[chucklesR]

If you have the $$ go bigger, as said before you'll be wanting a little extra for altenator/watermaker/yada yada's that will be taking HP directly off the top (before it gets to the prop).

I'm surprised the software indicated only a 38 to swing the 18x2 x 3, but you don't mention the transmission and the reduction ratio so that's hard to figure.

It's a tad to early in the morning for me to reverse engineer the math on the amount of HP required to swing that diameter at 12 pitch to your hull speed.
I'm no naval architect, didn't stay at a hotel of any type last night etc.. but:

My dinky little Irwin 38 at 20k runs with a 44hp

Putting a smaller motor on bigger and heavier seems like underpowered to me.