RPM surge at WOT

[legalfee]

I recently got my 95 Sylvan 824e pontoons cleaned, drained, patched, and pressure tested. The boat went from 5600 RPMs WOT at 18 MPH to almost 6000 RPMs and 22.5 MPH. Running at 5500 RPMs in 6" chop I noticed the boat would run at 5500 for 4 or 5 seconds then surge to 5600 for a second then back to 5500. Trimming did not seem to make a difference (I had it trimmed all the way down). I'm running a 15p prop. Would adjusting the prop pitch or motor height help?

 

[wahlejim]

It sure sounds like air is getting at your propeller in the 6" Chop and higher speeds. Motor height would be my first guess. Any other change to the boat other than cleaning? Has it done this before?

What size outboard are you running?

My only other guess would be to check a hub slip in the prop.

 

[legalfee]

It sure sounds like air is getting at your propeller in the 6" Chop and higher speeds. Motor height would be my first guess. Any other change to the boat other than cleaning? Has it done this before?

What size outboard are you running?

My only other guess would be to check a hub slip in the prop.

It's a Force 90hp with a 25" shaft. This is the first time it's done it. It just came from the shop and they did 100 hour service and worked on the lower unit (replaced clutch dog, etc...). I thought the same thing maybe the prop was coming out of the water and getting air. The prop and hub are only a month old.

 

[wahlejim]

Is it the same prop or a different one? Ported or not?

If this one is ported and the other was not, it may be the cause of your problem.

I would start at the prop. Do you still have the old one? Maybe put it back on and see if it still happens? Also, the prop does not need to be out of the water to suck air in. If your pontoon is shooting air down behind the boat into the prop, it will act similar to what you are describing. It doesn't take much, it happens to me once in awhile if my weight isn't distributed correctly (i have a 15 passenger toon and if the adults are in the front and kids in the back, it will happen)

 

[Scott Danforth]

if its prop coming out of the water, add a cooler of beer or have your buddy stand in the back. that should calm that down

if not, it could fuel related and you be running lean on the top carb

 

[legalfee]

It didn't do it on calm water and just me in the boat. It must be related to chop and weight up front.