What drive ratio & prop??
 

Ok, I know we have some smart guys here,,, so here it is:
Given:
1988 Sonic 31SS
twin 650hp n/a 6000rpm/full roller engines

Question:
I have both 1.36 & 1.5 Bravo Ones
Which drive would be best & what props to start with on each.:confused:
I realize the 1.5 would give better acceleration but must use a higher pitch prop also. I currently have 26" & 28" Bravo One props on hand, which may(?) work with the 1.36?? Lower rpm @ top?
Help!!

George,
How fast do you think it will go?

Example:
My old Top Gun flat hull ran 93mph and at that speed turned 6100rpm with 1:50 gears and Bravo I 30p out of box no work at all.

Definately The 1.5:1 Drives...
 

And then start with your 28" pitch Bravo One props, then dial the boat in with better props, taking in to cosideration what your prop budget is...Ed:)

Eric- I am expecting around 90 at least. I'm getting quite a collection of Bravo's & lowers now. Still interested in the gauges we talked about, I just need to get time to get up to see ya' !!

I was thinking of the 1.36's to keep the Bravo rpm's down since they don't really like over 5000.... maybe I'm wrong though....
Any other opinion's ??????????

A couple of us have run pretty big power on Bravo I drives (over 900hp) and actually had better luck turning them faster rpm than we did reducing gears and rpm, maybe too much torque I don't know. I only turned mine 6100rpm but had a friend turn his 7400rpm and did better than a lot of other guysturning under 5500rpm, in fact I think he only went through 2 drives in 5 years.

I would start with the 1.36 and the 26" and go from there. That will get you 80mph at 5000 and 90mph at 5500 with 10% slip. You will probably need the 28" or even 30".

As a genral rule you want to run the 1.5 gear ratio until you run out of available prop pitch (34-35). The lower gear ratio means the prop has a lower tip speed which can prevent some prop issues, especially for those who spin thier motors at high rpm.

You should be in about a 28-30 Bravo 1 or 27-28 Hydromotive prop. Your 28 would be a good prop to start with and then go from there.

Matt

Now were going!! Cobra- those #'s you threw up seem to be right where I need to be headed. Although the lower tip speed with the 1.5, I can see the need then for a 30"-32" prop. It just doesn' seem as efficient some how.

Any #'s for the 1.5 drive????:confused:

The pitch #'s I posted were for a 1.5 drive.

6000 rpm, 1.5 ratio, 12% slip, 28" pitch (Bravo 1 30), 93 mph

With the rpm you will be turning, going to a 1.35 ratio would not make sense.

A 5 blade prop would reduce the slip to about 7 or 8 % so you would be able to reduce the pitch to about a 27" pitch at the same speed.

Matt