Andy
The 50mm plastic prop on the model I made has a very fine pitch - possibly too fine, because I had to use 12 NiMH cells to get anything like a reasonable performance from the boat. Most of the bronze/brass props I've had are of a coarser pitch, so maybe you should start with a 45mm cast one. I can recommend Protean Design's range, specifically the General Scale pattern. PM me if you would like Simon's contact details.
Hope to see you on Saturday at StickyWeed when we can have a good old natter.
DaveM