Boat Speedometer – How It Works & How To Repair It On Your Own

If you own a boat, then you know the fun and relaxation you can have out on the water. However, if you own a boat, you might also know it is a thing of comprehensive mechanical knowledge. As such, its full functions require a bit of a learning curve.

To be a well-rounded boat owner you should have some awareness from the biggest boat mechanism to the smallest boat mechanism. I know what you’re probably thinking. No one has time to be learning everything about their recreational boat.

I definitely agree. However, that does not mean you should not pick up on basic know-how. But where is the best place to start?

A boat’s speedometer is one of the smaller, but still integral parts of a boat. Understanding its function is not as simple as you might think, but it is the key to keeping it in top shape.

As such, we will be talking about the functions of a boat speedometer. We will explore the real mechanics of a manual speedometer and how you can care for it on your own.

Boat Speedometer Basic Function

Many people assume that a boat speedometer works a lot like a car speedometer. However, that could not be further from the truth.

It is actually much easier to understand how a car speedometer works. A car is moving on a stationary road and measuring the tire revolutions makes for an easier calculation.

A boat, unfortunately, is not that simple.

First, a boat might maneuver like a car, but they are not driving the same roads. Water is a whole new ball game and involves extra environmental circumstances that a car does not have. A boat speedometer has more to measure than a car.

Second, a boat speedometer is not actually a speedometer. In reality, the speedometer is actually a pitometer.

A pitometer compares how fast your boat is going against the speed of the water. This works via a pitot tube mechanism, which you will learn more about later. Essentially, the pitometer measures the speed of the water coming through the tube compared to air pressure.


That’s alright. Let’s break it down.

How Boat Speedometer Works

The pitometer requires two unique pieces of data to accurately calculate the speed of the boat. The two pieces of data are:

  • Water speed
  • Air pressure created by water

This is where the pitot tube mechanism comes into play. The pitot tube passes through the underside of the boat and into the water.

The pitot tube has two holes in it. One hole will measure the speed of the water passing through it. The other hole will measure water pressure.

So as the boat glides through the water, water is pushed through the tube connected to the speedometer. As the water travels up the pitot tube toward the speedometer, speed is measured.

Now, between the pitot and the speedometer gauge, there is a tube of air. Depending on the speed, the water can rise to a certain level. In turn, this causes the air pressure in the tube to fluctuate.

Therefore, the speed of your boat determines the amount of water in the tube versus the air pressure in the tube. Presumably, at different speeds, there are different pressure levels. This combination of the pressure and water speed is eventually translated into your boat’s speedometer.

To put it simply: There is a direct relationship between speed and pressure. Essentially, speed and pressure are proportionate to one another.

The higher the air pressure in the tube, the faster the boat’s speed. The lower the air pressure in the tube, the slower the boat’s speed.

And that’s it. Granted, this is no comprehensive mechanical engineering explanation. However, this explanation can suffice for basic instructions.

Is There A Better Way to Measure Boat Speed?

So, here’s the thing about boat speedometers: they are not perfectly accurate. As such, some boaters tend to turn to GPS systems to help measure their speed more accurately.

A GPS system measures speed quite differently than a boat’s traditional speedometer. Whereas a traditional boat speedometer measures speed against water, a GPS measures speed through space. A GPS measures the real speed from location to location, which is why it is considered more accurate.

Another form of measuring boat speed is the impeller log. This a small propeller that is mounted on the boat and sits in the water.

As the boat glides through the water, the propeller spins. Much like car tires, the revolutions of the propellor help measure speed. However, the accuracy of the impellor log is not as good as a GPS or manual speedometer.

Granted, a GPS speedometer and manual speedometer can measure the same speeds. However, this is only if the boat is on totally still water. The slightest change in tides can show discrepancies between the GPS and the manual speedometer.

The choice of which speedometer depends on you and the age of your boat. Older boats tend to have a pre-cut for a speedometer, meaning a manual speedometer would be more ideal.

However, if you are a fan of accuracy, the GPS speedometer is for you. Moreover, if you have a more modern boat, the digital display of a GPS would be a nice addition.

Why Do You Need to Know About Boat Speedometer?

If you love boating, it is only natural that you understand how its different parts work. Not only does it better inform how you maneuver and drive your boat but it also contributes to your ability to care for your boat.

For example, boats will not stay perfect forever. Being out on the water invites a multitude of unforeseen incidents.

Now, you are probably thinking you could just go get a repairman for whatever dinks or damages your boat might get. However, there might be circumstances where a repairman is not an option and fixing your boat is an immediate necessity.

In that case, being able to fix your boat yourself could be your saving grace. Moreover, it will save you a lot of time and money.

And this all starts with you taking the time to learn about the functions of your boat. Developing a well-rounded understanding of your boat’s functions ensures you are a better boat owner in the long-run.

How to Fix Boat Speedometer?

Fixing your speedometer is a very quick repair and leaving it broken can cause more trouble down the line.

As such, here are the steps to repair your speedometer by yourself:

  • Step 1 – If your speedometer is off or not working, now it’s high time to do some fundamental repairs. But, before you go pulling wires, determine the cause of the damage.
  • Step 2 – In order to find the cause, first locate the speedometer tube. It is typically located underneath the dashboard right behind the speedometer. The tube itself is often black and way too thick than regular wiring on a boat.
  • Step 3 – So, if you have already found the speedometer tube, check it for damage. Look along its sides to check if there is any breakage that could have caused the damage.
  • Step 4 – Next, go ahead and detach the speedometer tube.
  • Step 5 – Now, take a closer look at the tube. If you see any holes, cuts or damages, it is time to replace the tube. If you need to purchase a brand-new tube, be sure to take the used tube with you so you can choose the right length of the tube.
  • Step 6 – Double-check to make sure your pitot tube is also intact. If it isn’t, you will need to replace it too.
  • Step 7 – If you find that the pitot and speedometer tube is not damaged, you must check the speedometer itself. There is an easy way to test it. First, disconnect the speedometer tube. Blow into it for 10 seconds while someone watches the speedometer.
  • Step 8 – If the speedometer needle isn’t moving, you may have dirt and debris stuck in the tube. Use a compressor to clean out the tube. Then, reattach it to your speedometer and try the blow test again.


Well, there you have it. A speedometer might not seem like a big deal compared to everything else on a boat, however, misreading your boat speed could lead to a number of hairy situations.

Not only is understanding your boat speedometer’s functions valuable for safety, but it also saves money. And this goes for all aspects of your boat. Learning the different functions of your boat gives you a leg up on any repairs.

Now, instead of spending hundreds of dollars on simple repairs, you have the knowledge to fix any damage yourself.

We hope this article helped you better understand the function of your boat speedometer. Moreso, we hope it helped you develop a new skill, making you a more competent boat owner.

If you have any more thoughts, please feel free to share them with us.

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