looking for my buddy.
he has a 21' 90 with a small block and alpha drive.
he had a 17p 3 blade on it, have no clue what kind but its stainless. got it free.
put it on the boat and it seemed to plane good and go good but only ran4,000 rpm.
i was thinking the tach may be off so we will test that but just wondering what people are running on these boats to give him a good base.
he hits rocks a lot so alum prop would be better.
his old out board had atleast 3 props in 1 year. he boats in a place called rock pond. lol

Just for comparison I'm running a 19 pitch Mirage on my '87 242 with a 350 mag. I'm at about 4300 rpms.

what rpm should his boat be running?

It depends on what engine he has but I think both the 350 mag should be between 4200-4600 rpms. He is low at 4000 but I find strange thats all he is getting on a 21' formula with a 17 pitch. What engine do you have?

On my Formula 242 with a Merc 260 I get 4600 RPM with a 17 pitch and 3600 with a 23 pitch. Trying a 19 and 21 this weekend and will report back