Calculate wavelength, period, length and boat size
In deep water the rule is: wavelength is 1,56 times the period squared, in metres.
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A wave with a period of five seconds is around thirty-nine metres long. For a ten-metre boat that is comfortable: she sits in the trough and is lifted by the wave. It becomes critical when wavelength and boat length are similar: the boat is left bridging two crests and pitches heavily.
Short, steep waves build up where the fetch is short and the water shallow, which is what the Baltic is known for. A long swell with a period of ten seconds is far more comfortable at the same height and puts less strain on gear and crew.
In insurance terms this is not a subject in its own right, but it does matter for avoiding damage: damage to the rig and the fittings mostly happens in a short, steep sea, not in a big swell. If you can judge the conditions, you plan differently.
Frequently asked questions
When does the sea become critical for my boat?
When the wavelength and the boat length are similar. The boat then pitches between the crests.
Why is the Baltic more uncomfortable than the Atlantic?
A short fetch and shallow water produce short, steep waves instead of a long swell.
Worked out for your type of boat
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