scabvzla

I have a 1991 21f scarab with a 7.4lts 454 carburated mercruiser with thru hull exaust an a bravo 1 sterndrive. Im looking to go a little faster with out spending a lot of money. Curently mi top speed is 54 mph wot @ 4800rpm with moderate trim, one person on board and 50galons of fuel... I want al least to top at 65 mph... Actualy i run a 14 x 23 stainles steel mirage prop. Thanks for the help!

throttleup

To see that type of speed increase, 11 MPH, you will need to crack the motor open. You will not see this thru propeller work solely.

Julie

D Harry

I do have a scarab 22 96 model and with a 7.4ltr it use to run 64/65mph with a mirage 25 . Now I do not know the diffrence in weight and hull between the 2 boats, but there is something wrong for sure, cause you are over revving the engine by 400 rpms. Did you try a 25 pitch prop.

throttleup

Yes, a newer model 22' Scarab with 454 Mag will run 64-65 mph.

Julie

scabvzla

I think i need more pitch in my prop, a friend is going to lend me a 4 blade bravo one prop 26 pitch to see how it behaves. This engine should top al 4300 at wot, so if i increase 3 in pitch i shoul lower at least 400 to 600 rpm and gain like 5 mph or so. What do u think?

scabvzla

also this is a carburated model, the fuel inyected ones have better performance, and higher rpms at wot, mabye i should upgrade my carburator to a 4 barrel and change the camshaft if i dont see an increased speed with this prop

Perlmudder

that does sound slow for your boat. I have a 20ft larson with a 5L and it does 55, and weighs 3400lbs dry. we spin a 14.25 x 19 4 blade, or a 15 x 21 3 blade. good luck on your quest!

D Harry

I would try a 3 bladed prop, cause it was the best prop I found with the standard set up. My 7.4 engine was a carb as well at it time. So in my opinion try a 25 mirage see how it goes, then give all your specs to Julie at Throttle up and may be they can get 1/2 mph more. Forget upgarding a 7.4l engine cause it would be better to sell that engine and buy another with the changes you have to do to add more power.
good luck
Herman

throttleup

If your motor is not a mag it needs to run at 4600 RPM.

(I have corrected the RPM thanks DHarry)

Julie

D Harry

Julie, if i remeber well according to the mercruiser specs for a 7.4l it should rev from 4400/4600.