i've run a roundnose hydroplane without the fin...and it's quite entertaining, boat kinda drifts - you've got to leave quite a bit of room for understeer. i'll try leaving the mount in and unscrewing the fin itself, it's one of the graupner flyback ones, so that's easy - worth a try! on closer inspection it turns out the prop's missing quite a big chunk....oops! hope the motor bearings are ok....

now as regards props, i tried a couple out, and here are the results:

30mm low pitch - cavitation, boat slow
34mm high pitch - not enough revs, boat slow
35mm high pitch - turns out is a left-rotating prop (the idiot! - ever seen a rigger going backwards?)
32mm medium pitch - fast acceleration, low top speed
34mm medium pitch - slower acceleration but decent top speed, as seen in the video
38mm, low pitch (eco type prop) - faster acceleration but slower top speed.

the results from the first four props make sense i think, but the 38mm has me stumped. It looks fairly different to the others but seemed to work quite well. Can't understand why boat went slower though....will try more rigorous testing to find out.