Gearbox ratio problem

[stuart23]

A few years ago I had to replace my Hurth HBW 100 gearbox, they had stopped making the HBW 100 but I was told the ZF 10m was it's replacement. What I didn't realise was that they had changed the ratio from 1.79 to 2.02 resulting in the boat going slower.

I'm now looking to resolve this problem.

I can try and change the propellor (someone at Uxbridge boats reckons they can add a bit to the existing propellor to compensate)

or I can replace the gearbox. ...either by finding an old original HBW 100 or trying to find a suitable replacement.

Both routes have problems.....to change the gearbox the engine has to come out...to change the propeller means lifting the boat out...and then hoping the propellor will come off

Has anyone any advise on this? If I wanted to replace the gearbox how would I find a suitable replacement?

[kris88]

If I wanted to replace the gearbox how would I find a suitable replacement?

eBay , personally I think I'd go for change of prop.

Regards kris

[furnessvale]

eBay , personally I think I'd go for change of prop.

Regards kris

Easiest option provided the engine remains within its optimum power and torque range at whatever the new revs will be.

 

George ex nb Alton retired

[BEngo]

Change the blade. Replacing an obsolete gearbox with another obsolete one is not sensible and all the work on mountings, controls etc. for the ZF will have been wasted or might even need to be re-done to get a Hurth box back in. The greater reduction ratio is probably a better idea for canal work anyway, if matched to the right blade.

 

You don't need to take the boat out of the water- just take the rudder off and push the prop and shaft backwards out of the stern tube with a dummy piece of shaft, leaving the dummy shaft in the stern tube to keep the water out. Change the prop on the bank and re-fit the whole prop and shaft, pushing the dummy shaft back into the boat.

 

N

[stuart23]

Thanks for the replies

[RLWP]

A few years ago I had to replace my Hurth HBW 100 gearbox, they had stopped making the HBW 100 but I was told the ZF 10m was it's replacement. What I didn't realise was that they had changed the ratio from 1.79 to 2.02 resulting in the boat going slower.

 

That isn't strictly accurate. The ZF10M is available with 1.5, 1.8, 2 and 3:1 ratios: http://www.bukh-bremen.de/files/346/upload/Hersteller/ZF/Reparaturanleitung%20u%20Ersatzteilliste%20ZF3M%20bis%20ZF30M%20mechanisch.pdf(Page 10)

 

Richard

[Alan de Enfield]

I do sympathise to some extent. We have a 3:1 ratio gearbox and we are at quite high revs to achieve 4mph (around 2000 rpm) - but - I don't see the 'problem', so maybe I am not understanding the mechanics of it.

 

Your prop is quite happy pushing the boat along, it is not over or underpropped (presumably) the only difference is the revolutions the prop is achieving compared to the engine.

 

At (say) 500 (prop) RPM the boat is doing 3mph.

It does not matter to the prop (or its capabilities) whether the engine is doing 1500rpm or 2500 rpm - the prop is still pushing the same boat and doing the same RPM

 

Why would you need to change the prop ?

 

My logic is that on my boat with a 3:1 gearbox running the engine at 1500 rpm is going to give the prop 500 rpm - using a 2:1 gearbox running the engine at 1000 rpm is going to give the prop 500 rpm.

 

In effect all that you are doing is running the engine faster than you need and burning up extra diesel.

 

Is the thought pattern flawed, and if so why ?

[RLWP]

Your logic is correct. I find the higher revving engine on a 3:1 box irritating, so personal preference is also a factor

 

Richard

[Alan de Enfield]

Your logic is correct. I find the higher revving engine on a 3:1 box irritating, so personal preference is also a factor

 

Richard

 

Thanks - I just wondered if I was missing something when the 2nd post said "change the prop"

 

 

eBay , personally I think I'd go for change of prop.

Regards kris

 

I have now sourced a rebuilt PRM150 at 2:1 so its just a case of swapping them over and then finding a home for a PRM 150 at 3:1

[kris88]

I did say "personally", out of the two options the op was offering changing gearbox or prop I'd still go for prop. Sorry for confusion.

Regards kris

[Timleech]

 

Thanks - I just wondered if I was missing something when the 2nd post said "change the prop"

 

 

 

I have now sourced a rebuilt PRM150 at 2:1 so its just a case of swapping them over and then finding a home for a PRM 150 at 3:1

 

Do't you have room for a bigger prop? You shouldn't need to have to rev the engine faster just because of the higher reduction ratio.

In general, if you have room to match prop to engine the higher reduction will give better and more efficient results.

 

Tim

Edited March 12, 2015 by Timleech

[Tony Dunkley]

 

Do't you have room for a bigger prop? You shouldn't need to have to rev the engine faster just because of the higher reduction ratio.

In general, if you have room to match prop to engine the higher reduction will give better and more efficient results.

 

Tim

 

Unless it's already been done he could have the existing prop re-pitched to put a bit more lift on it, and that would bring the revs back down to what they were before for the same boat speed.

[RLWP]

 

Unless it's already been done he could have the existing prop re-pitched to put a bit more lift on it, and that would bring the revs back down to what they were before for the same boat speed.

 

I suspect that is what 'someone at Uxbridge boats' is proposing to do. It makes more sense than 'adding a bit on'

 

Richard