The message
from Roger contains these words:

That would leave each flow tide on its own if there were
just 2 systems 90 degrees out of phase.

Just to set the record straight by the time I posted that I had edited
out my original comment about a 50% hole and rewritten much of the rest
on the basis of 2 systems 180 degrees out of phase. Unfortunately
something very obvious that should have be changed didn't get changed.

Sods law doesn't really fit but surely there must be a Usenet law about
that millisecond between pressing send and the realisation of a stupid
mistake.

--
Roger Chapman