Griff

The 1.36's are NOT the issue. I have 1.36's with 502mpi's. I spin my 24's to 5100. I borrowed a pair of 26's and spun them to 4700rpms.

Being the props are used and you don't know if they are stock specs, start there.
Are you seeing a speed difference with the 26's and 28's at 4900rpms?? You should see about 4mph more with the 28's.

Tinkerer

Griff - your boat has twins - You can spin a bigger prop or run a taller gear than a single engine boat.
I lost 600 RPM when I went from 1.5 to 1.36 ratio. I am spinning a stock Bravo 28 to 6100 RPM but I have 800+ HP.
I just bought a labbed for me Bravo 28 from BBlades to see if I can get on plane a little easier and gain the 300 RPM I need .
I also think that the 1.36 ratio is holding you back. It takes torque to turn the higher ratio.

F-2 Speedy

iTrader: (6)

There is a huge difference from a 1.5 to 1.36...........agree ^^^.............my bud's 35 Ol has 5 blade maximus and 1.36 IMCO's and idles about 12 mph

ToMorrow44

Agreed, the 1.36 gears is the problem. A 28p prop with 1.36 gears is like a 31-32p prop with 1.50 gears, to compare it to your old boat. You'd probably need to step down to a 24 or so if you wanted to keep the gears you have. To swap gears is ~$1000 I believe. I agree that you should be seeing a difference RPM wise between the two props, but only if they were both stock or both labbed etc. Since you don't know the history of the props, if the 28 is labbed, it will spin the same rpm as a stock 26. Should be about 200rpm between the 2 normally.

Chestah Cheetah

iTrader: (1)

Think I'll have to try this route first just to rule out any other problems.

Say I ran a Bravo 24P with the current 1.36 gears, can we assume the RPM goes back to normal 5250? Would a Bravo 24P get the top speed numbers? Sounds like it would be in my best interest to change the gears to 1.5 or trade the drive for a stock XR.

Tinkerer

Why don't you find someone with a 1.5 ratio drive that will let you try it.
I lend mine out here in west MI. As long as they pay for the oil change and I go for a ride.
But I am running 1.5 ratio IMCO -2 shorty ( spare ) and a 1.36 ratio IMCO -3 shorty.
So the list of people wanting to try it is real short. Most people can't run a -3.
I was told that I was one of few able to run my prop shaft at 2 inches above the low point of the bottom on a 26/27 Daytona.

ToMorrow44

Based on gouge numbers, it should pick up to 5100rpm or so with a 24p prop. From what I understand, you should always keep the lowest gear ratio (highest numberically) until you run out of prop, then go to 1.36. With 1.36 gears, the prop is spinning ~350rpm faster at WOT than the 1.50 gears, so there are hydrodynamic effects that come into play with having a smaller prop spin faster. I'd be willing to bet that 1.50 gears and the right prop will net the best speed.

Griff

General rules of thumb

2" of prop pitch change = 400rpms

1.5 to 1.36 change = 600rpms

ToMorrow44

You're right, 600rpm difference at 6000rpm (~10% difference). I was talking prop speed which isn't very useful.

The OP needs to spend some time with the prop slip calculator.

For instance, you said you had this boat with 1.50 gears, 28p prop, at 5250rpm (lets assume 15% slip) would be 79mph.

To run 1.36 gears, 5250rpm, 79mph, and same 15% slip assumption, you would need a 25p prop.

Like I said, these are all theoretical, just math formulas. More comes into play hydrodynamically, mechanically (driveline losses), etc. Post this question on Riverdavesplace.com. Theres several guys with 28 bullets over there that could steer you in the right direction.

Chestah Cheetah

iTrader: (1)

I'd bet you're right.

The last boat with 1.50 gears, 28P prop, 5250RPM ran 78.3MPH best. So you're very close.

This boat with 1.36 gears, 28P prop, 4900RPM ran 79MPH.

Just posted on RDP as well. I figured I would start on OSO since it seems there is a larger contingency with 525's. However, not very many with singles. Great problem to have!