Hi , i have a Scarab 29 from 2001 with a single 500 EFI , and standart Bravo drive.
Top Speed is only about 55 mls .
Prop is 25 Bravo lap finish turning it 5100 rpm
same speed with 26 Bravo one 4700 rpm

any ideas why it is so slow ??

Did you just buy this boat or have you always owned it and this is a new issue? What are you using to calculate speed gps?

i bought the boat two years ago . top speed i have seen was 57 mls .

gps or water pitot tube? i had a 29 with the 502 efi 415hp and i was running that you should be in the 63-65mph range

you have a blue motor with headers correct?

blue motor with CMI 500 EFI about 470 HP speed was on GPS

how low is the prop shaft below the transom? have you checked the tach to make sure it is accurate? 25 is a 3 blade correct? i would recommend trying a bravo one4 blade 24 pitch, i will save you a lot of time and money. i spent thousand upon thousands trying to get my 29 to run 75mph like my 26 scarab with a 500 hp did. after several 3 years of pulling the motor out every year and building from 415hp then 550hp to a 540 ci bbc 625 hp and the most i got out of it was 72 mph then i finall got tired of throwing money at it and put a blower motor in it making 825hp and a -1 inch imco lower unit and it ran 85mph.
my suggestion to you is to find out low your prop shaft is below the bottom of the hull and then more than likely you will need a -2 or -3 imco shorty lower drive unit. then see how it runs. trust me when i say the propshaft is more than likely 5 inches below the bottom and you want it 1-2 inches or even level with the bottom. you may need to get on plane with a five blade prop. i would go this way before trying to get more hp out of your engine.

you also need to get rid of the defuiser ring on the prop you will be amazed and how much more trimable the boat is.

I have a 29 single with a 525sc and i was run 68-70 mph gps, now i have build the engine with Bob Madara custom grind roller cam, ported alluminium head, 8:1 comp ratio, 468 ci block, BM 250 blower, twin QFT 850 marine carb at 9-10 psi boost with water/methanol injection estimate at arond 800hp runing now 80 mph gps turning a BBlade 26p lab at 5700 rpm.

With std height bravo drive that the center of prop shaft is at 7,375'' below the bottom of the hull.

In 2-3 foot chop the boat start to chine walk.

[QUOTE=supermx96;4143773]I have a 29 single with a 525sc and i was run 68-70 mph gps, now i have build the engine with Bob Madara custom grind roller cam, ported alluminium head, 8:1 comp ratio, 468 ci block, BM 250 blower, twin QFT 850 marine carb at 9-10 psi boost with water/methanol injection estimate at arond 800hp runing now 80 mph gps turning a BBlade 26p lab at 5700 rpm.

With std height bravo drive that the center of prop shaft is at 7,375'' below the bottom of the hull.

In 2-3 foot chop the boat start to chine walk.[/QUOTE

I HAD 825 HP 5900 -6000 RPM -1 IMCO LOWER AND HAD THE BOTTOM BLUE PRINTED AND DID NOT CHINE WALK AT ALL, YOU MAY WANT TO CONSIDER RAISING THE X DIMENSION OR BLUE PRINTING THE BOTTOM