Uncle Dave

hmmmm

exhaust restriction (stock marine exhaust maybe? )
Timing got bumped back.
Bad tank of gas.
Propshaft too deep

dynos 689Hp when you take the car exhaust off, put the accesories back on, misjet it because of the wet and different exhaust, and bump a degree or two of timing out of it likely isnt 689 anymore but you are right that prop isnt turning what it should.

Uncle dave

PatriYacht

The boat is overpropped. I bet the prop is deep in the water and getting a good bite. So with a low prop slip 5800 rpm and a 24 pitch prop, you will be going 80mph. You may even have to go to a 22 pitch. My old 30 Scarab went 72 at 5200 rpm with 23 pitch props. To do the same at 5800 rpm's would take a 20 pitch prop.

scarab542

I know most of the accessaries where on the motor including the water pump when it was dynoed. I have changed the pickup tube to 1/2in including the 90degree fitting on top of the tube so its a full half inch to the pump. I;m going to ask the builder about the timing. And i do have a 23p 3 blade i'm going to try this week to see if i can get a baseline. So hears a stupid question the more and more i hear about the props doesnt necessarily have to do about how much horsepower you have. Just because i have close to 700hp doesnt mean i have to run a bigger prop. Thanks for the schooling this is my first boat so i'm still learning lol!!! Also i just received my fuel pressure sending unit so now i can see my fuel pressure from the dash, any clue what it should be at full throttle?

fastscarab22

iTrader: (1)

With that much power you will need to run a bigger blade, a 26 to 30 pitch. A small blade you will turn it so fast it will run at 7000 rpm I have scarab 22 with 600 hp and i run a labbed 30 bravo 1 at 82 mph on gps

articfriends

iTrader: (1)

Just because your supposed to have 700 hp doesn't neccessarily mean you would run a taller prop in every instance BUT if cam is right and motor makes a decent amount of tq you usually can turn a taller prop. A prop is alot like a dyno, ie-if 500 ft lbs of tq at 5000 rpm's can turn a 25 pitch 5000 and propel your boat 75 mph, if you made 550 ft lbs tq (a 10%) increase at the SAME rpm you should be able to turn a taller prop the same rpm and see a gain in speed or turn old prop a higher rpm and see a gain in speed.
Like I mentioned in my previous post though, I have a friend who has a 540 in a 27 fountain, boat ran 72 mph with his old 502 mph (warmed over some), new motor supposedly made over 700 hp BUT at quite a high rpm (I think it was 6800). Boat would not turn his 24 pitch prop the 502 had previously turned and even go 70 mph, he dropped back to a 23 pitch and motor was still out of its power band, last I heard he was running a labbed 22 pitch and oil was getting hot, boat was sucking fuel like crazy, boat lost cruising speed, I never heard if he ever threw out the cam for something that worked better but he fully thought boat would be running well into the 80's when he was done and he never got there. I hope you find it and it's something simple,what size afr's and what was done to them?
FWIW too my 540 dynoed at 650 hp at 5600 N/A with no accesories (afr cnc'315's with extra porting) and my 272 Baja has ran mid 70's with a 26 labbed 4 blade/1.50 ratio drive gears on a good day

PatriYacht

6 lbs. is enough. don't go above 8 or it will overcome the needle and seat and overflow the bowls. I'm not saying there isn't something else holding you back but you need to get the rpm's up to where the engine makes power. Think of a prop being the same as a rear end gear in a car. You have to match the rpm's to the boats speed.