The patterns had to be there for the sailor to see them. It seems that "harmonics" was a poor choice of words. Wave interactions? Beats? http://www.redorbit.com/news/science/1112696643/wave-interactions-tsunami-091912/ 2 waves out of phase create beat waves which will have alternately higher and lower amplitude than either original. Please Register or Log in to view the hidden image! http://www.drhodum.com/html/waves/sound.html Please Register or Log in to view the hidden image! http://www.animations.physics.unsw.edu.au/jw/beats.htm see also: http://www.acs.psu.edu/drussell/Demos/superposition/superposition.html and: Please Register or Log in to view the hidden image! from: http://www.xyobalancer.com/xyo-balancer-blog/beat-frequency So wave interactions can create a new wave of greater amplitude. Does this hold true for gravity waves?