BOAT-COMPARATOR Guide
Nautical miles and knots: working out the real travel time for your rental
A nautical mile isn't a kilometre, and a knot doesn't scale linearly with a RIB's speed. The simple method to estimate travel time before you set off.
"The cove is 8 miles away" tells you nothing about travel time until you know how to convert it. The good news: the method is a simple ratio, and it keeps you from leaving too late or heading back after dark without planning for it.
The nautical mile, a unit of its own
A nautical mile equals 1,852 metres, about 15% more than a statute mile and 1.85 times a kilometre — not an arbitrary choice: it historically corresponds to one minute of arc of latitude on the globe, which makes it the natural unit for navigation. The knot, the unit of speed at sea, equals one nautical mile per hour: at 10 knots, a boat covers 18.5 km in one hour, not 10.
The one ratio that covers all your planning
Travel time (in hours) = distance in nautical miles ÷ speed in knots. For 12 miles at an average of 6 knots: two hours. The tricky part is never the arithmetic, it's estimating the right average speed — almost always lower than the boat's stated top speed, once you account for slow-speed harbour exit, a close-hauled sailboat losing pace, or a RIB easing off in some chop.
Why a motorboat doesn't speed up in a straight line with power
On a planing hull — most rental RIBs and motorboats — speed doesn't rise smoothly with the throttle: below a certain threshold the boat pushes its own bow wave without planing, burning a lot of fuel for little speed gained; once it's up on plane, each extra knot costs relatively less fuel. Daniel Savitsky's foundational work on planing-hull hydrodynamics, published in 1964 in Marine Technology, laid out the equations that still describe this behaviour today (see the study on Google Scholar). In practice for a renter: aim for the throttle setting where the boat "hooks up" onto plane and hold it, rather than oscillating between under-powered and full throttle.
Practical benchmarks by boat type
- Licence-free and small RIB: a realistic average of 5 to 10 knots.
- 150-250 hp RIB, once on plane: 20 to 30 knots, but factor in the 5-knot harbour exit into your overall calculation.
- Sailboat under engine: 5 to 7 knots; under sail, highly variable depending on point of sail and wind strength.
Build it into your day
Always add a 20-30% margin to your estimate, and plan the return well before dark — our fuel budget guide and our Mediterranean winds guide help refine the realistic speed to expect depending on the destination and the day's weather.
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