I'll offer a caution to anyone doing a swap from 3 to 4 blade props, because it's easy to get overpropped unless you do your homework well, and get good advice too.
I swapped my 38' Commander Experess with 427 power and 2.5:1 gear reduction from 23x25 3-blade to 23x24 4 blade, and I'm significantly overpropped now. It makes for a nice leisurely cruise but on top end I clearly can not reach max rpm or speed now.
4 blade props are known for producing less top end, but they are supposed to be smoother and offer some cruising advantages in the mid range. Mine are smooth enough allright, but I would have been better off with 23x23 in this particular prop type.
As you can see, they're stunningly beautiful props, but next time out I'm sending them off to be repitched so I can get the true efficiency I'm looking for, and the crusiing comfort zone I want to be in. On top end even the perfect 4 blade will still fall a half mile or one full mile per hour off the optimum 3-blade, simply because there is one more blade producing friction. Some of the studies I've seen suggest you actually pick up a bit of speed and fuel economy at mid range, and as we all know, mid range is fashionable these days due to fuel cost. Most of our cruises are in the cocktail speed range or mid-range when running a distance, and hardly ever top end except to impress the guys (or girls) now and then.
You can actually hear the difference in pitch, between the two videos below.
This video is with the 3 blade props and rpm is reaching up around 4000 rpm, as it should.
This video is with the 4 blade props, and rpm is down around 3700 rpm, a little on the low side for me.
Since I rarely run this fast, it's not a real problem, but I'm still going to have the 4-blades repitched and computer scanned before they go back on.
Any time you go from a 3 to a 4, you'll need to give up one or two inches of pitch, in order to keep your rpm in the same max range specified by the engine manufacturer.
