What it means
A knot is a unit of speed equal to one nautical mile per hour — and one nautical mile is approximately 1.15 statute (land) miles. So when the captain announces over the PA that the ship is cruising at 18 knots, that’s about 20.7 mph in land-driving terms.
The word “knot” comes from a literal piece of pre-GPS sailing equipment. Sailors used to measure ship speed by trailing a rope (with knots tied in it at regular intervals) behind a moving vessel and counting how many knots passed through their hands in a fixed time, measured by a sandglass. The unit stuck even after the rope didn’t.
Why this matters for new cruisers
Two reasons that make this more than trivia:
The math actually matters for itinerary planning. When a cruise line lists a port-to-port leg as “overnight sailing,” what they really mean is “we have to cover ~250 nautical miles overnight, and the ship can do 18 knots, so we’ll arrive in port the morning after we leave the previous one.” If you understand the conversion, you can look at a cruise itinerary and intuit which sea-day stretches will be calm overnight runs versus long ocean crossings, and which “port-to-port” hops will have the ship sitting outside the next port for hours waiting for daylight.
You’ll hear the term constantly onboard. Every navigation update, every PA announcement, the ship’s chart displays in public areas, the wake-watching info on cabin TVs — all use knots. Knowing the rough conversion (knots × 1.15 = mph) keeps you oriented.
Typical cruise ship speeds
| Speed (knots) | Speed (mph) | When you’ll see this |
|---|---|---|
| 0 | 0 | In port |
| 3-6 | 3.5-7 | Tender operations, leaving/entering port, narrow channels (Panama Canal, port approach) |
| 10-14 | 11.5-16 | Slow cruising — scenic glacier viewing, leisurely repositioning, fuel-efficient transits |
| 15-18 | 17-21 | Standard sea-day cruising speed for most mass-market ships |
| 19-22 | 22-25 | Faster sea-day cruising — used when the schedule is tight or there’s weather to outrun |
| 22-25 | 25-29 | Top speed for most cruise ships; rarely sustained |
| 26-30 | 30-35 | Fast ferries, expedition ships, and a handful of newer cruise ships in emergencies |
| 30+ | 35+ | Naval vessels and racing yachts, not cruise ships |
The fastest current cruise ship is the Queen Mary 2 (Cunard), purpose-built for transatlantic service with a top speed around 30 knots. Most modern megaships top out around 22-24 knots and cruise at 18-20.
Why ships still use knots instead of mph
Three practical reasons:
1. A nautical mile is historically based on one minute of latitude on the globe — meaning that knots work directly with maritime navigation charts in a way that mph doesn’t. If you’re going 60 knots, you cross roughly one degree of latitude per hour. This made pre-GPS navigation arithmetic vastly easier and the convention has stuck. 2. International standardization. All maritime traffic — cruise ships, cargo, fishing, naval — uses knots and nautical miles. Aviation, despite using mph in some places, uses knots for the same reason (and a nautical mile remains the standard aeronautical distance unit). 3. Maritime law and treaties specify distances and speeds in nautical miles and knots — territorial waters are defined in nautical miles, for example.
Quick mental conversion
You don’t need to do the exact 1.15× multiplication to keep up with cruise announcements. A close-enough rule of thumb:
- Knots × 1.15 ≈ mph (standard conversion)
- Knots + 15% ≈ mph (mental shortcut)
- 18 knots ≈ 20 mph (easy benchmark — most cruise sea-day speeds)
- Knots × 2 ≈ kilometers per hour (close enough for European cruise contexts — actual factor is 1.85)