Every 7th ocean wave is bigger
Some years ago I visited my friend Mountain Man in Oz. You might not guess from his name that he is a surfie. Watching the waves while he surfed, I checked out that the old story that every 7th wave is bigger is true. Also, as he told me, super big waves come at intervals of about 20 minutes.
The above graph shows data that Pete and I got from the Manly Hydraulics Laboratory. It shows clearly that waves have a main period of about 12 to 13 seconds and that there is a cycle of 7 or 8 waves.