How a boat speedometer works is not exactly an open-and-shut question. Speed isn’t measured on a boat the same way that it is in a car or truck, both of which measure speed based on how fast the wheels spin.
Boat speedometers are one of three types, with two being the most common. Unfortunately, there are a lot of terms applied to the first one—handheld, manual pitot, and manual speedos, are some of the names applied to the first one, while the second type is a GPS speedometer.
The third type is less common than the other two, and it’s called an electromagnetic speed sensor. All of these speedometers work differently, and they are either separate components you can purchase, or it’s built-in.
How Does a Boat Speedometer Work
Types of Boat Speedos
While there aren’t necessarily speed limits on the water, you are expected to power down when you come into a crowded channel or a small bay and slow the craft down. Most of that is because of the wake it causes when you have smaller craft and swimmers in the water.
Manual Pitot Tube
If you’ve never heard of a pitometer before, don’t worry, most people haven’t, including Grammarly and most dictionaries. The construct is basically a tube that sits below the boat, partially in the boat and partially in the water.
The tube captures the water as you move through it. As water enters the tube, it forces air toward the back of the tube, like an air bubble in an upside-down boat. As the water is pressed against the air, it can go no further.
However, as it presses harder against the air pocket, there is a gauge in the back that measures that air pressure. A small computer converts the strength of the air pressure into terms the captain can understand (otherwise known as speed in knots).
The problem is, it’s about the oldest speed-measuring technology on the planet. The invention of this type of speed-measuring device is often attributed to Henri Pitot, a French engineer from the 1700s.
Just because it’s old doesn’t mean it’s bad. A Pitot will give you a solid estimation of your speed, but it’s not the most accurate device.
GPS Speedometer
Chances are, if you have ever used a navigation app on your smartphone or a Garmin device on the road, you’ve seen a GPS speedometer in action. GPS stands for Global Positioning System, and that’s exactly how your speed is measured, via satellite tracking.
GPS speed is measured by how quickly you move from point A to point B. They are considered to be 100% accurate, however, there are times when it’s difficult to track you. For instance, in heavy cloud cover or during a heavy storm, your GPS might not be up to the task.
If you’ve ever been out in the middle of nowhere and listened to Siri or Google Voice Assistant prattle on about how you need to get back on the route, you know that the GPS has lost your position.
Another advantage of GPS speed is the tide, current, waves, and anything else on the water do not affect the GPS.
Electromagnetic Speedometers
These devices are more often known as EM logs or electromagnetic logs. These devices are small and tube-shaped, much like the Pitot. They measure the speed of your boat by how fast a current passes through a generated electromagnetic field.
When you think of the term “conductor,” it’s usually in terms of electricity. However, in this case, the ‘current’ is the water as it passes through the field. This process generates voltage and the voltage is both measured and converted into speed.
That speed is most likely reflected in ‘knots,’ unless you have a device that feeds you the numbers in miles per hour.
Other Speed Measuring Devices
There are at least two other ways you can measure speed on the water—an electric paddle wheel and by using your smartphone. Using your smartphone basically falls under the same category as GPS, since that’s exactly what it does.
Boat apps that help you navigate and measure speed do so in the same way that Google Maps, Waze, Apple Maps, or any other GPS navigation app. It’s a pretty accurate method for determining speed, though it’s subject to dead spots just like any GPS speed-measuring device.
An Electric Paddle Wheel measures speed through the use of a paddle mounted on a cylinder. The passing water spins the paddle and the speed of the spin is measured and converted into a speed measurement.
Electric Paddle Wheels are not as popular because of how easy it is to interrupt the spin through any kind of debris—seaweed, wood, plants, sand, and anything else you might pick up passing through the water.
Are they accurate?
Generally, all of the devices for speed measurement listed above are accurate. However, none of these devices is as accurate as the GPS options. GPS is considered to be 100% accurate, with the only drawback related to signal and how quickly the information is relayed to you.
The Pitot method is probably the least accurate. With that being said, Pitot tubes have been around for centuries, and that doesn’t happen when it’s a piece of unreliable garbage. They’re still around because they do a pretty good job, if not 100%.
If you want 100% accuracy, nearly all the time, go with GPS. If you want 95% accuracy all of the time, choose any one of the other methods.
Bottom Line
Boat speedometers are one of the last devices that any new boat owner thinks about when purchasing a boat. If you stop and think about it, how many customers have you seen climbing up on a boat trying to find the speedometer before making a purchase?
However, just because it isn’t the most popular feature on a brand-new boat doesn’t mean it’s not important at all. This is especially true when you are close to shore, and the area is crowded with kayaks, smaller craft, and swimmers in the water.
If you’re not all that concerned about the type of speedometer your boat uses, you can always bring your smartphone. It will do a good job so long as you are not heading so far out that you hit international waters.
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