how to replace a non-reduction gearbox?

[chal]

I am restoring an old gaffer on a very tight budget, and unfortunately the engine has died and I need to replace it (it's about 40 years old and not worth the expense of a rebuild).

I am thinking in terns of spending about £2000 on a recon diesel of about 25HP and installing it myself to save some money. Ideally, I'd like something as close as possible to a drop-in replacement for the existing engine (an Albin AD21, rated 22HP@3000rpm). I'd prefer not to have to make any changes at all behind the prop shaft coupling to try to minimise both complexity and expense.

The Albin has a planetary transmission and does not have the optional reduction unit fitted, so the prop shaft is normally running at engine speed afaik. It was all professionally installed when new by a marine engineer I know had a good reputation, so I presume the prop (fixed 3 blade) was chosen to run at the rev range you'd get with this transmission.

The only gearboxes I have seen have reduction ratios of mostly between 2:1 and 3:1. They also seem to have an output shaft that is a few inches lower that the drive shaft, which would give me an additional problem of having to mount the engine higher up (the Albin gearbox has output shaft at same height as drive shaft).

Can anyone suggest a solution (preferably bearing in mind that any additional costs would mean there would be something else I couldn't afford!).

 

[scottie]

There were a few moody s build before saildrives with direct drive 1:1 gearboxes to get the engine in maybe some thing from them

 

[caiman]

Are you sure there is no reduction through the 'planetary', presumably epicyclic gearbox? I'm not questioning your 'knowledge' but some gearboxes achieve their reduction epicyclicly(spelling) to maintain the same output shaft height as the engine crank.Have you tried turning the input shaft one revolution and compared it to the output shaft revolution?
Don't wanna start a fight,just possibly give you some help.What make gearbox is it?
Cheers

edit-count the turns when in gear

 

[chal]

thanks for your suggestion - I really have no knowledge of these things at all so just assuming same rotation speeds as that's what the documentation seems to imply

the gearbox is also an Albin and clearly was designed to pretty much "go with" the engine, as the engine manual talks about it as "the" gearbox, and the rear engine mounts are on the gearbox; there's no mention in the documentation of any alternative options except for the optional 2.04:1 reduction gear

one mistake I have made is in saying engine speed at max power is 3000rpm, but looking again at the manual it says 2400, so perhaps engine speed is normally quite low

I will certainly test for speed reduction as you suggest - it would solve one problem!

 

[caiman]

Had a quick 'Google'-my mistake,the F-N-R box on the Albin AD2 has a reverse 'planetary' gear,and an option of an 'external' reduction box.There are 2 reduction boxes available 2.1/1 and 2.7/1.If you fit the reduction box,as well as mounting the engine higher, you will need a new prop because the output shaft rotation is changed.
Good luck,I quite like 'gaffers'.
Cheers

 

[Tranona]

If it is running at engine speed then the prop will be tiny to run at a shaft speed of 2400-3000. Most modern engines run in the range of 3000-3600 and have 2:1 or 2.5:1 ratios giving shaft speeds in the range of 1400-1800 (some such as Yanmar offer 3:1 suitable for large heavy boats and slow turning props).

You will almost certainly find that you have to replace the whole drive train - not just because the old will not be compatible with the new, but when you get it all apart you will find wear etc.

In re-engine jobs, usually the only two things that are fixed are the stern tube and the space to swing the prop. Both of these are difficult to change so you need to look at what will fit without changing those. Anything else you manage to save and re-use is a bonus.

You may be lucky and find a suitable used unit, but normally they only come up for sale because the owner is fed up trying to keep an old engine running and refurbishing old engines is a mugs game - costs a fortune and you still end up with an old engine. There are, of course exceptions, but the chances of finding a unit that meets your requirements and is in good condition are very slim. You can end up doing a huge amount of work and still end up with an unsatisfactory installation.