High Reverse Cup fitting Mercury 8HP OB (Several Pictures)

[davidpbo]

Mercury 8HP 1989 vintage.

We have always had problems with controlling our boat when reversing. The outboard is offset to starboard which combines with the effect of prop wash. I have tried different props one was very good for thrust in reverse but unfortunately got lost when I left it on the car park at Largs having swapped it for one that was better for forward speed.

I have been doing some work on the engine and whilst ordering spares came across something called a high reverse thrust cup

It replaces the spacer between the prop and the prop nut. I believe it allows the propeller to slide on the splines such that when in reverse the prop moves back

and shuts off the exhaust exit through the centre of the prop meaning the prop is biting into less turbulent water, the exhaust would appear to exit from behind the prop.

When going forward the prop is pushed forward and the normal exhaust channel is open.

At least that is how I think it works, I am open to correction.

The fitting instructions are telling me to drill out the splines with a 13mm drill to a depth of 13mm. I can’t see why because when the prop is in reverse the cup would be seated tightly sealing the exhaust gasses anyway.

Anyone know whether I need to?

Is this the tool for removing the prop shaft etc. or the correct spacer with a fish line cutter?

Anyone know whether this part should be in there?

Not shown in manual, mechanic at outboard shop had no knowledge of it, I put it over what I know think is the line cutter last year but can't see what it does and wonder whether it is anything to do with this part of the engine.

Any input appreciated.

 

[rex_seadog]

In our previous boat, A Hunter Delta 25, we had a Mariner 8 (prettly well identical to the Mercury) with the high reverse thrust cup. We bought the engine new in about 1999 with the cup already fitted and so no modification was required. From what I remember, however, it was possible to revert to the standard fixed prop set-up by replacing the cup with a washer or spacer. Our engine was also offset and found the increased thrust in reverse to be quite effective.

We also found that many dealers were not aware of this cup and your best bet is to contact the distributors, E P Barrus at 01869 363636. We always found Tim Bennett, the technical expert, to be most helpful although it is several years since we had any dealings with him so I don't know if he's still there.

One problem we had with this set-up was that over a period of time the bronze splined tube tended to slide out a few mm relative to the rubber bush surrounding it. I put this down to the movement of the prop moving backwards and forwards when changing from forward to reverse. We had the prop replaced under warranty but even the replacement suffered slightly from this problem- perhaps they came from a faulty batch as I never heard of anyone else having a similar problem. At the time I posted several times on this forum but I'm not sure if you can search that far back.

Hope this helps.

 

[davidpbo]

I will be able to revert to the original with mine using the existing spacer if I need to. I am glad you found it effective. I hope I will.

My local dealer certainly did not know of it.

With reference to the bronze splined tube mentioned, I presume you mean the one inside the prop.

Was the cup the same as the one in my first picture, I am a little concerned that as supplied the metal collar which fits over the prop shaft splines is cracked slightly. I would have expected the inner to be splined as it must rotate slightly on the splines of the shaft when under way without.

 

[rex_seadog]

Have sent a PM with more info.

 

[davidpbo]

I have subsequently found out there are two designs for the gear box and prop shaft, types I and II mine is type I and various parts are different including the oil seall and bushing.

 

[gsh1]

Prop removal

Sorry to drag up this old thread, but I have been searching for information of how to remove the propeller on these engines.

I have removed the nut, but there is another hex spacer between the nut and the prop. Everything is a bit "stuck" and I don't know whether to pull or turn this spacer to remove it. I think it sits on the prop shaft splines, but I would like confirmation of this before giving it some welly!

Anyone?

 

[davidpbo]

I haven't got my manual to hand.

Any chance of a photo of what you talking about?

As far as I can recall anything on the spline slides up an down, |I may have removed the hex spacer when I fitted the the reverse cup.

I would suggest a good soaking with WD40 or (Apparently) better still Plus Gas or a penetrating oil. As I understand it WD40 is a water dispersant with penetrating and releasing properties. Plus Gas is a penetrating oil.

 

[xeitosaphil]

David I used to have a Merc 8hp when I had a Fin Keeled Hurley 22 and found it most useful. I converted it to a Merc sail 8 spec by adding a charging rectifier, and high thrust prop. The cup is indeed to stop the exhaust going through the centre of the prop which would counteract the thrust produced in trying to go astern. When in forward this exhaust aids the forward thrust.
It is some time ago now I had the H22 so memory fails a little when it comes to the details so have uploaded a schematic of the parts list for the lower end of the drive, so you can see the orientation and order of the parts assembly. If it doesn’t blow up so you can see the detail clearly, if you PM me your email address I will send you a Scanned copy which should be better? Hope this helps?

Mercury 8HP 1989 vintage.

We have always had problems with controlling our boat when reversing. The outboard is offset to starboard which combines with the effect of prop wash. I have tried different props one was very good for thrust in reverse but unfortunately got lost when I left it on the car park at Largs having swapped it for one that was better for forward speed.

I have been doing some work on the engine and whilst ordering spares came across something called a high reverse thrust cup

It replaces the spacer between the prop and the prop nut. I believe it allows the propeller to slide on the splines such that when in reverse the prop moves back

and shuts off the exhaust exit through the centre of the prop meaning the prop is biting into less turbulent water, the exhaust would appear to exit from behind the prop.

When going forward the prop is pushed forward and the normal exhaust channel is open.

At least that is how I think it works, I am open to correction.

The fitting instructions are telling me to drill out the splines with a 13mm drill to a depth of 13mm. I can’t see why because when the prop is in reverse the cup would be seated tightly sealing the exhaust gasses anyway.

Anyone know whether I need to?

Is this the tool for removing the prop shaft etc. or the correct spacer with a fish line cutter?

Anyone know whether this part should be in there?

Not shown in manual, mechanic at outboard shop had no knowledge of it, I put it over what I know think is the line cutter last year but can't see what it does and wonder whether it is anything to do with this part of the engine.

Any input appreciated.