size & pitch

[harv]

I am running a 1955 Chris Craft that weighs 1650# and is powered by a new 377" small block Chevrolet motor (400" block with a 350" crank) with appx. 400 HP and is cammed for 6200 RPM. At 6200 RPM I am only at 45 MPH(hand held GPS) with 1-1 ratio.... I now have a 12x13 NYBRIL prop on a 1" shaft , and it is 1" from bottom of boat ....WHAT TO DO ???

[FAST FRED]

Sounds like the prop slip is way to high . The origonal setup was probably great for 150 to 200hp , maybe. But with doubble the power , youre just warming water with the prop.

A 5 blade night help , but you will probably need to increase the diameter to eat all that power , new shaft strut and all thats involved.

There are sites that figure props on line , yours will be supercavitating.

FF

[dmcld77]

Originally Posted by harv

I am running a 1955 Chris Craft that weighs 1650# and is powered by a new 377" small block Chevrolet motor (400" block with a 350" crank) with appx. 400 HP and is cammed for 6200 RPM. At 6200 RPM I am only at 45 MPH(hand held GPS) with 1-1 ratio.... I now have a 12x13 NYBRIL prop on a 1" shaft , and it is 1" from bottom of boat ....WHAT TO DO ???

I run a 13" x 17" on a similar boat but only have 250hp. You are way low in pitch. I am moving to 19" for an easy cruise of 40 mph at 3000 rpm.
(40 years in boat racing and setup)

[Jango]

1650# sounds awfully light for a 55 chris craft unless it is a very small (15' or shorter) in which case CG will probably be too far forward causing the boat to "Plow",resulting in very high slippage. If you using a gear box, motor and trans weight alone is 700 - 900 lbs. (depending on how much alum. is used)

Correct Prop for a properly set-up 1650# boat (1850 w. driver and min fuel) is a 11 1/4 dia x 17 pitch. 400hp, turning 6200 rpm providing 90 mph @ 7% slip (55% D.A.R.)

Unless your boat is fairly long > 20 ft.This kind of power and speed usually requires a V- Drive set up to enable CG to be close to transom.

Note: Higher max RPM results in Smaller dia Prop requirements. i.e if Max rpm was (lower) say 5200 with 400 HP, recommended prop would be a 12 1/2 D.. x 20P. providing same top speed. If your boat were not "plowing" or subject to High Drag, current prop dia is addequate & motor would be overreving due to lack of pitch.

Hope this Helps - Jango