Merc 115 dog cluth or prop?

[Booyah Bass]

I have a Mercury (possibly mid to late 70's) 115 HP. I have been rebuilding boat and took to the water today. When I put it in forward gear the engine will rev up and boat tries to go but moves about as fast as the trolling motor. Do I have a dog clutch slipping? When I am on land - I put it in forward gear and it makes a stiff ratchet sound both ways. When I put it in reverse it will lock into place one way. I tried with it in reverse to turn the prop, I thought possibly I had spun the prop - it doesn't seem to move. Any words of wisdom??

 

[oldgradywhite]

Re: Merc 115 dog cluth or prop?

I haven't heard of clutch dogs in a lower unit yet? But it is normal to hear a ratcheting sound when spinning the prop when the motor is forward gear. Your props hub should be made of plastic I would remove it and take a look at it. Other wise it sounds like a classic blown lower unit.

 

[CharlieB]

Re: Merc 115 dog cluth or prop?

The engagement 'dogs' on the forward gear and shift slider are 'ramped' so they act as a one-way clutch allowing the motor to turn the prop shaft but when the prop shaft attempts to turn the motor the dogs 'cam' up and over into the next step, ratcheting, so to speak.

Reverse gear and the reverse gear side of the shift slider, the 'dogs' are straight cut, and positively lock.

This should help you understand why when you turn the prop in Forward gear, it will only turn one way, and ratchet the other.

Your problem with the motor revving but no, or little forward thrust is classic of a 'spun hub'. The rubber 'shock' hub between the prop and the prop splined hub, is slipping under power. It may hold sufficient that you cannot turn it by hand, but when you throttle up and start putting serious load, horsepower, against it, it then fails, the hub then spins 'inside' of the prop.

The easiest test is to draw a straight line across the prop and hub, run it briefly (in the water) then look to see if your line still lines up.

Once you see that it has spun, take the prop to a prop shop, they will press out the old hub and press in a new one, usually far cheaper than replacing your prop.

 

[Moody Blue]

Re: Merc 115 dog cluth or prop?

I haven't heard of clutch dogs in a lower unit yet?

Around here you learn something new every day

Take a look at this good video that shows how the LU gears and clutchdog operate.

http://www.youtube.com/watch?v=vuAcrDFvApA

 

[Booyah Bass]

Re: Merc 115 dog cluth or prop?

Thanks for the quick reply. I stopped by a boat shop on the way back and they seem to think that the prop isn't spun. I will take your advice first about how to check for a spun prop before I decide to tear into the lower unit.
Are you saying that when the motor is in forward gear that when i turn the prop it should make a ratchet sound turned both ways? and when I put it in reverse it should lock in when turned one way and ratchet 1/4 turn then stop the other way?

 

[Chris1956]

Re: Merc 115 dog cluth or prop?

If it rachets both ways (motor not running) while in forward, you either have a shift cable adjustment problem or a bad clutch dog.

I recommend you disconect the shift cable. Now slide the shift arm forward until it settles in forward. Now try to turn the prop (motor off) by hand. It should rachet in the CW direction only. If it does this, move the Mercontrol to forward and adjust the barren nut on the shifter to fit the clamp, with the shift cable reconnected. Do not move the shifter arm, adjust the shift cable.

 

[Booyah Bass]

Re: Merc 115 dog cluth or prop?

When Ilook at the prop I do not see anything on the hub that is plastic? I have a stainless prop.

 

[Moody Blue]

Re: Merc 115 dog cluth or prop?

From the center out ....

Prop shaft, bronze(?) splined sleeve, rubber (not plastic) bushing, prop hub.

Draw a line across the rubber bushing and onto the prop hub. You want to verify the position of the rubber bushing relative to the prop hub. If the mark on the rubber bushing and prop hub are no longer aligned after going for a run then it is a spun hub.

 

[Booyah Bass]

Re: Merc 115 dog cluth or prop?

Thanks for the help!! It turned out that the spring had broke on the dog clutch. This also caused it to round off the ears on the forward and reverse gears. I was lucky enough to find a lower unit off of a 150 and switched them. Works great now. Thanks for the info.
Have a great Father's Day!

 

[Moody Blue]

Re: Merc 115 dog cluth or prop?

Glad to hear you got it resolved. Thanks for taking the time to post back with the fix.

Happy Fathers Day to you as well.