In addition to my sonic I have a 1990 270 sundancer with a single 340 hp 7.4 1.50 Bravo 1 drive
WIth the 15 P 3 blade prop that the seller just
threw on there I am getting 5500 rpm wot in fresh water. It seems like I have to maintain 3500 just to keep it on a plane.

What are the best choices for a 5800# 20deg dead rise boat ? I will use it is salt and fresh water.

19P mirage? 4 blade?? Sea rays prop matrix doesn't list the 270 in 1990 It does however list a 290.

Any help will be appreciated very much

Think your tach is off, the 7.4 max rpm (unless work has been done) is 5000

Can you get a handheld tach and get your max speed at WOT using a GPS or one on a phone?

What limits the rpm the ignition module? The engine is an 86 v drive 340 hp std rotation version

You should have a TB-IV and of which does not have a rev limiter, but going above max WOT I can see the motor coming apart

It was still OK to see what the 15 would do. Now OP knows he needs a 19" 3 blade or such. For a heavier boat like that maybe a 4 blade 18.