To those who understands props, I have an old bass boat with an 80hp mercury (1980s model). The boat had always struggled to get up on plane. It has a 3 blade prop with the #'s 48 72500 a4 21p. What prop could I put on it to make it jump up on plane better? I care nothing about speed, if the boat is running fast enough to blow my hat off its running too fast.

It's a 21 pitch prop, so you can go down in pitch to help with your hole shot. Try something in a 19p.

Ok, I wasn't sure if I needed to go up or down. Would a 4 blade help?

4 blade will help hole shot but will hurt you on the top end

A 21 pitch on a old 80 hp? Damn what kind of boat is it on?

If it's something like a bass tracker it may not be the prop. Our tracker had issues getting on plane the foam in the back was soaked. After awhile that foam is like a sponge.

It's a mid 80's 15' Bumble Bee. What Pitch should it have? The motor was rebuilt a few years ago using a 90hp block so I guess it is now a 90.

How many rpms is the motor turning? I'd think you need more like a 15 or 17 but depending on boat weight you may could swing more

I don't have a clue on the rpms, it is an old heavy made fiberglass.

I bet your not turning very many rpms. I'd try a 15 or 17 and have a tach hooked up to watch rpms. The reason you can hardly plan off is that your prop is way too big.

21 is kind of tall for an 80hp. What RPm's does it turn when up and running wide open? I'm guessing your going to need around a 17 maybe a 15.