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The Accuracy of Boat Speedometers (By Type)

How often have you heard a proud boat owner claim that the boat is faster than anyone else’s, and they can prove it because they have a video of the speedometer showing the impressive number? Most boat companies don’t spend a lot of effort ensuring the speedometers are completely accurate.

If your boat has a pitot tube speedometer (the traditional type) or an electric paddlewheel speedometer, you should take the readings with a pinch of salt. Even when these types are properly set up, several factors will cause the displayed speed to be inaccurate.

For most applications, a leisure crafts speed through the water is purely for academic interest. For this reason, the most common type of speed measuring devices used are the pitot tube speedometer and the electric paddlewheel speedometer.

boat speedometer

A Boats Speedometer Accuracy Depends On The Unit (4 Types)

Four types of boat speedometers are fitted to marine craft, the accuracy of which will depend solely on the kind which is installed.

The four speedometer types are as follows.

  1. Traditional pitot tube speedometer. 
  2. GPS speedometer.
  3. Electric paddle wheel speedometer.
  4. An electromagnetic speed sensor.

Traditional Boat Speedometer

Veethree Lido Pitot Speedometer 65MPH Marine Boat with Pitot kit and 20 feet tubingVeethree Lido Pitot Speedometer 65MPH Marine Boat with Pitot kit and 20 feet tubing

There are two tubes in a pitot tube speedometer called – you guessed it pitot tubes.

It is a variation on the system used in an aircraft that also has pitot tubes.

The pitot tubes extend under the boat and into the water as it flows under the boat.

As the boat accelerates, water flows into the pitot tube and compresses the air inside. At the receiving end of the tube is a pressure sensor that measures the increasing pressure as it increases as the boat accelerates.

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Similarly, as the boat decelerates, less water pressure is imposed on the inside of the pitot tube, and so the gauge records a lower speed.

The gauge converts the pressure reading to a speed reading, which is the value shown on the speedometer.

The issues with a pitot tube speedometer are.

1. The Speed Is Always Relative To The Water Speed

As the measurement is made against the speed of the body of water relative to the boat’s speed, the value displayed is not a true measure of the boat speed moving from A to B.

If the current flows 10 miles per hour from the front of the boat to the rear, if the speedometer is reading 60 miles per hour, the actual speed against is 60mph – 10mph = 50 mph.

Similarly, if the boat is traveling in the same direction as the current, which is still going 10mph, and the speedometer is reading 60mph, the boat’s true speed is 60mph + 10mph = 70mph.

A pitot speedometer will only measure the boat’s speed going forward and not while it is reversing.

2. The Speed Which Is Displayed Is Affected By Altitude

As altitude increases, the air density decreases. It will directly affect the residual air pressure in the tube.

As the altitude increases, there will be less air pressure in the tube, which will cause an “under” reading of the speed.

3. The Pitot Tubes Can Be Blocked

If the boat is not stored on dry land, it is possible that algae or some other contamination will grow inside the tubes and prevent the water from flowing, resulting in a zero reading.

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GPS Speedometer

A GPS speedometer works the same way as the GPS you use for navigation in the car.

The GPS connects to several (8-10) satellites that send out a continuous location data stream. The GPS measures its distance relative to each satellite and triangulates its position. 

As the boat starts to move, the GPS constantly recalculates its position in time and space and can work out the rate of change (acceleration or deceleration)  in its motion.

As long as there are a sufficient number of satellites that the GPS speedometer can connect to, it will provide an exceptionally accurate speed reading on display.

Unlike a pitot tube speedometer, the GPS makes no reference to the water current speed, and it always references the speed of the boat in relation to space and time, as opposed to the body of water.

A GPS Speedometer May Not Register Small Increments

The only downside of a GPS speedometer is that it will not always immediately register small incremental changes in speed, so there may be moments when it is not completely accurate.

You Already Have A GPS Speed Sensor

With the explosion of devices with a GPS speedometer included, it is not always necessary to have one fitted to a boat; instead, you can access the GPS on your cell phone, which will provide just as accurate a reading as other devices.

Electric Paddle Wheel Speedometer

boat electric paddle wheel speedometer

An electric paddlewheel speedometer is positioned inside a housing that extends into the water.

As the boat starts moving, the wheel begins to turn a sensor, and the rate of revolutions of the paddlewheel is displayed on the speedometer screen.

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The wheel will turn either way, whether the boat is moving forwards or backward, and so it will record both positive (forward) and negative (reversing) speeds.

The Speed Is Always Relative To The Water Speed 

As with a pitot tube speedometer, an electric paddlewheel speedometer only measures the boat’s speed relative to the body of water.

Depending on the current water flow, the displayed speed will be overstated or understated and will not necessarily be the “true” speed.

It Contains Mechanical Parts

As with anything that contains moving parts wear happens over time.

The same is true for an electric paddlewheel speedometer, where the moving paddlewheel and the speed sensor are potential failure points.

An Electromagnetic Speed Sensor

yacht speedometer

An electromagnetic speed sensor is installed in the housing through the hull.

When a conductor (which is the water) moves relative to an electromagnetic field (created by the electromagnetic speed sensor), a voltage is generated, which increases or decreases as the relative speed of the water and the sensor change.

The sensor measures the voltage as it increases or reduces and converts this to the display’s speed.

The sensor does not contact the water, and there are no moving parts.

It is a very robust technology, and it is not affected by any contaminates which may block tubes from stopping wheels from turning.

The Speed Is Always Relative To The Water Speed 

Once again, it only measures the boat’s speed relative to the body of water. So the displayed speed will be overstated or understated depending on the current water flow and will not necessarily be the “true” speed.

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Conclusion

We strongly recommend that you never take a bet in the pub concerning your boat’s speed capability unless your boat is fitted with one of the newer technology speedometers. The older pitot tube speedometers and the paddlewheel varieties are notoriously inaccurate.