Also bear in mind the diameter of the prop also affects the RPM......
FWIW, I'm 60Hp, 5m and the difference in literally a tap or two of the trim switch can make or loose 4 knots.
E.G on Monday I had it running at something like 4800 rpm, getting about 20 knts. I trimmed up a tiny amount, didn't move the throttle, and next thing I'm doing 5300rpm, 25 knots and throttled back a bit for a quieter / more economic run!
If you were going with the tide on Etive shouldn't you have been doing about 108 knots?