How Does a Boat Speedometer Work?

How Does a Boat Speedometer Work

Is boat safety the same as car safety? Driving a boat may seem safer because there are fewer chances of collision but you might be surprised at the various problems that come with steering a boat on the water. This is why just like cars, speedboats come equipped with their own speedometers. A boat speedometer is a standard inclusion in any boat and helps guarantee the safety of its passengers. In this article, we’ll talk about how does a boat speedometer work and more importantly – how to fix problems that could happen on it.

Where is the Speedo Meter Located

Before you even peek beneath the body, it’s important to identify the speed gauge on the vessel itself. It can be quite confusing at first because it sits together with all the other meters used to navigate a boat. You’d want to remember exactly where the meter is located because you’ll be looking at it often during your travels.

How does a Speedometer Work on a Boat?

How does a Speedometer Work on a Boat?

Boat speedometers measure the boat’s speed. That’s not exactly brand new information since the name is pretty descriptive. Here’s a question though – how does a boat speedometer work compared to a car? Boat speedometers work at a different level because it takes into account the movement of the water. Before we delve into that, however, you need to understand that there are actually different types available. There’s the manual which works with water pressure, the GPS speedometer, the paddle-wheel type, and the magnetic one.

How are they different from each other? The difference really is how they approach measurement and therefore, how they accurately measure speed. We’ll explain more of that here:

How Does a Boat Pitot Tube Speedometer Work?

A manual way to measure speed is also known as the pitot tube speedometer. The mechanism basically relies on what is known as a pitometer. There’s this part called the pitot tube which takes in water as the boat moves on the surface. The pitot tube is located at the bottom of the boat and as it glides, water is pushed through the tubes and it becomes pressurized. The air pressure change gives a direct estimate of the speed and is displayed on the boat speedometer.

Note that there are two holes to the tube – one for the water flow and another for the pressure. A faster vessel means there’s more air pressure running through the tubes, resulting in higher speeds. Lower speeds means there’s less air pressure passing through the air inside the tube. This was actually invented by Henri Pitot who died in 1771. The invention was so good that most boats today still use a speedometer tube to determine speed of the water.

How Does a GPS Speedometer Work?

Next you have the GPS speedometer which has a more straightforward approach compared to measuring water pressure. It basically uses GPS to figure out how fast you’re moving by calculating how fast you move from one location to the next. It’s said to be 100 percent more accurate than a traditional boat speedometer because you’re using a modern global positioning approach.

The problem with this is that it measures your speed on the ground in the same way a GPS car speedometer works. On the upside, GPS speedometers can be used together with manual speedometers to give you a better idea of your speed. In fact, there are handheld GPS system today that you can buy online to help you accurate measure faster speeds.

On the other hand, there are also applications can be conveniently installed in your phone so that you can have both types of measure while on the water. The tube compared to these apps may be more reliable – it really depends on the quality of the software you have in your phone.

Electromagnetic Boat’s Speedo

There’s a third kind being used today – although not quite as popular as the other two. It’s called the electromagnetic speedo and it works with the help of ignited engines, eddy currents, and magnetic field at the same time.

Impellor Log Boat Speedometer

Another way to measure the speed of the boat is through an impeller log. This is basically a log that is mounted and sits right by the water. As the boat speeds up, the water hits the log and it starts to rotate. The speed of the revolution signifies the speed of the boat. Think of it as an old-school paddle wheel, or perhaps a wind mill that rotates depending on the speed of the wind.

Which Boat Speedometers are More Accurate? Tube Versus Others

Finally, which of these three types would provide you with the most accurate results? A GPS speedometer is typically better if you want an accurate reading. It will give you better results when measuring not just your speed but also the distance you’ve traveled while on the water. So why don’t all boats use GPS instead of other popular speedometers? Well, GPS also comes with certain drawbacks starting with the tech limitations.

To guarantee that the unit is working properly, you need to be constantly within satellite range. After all, global positioning functions through the help of signals and if you’re far off, the signals will be hard to send or receive. The small propeller or the paddle wheel are also accurate but not really the gold standard.

Fortunately, many vessels today follow specific routes when passing water. These routes make sure that they’re in contact at all times so the GPS units are always working for them. You really just encounter the problem during bad signals or when the weather is terrible. Otherwise, most professionals rely heavily on the GPS to accurate measure speed and then use pitot tube speedometers as a safety backup.

Benefits of Having a Speedometer

Benefits of Having a Speedometer

There are lots of reasons why you’d want to install one in your boat to accurately measure speed. It doesn’t matter whether you’re just a few miles off the coast or in the middle of the ocean – a boat’s speedometer works to your advantage in the following ways:

Speed Limit

Yes, there’s still a speed limit that you need to follow even while on the water. This is why you need to understand how boat speedometer works because you don’t want to find yourself being cited violating any marine law – especially if you’re within a country’s territorial sea. Keep things within legal speed limits and you shouldn’t have any worries while on the water!

