Outboard Motor Issue

[dennishoddy]

Thanks DS. They're close and I'll give them a shop.

Dennis, I'll double check, but it's likely the same place.

I don't know a ton about boats, but the guy I bought it from said he put a smaller prop on to help the boat launch faster vs higher top speed. It sounded weird to me, but I'm curious as to if the prop, even though it fits the motor, is too small for the weight of the boat/gear, thus causing the prop to spin on the hub? I think it's a 9.5 degree prop on a 25hp mercury and a 1652 lowe.

That is true. A lower pitch prop will get you out of the water and on plane a lot quicker, but the top speed will suffer.
The prop is vulcanized to the hub.(think glued) if it gets in too much contact with mud, trees or rocks, the prop breaks that "glue" and the prop will spin on the hub. Weight on the boat will have nothing to do with it. (simplified version)
It's either "glued" or not, not a slip joint. The hub then needs to be replaced with another one and you're good to go.

 

[makeithappen]

Hmmm. Well, it only feels like it slips if I'm more than half throttle or making a hard turn. Guess it's worth taking the prop in to get checked out at last.

 

[dennishoddy]

Hmmm. Well, it only feels like it slips if I'm more than half throttle or making a hard turn. Guess it's worth taking the prop in to get checked out at last.

Cost nothing to get it checked last time I was there.

 

[okie362]

That is true. A lower pitch prop will get you out of the water and on plane a lot quicker, but the top speed will suffer.
The prop is vulcanized to the hub.(think glued) if it gets in too much contact with mud, trees or rocks, the prop breaks that "glue" and the prop will spin on the hub. Weight on the boat will have nothing to do with it. (simplified version)
It's either "glued" or not, not a slip joint. The hub then needs to be replaced with another one and you're good to go.

FYI...New props (Mercury) are not vulcanized. They have a Hex (Could be oct can't remember) hub insert that can slip and recover. I know this because I bought a SS prop for mine and when I took it out of the box the hub fell to the floor and I was pissed util I took a closer look.

Edit: Mine is a 40HP

 

[DFX]

Do the engines RPM's increase when symptoms occur ?
Do you have to put it back into gear after it slips out.
What year model is the motor?
I'll ask the Tech at the shop I work at for a solution.
It is either shift cable adjustment, prop slippage or cavitation.

 

[makeithappen]

Do the engines RPM's increase when symptoms occur ?
Do you have to put it back into gear after it slips out.
What year model is the motor?
I'll ask the Tech at the shop I work at for a solution.
It is either shift cable adjustment, prop slippage or cavitation.

I'd really appreciate that! It's a 92 25hp mercury 2 stroke.

When it first started, last winter, the RPMs would increase with the prop feeling like it disengaged, but would engage again in a second or two without having to do anything. It's gradually gotten worse. Now I can't get above half throttle without it happening. I can sometimes ease it to full throttle, but the RPMs will eventually rev and forward momentum is lost. I can force the issue to occur by making a hard turn. I only have to ease back on the throttle to get the prop to reengage / gain forward momentum again.

Lower unit gears are good. The guy I bought the boat from put on a smaller prop for faster acceleration (9.5 degree I believe). I'm leaning toward the prop at this point. I checked the shift cables and everything looks good and lined up / tight.

 

[doctorjj]

I'd really appreciate that! It's a 92 25hp mercury 2 stroke.

When it first started, last winter, the RPMs would increase with the prop feeling like it disengaged, but would engage again in a second or two without having to do anything. It's gradually gotten worse. Now I can't get above half throttle without it happening. I can sometimes ease it to full throttle, but the RPMs will eventually rev and forward momentum is lost. I can force the issue to occur by making a hard turn. I only have to ease back on the throttle to get the prop to reengage / gain forward momentum again.

Lower unit gears are good. The guy I bought the boat from put on a smaller prop for faster acceleration (9.5 degree I believe). I'm leaning toward the prop at this point. I checked the shift cables and everything looks good and lined up / tight.

That sounds like a slipping hub for sure.

 

[dennishoddy]

I'd really appreciate that! It's a 92 25hp mercury 2 stroke.

When it first started, last winter, the RPMs would increase with the prop feeling like it disengaged, but would engage again in a second or two without having to do anything. It's gradually gotten worse. Now I can't get above half throttle without it happening. I can sometimes ease it to full throttle, but the RPMs will eventually rev and forward momentum is lost. I can force the issue to occur by making a hard turn. I only have to ease back on the throttle to get the prop to reengage / gain forward momentum again.

Lower unit gears are good. The guy I bought the boat from put on a smaller prop for faster acceleration (9.5 degree I believe). I'm leaning toward the prop at this point. I checked the shift cables and everything looks good and lined up / tight.

If all else fails, you could come up to Ponca with your boat and we could fish Kaw Lake in my boat while testing yours.
I have a 20hp merc on a smaller boat. We could try swapping props to see if it fixes your problem. I'd have to check, but think the output splines are the same.

 

[DFX]

It's not a shifter problem. More likely the prop.
A lower than normal prop can wash out like you describe or being an aftermarket it may need rebuilt or replaced.
I'll post what the tech has to say asap tomorrow.
If it's the prop go to Boat City Prop Shop 832 SE 44th in OKC.

 

[okie362]

Based on the fact it happens during a turn I would guess possible cavitation. Try lowering the trim pin one notch as I assume it doesn't have power trim.