How Does a Boat Speedometer Work? [Explained]

how does a boat speedometer work

The boat speedometer is a thing of wonder. Unlike a car that has wheels to measure speed, a speedometer on a boat can't use anything quite as direct since its gliding over water. So exactly how does a boat speedometer work and can it accurately measure speed?

You'd be surprised to find out that the humble technology behind the boat speedometer has become impressively sophisticated over time. Wondering how that little gauge on your boat works? Here's a quick explanation.

How Does a Speedometer on a Boat Work?

Boat speedometers can work in two different ways. They can either be manual or GPS powered. Generally, GPS speedometer units are more accurate. But even then, lots of boats still use manual speedometers to provide a rough estimate of speed.

Manual Boat Speedometers

A manual speedometer, by definition, uses an instrument that measures speed by the movement caused by the water itself. As of writing, there are two distinct kinds of manual speedometers.

Pitot Tube Speedometer

A pitot tube speedometer (also called a pitometer) uses a long, hollow, narrow triangle tube with two holes mounted to the transom to measure speed. It entails the movement of water to displace air pressure and cause movement on the gauge. Here's how it works:

  1. The pitot tube, situated under the boat, allows water to pass through as the boat moves across the surface of the water. This water is then fed into two different holes.
  2. The inside of the tube contains nothing but air pressure. When the water pushes up into the holes, it displaces this air in the chamber.
  3. As the water is pushed into and up the tube, it moves the air pressure thus causing changes in the gauge on the boat's speedometer.
  4. At faster speeds, more water enters the tube hole. This in turn displaces more air pressure, further pushing the needle and increasing the reading of the speed of the boat.

Lots of people appreciate the pitot tube speedometer because it's simple, durable, and cost effective. Manufacturers also install the pitometer on most low to mid range boat models. So it essentially comes with the purchase of a new boat.

The downside of course, is that the pito tube isn't the most accurate. While it provides a general idea as to the speed of the boat, lots of things can cause variations in air pressure inside the tube hole. Thus the speed measured by the mechanism won't always be reliable.

Changes in water conditions like the speed of the water under the boat can also affect pitot tube readings. On generally calm waters, the pitot tube may provide a more accurate measurement. But on rough water, pitometers may lose their credibility.


The impeller is a special kind of propeller that's attached to the hull of the boat, extending downwards. Considered more accurate than the pitot tube, the impeller works similarly - by measuring speed through direct contact with water. Unlike the pitot tube though, the impeller feeds data to the boat's speedometer by way of an electrical impulse.

  1. As the boat moves through the water, the impeller spins. The faster the water moves around the impeller, the faster it spins.
  2. The movement of the impeller is then fed to an electric apparatus that converts each spin into a speed measurement.
  3. The spinning movement thus produces an electric impulse that more accurately measures the boat's speed.

Impellers provide a more accurate reading than pitometers because they're connected by wires to the display for more reliable speed measurements. They also don't rely on such a fickle factor as air pressure. On top of that, the mechanism is also equally cost effective.

But they're also more prone to damage, especially if it extends far down underneath your boat. Debris and solid objects in the water can also hit the specialized propeller, causing malfunction and damage.

Another thing to consider is that boat impellers may move the speedometer gauge even if the boat is stationary, depending on the speed of the water.

GPS Boat Speedometers

Considered the most reliable choice, GPS measures your boat speed by locating your vessel relative to the earth's surface. Similar to the ones used in a car, these GPS's don't rely on water pressure and provide more reliable speeds by basing measurements off of your global positioning.

You can install a GPS on your boat's dashboard and download maps to match your locale. What's nice about a GPS speedometer is that its measurements are not affected by external factors like water conditions and obstructions.

Because they don't use a mechanical part that relies on air pressure or water movement like the pitot tube or the impeller, the GPS also isn't as likely to require repair. They're also typically much easier to install since all they need is to be connected to a power source.

