Ship Speed Test: Your Real Speed in Knots
Measure how fast your ship is actually moving. Your phone's GPS reads your speed over ground in knots, the same measure the bridge uses, with no app to install.
- Peak
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- Average
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- Distance
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- Time
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Tap Start GPS and allow location access
How the ship speed test works
Press Start speed test and allow location access. Your phone compares its GPS position from one moment to the next and works out how far it travelled in that time. That gives your speed over ground: your true speed across the earth’s surface, including any push from current or tide.
Speed at sea is measured in knots. One knot is one nautical mile per hour, which is 1.852 km/h or about 1.151 mph. The test shows knots by default because that is what ships, ferries and charts use, but you can switch to km/h, mph or m/s with the buttons under the readout.
Speed over ground versus speed through water
A ship’s own instruments often show speed through water, measured by a log impeller or pressure sensor sensing the water flowing past the hull. Your phone shows speed over ground. On a still sea the two match. In a current they do not: a ship making 15 knots through water with a 2-knot following current is making 17 knots over ground, and your phone will show 17.
That difference is the useful part. Speed over ground is what determines your arrival time, so the figure on this page is often closer to what matters to a passenger than the figure on the bridge display.
Getting a good reading at sea
- Go on deck or sit by a window. GPS needs a view of the sky, and a steel hull blocks the signal in interior cabins.
- Give it twenty or thirty seconds. The first fix is rough; accuracy improves as more satellites lock on.
- Keep the page open and the screen awake. Browsers stop location updates when a page is in the background.
- Ignore the first few readings as you leave a dock. Speeds under about 2 knots are noisy on any GPS.
How fast do ships actually go?
A large container ship cruises at 16 to 24 knots, though many now run slower to save fuel. Cruise ships sit around 20 to 24 knots. A passenger ferry on a short crossing typically runs 15 to 25 knots, and fast catamaran ferries reach 35 to 40. Bulk carriers and tankers are slower, often 12 to 15 knots. If you are on a smaller boat rather than a ship, the boat speed checker covers the same job in the units small craft use.
Ship Speed Test: frequently asked questions
How accurate is a phone GPS at sea?
Very accurate for speed. With a clear view of the sky a phone is usually within a few tenths of a knot, because speed is calculated from satellite Doppler shift rather than from position alone.
Why does it not work inside the ship?
Steel decks and hulls block satellite signals. Move to a window, a balcony or an open deck and the fix should appear within a minute.
Does the ship speed test need internet?
It needs a connection to load the page once. After that the speed reading comes from GPS satellites, so it keeps working at sea with no mobile signal.
Why is my reading different from the ship's display?
The bridge usually shows speed through water. Your phone shows speed over ground, which includes current and tide. In a strong current the two can differ by several knots.
What is a knot?
One nautical mile per hour. A nautical mile is one minute of latitude, which is why it has stayed the standard unit for sea and air navigation.
Does it drain the battery?
GPS does use power. On a long crossing keep the phone on a charger or a power bank if you want to leave the test running.