I'd be interested in more info on this if you find anything out, as I have the same engine (Johnson 6hp which is a Suz painted white!) and the same problem.
One thing I can't completely get my head around is the "hull speed" which is the theoretical maximum displacement speed of the hull or something, I'm not sure if I could get more than the current 4.5kt (approx) by finding the extra couple of thousand revs, or if it would just use a lot more fuel! At the moment it makes very little difference to revs (or speed) on my aux if you have it at half throttle or full throttle, it just sounds a lot more strained at full throttle and presumably uses more fuel too, though I haven't measured it. You only get a fraction of a knot extra by using full throttle.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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