Prop pitch change for lower fuel consumption?

[bdwernychuk]

I've got an '85 90hp on my 17.5' Campion Cuddy. I'm getting atrocious mileage - about 3.5 mpg at about 3/4 throttle, running 23-25 kts (GPS). This is a project boat I recently bought and the tach doesn't work so I can't do the usual speed runs with the tach.

Acceleration now is excellent, boat gets up on the step very quickly. I use this boat for cruising and overnighting and would gladly trade some speed and/or acceleration for better mileage / range.

My question is: If I replace the existing 13 1/4 x 17 prop with something coarser, like maybe a 19" pitch, will I see better milage? If I'm picking an aribitrary prop pitch for the change, is 19" a big enough increment?

Thanks,
BDD

[Ken]

BDD,
You really need to know what your rpm is at full throttle and trimmed out for max speed. I believe the max rpm range for your 90 hp is 4500-5500. For max fuel economy, I would go with a prop that gives me close to 5000 rpm max. Your best throttle setting for max mpg will be around 4000 rpm - but you need a fuel management system to determine that exactly.
Good luck,
Ken K