improving fuel economy on 5.0 mc with alpha 1.62,poss. switch to 5.7

[telstar1]

Have a new to me 2359 Trophy with 5.0/alpha,1.62 gears. running the 14.5x18 4 bladed prop I got with boat,havent tried any other props yet. Getting about 9 gals imp/hr 27 mph. Need to run 3650 rpm to get that. The boat is likely overpropped a bit as max rpm is 4300.Merc recommend 44-4800 max. Wondering if its likely Ill burn less fuel by going with a lesser pitch,freeing up motor a bit, but Id then have to run at a higher rpm to get my 27 mph.27 mph seems to be the sweet spot for this hull,Id like to target that speed.I figure a 15x17 3 blade will up my peak rpms a few hundred,but id have to cruise at3850 rpms or so.Anyone have ideas or experience that would indicate likely outcome of this switch?
Also,thinking of upgrading to a 5.7 litre which I have.This would assumably swing a higher pitch prop which could allow me to drop revs back into mid or low 3000s.Is this larger engine,running at a lower percentage of its available power, going to be more fuel efficient I wonder?Again,ideas anyone?

 

[HT32BSX115]

Re: improving fuel economy on 5.0 mc with alpha 1.62,poss. switch to 5.7

Howdy,

This is probably better asked in the prop forum......but I will say that you ARE over propped if your MAX RPM is 4300.

Just about all engine manufacturers suggest best performance will be obtained if you prop for top of the MAX RPM lightly loaded.

select a prop to give you 4800 RPM at the pressure altitude you'll operate with the lightest load you expect to run. That way if you load it to capacity, it shouldn't drop below min (4400)


If you want to switch to a 5.7L engine, you';ll have to experiment with a couple of higher pitch/larger dia props.

In either case, select a prop pitch that gives you MAX RPM at WOT.


Regards,


Rick