I thought you need less RPM with a 4 bladed propeller (!??).

You are right about the Prop size, the P-47D Razorback had an Curtis Electric C542S with 13 foot diameter – this would be on a 80 inch (1:6 scale) a 24 inch airscrew.

By searching about the hubs and 4 blades I came across of a interesting website…

[link=http://www.warbirdpropdrives.com/18422.html]here is the link[/link]

Happy landings[sm=thumbup.gif]
G-Pete