Matching Your Speed to a Nautical Chart
A boat bought in the United States often shows speed in miles per hour, because that is what the buyer understands. The chart under your elbow, the buoy list, the tide tables and the coastguard on channel 16 all work in nautical miles and knots. Navigation gets easier the moment those two agree.
The reason is arithmetic, not tradition. A nautical mile is one minute of latitude, so distance can be stepped straight off the side of the chart with dividers. Pair that distance with a speed in knots and a passage time falls out with no conversion factor anywhere in the sum.
Who Converts in This Direction
New Boat Owners
Licence Course Students
Passage Planners
Knock Off a Seventh
Knots are the smaller-looking number: subtract roughly a seventh from any mph figure and you are close. 35 mph becomes about 30 knots against a true 30.4, and 21 mph becomes about 18 against 18.2. Good enough for a mental cross-check of what the plotter is telling you.
Setting Speed Units Before a Passage
Most of this work happens the evening before, at the table rather than at the helm, and it is mainly a matter of writing down the three or four speeds your boat actually runs at.
Enter your usual cruising speed in mph
The knot equivalent appears as you type, so trying displacement, half-plane and wide-open speeds takes moments.
Write the knot figures onto the plan
Copy each result as a plain number and put it beside the leg distances in your passage notes, where the chart's own units live.
Reverse it for the tow home
Swap the fields when a marine figure has to become road-friendly again, such as explaining a day's run to someone who thinks in mph.
Pick metric units for a foreign chart
The dropdowns are searchable, so km/h or m/s is one keystroke away when the documentation you are reading came from Europe.
Passage Time for a Twelve-Nautical-Mile Run
Twelve nautical miles is a common coastal hop — a headland to a harbour entrance, or a run out to a fishing mark. The times below assume the speed is held over the ground, which is what a GPS plotter shows.
| Speed in mph | In knots | Time for 12 nm | Typical boat |
|---|---|---|---|
| 10 mph | 8.69 kn | 1 h 23 min | Trawler or sailing cruiser |
| 15 mph | 13.03 kn | 55 min | Small displacement launch |
| 20 mph | 17.38 kn | 41 min | Pontoon at cruise |
| 25 mph | 21.72 kn | 33 min | Bowrider on plane |
| 30 mph | 26.07 kn | 28 min | Centre console cruising |
| 40 mph | 34.76 kn | 21 min | Offshore boat running hard |
Currents rewrite the whole table. Two knots of foul tide turn the 21.72-knot row into 19.72 knots over the ground and add roughly three minutes to the leg, while two knots under you take about the same off. Planning in knots keeps that adjustment a simple subtraction rather than another conversion.
Speed Sweeps in Seconds
Because the result follows every keystroke, running through five candidate cruising speeds is faster than writing them down.
Log-Book Ready Numbers
Each field copies without its unit, so a figure drops into a log entry or a text to the marina exactly as you want it written.
Two-Way Between Dock and Road
The swap button covers the return trip, turning a knot figure back into the mph most passengers still picture.
Metric Units for Foreign Charts
Searchable dropdowns reach km/h and m/s for European pilot books, engine documentation and weather bulletins.
Navigation Questions From New Skippers
Why are charts ruled in nautical miles instead of statute miles?
Because one nautical mile is one minute of latitude, so the scale you need is already printed down the side of every chart. Dividers stepped along a course can be read against it directly, with no ruler conversion and no scale bar to trust.
Should I switch the boat's display from mph to knots?
If you navigate from charts, yes — matching the units removes a conversion from every estimate you make under way. Many plotters can show a second data field, which lets the crew keep mph while the navigator works in knots.
What is a 5 mph no-wake speed in knots?
4.34 knots. Bear in mind that most no-wake rules are written around the wake itself rather than a number, so a heavy boat may need to go slower than the posted figure to comply in practice.
Why does a trip take longer than the mph figure suggests?
Usually because the distance was in nautical miles while the speed was in mph. A nautical mile is about 15 percent longer than a statute mile, so mixing the two quietly understates every leg on the plan.
Why do pilots use the same unit as sailors?
Aviation inherited marine navigation wholesale: latitude and longitude, the nautical mile, the knot and the compass rose. Keeping one unit across both domains means an air-sea rescue coordinator never has to translate between the vessel and the aircraft.
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