The cargo ship Swanland sank off the Llyn peninsula on Saturday night. Today's [Monday 28th November] Guardian newspaper reports on the tragedy where 5 sailors are feared lost and quotes a survivor describing " a huge wave rolling the ship, and she broke her back....and sank very quickly after that". There were gale force 8 and higher winds blowing and I think 6 metre waves but this wave seems to have been exceptional. I recall some years ago that something similar happened to a Stena ship [was it the Hibernia?]. What size waves are these, how do they come about and why off the North Wales coast?