115 4 Stroke surging in midrange RPM

[Shawn200m]

2020 115 4 stroke, 2B719706.

Recently when I've been running in the mid range 4000-4500 RPM range with the engine just trimmed enough to keep from plowing water it's been acting like its cavitating. The RPMs jump to 5000 and the engine jumps around. The only way I can get out of it is to trim all the way down or slow down. If I run WOT and trim all the way up this doesn't happen, only in that mid range around 30-35 mph. Is it actually cavitating? Should I be running a different prop? I'm running the factory 4 blade spitfire 13x17.
Link to video of it running yesterday from the beginning of it happening through to recovery.

[fishon396]

I have a 2016 Mercury 75 hp 4 stoke and it came with the same prop you have. I would think the 115 would have a bigger prop.

[ourflat]

Prop blowout! My pontoon boat had the same problem when purchased new a couple of years ago. I ended up with a four blade Merc prop on my 115. I sacrificed speed for control and no prop blowout!

Frank

[SilverFox579]

Engines are shipped without a prop so whatever prop you have was chosen by the dealer who rigged the boat. I'd contact them about options. If you provide boat information and the optional data from the Announcement at top of forum someone here may be able to help with prop selection.

[Shawn200m]

Engines are shipped without a prop so whatever prop you have was chosen by the dealer who rigged the boat. I'd contact them about options. If you provide boat information and the optional data from the Announcement at top of forum someone here may be able to help with prop selection.

Thanks, I didn't realize that the dealer were the ones who select and rig the props. It's on an 18' Lund aluminum Impact. Currently running the Spitfire 4 blade 13x17. WOT RPMs are around 5700, up to 6000 if I really trim up as far as I can. The cavitating/blow out always happens around that 4500 RPM range when I'm not trimmed all the way down. WOT trimmed all the way up there's no issue.

[EuropeanAM]

Engines are shipped without a prop so whatever prop you have was chosen by the dealer who rigged the boat. I'd contact them about options. If you provide boat information and the optional data from the Announcement at top of forum someone here may be able to help with prop selection.

+1

This does NOT mean the prop you have is a bad choice for your rig. Different blade styles or prop models will perform differently- the symptoms you experienced in the above video are relatively normal for this prop under the conditions you commanded.

You will find that if you drop the trim a bit more, this becomes less likely when accelerating out of a turn. Trimming down keeps the blades in "cleaner" water (less air or aeration) and provides a better bite.

Hope this helps!

[The gman]

My 2020 tracker with a 115 and a spitfire x7 19 pitch does the same thing. Could dropping the motor help this condition? It is mounted in the 2nd hole at 4 inches pad to prop.

[EuropeanAM]

Possibly... depending on what Tracker it's on.

[The gman]

My 115is on a 2021 pro team 195 , I dropped the motor down to the lowest hole and got rid of the prop chop and lost nothing on speed.

[Shawn200m]

Wanted to update this. I found an old Vengeance 18p SS prop in the garage from an old boat and decided to swap out the spitfire 17p that I'd been running and it is a night and day difference. I can now run all throttle ranges and the full trim range with zero blow out. I gained 3 mph and run right at 6000 rpm WOT. I'm blown away that there's this dramatic of a performance difference. I'm sure the aluminum Spitfire has applications where it works otherwise it wouldn't get installed by dealers, but on my rig it really was a dog and dragged down the performance of the whole set up except for slightly improved hole shot. It's like a new engine now.
.

[EuropeanAM]

And just think- the Vengeance is essentially an "entry level" stainless prop.