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.
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.

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????

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