By: Olivia Hudak on March 30th, 2022

How Do Boat Speedometers Work?

Have you ever thought about what kind of mechanics make a speedometer function? Maybe you’ve wondered, how does a speedometer reports speed to the gauge on your helm? These questions draw quite a bit of curiosity from boat owners. Especially if they ever stop working.

Speedometers are important for a good number of reasons. They’re critical if you’re boating on a body of water that has a speed limit. Another great reason speedometers are utilized is for towing watersports. 

For example, say you’re pulling a water skier. Typically, someone who water-skis will have a preferred speed they’ll want to be going. This is where the speedometer comes in handy. It’s especially convenient if your boat is equipped with cruise control. The cruise control function works alongside the speedometer to maintain a constant speed on the boat. 

Regardless of the reason, knowing how fast you’re going while on the water will be important at some point in your boating lifetime. So, how and where do you read the speed that’s clocked by a speedo? I’m going to give you the speedometer 101 so that you can understand how this works on boats.

Where to Read the Speed

Speedometers can report to different devices on your boat to record the speed. The most common place to find a speed is on a tachometer. Many boats are equipped with a tachometer which is normally housed on the helm of the boat.

The tachometer typically displays speed and RPM on a gauge to make it easy for the driver to see. Speedometers can also report to devices such as Simrad systems, cruise control systems, and other control panel systems. 

Different kinds of Speedometers

There are several different kinds of speedometers. Each kind shares the same goal of reporting the speed of the boat to the driver. Although they are similar to a car’s speedometer, they are slightly different because of how the mechanics work and how the device reports the speed to the driver. 

A boat’s speedometer is often controlled by water displacement in one way or another. Exactly how it is measured depends on the specific speedometer your boat has. Each model of speedometer has a different way that it measures speed. How exactly each speedometer works will be explained later in this article. 

There are four common types of speedometers that you will find on a boat. Each is built in a slightly different way to calculate the speed of the boat while it is moving across the water. 

Manual Pitot Tube

A manual pitot tube speedometer 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 what kind of boat you have. 

There are two holes in the tube which is attached to the bottom of the boat. As you travel across the water in your boat, water goes into one of the holes in the pitometer. The speed is then measured. The second hole on the pitometer is used to measure water pressure. Both measurements are used to calculate a speed.

How does this measure the speed? As water enters the tube, it forces the air in the tube upwards. The pressure change is recorded on the tachometer as speed on your helm. The pressure in the tube changes as the boat's speed changes, therefore allowing a speed to be reported. And there you have it, the mechanics of a manual pitot tube speedometer.

GPS

Another kind of speedometer utilizes GPS systems to record the speed of your boat. These speedometers utilize satellites to determine the speed of a boat. Comparable to using a GPS on your phone, these types of speedometers will be able to report your speed in no time. This kind of speedometer is very accurate and rarely malfunctions which is a huge pro for these systems. 

Electromagnetic Speed Sensor

Another type of speedometer you may come across is an electromagnetic speed sensor or simplified to be called an EM speed sensor. These types of speedometers measure speed as your boat passes over water and through an electromagnetic field. The electromagnetic field is the actual speedometer that is on your boat. As water passes through the electromagnetic field, a voltage is produced. 

The voltage changes as the speed changes. Therefore, it can report the speed to the boat driver as the voltage changes in the sensor. Put simply, as your boat increases in speed, the voltage produced increases as well. The changes in voltage will report to the tachometer on your helm. 

Electric Paddle Wheel Sensor

This type of speedometer is a bit different than the previous three in the way the mechanics of it work. The way this works is by measuring the force of the water that passes through a paddle wheel. How fast the wheel is rotating determines the speed the boat is traveling. 

The electric paddlewheel sensor is also one that is placed below the hull or toons of your boat. It is located in that spot to give the best measurement of the speed of your boat.

Speedometer Malfunctions

Each type of speedometer comes with pros and cons. So, let’s go over some of the more common problems that people see with each type. To start, they can all be influenced by different factors that can cause inaccuracies in the speed that is reported. Although, some are overall more accurate than others. 

The manual pitot tube speedometer falls into the category of often reporting inaccurate speeds. This type of speedometer can be greatly influenced by current or other water movements. Because of this, the reading you will see is not always completely accurate. If your boat is equipped with this type of speedometer, just be aware that it may not be an accurate speed that your speedometer is reading depending on different factors. 

The GPS systems are typically more accurate than the pitot tube. Speedometers powered by GPS systems are not subject to influences from natural factors such as water movement. The only thing that can cause some problems on this end is a down satellite. If a satellite is not reporting properly, or at all for that matter, you will not be able to get a reading on your speed. This is rarely an issue but it is something to keep in mind. 

Next, let’s talk about inaccuracies involved with EM speed sensors. This type of speedometer is fairly accurate. The reason it is more accurate than the pitot tube and electric paddlewheel is that there are no moving parts on this speedometer. The lack of moving parts reduces the probability that errors occur when reporting the speed. Although, windy conditions or current can still cause inaccurate readings on the speed as well.

Finally, the electric paddlewheel sensor is accurate in reporting speed as long as nothing is blocking the wheel. The wheel can be clogged or blocked by things such as weeds or marine growth. If the wheel gets clogged or blocked, it will not be able to give an accurate reading of the speed of the boat. Therefore, if you feel that your speedometer is off at the speed it is reporting, make sure it is not clogged by anything.

All types of speedometers have their flaws. Whichever one you choose to put on your boat or your boat is already equipped with, it is important to know what can cause inaccuracies. It’s also important to know how to minimize the number of inaccurate speeds reported. 

The Need for Speed

Speedometers are an essential tool for boating. They are now a standard on most motorized boats and included during the build process. It is so important to understand the mechanics of the specific speedometer on your boat. Understanding how the speedometer functions will allow you to understand what factors can cause a false reading, and how to avoid it. 

No matter what kind of speedometer you have on your boat, it is always important to inspect it every so often. Make sure it’s not loose if it is placed on the bottom of your hull. Also, be sure there is no debris stuck on it that could be influencing the speed that is being read on your helm.

Another important fact to consider is that you should have a general understanding of speed while driving your boat. Say, for instance, if you’re cruising around and your speedometer malfunctions you should be able to have a general understanding of your speed. This knowledge will make you a strong boater and allow you to manage situations that could be potentially stressful with ease.