, divided by the density of the water.

The first method is used by turbine-type gauges (those that are under the hull and have a small wheel with blades, being very common on sailboats). It is also the method used by GPS. In the second, more used in boats, the “pitot” tube is located inside the tail or, in boats with a crow’s-foot axle, coupled externally. Let’s learn more about these very important instruments in the three questions below.

Where is the speedometer sensor located on a boat?

There are two main speedometers that you will find on modern boats. These speedometers include the electric paddlewheel and the pitot tube style speedometer. The pitot tube speedometer looks like a triangle that is long. It is made of plastic and the triangle’s long end sticking down into the water just below the bottom of the main body of the ship. You will be able to find the speedometer sensor under the transom of the boat or at the lower section of the outboard motor. 

The speedometer itself is a device for displaying speed information. There are analogue and digital speedometers. Analog data is displayed with an arrow, while digital data is displayed in numbers. The accuracy of the speedometer does not depend on this because the speedometer only displays the information that comes from the sensor.

Sensors are used to determine the speed of the boat. The methods of obtaining information about the speed of a boat are different for them. Below are some of the boat speedometer sensors:

Impeller Sensor

The rarest type of speedometer sensor used on boats and motorboats is the impeller sensor. The higher the boat speed, the faster the impeller rotates. A magnet is installed in the impeller, which, when the impeller rotates, passes near the reed switch. The speedometer reads impulses and translates them into speed. The sensor shows the speed of movement relative to the water. If your boat is going against the current, then the displayed speed will be higher than the real one, and if it is going downstream, then the displayed speed will be lower than the real one. Such speedometer sensors usually operate at low speeds and are used on yachts.

Sensors of this type are rarely used for the following reasons:

 •. The more a part is moving in the device, the higher the chance of breakage – this is an old truth. And if the device is constantly in the water, then the risk is even greater. In addition, there is a possibility of algae and debris winding on the shaft/screw, which will instantly disable the speedometer.

 • The sensor shows the speed of movement relative to the water, not the shore. In places with strong currents, readings may differ significantly from real ones. If you are swimming against the current, the displayed speed will be higher than the actual speed, and if it is downstream, it will be lower.

But the last drawback for certain situations can turn into advantages. If you wish to test the speed of the current, then you simply cannot find a better sensor than an impeller. The high measurement accuracy helps to determine the speed even when driving very slowly.

Sensor with pressure gauge – Pitot Tube 

The speed is determined using a Pitot Tube, with an inner diameter of 1–2 mm, directed by the hole towards the movement of the boat, which registers the pressure of the incoming water. The higher the speed, the more pressure is created. A pitot tube speedometer is basically a simple gauge. It works by changing the air pressure in the speedometer’s hose. Water presses through the pitot into the hose and compresses the air. The air pressure increases at the speed of the boat, as a result of which the gauge (gauge) shows a higher speed. 

The approximate speed of a watercraft, boat or motorboat can be determined by the formula: 

V = 50.4√P, km / h

Where P – pressure gauge readings, kgf / cm 2 

Pitot sensors are the most common, as they are much more reliable than impeller models (due to the lack of moving parts). As such, many outboard motors already have a built-in sensor of this type. 

Disadvantages of Pitot Tube Speedometer Sensor

The sensor must be installed exclusively perpendicular to the incoming water flow. When the branches/boards are caught, the tube leans up, it can get clogged and can catch debris. In these cases, the speed of the boat will not be indicated. On the gearbox, the hole is also constantly clogged. Sometimes the readings are inaccurate but work fine for the purpose of selecting a propeller to achieve the maximum speed.

GPS speedometer  

The GPS speedometer provides an accurate reading of your ground speed. The GPS speedometer can be connected to an existing NMEA network. Ideal for measurements, especially both at the slow speed of a powerboat and at high speed, where traditional speed sensors such as pitot tubes and wheel speed sensors may not work well. An added benefit is that there is nothing to rust in the water or break down, no moving or rotating parts. It is the most accurate speedometer gauge. The only drawback is that you need a power supply and a higher price.   

Why would a speedometer stop working on a boat?

The main cause of speedometer failure is the clogging of the intake hole that can be either on the front side of the lower unit or in a separate pickup that is attached to the transform. The gauge is used to measure pressure. To remove whatever is clogging the boat speedometer, you need to disconnect the hose on the gauge and then blow it backwards.

How to test boat speedometer?

The problem is much more common than you think. Speedometer failure is one of the most frustrating things that you will ever encounter. Speedometer malfunctioning is a more common problem than you may think. Below is a guide on how to test a boat speedometer.

Identify the source of the problem.

The first thing is to identify the source of the problem. Carefully examine the boat speedometer and identify signs of damage. Then, you can easily fix the problem if it is a basic problem.

Check the speedometer tube.

Sometimes the speedometer tube might be the source of the problem. This tube is usually located under the boat’s dashboard. It is thicker than wires, and it is black in color.

Check if the tubes are damaged externally

Check if the speedometer tube is damaged. Confirm that the tube has no cuts or holes since they can be the cause of the problem. It is very easy for you to inspect the pitot tube and wire located on the boat.

Install a new tube if the old tube has a problem

If you find some visible damages on the tube, there is a great chance that the cause of speedometer failure is the tube. Repairing a tube is quite difficult. As such, the choice is only to replace it with a new one. When buying a tube, it is good to ensure that it has the same length as the old one.

Change your pitot probes

Pitot probes can also be the source of the problem. Inspect them, and if you find that they are damaged, it is good to replace them. Replacing pitot probes is easier than repairing them.

Inspect speedometer.

If you find that the tubes and probes are working well, then you need to check the speedometer. Inspect it thoroughly and try to detach the outdoor motor tubes from it. After that, spend 5-10 seconds blowing air into it.

Repair the speedometer

If the speedometer’s needle does not move after blowing air into it for 5-10 seconds, then you will be required to detach the tube that is attached to the speedometer. Ensure that the tube is not lodged by dirt. If the tube is lodged by dirt, clean it, and the speedometer will work properly again.


So which speedometer should you choose? It all depends on your needs. If you need more accuracy and comfort in use, choose a GPS device. If these criteria are not so important, then it is more rational to use a cheaper sensor with a pressure gauge. A version with an impeller cannot be recommended (except for the specific cases described in the article) due to its many disadvantages.

Nicholas Finn

I've been the captain of a fishing boat for over 20 years, and I created Pirateering to share my knowledge of and interest in seafaring.

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