How it works
How your route becomes one number.
Bonasea does not tell you what the waves will be. It tells you how those waves will feel, on your boat, on every leg of your route. Here is exactly how it gets there.
The number
One score, four levels.
A number from 1 to 10 for a route on a given day. The same scale colours every day in the 10-day outlook and every moment on the navigation screen.
The scale reads left to right, cold water to warm sun.
| Score | Level | What it means |
|---|---|---|
| 9.0 — High comfort8 to 10 | High comfort | An easy day. Crew relaxed, coffee stays in the cup. |
| 6.5 — Medium6 to 7 | Medium | Noticeable motion on some legs. Fine for an experienced crew. |
| 4.5 — Low4 to 5 | Low | Uncomfortable. Doable, but nobody will enjoy it. |
| 2.0 — Discomfort1 to 3 | Discomfort | Pick another day or another route. |
Each badge shows a score from the middle of its level, in the colour band the app gives it. Below 5 the whole scale is the same pale water, which is the point: there is nothing to choose between a bad day and a worse one.
Step one
Your boat’s limits.
Every boat has a point where the ride stops being pleasant, and that point is different for a light planing hull, a heavy cruiser and a displacement boat.
This is where our own model comes in: from the boat’s length, hull type and displacement it works out how much sea your boat can take before comfort starts to go, and then shifts those limits according to how much your crew tolerates, from cautious family to sporty.
You never have to work any of this out yourself. Set the boat up once and Bonasea takes it into account every time it scores a trip against the forecast.
- From the model database. Pick your boat and the length, hull type, displacement and cruising speed fill themselves in, or set them yourself.
- Crew tolerance. Family, normal or sporty, and every limit shifts with it.
Step two
The effective wave.
Wave height alone is a poor predictor of comfort, so Bonasea converts it into an effective wave: where the sea meets your course, how closely the crests are spaced, and whether it is a short chop or a long swell you ride over.
A wave from the side is a rough, rolling ride, so beam seas count most and a following sea counts least. A short period raises the effective wave, because closely spaced crests hit harder. A long, slow swell lowers it, because you ride over it rather than into it.
The same wave, five directions. What changes is not its height but where it meets your course.
Step three
Your route, simulated in time.
Bonasea walks your boat along the route every 30 minutes at its cruising speed, and asks for the forecast at the position the boat will actually be in, at the time it will be there.
Every leg, its own sea
Waves, wind and weather are read leg by leg, not once for the whole trip. You are in an open channel at 10:00 and in the lee of an island at 12:00, and the score knows it.
Stops are part of it
Anchoring stops and overnights sit in the timeline, so a 90-minute lunch stop in a bay changes everything that comes after it.
Step four
The lee of the islands.
In the shelter of an island the sea is calmer than any open-water forecast suggests.
- A high-resolution sea mask. Bonasea measures how much open water lies in the direction the waves are coming from.
- The wave is damped by the shelter it has. This is why the channels between islands get realistic scores instead of pessimistic ones.
- Nothing is hidden. Switch the shading off and you see the raw model values.
Step five
The score for the day.
Our model then weighs the whole route together with its worst stretch, so one bad leg pulls the day down instead of being averaged away.
That is exactly how a crew experiences it: nobody remembers the four calm hours, they remember the rough one.
At anchor, less penalty
In a bay the wave and wind penalties are reduced, because the boat is sitting there rather than pushing through the sea.
Sources
The models underneath.
Where the data comes from, in full. Global models everywhere, and a higher-resolution one wherever it exists.
Waves
Open-Meteo Marine (MFWAM) worldwide, with Copernicus Marine (CMEMS) at higher resolution where available.
Wind and weather
ECMWF, overlaid with high-resolution regional models where they exist: ICON-2I in the Adriatic, AROME in France and the western Mediterranean, HRRR in the United States, ACCESS in Australia.
Tides
NOAA in the United States, LINZ in New Zealand, BOM in Australia, and a global model elsewhere, in the regions where tides matter.
- Forecast reach
- 10 days
- Refreshed
- Every 2 hours
Honestly
What the score is not.
It is an estimate from weather models, not a promise.
Model uncertainty grows with every day ahead, local effects such as gusts off a mountain or a squall line are not fully captured, and no score replaces looking at the sky and at the sea.
Bonasea is a planning and decision-support aid. It does not replace official nautical charts, regulations or the skipper’s judgement.
Bonasea does not just tell you what the waves will be. It tells you how your crew will feel them, on your boat, on every leg of the route.
Find your day on the water.
Set your boat up once, and every route you care about gets a ten-day answer.
Three days free when it lands. No card to start.