Test 1 is with small upper def plate as to water flow passes slight under which is the sweet height spot for Tohatsu, Merc, Mariner engines as many boaters having splash over transom issues have solved forever.
Test 2 is with raised engine as to water flow passes slightly over anticav plate.
Both water flows are being cut nicely through the edge angle. The speed difference is about 1Knot in straight flat water courses for both tests. Trin angle is same, being a mechanical trim engine.
The rib used for test is a shallow deep V light hull if would have used a 30 HP for same test would achieved more top knots, but with slight speed difference between both settings as both 18 & 30 have same lower leg shape.
On test 2 find that water flow doesn't pass even through plate as on test 1, on left side side has slight more water and bit of air than on right side, that's the spot where it's probably loosing grip and achieving less top speeds.
This test probably doesn't work on other brand engines, a matter to check under trial and error. Usually stay with factory delivered prop as find them delivering near it's max rpm at wot while solo speeding and rpm drop decrease accordingly while adding more mates or weight on rib.