Quartering seas, and your boat

Why the Boat tab asks which wave angles and heights bother your boat, and how to choose the numbers. This is what makes the app's warnings fit your boat and your crew, rather than one rule for every boat.

What quartering seas are

Sailors describe waves by where they meet the boat. Waves from dead ahead are a head sea, from the side a beam sea, and from dead astern a following sea. Waves in between, at an angle on the bow or on the stern, are on the quarter. The classic quartering sea comes from about 45° off the stern, and the International Hydrographic Organization's dictionary defines it as waves "moving in a direction approximately 45° from a vessel's heading, striking the vessel on the quarter".

Waves at an angle are often harder going than waves of the same height from dead ahead or dead astern, because they lift one corner of the boat at a time. The boat pitches and rolls together, and the bow or stern is pushed sideways.

How different boats react

• Monohull. Rolls and yaws when the waves come from behind at an angle. The autopilot works hard, the boat can round up or broach, and running downwind there is a risk of an accidental gybe. Waves on the bow quarter make it pitch and pound, and slow it down. A heavier, long-keeled boat usually holds its course better than a light boat with a fin keel.
• Catamaran. Rolls very little, which is why many people choose one, but a wave on the quarter lifts one hull before the other, so the boat slews and corkscrews. The autopilot can struggle to hold the course. Short, steep waves on the bow quarter can slam under the bridgedeck.
• Motor boat. Waves on the bow quarter cause pounding and corkscrewing, especially at speed. Waves on the stern quarter push the stern round and make the course hard to hold, especially at low speed. A displacement motor boat can roll heavily in a beam sea.

The crew matters as much as the boat. A sea that an experienced crew shrugs off can make a charter crew or a family seasick and anxious, so it is worth setting the numbers for the people on board, not only for the boat.

Wave height and the Mediterranean

Mediterranean waves are usually wind waves with a short period, which makes them short and steep. One metre of short Med chop on the quarter can be more uncomfortable than a long two-metre ocean swell. The app shows the wave period next to the height so you can see how steep the waves are likely to be: the shorter the period, the steeper the waves.

The forecast height is the significant wave height, the average of the highest third of the waves. Single waves can be nearly twice that height.

Choosing your numbers

1. Think back to a passage that was uncomfortable on this boat. How far off the bow or the stern were the waves coming from? Put that range in Angle band min and max.
2. How high were they when it started to matter? Put that in Height threshold.
3. Add anything worth remembering in Notes, such as the autopilot model or how the boat behaved.

The band works from both ends and on both sides: a band of 30° to 60° means waves 30° to 60° off the bow or 30° to 60° off the stern, port or starboard. The app then marks a forecast CONCERN only when the waves meet your boat inside the band and at or above your height, and WATCH when one of the two is true. See OK, Watch and Concern.

Some examples of how skippers might set it, only as a starting point:

These are examples, not recommendations: only you know your boat and your crew. The app is for coastal sailing and is one input among several, so always check the official forecast as well.