Originally Posted by bosun
based on a quick google search 2.42-1 same as my evinrude
Ah ok, thanks - quite a low ratio.
A 15" prop at 5500rpm with zero slip should make around 32mph (28ish kts), so allowing for 12% slip, you'd be down to around 25 kts at 5500 RPM. As the boat is only making 19kts, then there's theoretical slip of 33%, which is a lot, but not as much as you would expect with a spinning bush.
Where's the cav plate relative to the underside of the keel and is it definitely a 15" pitch prop?
If the boat used is a 4m+ Zodiac Pro as in the OP's profile, then I'd expect the boat to make 33-35kts with 50hp, which suggests it needs a 19"-21" pitch prop (assuming 12% slip and a max 5500rpm)