I/O Performance Advise

[Blue_Heron]

Originally Posted by Ripcurl

It's a Bravo One 1.65:1 outdrive with a Bravo One 26 / 15.25 wheel.

I think the issue is your drive ratio. I'm running the same ratio (if memory serves) on a 350 mag mpi that hits a rev limiter at 5200 rpm.

Take your prop pitch relative to top speed and compare it to an outboard or Gas I/O on the same or a similar boat. I think you'll find boats topping out at 35mph running 15P to 17P max props.

Now ask yourself why. Engines are typically geared so they'll turn the most efficient prop at the right rpm. It's generally more efficient to turn a lower pitch prop at a high rpm than a high pitch prop at a low rpm. But you've got a diesel with a ton of torque delivered at low revs. To convert that torque to propulsive power, you've got to have a prop with a lot of bite turning fast, or a prop with a lot of slip turning slow.

I'd check with a trusted Mercruiser dealer and see what he thinks is the right drive ratio for that engine. I'll bet he'll recommend a ratio that will allow you to prop down to a 15p or 17p.
Dave

[FishStretcher]

So the drive ratio on my bravo 3 is on a sticker- and it is bleeding away at that. it makes me wonder if yours might be a 1:2.0 labelled as a 1:1.65?

BTW, I have a 1:2.20 on a 350 MAG MPI bravo 3 at 22" pitch both props on the counter rotating pair. It would seem that 26" pitch is ambitious for your setup.

[Blue_Heron]

Originally Posted by FishStretcher

So the drive ratio on my bravo 3 is on a sticker- and it is bleeding away at that. it makes me wonder if yours might be a 1:2.0 labelled as a 1:1.65?

BTW, I have a 1:2.20 on a 350 MAG MPI bravo 3 at 22" pitch both props on the counter rotating pair. It would seem that 26" pitch is ambitious for your setup.

Bravo 1 drive ratio info here:

http://www.go-fast.com/sterndrive_gear_ratios.htm

It's a reduction gear ratio, so the higher the number, the lower the prop rpm compared to engine rpm. I think the diesels usually run the 1:1.36 ratio. I checked again and mine is a 1:1.50.

The Bravo 3 ratios are higher because the counter rotating props are more efficient even when turning a high pitch prop at a lower rpm.

Dave

[bitsamonkey]

Hey Blue Heron, I think you got it backwards. A higher pitched prop turning lower rpm is more efficient than the other way around. OP, you're propped for a theoretical max speed of 54mph which you know won't happen. I remember towing my swamped seacraft with my big block powered inboard and the motor was able to "slip" the prop with no increase in speed. Maybe you should try a prop pitched closer to a realistic top speed and see what happens.

[Ripcurl]

Back on this... Thanks to all for the insight.

My mechanic has a B3 laying around that he's going to let me try out. I don't care so much about the top-end, the issue is as I load up or run in wind / chop, the wheel doesn't bite enough and the change in performance is material. Assuming the B3 bites alot more, this effect won't be as great and I'll get better economy and stress the engine less when running offshore with a load.

One issue I'm keenly aware of is that if I don't have enough slip w/ the B3 when I throttle up, the turbocharger won't spool... basically the opposite problem to the one I have now. This can be solved with less pitch, but then I'm increasing slip again.

Clearly a narrow potentially expensive corridor to navigate.

[FishStretcher]

I suspect you will be ok. I don't know if those are waste gated turbos or not, it sounds like maybe not?

I did a little counter rotating prop specification last year, working with an OEM. I seem to recall that extreme pitch ratios are less efficient, generally speaking. I seem to recall P/D in the 1.2-1.6 as desirable. I think that a 26P hasn't gotten so far in P/D ratio as to be a concern.

Bravo 3 props are stainless only, I am told. Based on the local prop shop, he isn't a fan of stainless for modification, he has seen too many shed blades!

But it seems like a bravo 3 at the standard 22" pitch probably is a good starting point. But it might still be slow over the hump, depending on the turbocharger trim.

Edit: I strongly suspect you have the 230HP waste gated model. I still think 22" is a good place to start for pitch.