Eek! Huge waves off Lewis

[Nick]

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Waves over 15m in height - said to be twice the normal height expected - were recorded off NW Lewis on Monday.

BBC article

[mm5aho]

...and I thought the 5m swell I came into west of Islay 3 weeks back was big enough.

I was reading in "Rock Lighthouses of Britain" (C Nicholson), about the building of Skerryvore...

"The dawn of April 17th 1842 saw Stevenson making his preliminary inspection of the season, and finding large boulders in the uncovered room at the top of the tower, 60 ft above the level of the highest tides."

They only worked the summers, and leaving the top of the then 60ft tower open at the top that winter, waves had deposited boulders at the 60ft level. So what height of wave leaves boulders 60ft above high tide mark?
Elsewhere in the book it records keepers in the finished tower reporting green water over the top of the light at the 141ft.

Easy to underestimate the power of the sea!