Turning the Marine Forecast Into Speeds You Feel on a Boat
American weather reaches the water in two dialects. The land forecast on your phone is in miles per hour; the coastal waters forecast, the NOAA weather radio broadcast and the warning flags at the ramp are all in knots. Anyone launching a trailer boat ends up translating between them before every trip.
One knot is a little more than one mile per hour, which is exactly the trap: the numbers look close enough to ignore, and a 30-knot forecast quietly means 34.5 mph of wind rather than 30.
Who Converts Knots on the Way to the Ramp
Weekend Boaters
Inshore Anglers
Paddlers and Kayakers
The 15 Percent Rule
Knots run about 15 percent above miles per hour, so adding a seventh to any nautical figure gets you close: 20 knots becomes roughly 23 mph, 40 knots roughly 46 mph. The estimate is good to a fraction of a mile per hour up to gale strength, which is more than enough for a go or no-go decision at the dock.
Reading a Wind or Boat-Speed Figure Off the Bulletin
Marine bulletins give wind as a range with a separate gust value, and boat speed as a single number from the chartplotter. Both go through the same two fields.
Enter the knot figure from the forecast
Type the number on the left and read miles per hour on the right; there is no button to press, the result forms while you type.
Check the gust against your comfort limit
Convert the gust separately. Most small open boats become unpleasant well before the sustained wind reaches advisory strength.
Reverse it before a radio call
If you need to report a speed in knots but only know it in mph, one press of the swap button turns the pair around.
Marine Warning Levels in Knots and mph
The National Weather Service issues its marine products in knots, and the thresholds below are the ones behind the flags, the radio broadcast and the wording of the coastal waters forecast. The mph column is what the same wind would be called on the land forecast for the same town.
| Product | Sustained wind | In mph | What it usually means |
|---|---|---|---|
| No advisory | Under 20 kn | Under 23 mph | Workable for most trailer boats |
| Small Craft Advisory | 20–33 kn | 23–38 mph | Hazardous for smaller vessels |
| Gale Warning | 34–47 kn | 39–54 mph | Recreational craft stay in |
| Storm Warning | 48–63 kn | 55–72 mph | Dangerous even for large vessels |
| Hurricane Force Wind Warning | 64 kn and above | 74 mph and above | Ports close, vessels seek shelter |
Boat speeds convert the same way and are worth knowing in both units. A displacement sailboat at hull speed, 6 to 8 knots, is doing 6.9 to 9.2 mph. A pontoon cruising at 20 knots is at 23 mph. An offshore centre console running 45 knots is travelling 51.8 mph — highway pace over water that is never as flat as a road.
Answers Arrive as You Type
No submit step, so running through a forecast band, its gust value and your cruising speed takes seconds.
Float-Plan Friendly Copy
Each field copies as a plain number, ready to drop into the float plan or text message you leave with someone ashore.
One-Press Direction Change
Swap turns the card into an mph-to-knots tool, which is the direction you need when reporting conditions on the radio.
Every Marine Speed Unit Listed
Both dropdowns are searchable and cover the full unit set, including the m/s used by many anemometer and buoy specifications.
Questions From the Helm
What wind speed triggers a small craft advisory in mph?
Sustained wind of 20 to 33 knots, which is 23 to 38 mph. Some offices issue at a lower threshold in sheltered waters or on the Great Lakes, and hazardous seas alone can trigger an advisory in light wind.
Why does my chartplotter speed differ from the boat's speedometer?
They measure different things. A GPS plotter gives speed over ground, while a paddlewheel or pitot gauge gives speed through the water. In a two-knot current the two readings differ by two knots, or about 2.3 mph, in whichever direction you are heading.
Is trolling speed usually given in knots or mph?
Freshwater tackle advice tends to use mph, saltwater advice tends to use knots, and lure packaging can use either. A typical walleye troll of 1.5 to 3 knots is 1.7 to 3.5 mph, so the gap is real enough to matter at those speeds.
Why do marine broadcasts stay in knots at all?
Because charts are ruled in nautical miles. With speed in knots, a distance measured off the latitude scale converts to time with no arithmetic at all, and the convention is shared internationally so a bulletin reads the same to any vessel.
How rough is 25 knots for a 20-foot boat?
25 knots is 28.8 mph of wind, advisory conditions on most coasts. Open water at that strength commonly builds a short, steep chop that soaks the crew and slows the boat well below its comfortable cruising speed, long before anything becomes dangerous.
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