I'm working on an outrigger set up for a .40. So I'm debating between O.S. .21 VZ-M and the O.S. .46 VX-M. They are both claiming 2.5 horse. the .46 will make that at 23K RPM and the .21 will make it @ 33K RPM. The .21 has a mass of 12g and the .46 is 15.7g. So I'm thinking that the .46 will have more torque inturn, I can mount a larger prop. Though, the .21 will turn a max of 40,000 RPM so less torque but more horse and a smaller prop? I don't know some one friggin' help me here!

I don't think the .21 will have enough torque to swing a big enough prop to get a .40 size rigger running right.

There is not replacement for displacement

The boat is classed as a 40 for a reason. The small .21 may have 2.5 horses and 40K RPM, but horses and RPM don't spin props, torque does. Stick with the .46, it can do the job, the .21 can't