Boat Speed: Knots vs mph vs km/h — And the App That Shows All Three

Understanding Boat Speed Units: Knots, mph, and km/h

Speed on water is measured differently depending on where you are in the world and what type of boating you do. The three most common units are knots (nautical miles per hour), miles per hour (mph), and kilometres per hour (km/h). Understanding how they relate to each other is essential for safe and efficient seamanship.

Here are the exact conversion factors:
1 knot = 1.852 km/h
1 knot = 1.151 mph
1 mph = 0.869 knots
1 km/h = 0.540 knots

Rather than memorising these conversions or carrying a calculator, Boat Knot Meter: Anchor Watch displays all three units simultaneously in real time on your iPhone. There is nothing to convert — just look at the screen.

Download Boat Knot Meter free on the App Store and never worry about speed unit conversions again.

Why Knots Are the Standard for Marine Speed

The knot has been the standard unit of speed at sea since the 17th century. A nautical mile — the distance covered in one hour at one knot — equals exactly one minute of latitude on Earth's surface (1,852 metres). This makes nautical charts, position fixes, and voyage calculations all interoperate naturally. When you say you are doing 6 knots, you know you are covering exactly 6 nautical miles every hour, which translates directly to position planning on a chart.

Most professional marine instruments — chartplotters, VHF radios, AIS transponders — report speed in knots. Marine weather forecasts report wind speeds in knots. Harbour regulations and speed limits are posted in knots. Knowing your speed in knots is not just tradition; it is practical necessity for communicating with other mariners and authorities.

When mph and km/h Matter on the Water

Not everyone on your boat thinks in knots. Crew members from countries that use the metric system may find km/h more intuitive. Passengers from the United States may prefer mph. When explaining your boat's speed to someone unfamiliar with maritime units, being able to say 10 knots is about 11.5 mph or 18.5 km/h makes the information immediately meaningful.

There are also specific applications where non-nautical units are used. Some inland waterways in certain countries post speed limits in km/h. Jet ski and powerboat rental operators often use mph for marketing purposes. Boat Knot Meter shows all three simultaneously so you are never without the right number for the right audience.

Boat Knot Meter: The GPS Speed App That Does It All

Real-Time Display of All Three Units

The main screen of Boat Knot Meter shows your current GPS speed in knots, km/h, and mph simultaneously. The display is large and designed to be readable in bright sunlight — the conditions you most often need it in. There is no need to switch between modes or perform manual conversions.

GPS-Based Speed: Speed Over Ground Explained

Boat Knot Meter measures speed over ground (SOG) — your actual movement across the Earth's surface. This is different from speed through water, which is what a paddlewheel or pitot tube measures. SOG accounts for current and leeway, giving you your true progress. For passage planning and ETA calculations, SOG is what matters.

High Update Rate

The app leverages the iPhone's GPS chipset for frequent position updates. Speed calculations respond quickly to changes in velocity — when you accelerate through a puff of wind or ease the throttle, the displayed speed adjusts within seconds.

Trip Log With Speed Data

Beyond the live display, Boat Knot Meter records your trip statistics: maximum speed reached (in all three units), average speed, and total distance covered. This gives you a performance record for each outing.

Anchor Watch Built In

When you stop for the night, Boat Knot Meter transitions from speed meter to anchor watch. Set your GPS anchor point, configure the alarm radius, and the app monitors your position while you sleep. One app for the whole voyage — free and offline.

Practical Speed Reference for Boaters

Here are some common reference speeds in all three units to give you a feel for the conversions:

• Hull speed of a 30-foot sailboat: approx 6 knots = 11.1 km/h = 6.9 mph
• Typical cruising speed of a small motorboat: 15 knots = 27.8 km/h = 17.3 mph
• A brisk walk: 3 knots = 5.6 km/h = 3.5 mph
• Gale force wind: 34+ knots = 63+ km/h = 39+ mph
• Calm paddling in a kayak: 2-3 knots = 3.7-5.6 km/h = 2.3-3.5 mph

Why Offline GPS Speed Measurement Matters

At sea, internet access is unreliable or absent. Any speed app that requires a data connection — for map loading, weather overlays, or basic functionality — is a liability offshore. Boat Knot Meter uses only the iPhone's GPS receiver. GPS satellites broadcast signals that work anywhere on Earth, independent of cell towers or Wi-Fi. Your speed data is always available, regardless of where you are.

Download Boat Knot Meter and Know Your Speed in Any Unit

Whether you speak in knots, km/h, or mph, Boat Knot Meter has your reading on screen at all times. It is the simplest and most reliable way to monitor boat speed on an iPhone — and it costs nothing.

Download Boat Knot Meter free on the App Store and see your speed in knots, mph, and km/h simultaneously the moment you get on the water.

No setup, no account, no internet required. Just download it and go. Get it free now.