The general rule is maximum rated engine rpm should be achieved at wide open throttle. This means, that when you factor in your engine type (I think you have the 427, which is rated at 4000, and the GM 454 may be 4200 better verify) you will find it is rated at a particular max rpm. With the gear reduction (I believe you have 2.5:1) and prop size, weight of the boat, hull type, etc., you should be able to reach that max rpm at wide open throttle in realtime operation on the water.
If you are only able to reach 3700 rpm as I am now with my overpropped 4-blade props, then you are clearly running too much pitch. If you are able to run a motor at higher speeds than the rated rpm, then you are underpropped. Chris Craft did exhaustive testing on the water with each hull, changing out props to achieve what they thought was the best top end and best efficiency. It is very difficult to beat them at their own game with second guessing what prop is best for a given boat.
The rule of thumb I'm (painfully) experiencing now, is the fact that my 23x24 4-blade props should actually be 23x23, because the specified prop for a 38 is the 23x25. That is why I cautioned about the need to compensate between the pitch of a 3 blade or 4 blade.
To restate this for you again to be clear: a 23x25 three blade will limit the rpm of a 38' Commander Express to real close to the 4000 rpm max allowable by the engine manufacturer; while a 23x24 four blade will cause the rpm to drop around 3700, and therefore should have been a 23x23.
You always give up a little top end with a 4-blade prop, but you (supposedly) get better mid range cruising and less vibration. I didn't really notice all that much difference in vibration, but the boat sure gets onto a plane nicely at lower rpm as you might expect. Under some engine speeds I'm causing the motors to work a little harder than they should, and I suspect fuel economy is suffering a bit. In the springtime I'm going to swap back to 3-blade, and have the 4-blades repitched to 23x23.
Hope this helps you,
Regards,
Paul
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