Bob - I'd like to think you could identify whether or not the tranny is slipping while under load with your hand-held tach by knowing the gear ratio. I haven't done this but, I can't imagine why this wouldn't be accurate.
On the Hurth / ZF transmissions the gear ratio is shown on a label attached to the top of the tranny near the shift lever. If its a hurth HBW50 or ZF 5 its probably labeled as 2.05. According to the manual I have, the "A Ratio" is either 2.045 or 2.722 on a ZF 5 M. If the label shows "A=2.05" on your tranny, multiply the shaft RPM by 2.045 while in forward under load. This should be the rpm of the engine assuming the connection between the tranny and shaft is good. If so, the tranny isn't slipping and your problem is not the engine or tranny.
After that, the following guesses come to mind. Is anything causing the prop to be less efficient than it was when you first re-mounted it after the prop pitch change? Did it become bent, is there slop in the cutlass bearing or strut, is the key in the prop doing its job or does the prop slip while under load?
On the Hurth / ZF transmissions the gear ratio is shown on a label attached to the top of the tranny near the shift lever. If its a hurth HBW50 or ZF 5 its probably labeled as 2.05. According to the manual I have, the "A Ratio" is either 2.045 or 2.722 on a ZF 5 M. If the label shows "A=2.05" on your tranny, multiply the shaft RPM by 2.045 while in forward under load. This should be the rpm of the engine assuming the connection between the tranny and shaft is good. If so, the tranny isn't slipping and your problem is not the engine or tranny.
After that, the following guesses come to mind. Is anything causing the prop to be less efficient than it was when you first re-mounted it after the prop pitch change? Did it become bent, is there slop in the cutlass bearing or strut, is the key in the prop doing its job or does the prop slip while under load?