impeller size vs rpm vs weight

[PLACECRAFT20]

i have been reading the impeller discussions. i also have studied the impeller charts and have a copy of the jet calculator. i have a queastion to throw in to the discussion. my boat is a 20ft placecraft day cruiser. i think it ways 3000 or 3500lbs. with the combination i have now i can run about 67 on the gps at 5000rpm. i have a full hydraulic roller bbc that realy should turn about 5500. i have been told a (b) impeller will make this big boat a dog on take off. anyway, i dont see how you can make rpm and impeller size references on the jet calculator without taking weight into consideration. anyone have any ideas? all i want is a honest 70mph boat on the motor.

[rivercrazy]

Weight will not impact how many rpm's your motor will turn with a jet boat. Rpms are determined by the HP/Torque of your motor and the impeller cut you are using. An impeller basically acts like a dyno to load the motor.
I bet if you turned your impeller down to an AB (1/2 way between an A and B) will might pick up some RPM and allow your motor to produce more power and higher rpms beyond just the change in impeller cut (assuming the motor does make more power at 5500rpm versus 5000rpm).
Good luck with your project

[flat broke]

IMO Rivercrazy is right on this topic.
RPM is independent of weight. Now MPH is a different story. Because the load or work seen by the impeller is only the amount of water it is trying to move, the weight of the vessel doesn't factor into the equation.
As far as the motor supposedly being able to turn 5500 as opposed to the 5k, what leads you to believe that the motor makes enough HP below 5500 to turn the impeller to 5500? Unless the measured Dyno numbers show the required HP being available at and below the 5500 mark, your mill might make its peak power above the power curve to work/load/power absorbtion curve intersection point. One last thing to remember is that detailed impellers/pumps will require more HP to turn RPM. The tradeoff if worked correctly is that the loss in RPM keeps the same MPH which would be indicative of a more efficient setup.
Chris

[quiet riot]

Just one thing I thought I'd throw in here. When I went from an aluminum legend A to a al leg B imp, I picked up 400 rpm's but no speed. I have a sbf turning about 5k with the A and with my engine setup (not dynoed) peak hp was supposed to be 5600 so I thought stepping down might help but it didn't. I also found the B tended to cavitate much easier when I sucked up weeds, rocks, sticks, etc. And the A pulled much better outta the hole.
jd