Prop Selection, 125HP 20' Seafari

[Max_Florida]

Well, after messing around with multiple older engines that were just problem after problem, I finally decided to just spend the money on a new motor.

I ended up going with a Mercury 115 Pro XS(rated ~125 at the prop)

The dealership that put it on gave me a 13-1/8 x 16 stainless prop, and told me I could switch it out with them until I found one that worked for me.

With 3 big guys, a full tank of gas, big cooler full of ice and fishing gear these were the performance numbers:

WOT: 5700 RPM, 37MPH
Cruise: 4000 RPM, 26 MPH

In sloppy/choppy 2ft seas it fell off plane at 3500 RPMs, or around 17mph.

The max RPM on this motor is supposed to be between 5000-5700.

I really only use this boat for spearfishing in the Gulf, typical run is ~20-25 miles and the usual conditions are 1-2ft slop. My primary concern is cruise speed/efficiency and being able to stay on plane at lower speeds when it's sloppy. I'm thinking I should try something around 14x17. Keep in mind this is the "Pro XS" model of the 115 so it's supposed to be closer to 125 at the prop.

What do you guys think?

To add:
I have the Smartcraft gauge so the tach is 100% accurate(I hope)
I will double check that the AV plate is 1.5" above the keel

[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

[mrobertson]

Max_Florida -

I just went through this as i am running a 115 2 stroke on a 20 CC. IF - you want to try either i currently have :

13 1/4 x 17
13 3/4 x 15

Both of them are Quicksilver Black Diamond aluminum props. They are like $115 new and ill cut you a deal on either or both of them. You would need the associated hub kit for your engine. Both props mint condition, the 17 pitch has probably 2 hours run time, the 15 pitch about 5-6 hours. No knicks, they are brand new

BTW - i settled on a stainless Stainless Powertech 13 3/4 x 15. For my rig the stern lifting properties are fantastic. I come out of the hole with almost zero bow rise. My boat is quick and nimble and i can cruise all day long 27 mph @ 4200 - 4400. I dont get quite the top end that you do.........33 - 34 mph is about it but i am super happy with performance of my yammie 115 2 stroke.

Let me know if you want to make a deal on either of those props and ill be glad to work with you.

-mike

[Max_Florida]

I checked the motor height today and it was about 1/4" above parallel with the bottom of the keel.

I talked to the Merc dealership and they are going to raise the motor a hole and let me try out a Trophy(4 blade) 13.75x17.

My initial thought about the 4 blade was that it would be too much prop for the motor, but if the motor gets raised a little I think it might be okay on RPMs. If the Trophy is too much prop I'll go from there.

I won't have a chance to run it until Friday. I'll report back then.

Mrobertson- Thanks for the offer. I may take you up on it depending on how this next trial goes.

[mrobertson]

Although my engine is a 2005, i bought it new in crate so it is not a rebuild. Point being, its fresh and has plenty of pep. I can turn the 17p just fine. The 15p gave me a little better holeshot and my yammie makes it power in higher RPM's so the 15p helps me get there a little quicker.

I think an opti or etec will have the balls to turn a 17p as my engine is still a traditional 2 stroke carb'd engine.

I think you'll be happy with a 13.75 x 17p prop, not sure about the 4 blade but only 1 way to find out. In any case, if you want to try out either of my aluminum props let me know and we can strike a deal.

-Mike

[mrobertson]

Also -

i have my engine cav plate mounted a full inch (maybe even a hair more) above the bottom. The only negative aspect i saw was that i can't trim the motor up as much or the prop will blow out. Other than that, i like how the engine sits up high out of the water and again my rig is very "snappy". It will surprise the hell out of you to be honest.