WOT going straight to 7k

[PartyBarge]

Good evening all. I’m new to the forum and also the Bennington pontoons. Recently picked up a 2013 Bennington 2375 GCW with Mercury 115. Cruise around the lake all day no problem, wanted to open her up, going full throttle RPM gauge goes right on to 7k. Download a RPM app, got 16MPH max. All this was with 4 adults and 4 small kids 1/2 to 3/4 tank. Boat would not plain out whatsoever. Pulled the prop today to check bushing. All seems solid and no spinning (basically corroded together). I’m stumped. Prop is a BlackMax 77342A45 15P. Engine is clean, fires up perfectly, idles perfectly, accelerates great. Keeping it between 5-6k rpm, I’m running 13mph. Am I missing something?

I haven't looked through the entire thread, so maybe this has been covered: have you check the tach setting? Most have a selector on the back for different motors (2.3.4 cyl, 2 or 4 cycle). Wrong OR corroded selector/contacts will give false RPM readings. As mentioned before, 15p for your setup should be too much pitch. IF the RPM real, there is something wrong with the prop, as in modified, poor choice for a pontoon or damaged.

 

[JJmathis]

I assume you have a 115hp 4 stroke? Its not a 2 stroke is it? They still made the optimax DFI back in those years. Reason why i ask is RPM range. 4 stroke is 5300-6000 WOT and the Opti is Up to 5750 or something like that.

SO it your hitting 7000, you well over-revving the motor and shouldn't be anywhere near that with a 15 pitch prop. I would think you have a bad hub that won't slip under light conditions but will fail under any heavy load / torque of the motor.

If you leave the trim all the way down or 3/4 of the way down so the nose of the boat is in the water, can you still hit those RPM? That can help rule out any cavitation / ventilation issues. Just make sure you do it in calm water as you don't want to take a wave over the decking and dent any of your panels or gate.

Do you know what hole the motor is mounted in?

It is the 4 stroke engine. I took the original prop off, pressed out the splined sleeve, don’t think it’s ever been apart, hoping the bushing was in pieces, it’s solid, corroded in place, was going to press it out but figured I’d wait. It doesn’t look to be spun or has even spun. I played with the trim a good bit on the water and it would hit 7k each time. Like automatically go straight up to 7 as soon as the throttle is pressed forward. The engine is mounted in the top hole, it is as low as it can go.

 

[JJmathis]

I assume you downloaded a boat speed (mph) app not engine speed (rpm). Your boat speed is somewhat proportional to engine speed. This would cause me to rule out propeller aeration (cavitation). Are you sure you got a prop with a 15 inch pitch. That does not seen to add up.

I did download a mph app. Not sure if it was strictly for boats, but it was dead on with my truck on the way there. I’m positive it is a 15p prop.

 

[Potomacbassin’]

If the prop is not sitting on the shaft properly (i.e. incorrect thrust washer etc) exhaust gas can escape through the gap and then aerate the water around the prop. This will spin up the RPM without providing maximum thrust.

Just a possibility. The parts shotgun approach is loathed by many, but it would eliminate a lot of possibilities if you buy a new prop, hub and washers.

 

[Joeb14]

I haven't looked through the entire thread, so maybe this has been covered: have you check the tach setting? Most have a selector on the back for different motors (2.3.4 cyl, 2 or 4 cycle). Wrong OR corroded selector/contacts will give false RPM readings. As mentioned before, 15p for your setup should be too much pitch. IF the RPM real, there is something wrong with the prop, as in modified, poor choice for a pontoon or damaged.

Yes, check this. When I got my boat brand new the revs were extremely high, much more than max revs told to me by Mercury factory rep. Called service dept, they had me adjust the dip switch on back of tach.

 

[PartyBarge]

The prop you listed has a user-replaceable insert (not a pressed-in hub). Replacement inserts are readily available, although it sounds like you don't need to do that. Still appears you need a reliable RPM reading, it's possible the existing tach has some internal failure.