gofast.com, BAM prop slip calculator,
50 hp 3 cyl looper,
1.64 gear box
4800 rpm
15P prop
Screw in wood 42 mph
Recorded actual 29 mph
Slip 31%
If your prop was slipping, your rpms would be up and they aren't according to your input. If in doubt , take it to the limit of the function......take all the pitch out of the prop. What would the engine do???????? Rev toward infinity rpms, just like it does in N with no load on the engine! According to your tach data you are lugging at 700 rpms below the published recommended rpm range of 5000 to 5500, upper limit which is where 2 strokers like to run....usually 2 strokers have low cubes (1 per hp or less), giving you low torque so to get HP you need rpms.......HP is (torque x rpms)/5252.
5500 is the max rpm for that engine
Assuming you were running 5500:
1.64
5500 (if your 4800 is a bad tach and this were your real rpm)
15P
Screw in wood 48
Assume 15% slip (which is reasonable on that boat....my last boat was that boat, different brand, different bottom, 90 hp slip running 12-15 area).
41 actual (or if your speedometer were off with your reading 29)
Assuming the engine was loading up with that prop and you dropped to a 12P
1.64
5500
12P
Screw in wood 38
15%
32 actual
Slipping hub unloads the engine allowing it to spool up in rpms. Slipping hub increases the calculated slip as Jimmbo stated. Your 31% is indicative of that in the first calculation. Fixing the hub should have reduced your slip number, pulled your rpms down more, while possibly increasing your speed.....whether or not depends on the torque of the engine under higher load at lower rpms. You didn't publish your data after the "fix".