The F6 is higher pitch and the F4 is lower pitch. You'd want an F6 for more potential speed. The F4 would give a faster hole-shot, but higher RPM at any given speed.
The F6 has more pitch and will, therefore, likely lower the WOT RPM by a couple hundred RPM, and may give a couple/few MPH. It will also lower your RPM at any given cruise speed. If you're doing 4800 on an F5 set (fully trimmed for higherst RPM and speed), I'm not sure that you want to lower the WOT RPM any lower than the curent 4800 rpm.
Four Winns states that the 7.4 in this year should top out at about 5000 RPM and be running in the low 50 mph range. Both your RPM and your speed seem a bit low. Are you sure that you are properly trimming the drive for best speed .... when that is the goal?
Your boat weighs just 300 lbs (more or less) than our 245 Sundowner and we had the 5.7Gsi-DP (280hp) with an F6 prop set. I could just tickle 50mph with the canves down, four people, smooth water and a proper trim.
I would be looking at why your boat seems to be, maybe, a bit sluggish. How many hours? Any recent "tune-up" work? Are you carrying a bunch of gear that would make the boat a lot heavier than normal (we had quite a bit of gear on our 245)?
If there are now composite sets for the DP, I can only recommend them as emergency spares on a heavy, high HP, boat.
Edit - In ref. to the DP composites, I found this too:
http://www.piranha.com/show_article.php?id=12
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Gordon Arnold
New Hampshire
2003 268 Vista ..................................................................Prior: 97 245 Sundowner
