better late than never
 

So many ? I have a 1990 21ft Scarab,Have had the boat for 11 years on my third motor.This motor was prof. built 462.8ci,.603in lift intake,.615in lift exhaust,hydralic roller.builder has givin me a dynosim saying HP is 575 at 5800rpm and torque # of 610 at 4000rpms.Oh 11.01compression.I have an imco upper drive with the bravo 1 lower.I was reading one of the other post so now I know how to check my gearing.The boat runs 62mph on gps at 5900 rpms with a 3 blade alum 19p prop,boat gets on plane good with this prop.have a 22p bravo 1 when I tested this prop had another 175lbs in boat and could only get 55mph at 4200rpm.tried a 23p mirage over the weekend and ran 61mph at 4100rpms.ok time for questions,am I crazy for running the boat at these speeds,I want to see 80 but would be happy with 70 if the hull is capable.Is my builder blowing smoke or do the engine # compute.If so what can I do to get 70 or 80 this side of buying a new boat.PS these test have been run at 3000 ft above sea level.Also where do you think the # will be if I go to havasu not sure what the elevation is there

You got serious problems somewhere , IF you actually have 575 hp in your 21 foot boat and everything is correct/working right I would expect 70 plus mph from it, not 55 mph, IMHO. Now, to your prop numbers-
23 pitch/61 mph/ 4100 rpm's tells me -.02% slip with 1.50 gears
and
22 pitch/55 mph/4200 rpm's tells me .05% slip (impossible) with 1.50 gears
http://www.rbbi.com/folders/prop/propcalc.htm
now, if I put in .07% slip in the calulator with your first set of numbers and let it telll ME the gear ratio It says you don't have a 1.50 (according to your numbers)


Are you 100% sure of your data? Do you know for sure your drive ratio, the numbers you are giving us say its a 1.36 as your slip isn't going to be negative or near zero, Smitty

I had an 89 21 foot scarab with a stock 260hp small block (alpha one drive)and it ran low 60's (GPS) with a 21p 3 blade Mirage prop....... Something isn't adding up with your boat...

I think either the Tach or the Speedo must be off.. considerably. 4000 rpms sounds like "cruising" rpms with 575 horses and peak torque at 4000 rpms. Now cruising comfortably at 55-60 is Nice, the slip % is virtually nil. If you've got .02 prop slip, I'd like to Buy your Prop!! That boat probably ran better from the Factory than what he's got posted here. There are variables here that aren't being calculated. I'd say run a Digital Tach and GPS speedo and then report back. As it is, you have enough motor to spin a 23-25 and your wot should be somewhere in the 4800 to 5100 range. As for the Hydrodynamics of the Hull, from what I've read and studied, control is compromised considerably after 72-75 mph depending on weight distribution within the craft but your Scarab is designed to handle speeds in excess of 80 MPH. That's my two cents. Double check all of your info and keep us posted. You should be seeing speeds in the high 60's to mid 70's with your setup and a larger prop.