Boat Speedometer Works on Control

Faster speeds usually mean more difficult control. Have you ever tried driving a car going at 100 miles per hour? Boats are just as hard and you want to make sure that you stay within speed limits that allow for easy control. Even with a wide open sea, you need to be aware that there could be reefs or shallow areas lurking underneath so you need to go slow and make sure your boat speedometer work to your advantage.

Avoid Accidents

If you’ve been navigating a vessel for a long time, you should already know that it takes a lot of work to maneuver it. You could be working with an engine that’s the size of a house and the last thing you want is to hit something on the water. A speed gauge will give you enough time to maneuver and watch your surroundings before moving ahead. Keep in mind that boats need a lot of leeway in order to avoid blocks. Remember the Titanic? The boat couldn’t just change directions because there’s a point of no return when it comes to boats. With a slower speed however, it should be easier to avoid collisions.

How do you Fix a Boat Speedometer?

How do you Fix a Boat Speedometer

What happens if your speedometer needle stops moving? Worse, what do you do if it is moving but the reading looks like it’s completely wrong? For safety reasons, checking if your speedometer works properly is crucial at every stage of the travel. Just like with your car’s speedometer, it’s important to make sure that you’re cruising at a safe pace. Here’s what you need to remember:

Check the Meter Itself

The best starting point is the speedometer itself to see if it’s working. Perhaps the water speed and the increased air pressure are there – it just so happens that the speedometer isn’t sensitive enough to reflect that. Troubleshoot the problem by detaching the manual speedometer from the dashboard and checking the other hole for any foreign material. Any dirt lodged inside could prevent accurate speed readings.

Once you’ve cleaned the speedometer gauge, it’s time to blow into it to check if it is still working. If it does, then the problem could be in another part of the boat. Note that it’s best to do this step first because the other steps take some time and could include detaching larger parts off your boat’s hull.

What if the needle doesn’t move at all? If that’s the case, this is the best time to replace the boat’s speedometer before the boat travels again. This would be the cheaper if not the safer option.

Problems with the Tube

What if the boat speedometer works? Now you have to look into the tube to verify the boat’s speed. This should be done after you’ve checked the speedometer itself because any damage to the tube necessitates removing it and replacing it with a new one. It doesn’t matter how big or small the hole measures – patching it up may not give you good results.

Comparing pitot tube and global positioning

Is the reading on the boat’s speedometer off? This is why every vessel – even a recreational boat – must come with both a manual boat speedometer and a global-positioning-unit. This is by far the easiest way to check if somethings wrong with the speedometer on a boat. Since the global-positioning would give you a near-perfect reading each time, it’s best to make a comparison between the two. Once you’ve confirmed that there’s a huge variance between the global-positioning and the boat’s speedometer, it’s time to go to the next step.

Checking the tube.

If the boat speedometer doesn’t work, there’s a good chance that the problem is with the tube. Remember how the speed is measured by air pressure? Well, any damage to the tube could compromise the air pressure inside it. Look for any sign of damage. Even one hole on the tube could prevent the buildup of air pressure – creating an incorrect speed reading.

Note that damage is just one of the reasons why there’s a malfunction. For example, there are kinks on the tube that could be impeding the flow of water. Imagine a garden hose that’s suddenly bent in one place – this stops the water from coming out of the other end, right? The same principle follows with tubes so you want to make sure that everything is straight and even. Watch out for any clamps also because it could be creating a blockage – even a small one. If those are the only problems, there’s really no need to buy new tubes.

Changing with a new tube.

So let’s say you’ve noticed one or two holes in your tube, how do you fix this problem? Unfortunately, patching the tube may not be good enough. The best method is by changing the tube entirely. You can detach the gauge from the pitot tubes and buy a new one. Note that a boat speedometer has to work together with all the other boat parts so you need to find a compatible replacement. It’s usually a good idea to bring the old tube with you when buying a new one. This will guarantee that you’ll get the perfect match including the length.

Final Word

To wrap it up, there is no one best way to measure boat speed while you’re on the water. It’s typically best to have at least two units that work for you to test the speed of the water. Remember that faster speed isn’t always better and if you’re careful driving on land, you have to be doubly more careful while steering a boat on the water. Always check local guidelines when driving your boat because speed limits can easily change from one city coast to another.

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Lisa Hayden-Matthews

An avid Skier, bike rider, triathlon enthusiast, amateurish beach volleyball player and nature lover who has never lost a dare! I manage the overall Editorial section for the magazine here and occasionally chip in with my own nature photographs, when required.
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