Prop Slipping? Maybe?

[Stan needs help!]

I've got a 1981 Johnson 15 electric start that recently started acting like a slipping clutch after I hit something under water. I can be underway at WOT and suddenly the engine will rev away. If I back off the throttle it'll resume until some random point it'll start to slip again. My first thought was the prop hub is slipping but after just basic eyeballing the rubber hub appears intact. I noticed that the prop nut was basically loose and the front and rear washers show signs of spinning against the prop I figure that it might be normal considering that the prop is designed to slip in some situations. I was wondering if there is a proper setup tension for the prop nut or is the prop just worn out after 29 years.

 

[Tacklewasher]

Re: Prop Slipping? Maybe?

Sounds like the hub is slipping, but it could be bad ventilation.

How I've checked it in the past is to make a chalk mark on the prop where the bend of the cotter pin is. Run for a while and look to see if the chalk mark has moved in relation to the cotter pin. If it has, you need your hub fixed.

 

[jasper60103]

Re: Prop Slipping? Maybe?

I second spun hub. Likely gave way when you hit something in the water.

 

[QC]

Re: Prop Slipping? Maybe?

I agree, but the fact you hit something tells me the blades are probably damaged. Ventilation can act very strangely and be inconsistent, damaged blades could easily result in Ventilation that acts like what you describe. A simple wake or other random event could start it and then slowing could stop it.

I guess the point is that I would not jump off a building if the hub tests good (chalk or scribe method) . . . Pics?

 

[Stan needs help!]

Re: Prop Slipping? Maybe?

Thanks to all who responded. I think I'm looking at a new prop but was still unsure about the tension setup on the prop nut.

 

[tashasdaddy]

Re: Prop Slipping? Maybe?

tighten it down firmly, the back off so the cotter pin fits the hole. spun prop, or clutch dog. do the spun prop test before buying a prop. no need to buy a prop if the clutch dog is slipping.

take a permanent marker, draw a straight line, to match cotter pin. like the picture. take the boat for a spin, make it do what it is doing. if the lines are still line up, it is not the prop, if misaligned, the hub in the prop has slipped, and need to have a new hub installed by a prop shop, or a new prop.