How Does a Boat Speedometer Work? Know With Video

Boat speedometers are one of the most important instruments in your dashboard panel. Just like a car, you can keep track of your speed on waterways. It is also a useful indicator that can allow you to predict the duration of your journey.  

Most modern boats make use of a pitot tube to measure speed. The small triangle shaped mechanism sticks out of the boat and takes in water as the vehicle moves through the water surface. The pressure difference causes the speedometer needle to move, indicating changes in speed.

But how does a boat speedometer work? We’ll know that in this artcle. We will also go through other alternatives to the basic speedometer, which may help you get better speed estimates and make your journey even better!

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Let’s begin.  

How Boat Speedometer Works?

You might be thinking that boat speedometers would work the same way as they do for cars. However, there is a distinction between the two. 

When it comes to measuring the speed of the boat, the term speedometer is quite commonly used. Unlike cars, where the speed is estimated by the number of revolutions made by the tires, boats make use of a pitot tube instead.

Boats make use of a pitometer to get an estimate of the speed. The pitometer refers to the built-in Pitot tube mechanism on the boat. It is very similar to your average car speedometer but works in a different way.

So how does a Pitometer work?

In order to estimate the speed of a boat, the device requires a continuous feed of two different data. The two factors are the velocity of the water and the air pressure created from the water.

Here is how the mechanism works:

1.  Pitot probes that are connected to the mechanism go through the underside of the boat and stick out into the water.  When the boat travels along the water surface, the water is pushed through the tube. The tube is connected to the boat speed gauge.

2. The probe contains two holes, one of them estimates how rapidly the water travels through the tube and the other hole estimates the water pressure.  The pressure changes as the boat travel at speeds up or slow down.

3.  The water that goes through the tube has no place to escape. As a result, this creates a change in air pressure inside the tube. This change in pressure along with the speed of water shows the speed of the boat.

4. Increased air pressure inside the tube means faster speeds. Reduced pressure in the tube translates to slower speeds.

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Is Boat Speedometer Accurate?

Boat speedometers are notoriously known for being inaccurate. Since it works primarily on water pressure, if any foreign material gets inside the Pitot tube, it will register an incorrect speed. The tube needs to be cleared of any foreign material so that it gets a proper speed estimate.

At lower speeds, the boat's speedometer is quite accurate. Compared to a GPS, the speedometer falls short when it comes to accuracy. However, depending on the manufacturer, some water-pressure based speedometer do perform well. 

The accuracy of the speedometer also comes down to the pitot placement. Pitot tubes that are transom-mounted receive cleaner water. This makes them more accurate. Also, the pitot that is integral to Bravo drives estimates accurate speeds depending on the drive height. 

Boat speedometers are not the best instruments when it comes to indicating speeds. If you are purchasing or trying out a boat, don't rely on its speedometer. It is best to bring a handheld GPS that will give you close to accurate speed readings.

How To Measure Boat Speeds More Accurately

Pitot Meters can be a good indicator of your boat’s speed, but it is far from accurate.  There are two better ways to measure the speed of the boat. They are:

1. GPS

One of the most accurate devices which can be used to read the speed of the boat is a GPS or global positioning system. GPS is regarded to give better reading compared to the speedometer and Pitot tube arrangement.

GPS can be bought and fixed on your boat. You can also purchase a small, handheld GPS which you can use it anywhere. Downloading GPS apps on your smartphone can be a good alternative as well. However, it is best to get a mounted GPS hardware that you can permanently fit on your boat.

GPS makes use of your satellite data to measure speeds. The speed is measured by determining the distance between the positions of your boat relative to the earth's surface.

The data that picked up from the satellites allows a GPS to calculate its location on the Earth.  Some GPS units are even more accurate as it determines the distance between itself and various cell towers.

2. Impeller

Another mechanism that can be used is called an impeller. This is a small propeller that projects from the hull all the way to the bottom of the boat. The working of the impeller is simple. As the boat moves through the water, it causes the impeller to rotate.

The movement of the impeller produces an electric impulse directly proportional to the revolutions of the impeller. This is then converted into speed and distance.  An issue with the impeller is that it can be damaged by any debris on the water since it is projected far below the boat.

How To Fix And Repair A Boat Speedometer?

One of the most frustrating events is when you are cruising on your boat and the speedometer starts to malfunction or stop working altogether. The problem is much more common than you think.

It is possible that the speedometer needs to be checked by technicians, but most of the time, the fix is actually quite simple.  If the issue is not looked after immediately, you might face bigger trouble later on. Let’s some of the steps you need to take in order to fix your boat’s speedo.

1. Identify The Cause

The first thing to do is to find the cause of the issue. Inspect the boat speedometer to find any signs of damage.  If the problem is basic, then it can be fixed easily.

2. Find The Speedo Tube

There is a high chance that the problem is caused by the speedometer tube. The tube is often situated under the dashboard and runs all the way to the middle of the speedometers back. It is usually colored in black and is thicker than most wires.

3. Check For Exterior Damages On The Tube

After you have found the speedo tube, inspect for any damages on the tube. Look for holes or cuts as they might be the cause of the speedo’s problems. You can easily inspect most of the wire starting from the instruments back to the Pitot tube.

4. Detach The Speedometer

Detach the tube that connects the Pitot tube by unscrewing it. If the tube is connected to the motor, simply disconnect the joint. 

5. Install A New Tube

If there is visible damage on the tube, chances are that it is causing the speedometer to malfunction. If there are holes in the tube, the entire tube needs to be replaced as it might be difficult to repair it. When purchasing the new tube, ensure that the new one has the same length.  Refer to the old tube when purchasing a new one. 

6. Change The Pitot Probes

A pitot tube can be the source of the malfunction. Damaged pitot tubes can be difficult to repair. It is much easier to replace the tube with a new one.

7. Inspect The Speedometer

If the tube or the pitot probes aren’t damaged, it might just be the speedometer that is malfunctioning. Inspect the instrument thoroughly. Try detaching the tube on the outdoor motor and blow air into it for 5-10 seconds. Tell someone to check if the speedometer is working while doing it.

8. Repair The Speedometer

If the needle on the speedometer doesn't move, detach the tube leading to the speedometer. Check to see if there is any dirt lodged into the tube. This might cause the tube to be stuck.

Clean the tube and connect it back to the speedometer. Then blow air into the tube to check if the speedometer is working properly.

Conclusion

Pitot tube speedometers can be found on most boats today. The concept was first engineered by Henri Pitot back in the 18th century and has gone through several iterations.

The modern-day speedometer has improved over the past years. However, the concept remains the same. The use of water pressure to measure speeds is good, but these days it can be far from accurate.

When it comes to speed, GPS is much more accurate and reliable. It continuously takes reading from satellite and recalculates your position. It can accurately measure the distance between fixed positions and provide an estimate of the speed.

Whichever method you prefer, there is always an option to use both at the same time. We hope this article helped shed light on how boat speedometer works. Have fun in your next boating journey and remember to sail safe!

Last Updated on September 8th, 2020

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