I have this tiny tach on my 20hp 2stroke Merc, works perfectly.
Readings that I got are:
2 = 2 persons on board, 4 = ..
Boat is a Mercury Ocean Runner 340 RIB, prop is BlackMax 10 3/8 x 13P.
This year I have raised the engine approx. 3cm and maxed out at 6200RPM and 28kts (GPS). With 44 liters I did exactly 100 NM, average speed of 17kts.