"Hydrodynamics are a powerful thing, relatively small foils create a lot of lift. the effect of a wing is more pronounced in water than air.
Check out the size of the foils on an AC45"
Without actual figures it is difficult, so I'm not going to stand on a soap box to get shot down but.......
I figure the "effect" is roughly equivalent to about 50Kg sitting in the bow and if we assume the effective centre of gravity to be about a third of the way down the length from the stern then, the "lift" force required to produce the same effect would be 1000N. As the engine probably already weighs around 100Kg, 1000N is simply supporting the engine's own weight leaving the imposition on the bracket at zero. Ask yourself...would I lift the engine by these plastic fins, how much more weight would they lift before they broke.
IMO they would 'fail' long before the engine mountings would.
As I say, all sort of guess work and I'm not advocating that they're the solution to his problem.............the 4 blade prop sounds interesting.