Ok guys I have a Nortech 3600 supercat with merc 700s that have the stage three whipple upgrades the boat currently has hering 16.5x36-16 props when I ran it in havasu it was spinning to high of rpm what would be a good size prop to move into from these a 16.5x38-16?

If you like the way the boat planes and carries. The overall attitude is good.
Go up in pitch till you gut to your 5000-5400 range.
What is the current rpm with the 36's?

Brett

it will pull over 6k full of fuel and three people on board I don't want to run it that high Id like to be able to only pull 5800 ish

1 inch of pitch will typically reduce rpm by 200. If you are over 6000, you would theoretically need to go to 38 pitch props or even higher.
I'm not a big fan of propeller pitches above 38. The large angle of attack can create stern lift and stern movement issues do to what I refer to as a paddle wheel effect. Some larger cats with extreme X dimentions can handle the higher X better than others and therefore the 39 and 40 pitch props work fine.
You should have a rev limiter adjustment with the Whipple kit. I'm surprised it isn't kicking in prior to 6000.
Sometimes it is time to also consider a gear ratio change.
I am sorry I do not have any 38 pitch props in our current test inventory.

Brett