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