i learnt it from other boaters too, this is the method of setting up tuned pipe, start by measuring from glow plug to the widest part of the pipe maybe 13-14 inches to be safe then run the boat if the engine keep boggin' down or won't rev high cut the coupling by 1/2 until engine giving good rpm,,mostly i know 3.5 engine needs around 10-11 inches, i do no with 7.5 but mine i found it somewhere 13-14 inches.

thanks zapster...