# Guide to the Nautical Speed Converter and Beaufort Scale
Speed is a fundamental piece of data in maritime navigation. Knowing how many knots your boat is making, how that translates into kilometres per hour for planning a passage, or what Beaufort force represents the wind you are experiencing, are calculations every sailor needs to master.
This nautical speed converter lets you transform between knots, km/h, m/s and mph instantly, while automatically identifying the corresponding Beaufort force with its sea state description and observable wind effects.
# Why is speed measured in Knots?
The knot (kt) is the official unit of speed in maritime and air navigation. Its definition is precise: one knot equals one nautical mile per hour, where the nautical mile (1852 m) is defined as one minute of arc of the terrestrial meridian. This direct relationship with Earth geometry is what makes the knot a particularly practical unit for navigation, allowing distances to be estimated directly on a nautical chart without any conversion.
Historically, sailors measured vessel speed using the chip log: a triangular piece of wood thrown into the sea attached to a rope with equally spaced knots every 14.4 metres (the proportional fraction of a nautical mile in 30 seconds). Counting the knots passing through their fingers during that time gave them the vessel speed, giving rise to the term we use today.
# The Beaufort Scale: The Sailor's Eye
The Beaufort Scale was developed in 1805 by British Admiral Sir Francis Beaufort as an empirical method to classify wind strength from observable effects, without instrumentation. Originally conceived to estimate how much sail a ship could carry, it is today a universal reference in maritime meteorology, adopted by the World Meteorological Organization (WMO).
The scale ranges from Force 0 (absolute calm, sea like a mirror) to Force 12 (hurricane conditions with waves over 14 metres and air full of foam and spray). For the recreational sailor, critical limits are usually Force 6 (strong breeze, 22-27 knots) for light craft and Force 7-8 for ocean-going yachts.
| Beaufort Force | Description | Sea Conditions |
|---|---|---|
| 0 | Calm | Sea like a mirror |
| 3 | Gentle breeze | Scattered whitecaps, crests beginning to break |
| 6 | Strong breeze | Large waves, white foam crests, possible spray |
| 9 | Strong gale | Very high waves, visibility reduced by foam and spray |
| 12 | Hurricane | Air completely filled with foam, sea totally white |
# Nautical Speed Glossary
- Knot (kt)
- Unit of speed equal to one nautical mile per hour (1852 m/h). International standard in maritime and air navigation.
- SOG (Speed Over Ground)
- Actual vessel speed over the seabed, measured by GPS. Reflects the effect of marine currents on boat speed.
- Log Speed (STW)
- Speed of the vessel relative to the surrounding water, measured by the log. Does not include current effects; key for calculating leeway.
- Apparent Wind
- Wind experienced by the crew on board, resulting from the vector sum of true wind and the wind created by the vessel advancing. Always differs from true wind in magnitude and direction.
# Quick Conversion Formulas
To convert speeds manually when you do not have access to a digital converter, use these quick approximations every sailor should know by heart:
- Knots to km/h: Multiply knots by 1.852. Quick approximation: knots x 2, minus 8%. For example, 10 kt ≈ 20 - 1.6 = 18.4 km/h.
- Knots to mph: Multiply knots by 1.15. Approximation: knots + 15%. For example, 20 kt ≈ 23 mph.
- Knots to m/s: Multiply knots by 0.514. Quick approximation: divide knots by 2. For example, 10 kt ≈ 5 m/s (exact: 5.14 m/s).
Safety and Climatology
Before setting out, always consult the maritime weather forecast. Beaufort 4-5 is manageable for most recreational vessels; from Force 6 upwards, seriously consider postponing departure if you lack sufficient experience or your boat is not prepared for those conditions.# Equipment Comparison
Digital Anemometer
Electronic instrument for precise measurement of wind speed.
- Accuracy of ±2% under normal conditions
- Requires electrical power or batteries
- Can fail in extreme conditions
- Instant reading and data logging
Beaufort Observation
Visual estimation of wind force from observable effects at sea and on land.
- Requires no instruments whatsoever
- Available in any condition
- Sufficient accuracy for safety decisions
- Fundamental skill of the expert navigator
# Navigation and Passage Planning
When planning a passage, your vessel speed determines estimated times of arrival (ETA) and fuel consumption for motor vessels. For a sailing yacht, speed also depends on wind strength and direction: knowing the forecast Beaufort force lets you anticipate whether you can sail or will need the engine.
In racing, the conversion between knots and metres per second is especially useful for calculating VMG (Velocity Made Good) and for analysing sail polars. Meteorologists and numerical forecast models typically give wind in m/s or km/h, so conversion to knots is a routine step in tactical planning.