Start with knots, not miles per hour
Marine forecasts give wind in knots. One knot is about 1.15 mph, so 10 mph is about 9 knots, 15 mph is about 13 knots, and 20 mph is about 17 knots. The NOAA knot explainer is the conversion I keep handy when a friend sends a phone screenshot in miles per hour.
The marine wind in an NWS forecast is the expected wind 10 meters above the water, averaged over two minutes. Your phone's weather app may be reporting something else, and the dock behind a tree line will feel calmer than open water. If the forecast says 15 knots and the marina feels like 8, believe the forecast for the water you are about to enter.
What the wind looks like on the water
The Beaufort scale is still a useful sanity check. These rows from the NOAA Storm Prediction Center Beaufort scale cover the range most recreational boaters actually deal with. Treat the wave notes as open-water descriptions. In a bay, fetch and depth change what you get.
| Beaufort force | Wind (knots) | Name | On the water |
|---|---|---|---|
| 3 | 7-10 | Gentle breeze | Large wavelets, crests begin to break, scattered whitecaps |
| 4 | 11-16 | Moderate breeze | Small waves becoming longer, numerous whitecaps |
| 5 | 17-21 | Fresh breeze | Moderate waves, many whitecaps, some spray |
| 6 | 22-27 | Strong breeze | Larger waves, whitecaps common, more spray |
| 7 | 28-33 | Near gale | Sea heaps up, white foam streaks off breakers |
Our guide to wind direction, fetch, and protected harbors is the next read when the same Beaufort force feels completely different at two harbors on the same bay.
Set three limits: wind, gusts, and seas
A single "too windy" number does not capture how a day feels. I think in three limits.
- Sustained wind limit: the steady wind you are comfortable running in with your usual crew.
- Gust limit: the peak you will accept. The NWS glossary defines a gust as a rapid change of 10 knots or more between peaks and lulls. A 12-knot day gusting 25 is a different day from a steady 12.
- Sea limit: the forecast wave height you will accept. Wave height vs. wave period is why the period belongs in the same sentence as the height.
Boat or Nope ships with a few starting profiles. They are defaults to adjust, not safety standards.
| Boat or Nope profile | Wind limit | Gust limit | Seas limit |
|---|---|---|---|
| 17-ft skiff | 12 kt | 17 kt | 1.5 ft |
| Pursuit 235 (23-ft center console) | 15 kt | 20 kt | 2.5 ft |
| 40-ft trawler | 22 kt | 27 kt | 4 ft |
If you only set a wind limit, the app uses your wind limit plus 5 knots as the gust limit. Lower these numbers if you are new, have kids aboard, or are running somewhere unfamiliar.
When the official forecast says small craft
In the NWS Eastern Region, from Maine to South Carolina, a Small Craft Advisory is issued for sustained winds or frequent gusts between 25 and 33 knots. The lower bound drops to 20 to 25 knots in some harbors and bays, and seas of 4 to 7 feet or more can also trigger one, depending on the area. Those thresholds are in NWS Instruction 10-303. Above that come Gale Warnings (34 to 47 knots) and Storm Warnings (48 to 63 knots).
NOAA also says there is no precise definition of a small craft. Any vessel that could be adversely affected by advisory conditions counts. In practice, if an advisory is up for your zone, assume it is about you. The marine warnings cheat sheet covers the whole ladder, including Special Marine Warnings.
Direction and current change the answer
The same wind speed can be fine on one heading and miserable on another.
- Fetch: wind blowing across a long stretch of open water builds bigger waves than wind blowing off the nearby shore.
- Funneling: the Coast Pilot notes that Buzzards Bay's northeast-southwest shape makes southwesterlies strengthen as they funnel up the bay. Speeds there are often double those at nearby land stations.
- Wind against current: the Coast Pilot calls the west entrance of the Cape Cod Canal one of the worst spots locally, because rough seas build when the ebb out of the canal meets a brisk southwest wind.
That is why a "15 knot" day can feel like two different days depending on where you are and when. Woods Hole is the other local passage where current timing belongs in the plan, not as an afterthought.
How Boat or Nope handles wind
For each hour of the trip the app checks the forecast against your boat's limits and gives a Go, Caution, or No-Go, with a one-line reason.
- Sustained wind over your limit is a No-Go.
- Gusts over your gust limit are a No-Go, even when the sustained wind looks fine. The default gust limit is your wind limit plus 5 knots.
- Forecast seas over your limit are a No-Go.
- A Gale Warning, Storm Warning, Special Marine Warning, or Hazardous Seas Warning means No-Go. A Small Craft Advisory, Dense Fog Advisory, or a gale or storm watch means Caution at best.
- Close to your limits means Caution. That is wind or seas above 75 percent of your limit, thunder in the forecast, or wind opposing a strong tidal current.
- Estimated seas never produce a Go. If the official marine forecast has no wave height and the app estimates seas from a nearby buoy, the best you will get is Caution. Stale or distant buoy readings are thrown out.
The reason line spells it out, for example: "Caution: Small Craft Advisory NW 18 kt gusting 27 at 2 PM." The call is driven by the advisory and the gusts, not by a black box.
See the same idea on a real bay at the Buzzards Bay marine forecast, or read how the call is built.
A simple pre-departure wind check
- Read the marine zone forecast and any headlines first. Here is the order I use.
- Compare sustained wind and gusts to your limits for the whole trip, including the ride home.
- Look at wind direction against your route and any tidal current at inlets, canals, and passages.
- Check a nearby buoy to see whether the forecast matches what is actually happening.
- If the afternoon is forecast to build, go early or go another day.