Sorry sundogz, I guess I didn't explain it very clearly in my last post, the problem is that if you dig too deeply into the hydrodynamics of water jet pumps it can become really dry reading.The essence of what I'm saying is that if you increase the actual RPM you'll increase thrust, the relationship between actual RPM and thrust is fairly linear. Vibration and friction absorb energy so you achieve less RPM and thrust. To increase RPM you can apply more power or reduce friction and vibration. The problem with going for more power is that to achieve it you have to use a bigger motor and or ESC and or battery, any and all of which will increase weight and excess weight is the mortal enemy of jet boats. Because the power to thrust relationship is very non-linear and the fact that jet boats are so weight sensitive going for more power can be counter productive, you're much better off making the system as efficient as possible.You're right in that vibration in and of itself doesn't cause cavitation, but in the stock NQD jet unit there's so much play between the shaft and it's bore that any misalignment in the motor/coupler/shaft causes the impeller to spin in a non-axial fashion (it wobbles). This causes the tolerance between the impeller blade tips and the tunnel wall to vary during rotation. When the tip moves away from the tunnel wall it creates relative low pressure cells which cause the cavitation. The tolerance between the trailing edges of the impeller blades and the leading edges of the stator blades is another area to look at, if the gap is too large you lose pressure and thrust but if it's too small it'll cause cavitation in the same way as a wobbling impeller. A worn stator bush is another major cause of cavitation because it too allows the impeller to spin out of true.Air ingestion is self explanatory and obviously unrelated to cavitation... BUT it has the same net result as cavitation... a loss of thrust, so it's just as undesirable. There's nothing you can do to stop air ingestion when your boat parts company with the water or when you're running in highly aerated water in rapids or the surf but if you experience sudden power loss on flat water then you're probably getting air ingestion caused by the poor tolerance between the stock shaft and bore. If you get any water seeping in through the impeller shaft bore then you can be sure that air is getting sucked into the jet unit.Running a flex shaft eliminates vibration but that's only one benefit it provides. The tolerance between it and the inside of the brass tube it runs in and the stator bush is negligible so the impeller runs extremely smoothly allowing it to operate very efficiently.It sounds like you've done a good job on your NQD and getting the drive train running smoothly is the single most important thing you can do but there are many other factors that need to be addressed to achieve the maximum performance from these little pumps. Building and improving these pumps and developing my jet sprint hull kit has been my full time job for over 2 years and as I've said, I've built hundreds of them, believe me I've tried it all and tweaked them in every way imaginable.Maybe the unnamed 'customer' you mentioned went real quiet because after installing my system he achieved the results he was after, one thing's for sure though, he's actually tried one
