Prop sizing

I have been playing around with different props lately and now I am stumped on where to go next.

Boat is a 1989 Invader 25 foot, 6000lbs, merc 5.7 with an alpha one drive


Stock (what I believe was stock)
15.25 x 15 aluminum prop

Run 4900 WOT

cruises @ 4200 RPM's at 3800 rpm's I cannot keep it up on plane, even with trim and tabs all the way down

Tried a 15x17 prop, would not work.

16 X 16 prop seems to be perfect or close to it.
3000 to 3200 rpm's and it stay's up on plane and it gets out of the hole allot faster then the 15.25, but, I can only achieve 4200 to 4300 RPM's

What should I do next?

My goal is to get the best fuel econ as possible with out effecting cruising speed much.

Stuck

 

sounds to me like you've done good the boat is in the propper rpm range at 4200-4500rpm
pat yourself on the back and have fun.

 

Prop

I was hoping someone would say that. I don't have anyone to lend me props so I am now the proud owner of 4 props I will never use. EBAY I guess.

What is optimal cruising RPM?

 

veries buy load and conditions to say one speed and rpm isn't right best way to find this out spend some money on a flow meter (some fish finders have this as an option) best rate of speed and lowest gallons per hour