ERROR: there is an error at 54:04. The significant wave height Hs is the mean of the wave heights between the highest wave height and the H1/3. The H1/3 is the wave height value for which 33.3% of the waves have equal or higher heights.
In another words the Hs is s the average of the highest one-third (33%) of wave heights.
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Reading material:
-Chapter 6 (from section 6.1.4) from compendium [ Ссылка ]
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