I have been wanting to go to a 21 from my 23. I have a 87 formula 272 with 350 mags bone stock 600 hrs. 2 weeks ago flat water 2 people 1/2 tank 56.8 GPS@4200. now going to a 21 should put me in the RPM range. I was told that even though i would be in the right rpm range i may lose speed. I want the same "bite" as my 23's but more rpm. What do you guys think?

You'll get better "bite" with the 21's. 4200rpms is way to low and you're not getting into power band of the engine. You will probably gain 500-600rpms by getting it propped right and a minimum of 400rpms.

Check the prop slip calculator and plug in some numbers. Yor slip% should remain the same.

http://go-fast.com/Prop_Slip_Calculator.htm

According to the calculator i'm at 8% right now and says i should be running at 61MPH

Just because you go to a lower pitch does not mean you will lose speed!

With 21 pitch props the motor and drive will be happier as well. The boat will feel more responsive and fuel mileage may also improve.

Go with the 21's and you will notice a difference across the rpm range and may pick up a little speed as well.

Matt

Thanks for the help TU and GRIFF!

You need to get the r's up for the sake of your engines. The drives would be fine at this HP level.

You will lose some mid range cruising speed, you will most likely lose some fuel economy with your hull. The top speed will most likely rise or fall slightly. What you will get is a boat that gets on plane easier and has much better throttle response. Ultimately, the engines will like the change.

Would lab finishing the props be a solution? Labbing the prop (or so Ive heard) will raise your RPM's by anywhere from 200-400 would that not accomplish getting him into where he should be with the RPM's but also accomplish more speed?

Labbing the propellers is option. To maintain good mid range, we may only see a 200-250 rpm gain.

I am trying to go from 21's to 23's or 25s. If you want to try mine out let me know.

I have alphas so i would need two RH props. I would buy you 21 from you let me know thanks