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How Accurate are Boat Speedometers?

Motor boat gauges zoom in photo.

Boating is a favorite pastime of many people. I’ll never forget the first time I was on a boat. The wind blowing in my hair, the sun shining on the water’s surface, the close-up look at water life – it was pretty incredible.

I also remember the family friend who was driving the boat. It was young and it was my first boat ride, so I wasn’t sure, but I thought he might be going a bit faster than he should be. When I said something, he responded that we were on the water – the speed didn’t matter.

The truth is, though, that it does. It matters very much how fast you’re going on the water. There are speed limits there, too, so it’s important you know how fast you’re going.

The trouble is that there’s a lot of talk about just how inaccurate a boat’s speedometer is. Is that true? Well, it depends on your speedometer. Let’s take a look at the basic options and just how accurate they are.

Manual Speedometer

The manual or traditional boat speedometer uses a pitot tube. You might hear this version a pitometer. The tube is beneath the hull.

Water flows into one side of the tube and gets measured. The speed at which this water flows through it is the speed you’ll see on the speedometer.

If you’re driving along fair or still water, this can provide a pretty accurate estimate. The trouble is that the water isn’t always still or fair. If you’re driving through a current, water is going to be traveling through that tube faster than your boat actually is.

In that case, this speedometer is not going to be accurate. In fact, some people have noticed a discrepancy of up to 15 mph faster than they’re actually going.

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Electric Paddle Wheel Speedometer

Boat control board of a motor boat parked at the shore.

This is a pretty simple speedometer option. As you move, the water turns the paddle wheel. The force of this water is measured to provide a speed reading.

In most cases, this reading is pretty accurate. However, if something gets caught in the paddle, like weeds, it’s going to keep it from moving. Therefore, you will not get an accurate reading.

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Electromagnetic Speed Sensor

You might hear this version referred to as an EM log or an electromagnetic log. It is one of the more accurate speedometer types. The water acts as a conductor passing through an electromagnetic field created by the sensor.

The voltage is measured as you drive, providing a speed reading. As there are no moving parts, you don’t have to worry about weeds or debris interfering with the accuracy. However, some water conditions can impact the reading.

GPS Speedometer

A man sailing on a summer season with his small yacht with gps speedometer.

It’s hard to argue with the accuracy of GPS readings. These utilize satellite readings, like the kind you use when driving your car.

Satellites track both your speed and your position accurately. You don’t have to worry about water conditions or weeds interfering with the reading at all. As such, this is one of the most reliable options.

However, you can’t overlook the fact that other issues can interfere with satellite capabilities. While many of these have been fixed over time, there are still some potential problems.

Phone Speedometer

They say that there’s an app for everything – and so far, I have to agree. I have yet to find anything that an app hasn’t been designed to manage – including boat speeds. These are pretty awesome as they can do more than provide a speed reading – a very accurate speed reading.

They can actually also recommend routes, keep you informed on weather conditions, and so much more. Even better, these can provide double duty – both on water and on land.

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The important thing to remember here is that you’ll need data for all of these apps to work. Most of the apps themselves are pretty cheap though – $10 or less – so you can probably splurge on some extra data.

So Which One Should I Choose?

Motor boat controllers, speedometer, gauges, steering wheel.

By now, you can see that there are advantages and disadvantages to every speedometer option there is. And thanks to me pointing these out, you probably don’t know which one to choose now. Sorry about that, but I’m not going to leave you hanging. Let’s work this out together.

In my humble opinion, I don’t think it’s wise to choose just one speedometer. Every good plan has a backup plan, after all.

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So let’s start with the first one we talked about above – the manual pitometer. The downsides are that it can potentially get clogged and that water conditions can impact your reading.

However, you don’t have to rely on satellites or data. If there are any issues with either of these things, your pitometer will still operate. So starting with this option is never a bad idea.

It’s still important that you have another option as a backup, though. Or better yet, let the pitometer be your back up. Investing in either a GPS speedometer or a boating app is a great way to go. Either is a good choice, but remember that you can use the apps when you’re traveling on land, too.

Also, they point out landmarks and other things. This means if you’re taking a long boating trip, you can find some things to check out. So consider using the app as your primary and the pitometer as your backup.

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If you don’t like the idea of the pitometer, the electromagnetic speed sensor can provide another great backup. You’ll only need to worry about interference during certain water conditions, but they are popular even on commercial and research vessels. Therefore, you can count on them to be pretty reliable.

In fact, you might choose to use it as your primary speedometer, but it’s still a good idea to have a backup. You don’t want to cause an accident, have to pay fines, or lose your boating license.

An Italian luxury motor boat sailing through the seas of Italy.

Remember, you’re probably not the only one on the water. One of the things I try to teach my kids is that they don’t just have to watch how they drive – they have to watch how other drivers do, as well.

Even if you’re operating your boat safely, that doesn’t mean that another boater will. You can’t control how fast they go, but if you’re both speeding, it can lead to a lot of trouble – potentially even death.

If you’re driving the speed limit, you have a better chance of getting out of the way if another boater is flying toward you.

Another thing to remember about speed limits on the water is, just like driving on the road, those limits are set according to ideal conditions. You should definitely drive slower if there is inclement weather or something else going on.

The same thing goes for boating. You should definitely go slower if it’s storming or if anything is impairing your ability to see. Just because you’re in the middle of the water doesn’t mean you are protected from accidents.

On the contrary, you never know what you might run into – sometimes literally – above or below the water. You have to drive responsibly, which starts by following the speed limit with an accurate speedometer – and a backup.