the 4th blade, apparently, is more costly than the first three. In a three blade I have to go with 20", 19 is already pushing the limits with 4 blades. Volvo don't make a 20" three blade. I would have liked to try the Volvo, its high skew design is supposed to be quieter running. But no one has a 20" saildrive version in stock, and are not willing to take back a special order if I don't like. In published tests, the Volvo comes in middle of the packish, undistinguished in any category (except, in one test, noise).

Astern thrust tests pretty good on many of the 3 blade folders, but the tests are always done at max rpm. I'm not a big fan of max rpm reverse in docking maneuvers - it seems a little frantic. The thrust of them seems modest at say 1000 rpm compared to a feathering. Your tri is a lot lighter than my monohull, maybe that makes all the difference?