94’ 15hp

[carsonmajure]

I have a 1995 Mariner/Mercury 15 hp that is having symptoms of a bad prop hub, but I replaced it and it’s still doing it and the hub is in perfect condition.

The motor will work for a little then begin to slip and rev up and lose traction. I can drive slightly above idle but as soon as I give it some throttle it takes off for .00001 seconds then starts revving up but doesn’t move the boat anymore.

The prop shaft splines are in good shape as the propellor itself, the hub is brand new and even after these symptoms still has no damage internally to the plastic/rubber. I have not checked the fluid yet, but I am just curious what the hell could cause this inside the LU? It seems that if it was slipping in and out of gear I would feel the boat jolting/shaking or hear a noise, or if a gear/driveshaft was stripped I wouldn’t be able to move the boat at all, let alone under any load.

 

[racerone]

Is it a short shaft motor on a 20" transom?----How did it run for you last season?

 

[carsonmajure]

Is it a short shaft motor on a 20" transom?----How did it run for you last season?

It’s a short shaft. I used this motor on my Jon boat all season and it worked great. However, this boat is a new boat I rebuilt its fiberglass but still the same dimensions 14x4ft. The transom is a tad bit taller, but the cavitation plate still sits about 1-2 inches below the bottom of the hull.

I would think it’s cavitation but then how did the boat work fine for about a mile or so at 20mph then all the sudden start having issues? I stopped the boat and put it in neutral several times before moving again but the issue persisted and I couldn’t get it past maybe 5-10 mph after that without it revving up super high.

 

[flyingscott]

What do the splines on the propshaft look like? What do you mean new Hub?

 

[carsonmajure]

What do the splines on the propshaft look like? What do you mean new Hub?

The splines inside the prop and on the proppeler shaft itself are undamaged. The new hub was hammered in, it’s a small rubber fitting that goes inside the prop with 3 grooves that fit in the prop.

 

[tphoyt]

Mark the hub and the prop and take it for a spin to confirm or deny the prop is slipping.

 

[carsonmajure]

Mark the hub and the prop and take it for a spin to confirm or deny the prop is slipping.

I did that, it ain’t spinning

 

[racerone]

Seems to me that the boat / height of the transom is the issue.---As per post # 2.-----

 

[carsonmajure]

Seems to me that the boat / height of the transom is the issue.---As per post # 2.-----

That’s what I was thinking, but hear me out on this. #1 The boat ran great for about half mile at WOT and then started having the issue and I had to idle back to the ramp. If the transom height was the issue, wouldn’t it start revving up/cavitating/ventilating immediately?

#2 The cavitation plate still sits about 1-2inches below the bottom of the boat so I know it’s underwater. AND, the prop acts like it loses traction even at slower/medium speeds when the boat isn’t planed out, in other words when the motor is still farther under water than when planed out and the issue persists.

 

[racerone]

I do not believe that gears . splines and clutch dogs will STRIP and fail and not recover to push the boat at slower speeds.