Very common "feature" my old Quintrex with a 2 stroke Merc was exactly the same, mine did it at the speed limit in my crabbing lake, I either had to go slower, speed up and risk a fine, or just put up with it.
Hi, I've got a 12 foot tinnie with a 15hp 2cylinder 2 stroke, at just below half revs I get harmonic, high pitch vibrating noise through the hull, it goes away with less or more revs, any ideas how to get rid of this annoying trait?
Very common "feature" my old Quintrex with a 2 stroke Merc was exactly the same, mine did it at the speed limit in my crabbing lake, I either had to go slower, speed up and risk a fine, or just put up with it.
Yeah, mine does it just on the plane, maybe I should just go faster lol
Best bet is to try a new prop with a different number of blades.
Rubber sleeve over the transom to take out some of the motor vibration would be a good start. The frequency of the motor at those revs is obviously the harmonic frequency of the hull so if you can reduce the strength of the vibration it may fix it.