Keep in mind though that the measuring capacity of a GPS remains restricted to speed on ground. As it measures the distance traveled from point A to point B via your coordinates, the GPS can determine your speed relative to your position in space.

However since it doesn't have any instrument in contact with the water, it can't display an estimate of your speed through water. For more casual boating enthusiasts, speeds measured through the water might not be an essential. But boats used for racing and related sports and activities may want to consider this.

How Accurate are Boat Speedometers?

boat speedometer

Interestingly, boat speedometer accuracy has long been debated by experts and enthusiasts in the boating field. For the most part, pitot tubes get the most flak because of the way their readings are affected by external factors like air pressure and sea conditions.

Things like water conditions, water pressure changes, and even the presence of objects and obstacles underneath the surface of the water can affect the performance of pitometers and impeller speedometers. 

That's why if accuracy was your main concern, you might want to consider getting a GPS instead. These tools measure your boat's speed through space with significant accuracy because of the method it uses.

If you have a pitometer though or an impeller and you want to find out how accurate it is, you can download a GPS app on your phone and see if the reading on your gauge matches the speed on the app. If there's more than a few miles per hour of difference, then you might want to consider upgrading to a GPS speedometer instead.

What's the Best Way to Measure a Boat's Speed?

Again, you can't go wrong with a GPS speedometer. These tools provide accurate measurements that don't rely on a little propeller spinning in the water. Since a GPS measures your speed by calculating the time it took for you to move from point A to point B on the map, it can measure the speed of your boat more reliably.

But that doesn't completely phase out the need for pitot tube speedometers and impeller speedometers. On smaller vessels or personal watercrafts where speed isn't much of a concern, a pitometer or impeller speedometer should work just fine.

Perhaps the only upside that a GPS has over pitometers and impellers (aside from accuracy) is that these tools provide a map of your general vicinity. This way, you can picture out your route more precisely, as opposed to speedometers that solely provide speed measurements.

Frequently Asked Questions

Why is my boat speedometer not working?

It depends. There are a number of things that could go wrong with a boat speedometer. So if yours has stopped working, then you might have to check all of its different parts. To check, start your engine and move along the water to see if the boats speedo provides a reading.

If you have a pitometer or impeller speedometer, it could be the speedometer tube underneath the dashboard. It might also be the result of damage to the pitot tube itself or the impeller, depending on which one you have. An inspection of both should give you a better idea.

If your GPS speedometer isn't working, then there may be an issue with the software. Check for any software updates or patches to get your GPS working again.

How do you fix a boat speedometer?

Fixing a broken pitometer or impeller speedometer may require some knowledge of its different parts. If you're not confident that you can perform the repair yourself, then you might want to have a professional take a look and fix it.

Nevertheless, if you inspect the speedometer tube and find exterior damage, you may replace the tube on your own. The same goes for the pitot tube and pitot probes.

Another factor worth considering is debris. Dirt and dust stuck in the speedo tube hole may interfere with the movement of the needle on the gauge. Sometimes, a proper cleaning is all it takes to get your speedometer work again. To achieve that, press on the speedo hose junction and pull it out to open the tube and access its insides.

Is GPS better than pitometer?

It really depends on what you need. The GPS speedometer does provide more accurate speed measurements, and it can show you a general idea of your location and any surrounding landmarks. But because they're expensive, not every boater has the budget to shell out on such a sophisticated piece of tech.

If you don't really need pin-point accurate speed measurements, then a manual speedometer should do. Although they can only provide speed readings to an extent, they get the job done and work just fine for causal boaters who only need a rough estimate.

Over to You

Your boat's speedometer gives you insightful information on your boat's performance. Using clever manual apparatus or intuitive electronic data gathered via GPS like in a car, it's truly interesting to imagine how does a boat speedometer work.

Depending on your needs for accuracy, you might find yourself more satisfied with one versus the other. Whatever the case however, you can rely that these trusty tools will perform to provide you a sufficient measurement to guide your boat use.

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