Trying to figure out a good starting point for prop selection. I bought a 625 hp 540 to put in a Donzi 25zx. Right now boat runs 60-62 with a 454mag and either a 21p mirage plus or a 21p rev4 at 4900rpm. Owner of the 540 states that he was running 90 mph at 5600 rpm with a 28p bravo1 in a 253 checkmate.

Prop guy says add 1" pitch for every 100hp. He also states that for every 200rpm increase I should figure on 1" less pitch...so basically I should be pretty close with staying at 21p. Sound right?

No

I am not sure I believe what the guy said he got in his checkmate, I don't at all actually. For one a. 6.75% slip is completely unrealistic, but 650hp in a 24 outlaw (which is very similar type hull) will spin a 26p bravo 1 to about 5600 at about 78-80mph. I would say a 26p is probably a good starting point depending on what rpm the motor makes peak power at

Bravo 1 props are different. I run a 30p Bravo 1 labbed by Brett. My boat could not pull a 30p of most other prop types.

I own a 253 and you will not go 90mph with 625hp 540. not even close.

The checkmate is going to run around 80, not 90 with 625hp.

is your drive ratio 1.36? cause with the numbers you gave 1.36 ratio would be around 14% slip and 1.5 would be only 5% slip.

you can start with the 21 and get a baseline, it will probably be too small, then go from there, depending on top speed you shouldn't need more than a 24p with a 1.36 drive.

1.5 drive

Bravo 1 props aren't true to stamped pitch, they are under pitch. 24 is 23.3, 26 is 25.3, etc...100hp took me from spinning a 23p mirage at 4800-4900 to spinning a 26p bravo 1 to 4800-4950. 625hp I think the 26p bravo 1 is a good starting point to get a baseline and very well may be the prop you end up needing

My 26ZX spins a Bravo 1 26P to 4900rpm at 69mph.... This was into the wind and against the current so I think it has a few more mph in ideal conditions....Motor is a new 502 with ported aluminum heads and 10-1 C/R and was not dyno'd but I would say around 600 ft/lbs torque is pretty close..... cruise is awesome @50mph at 3,500rpm so I think this is the perfect prop...... Your motor with a little more power and a little lighter hull should get you up to the mid 70's I bet....... I also think the 26P will work the best for you... TBF

tnc110 something isn't adding up, with the numbers you gave that's only 5% slip, your going to have at least 10% slip. That being said, either speed, rpms, pitch or gear ratio is different than what is stated.

run the 21 or borrow a 24 or 26 and get some baseline numbers and go from there.