"Clutch Slipping" - Mercruiser Alpha 1 Sterndrive

mlines

mlines

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31 Aug 2009
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Location
Finchampstead, Wokingham, Berks
www.sportsboat.org.uk
Went out yesterday and the sterndrive has been giving some worrying problems.

Its ok at speed and effort (towing a ringo) for about 5 minutes. Then something happens and the engine can rev freely on the throttle with no drive. Throttle right down and it will drive ok at low speeds. Leave it to rest (cool??) and you can then ringo/speed for 5 minutes until it happens again.

There are no strange noises, no heat and no smells, no gear meshing /grating noises at all. A short rest and it will work for 5 minuts. Symptoms are exactly the same as a clutch slipping/failing in a car.

There was lots of weed in the lake but no weed wrapped on the prop but would have been enough to make it heavy going.

It has been suggested that the flo-torque hub may be worn/slipping so I have taken the prop off for a look.

Before I did this I put it it gear and tried to turn the prop by hand. In one direction it was locked solid and could not be turned, howver it was free to turn in the other direction with a loud clicking from inside the leg. I assume this a bit like the "freewheel" action on a cycle wheel???

Do these pictures show anything suspicious to anyine, if not it sounds like an expensive problem, any ideas anyone??

hub1.jpg


hub2.jpg


hub3.jpg


hub4.jpg


hub5.jpg


Not 100% sure what I am looking for here, was expecting to see some kind of polish/spin marks.

flohub1.jpg


flohub2.jpg


flohub3.jpg


flohub4.jpg
 
Well not an expert on these but i would have thought that the piece that looks like rubber and the steel insert should be bonded to the prop ? or how else does the shaft drive the prop ?
 
alpha 1 doesnt have a clutch that why it grinds and clunks when it engages gear

you need a new prop by the looks of it
 
Of course the Alpha one has a clutch! A dog clutch. !t is what engages/disengages the gears. It is part #45 in THIS DIAGRAM

It does appear to be designed so that it will "free wheel" see THIS PICTURE. That explains why the prop can easily be spun one way with a clicking sound when in gear.


The photos show nothing obviously wrong with the prop assembly but only close examination will show if the various parts fit together tightly and with the prop itself, in particular the 8 ? sided sleeve in the prop.

If the prop is OK maybe the teeth on the dog clutch ( and the forward gear ) are worn so that it tends to slip out of engagement under high loads.
A not uncommon problem with outboard engines, which have a similar dog clutch mechanism.

Personally I would examine all parts of the prop hub critically. If they are all fine I'd resign myself to the possibilty that the clutch is worn.
 
I was only refering to "slipping clutch" as being the best way to describe the symptom rather than the actual cause.

Thanks for the pointers, I would have assumed that if the dog clutch engagement was slipping out under load then there would be a clunk as it re-engages, the whole effect I am experiencing does not have any mechanical noise associated with it, the slipping starts and stops "silently".

I think I will buy a new flo-torque (now an XHS kit) as its £32, seems like the cheapest place to start.

Martin
 
I think I will buy a new flo-torque (now an XHS kit) as its £32, seems like the cheapest place to start.
Click to expand...
Before you do that you should be able to verify that it is the prop hub slipping.

Scribe, or use a waterproof felt tip pen, a mark on the end of the prop shaft and put a corresponding mark on the prop.

Go for a spin until it slips then check your marks.
If they no longer line up it confirms that the prop hub is slipping and you can spend your £32 knowing that it wont be wasted.
 
Every prop that I've seen to date with the rubber 'cush drive',rather than(or as well as) a shear pin,has the rubber 'bush' imoveably bonded into the prop hub.I have not yet seen a properly working prop bush that is removeable without a hydraulic press.In my humble opinion,you need a new bush pressed into the prop.Try Castle Marine,Caernarvon.They will probably have an exchange prop in stock.If not,their turn around time is quite good.
Worn gear dogs go clunk,clunk,clunk etc.You would hear it.
Cheers
 
The rubber bush on the prop hub is designed to fail if the propeller hits something. As it gets old it can also start to slip as described in this post. Its highly unlikely the clutch itself is failing as its either engaged or not due to the dog-tooth design.

The other possible place the drive can fail in this way is the engine coupler. If it is out of alignment the rubber can fail, losing drive - but this is normally accompanied by a burning smell and complete loss of power.

In your pictures the rubber bit in the middle of the hub should not come out of the prop like that - it should be bonded in - time for a new prop by the looks of it.
 
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The parts of the Flo torq hub can easily be replaced without a press etc
See

http://www.mercurymarine.com/propellers/advantage/technologies/flo-torq/

http://www.mercurymarine.com/media/mercury/pdfs/service-and-support/props/Flo-Torq.pdf

http://www.youtube.com/watch?v=XC2iDbtJW8o

Not bonded in or anything like that either!

BTW Dog clutches do wear and slip, or at least disengage.
It is not uncommon in outboard gearboxes.
The dog becomes worn especially if it has been regularly engaged gently. Then under power jumps out of engagement.
 
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The rubber bush is shagged:

compare the state of the splines on the left ~3/4 of its length with that on the right hand ~1/4 in this pic:

flohub1.jpg


By all means verify that it's slipping by marking prop / drive spline, but I'd be 99% sure that this was your problem.

0.02p

Andy
 
The new insert kit arrived today.

Looking at them side by side and using a Vernier to measure them, the old one is between 1mm and 0.5mm smaller across various dimensions.

Also the design has changed at one end. The old one has a large internal open space in which to flex so the new one should be more rigid.

comparehub1.jpg


Here's hoping this is the solution
 
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