Prop Heigth Question

[T6X7]

I have a 16' Alumacraft 1970's. Just picked up a Johnson 20 hp 1966.
Took it out for run and noticed when I have it wide open, I get to a certain speed and feel a jerking sensation. Like it's missing or maybe the prop isn't low enough in the water, and it's slipping. Is this possible? Is there a certain depth the prop and water inlet has to be, below the bottom of the boat?

Thanks folks. I'm new at this boating thing.

 

[Fireman431]

Re: Prop Heigth Question

The anti-cavitation plate (large flat thing above the prop) should be just below the water line while on a plane. Then you can fine tune it with trimming the engine up or down. Good place to start.

 

[T6X7]

Re: Prop Heigth Question

Thanks for the help.

 

[steelespike]

Re: Prop Heigth Question

I have a 16' Alumacraft 1970's. Just picked up a Johnson 20 hp 1966.
Took it out for run and noticed when I have it wide open, I get to a certain speed and feel a jerking sensation. Like it's missing or maybe the prop isn't low enough in the water, and it's slipping. Is this possible? Is there a certain depth the prop and water inlet has to be, below the bottom of the boat?

Thanks folks. I'm new at this boating thing.

If the jerking feels kind of like you hit something you may have a worn clutch dog.

 

[T6X7]

Re: Prop Heigth Question

It does feel like it hit something. The first time it happen, I thought I had run over something.

Is this the animal?

http://forums.iboats.com/attachment.php?attachmentid=13674&stc=1&d=1213142451

 

[steelespike]

Re: Prop Heigth Question

It does feel like it hit something. The first time it happen, I thought I had run over something.

Is this the animal?

http://forums.iboats.com/attachment.php?attachmentid=13674&stc=1&d=1213142451

Thats it.A relatively simple repair On some motors you can reverse the dog
so the less worn reverse side is in the forward side.
Be sure the motor idles down well and try to shift crisply neither jamming it into gear nor dragging it in.This will extend clutch dog life.

 

[T6X7]

Re: Prop Heigth Question

Thanks alot steelespike. I believe you have nailed this right on. I haven't tried looking for parts for this thing, but I imagine its not going to be easy.
You don't know of any place that sells part for this old of a motor do you?

FD20hp Johnson 1966 seahorse