We are assuming your tach is not accurate.We now know your speed is accurate.(gps)
Your actual rpm is about 4200.
It is highly unlikely your rpm is at 5000 and only 40 mph
At 5000 your slip (prop slip) would be about 26%.Like pushing a really big pontoon boat.
Slip is the actual distance the prop moves forward in one revolution.
Slip varies based on speed, hull design/weight,prop design, and condition,rpm,
Typical slip on your cuddy is probably in the low double digits.
The calculator uses your gear ratio 1.66,prop pitch 19,speed(40) and rpm. to figure slip.
You can also use any 4 of these 5 numbers to figure the 5th.
We know the ratio 1.66 is accurate from a chart, we know the prop is accurate you see the number,, we know the speed (gps)
We don't know the "true" rpm except by the calculations. We start with your 5000 and we calculate for slip.
When slip is wacky, 26%, we go to the only remaining number that is unknown" rpm".
Do a search for" prop calculator". Put in your numbers and calculate for slip.
I feel your motor may be a little tired because typical speeds with a 5.0 are usually close to 50 mph.
Your cuddy may be a little less. If your 5000 was accurate speed would calculate at about 47 mph.
A typical healthy 3.0 I/O in a 19 ft cuddy would run close to 40.
Lost in the jumble I created is the need to varify your tach. Perhaps borrow a shop tach or use an induction tach like a tiny tach.
just follow the directions carefully.