The amount of thrust(water coming out of the
back of the) is based on three things; them being the propeller pitch, the RPM,
and the combination of gear ratios and horsepower since they directly effect
each other. The RPM is the rate at which the engine is cycling. The gear reduction
ratio is based on the amount of horsepower the engine has. The propeller pitch
is the distance between the inside and outside of the propeller blades.
The RPM divided by the gear ratio gives the
value for the turning rate of the driveshaft of the impeller. To calculate the
theoretical speed created by the jet take the Driveshaft RPM and multiply it
by the propeller pitch this gives a value for how far the propeller will push
the PWC based on the RPM. So it gives a value of distance per time or simply
speed.