Many people, me included, say 8 knots because it is easier than saying 7.2 or 7.6 or 8.3.
In most cases 8 knot is good enough unless there is a specific reason for being picky such as fuel use comparsions which I have also done and apparently so has O.C. Diver.
The rule about w/l length does stand although modified somewhat sometimes for power boats depending upon planing, semi planing, full displacement..
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