Diameter doesn't enter into the equation as much at the pitch. Usually as pitch goes up diameter would go down
and viceversa.
Yes you could test with the larger diameter and adjust the pitch to compensate for any rpm problem.
If I remember right a 1" increase in diameter lowers rpm about 100.But in props diameter and pitch changes aren't carved in stone
results can vary widely.
When changing diameter you need to be sure you stay with a prop intended for your motor.
There is a strong possibility that going from 14 to 15"diameter prop that fits the prop shaft could be too big for the gear case.
And of course going from 15 to 14 may be too small for the gear case.
If you were going to try to force fit a prop I'd be looking for a diameter 1/4 to 1/2'" from original.
Ayuh,..... I agreed with most everything SSpike says,..... The bolded line I'll get to, but wanta expand on the prop hub diameter/ length thing,....
With Merc.s,... the mid-sized gearcase outboard motors, 'n the Large gear case outboard motors, use either the 4" prop bodies for the mid-sized motors, 'n the 4, 1/2" prop bodies for the large gear case outboard motors,...
Both of the prop body sizes will mount on the Large gear case outboards, but blow out because of the big air gap at the prop's leadin' edge,...
Obviously, the large bodied prop will slide onto the prop shaft of the mid-sized gear case outboards, but hits the gear case, regardless which thrust hub is used,...
So again, it comes down to what prop ya wanta try,... on what motor,....
Btw,..... The props that fit the Large gear case outboards, are also direct fit to the Alphas, 'n Bravos,...
'n for the bolded line, I gotta story,....
When my tinbarge had the ole 3.0l motor, 'n after I'd trashed the drive, I'd gotten a used drive that was for a V8,.....
On my last attempt at re-engineerin' this thing, after tryin' every prop 'round here, includin' a mid-sized gear case 13" prop that blew out wildly,...
I headed out with a 15"x15" for an Alpha,.... 2800 rpms,.... luggin', Badly,....
I pulled up to a Bud's dock, tied up, pulled the prop, 'n headed into his workshop,.....
I found an ole jigsaw that ran, 'n some blades for it, scribed a line, 1" from the blade tips inward,...
I sat there, 'n carved a full Inch off all three blades,... that's a full
Two Inches total diameter, cut right Off,....
I cleaned up the cuts, 'n put it back on my barge, 'n headed out, thinkin' it'll Rev, Now,.....
Guess what,.... 2800 rpms, Wot, luggin to beat 'ell,....
I headed for the launch, 'n found me a 4.3LX,...... 'n another drive,.......