One more thought on long, low swells:
In extreme cases, such swells can be almost imperceptible but nevertheless contain huge amounts of energy, giving rise to strange phenomena.
once sailing out the local harbour towards the open sea, alternately catching up with for what seemed like several minutes and then dropping behind a boat ahead, so that we went from what looked nearly to be half a mile behind to not much more than a quarter. After a few cycles of this I was beginning to suspect I was hallucinating, but as we got out into the open sea we started seeing huge rafts of kelp (including one with a boat stuck in the middle).
Then we noticed that the ocean was piling up, slowly, in turn, against all the coastal cliffs angled at a certain aspect to the sea.
This 'mounting' was extraordinarily slow, but on a couple of occasions the water
eventually overtopped a cliff face, and some of the cliffs were of the order of 30m high.
We inferred, and it was later confirmed, that an extraordinarily powerful tropical cyclone had become 'extra-tropical' and travelled down into the South Pacific
nearly as far as 40 South, some thousand miles or more to the east from us.
Despite the visual effects, with our eyes closed we could not feel the swell. However in certain bays within the main harbour, we later heard that the energy refracted and curled in, and the child of a neighbour was pitchpoled in his Opti. This probably doesn't happen that often away from a surf beach (which this was not: generally it's sheltered from any swell)
Another story from around the side of the same bay, before people realised what was happening: a guy with a fancy jetboat (matching mag wheels on the trailer with the towing vehicle, matching paint
jobs etc) was backing down the ramp
. He got to the water's edge, got out, went back to take the chain off, looked puzzled because he wasn't as close to the sea as he thought - in fact, nowhere near it. He repeated this entire performance a couple of times - and then .... the sea came back in, rather a lot faster than it had gone out, and dumped the whole shooting box off the side of the ramp