Prop Selection, 125HP 20' Seafari

[McGillicuddy]

PowerTech MQF3 15 (13-3/4" x 15p) SS. Big blade area but easy to turn.

MQF comes in a 4 blade too. That will lower you planing speed a little. Not sure what model your thinking of but in most cases 14x 17 will be a little too much prop.

[Max_Florida]

I think you are right. I knew that 1" increase of pitch lowers your RPM by 200rpms, but I didn't think to see what the calculation for diameter was.

Apparently 1/4" of diameter increase lowers your RPMs by 200.

If I go to a 13.75x15 that should put me at 5400 RPMs, which would be good. 14x17 would drop me to 4800, no bueno.

[Bushwacker]

It looks like you're ok on max rpm, but does Merc post an OPTIMUM rpm for your motor? BRP does that for the E-Tec's. The "Operating Range" for mine is supposed to be 4650 - 5850, but the OPTIMUM range is 5400-5500, and it'll use the least fuel and oil if propped for that speed.

Sounds like you need a little more stern lift to get your min planing speed down. I think you'd generally want more blade area, so go up in diameter and down in pitch to get better load-carrying ability. A 4-blade will do that for sure, with a loss of a couple mph top end. Some 3 blade props with a lot of blade area like the Merc Mirage and BRP Rebel will also help with min planing speed. I tried a 15.5 x 17p Mirage that would plane at about 14 mph, but I could only turn it about 5100 rpm with the load I was running, so it was too much prop. My 4B 14.125" x 20p Michigan Apollo turned about the same rpm but planned at even lower speed. Finally ended up with a 4B 15x15 PowerTech that turns 5400-5500 with virtually any load and planes at about 12-13 mph. Another option would be to add something like a Doelfin to the AV plate, which will also add stern lift, although it costs ~ 0.2 mpg on mine.

As far as motor height, check to see where AV plate is when you're up on plane; top of it should be out of the water. Denny