Today I did some investigating and thought you would be interested in the results. A friend had the brilliant idea of using an optical tach on both the engine and the gearbox output, so I borrowed one from work. The Hurth transmission manual on this website indicated that there is a label on the trannie to show what the ratios are forward and backward. Great stuff. I should have mentioned before, that I have a 25XP engine.
After a number of interesting contortions with mirror in one hand and flashlight in the other, I concluded there was no part number and no ratio label on my trannie.
I went ahead anyway with runs in neutral, and in gear, loading the engine by trying to pull my helical mooring out of the ground, first in reverse, and then forwards. I didn't have the courage to pull against the mooring at max rpm. I concluded that the boat tach was reading a bit low, and assuming no slip, calculated the gear ratios. It just so happens that the calculations closely match the example shown in the manual (1.86 reverse, 2.05 forward), so I'm concluding no slip. Max true engine rpm in neutral was 3217.
Then I went out and did some motoring. There was just me, and I wasn't prepared to go below for long enough to make measurements, so I had to content myself with using the measurements on the mooring to estimate true rpm from the ship's tacho. Two main conclusions were my knotmeter is under-reading by about a knot, and the max rpm under load is 3125 indicated, 2952 estimated actual (i.e. a loss of 265 rpm due to load). Running at 80% of this value gives a true speed of about 5.1 kt. I'll try to attach an excel spread sheet with all the details - I may be successful or not, but thank you for your interest and help.
Moored neutral Tach Eng rpm Tran rpm GPS knotmeter tach o'read
1130 1076 idle 5%
1500 1445 4%
2000 1881 6% Tach over-reading about 5%
2500 2380 5%
3350 3217 max rpm 4%
Moored reverse 1100 1016 546 idle 8% Gear ratio 1.86
2000 1896 1013 5% Gear ratio 1.87
Moored forward 1100 1034 508 6% Gear ratio 2.04
2000 1900 932 5% Gear ratio 2.04
Motoring forward estimated % max rpm
2400 2286 5.0 4.0 77%
2700 2571 5.5 4.5 87% Ktmeter under-reading by ~1 kt
3100 2952 6.3 5.0 max rpm 100%