Carl,
The Aquacraft 42 x 55 is a two bladed propeller made by ABC
Propeller in Columbus, Ohio. It is not offered in a three bladed
version that I know of. If it is for a 21-tunnel hull he should try
an ABC 1515/3-23*-47% or the 1615/3-23*-47%. These would
be a better choice. I finished a 1515/3-23*-47% today in my
propeller shop and it will have great acceleration for sure. Walt
tried the 1615/3-23*-47% on his 21-Speedmaster mono and he
picked up 3 to 5 mph I believe. You would need to raise the outboard motor up a little, but it may pull this one really well.
See if Walt has any of these still in stock?
Thanks,
Mark Sholund