How fast should my 20ft 454 be?

[Starcraft5834]

I agree, based on your own test you could use more prop, I'd go up 2 from where you are now, I'm also a 4 blade fan, that's just me. .. you will like the electronic ignition upgrade, I did it, bought Pertronix

 

[frantically relaxing]

I had a 21" pitch prop on my 17' 4-banger Bayliner, you should have at least a 23" on that thing. Cruise speed-to-rpm numbers matching up (3000 rpm=30mph) is a good basis to go by. Since your first test was 25mph at 3200 rpm, that's a dead giveaway your prop's not right. My Regal runs 30mph at 3300. My last boat, Chaparral 260ssi 8.1 Volvo would cruise roughly 32 mph at 3000 rpm with whatever pitch the DP's were. IMO, that's about where you should be with that boat.

Anywhere nearby that rents props? Hitting 45 at 5500 rpm with a 21 almost sounds like the drive has 'mountain' gearing. I'd put a 25 on that and try it out, and I wouldn't be surprised if a 26 wouldn't be just about perfect...

 

[roffey]

I had a 21" pitch prop on my 17' 4-banger Bayliner, you should have at least a 23" on that thing. Cruise speed-to-rpm numbers matching up (3000 rpm=30mph) is a good basis to go by. Since your first test was 25mph at 3200 rpm, that's a dead giveaway your prop's not right. My Regal runs 30mph at 3300.

on my boat the RPM will match the speed as you say, until I hit WTO, trim the motor and then you can feel the speed pick up.

 

[Oshkosh1]

Yes, I expect to see 10mph per 1kRPM on my setup; 1991 23' Celebrity cuddy/Mercruiser 5.7L(Quadrajet)/Alpha I Gen II/19P "HighFive" prop. Last time I had it out, it was slightly below that, but I had 3 people aboard. At 4k I was running 39 or so. I put the hammer down a couple of times and saw 43 @ 4500k. I had more left but it was only the 2nd time the boat's run in 7 years so I'm not going to jump on it too quickly.