First, the pump inlet is to small..goes back to cant scale down volume..a bigger pump, such as the KMB 28mm, will of course put out more water, so, you CAN run lower rpm for = boat speed..
Real problem is exactly what Rim was talkin about, its too short, and not a good hull design, but, its a TOY..wasnt ment to run fast, or, have big $$ spend on design..
Nozzle outlet size is a tuning tool, on the real boats, we use .030 at a time down, on a lot bigger exit hole..wouldnt even guess on how you could tune this small outlet.
Also, real jets have wear rings inside, to hold the tolerance around the impeller..we have a gap thats huge, even for a real one..playing with a clearance ring between the impeller and the drum would work here, well..
I will be doing that on my KMB.
A jet boat, has to have a semi v setup, with a flat pad at the stern to work correctly..trying to mount em in a deep v without a pad will always give some cavitation on turns, and some as the boat rises up on the chines..
Steve at sic rc is gonna be doing a 22 inch boat after the holidays, he tells me, I think he does nice work, hes into it, AND at very fair prices..
The bigger hull will be on the size of the KMB jet sprint, and also use the KMB 28mm, with the option of the 40mm KMB..the maniac version, which I gotta do..