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Old 07 December 2009, 16:40   #1
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Help me understand how outdrives work!

Further to my post about small outdrive units (post 32679), I have been thinking about how or whether I can connect a Mercruiser Alpha 1 leg by chain drive to a motorcycle engine.

Having bought a leg I'm beginning to realise that the possibilities of connecting power to it may be much more complex than if it was a standard propshaft drive or a jetdrive with a simple propshaft coming out of it (which I am more used to).

Because I have not yet taken delivery of the sterndrive bits I'm trying to visualise exactly how stern drives work from static photos which is actually quite hard!

Is the chain drive thing possible? I see the alpha 1 has a splined shaft with a UJ coming out of it on the input side. How is this shaft supported at the engine crankshaft end? Does a Mercruiser engine have special stuff going on where its driveshaft comes out or can I support the free splined end of the leg's driveshaft in a wheel bearing and hub mounted on a strong spaceframe and simply weld a sprocket for my chain drive onto the leg's driveshaft somewhere in the middle between where it comes out of the leg and where it is supported in the bearing and frame? Would the driveshaft be adequately supported at the leg end for the increased lateral loading a sprocket and chain drive would put on it?

I think the crux of my question is: Does the driveshaft and universal joint I can see in pictures of these legs which comes out of the cylindrical casting in the back of the leg move anything other than round and round (with the engine) Or does it move in and out (on the splines) or up and down as the leg is trimmed up and down?

The other issue I suppose is whether when close coupled to its correct engine as Mr Mercruiser intended a sterndrive takes some of the propulsion load through the driveshaft or other components to the engine block, in effect pushes the boat along by its engine mounts and bearers rather than by its transom, so when you connect it by chain drive to an offset engine it could collapse or deform the transom under power and break something.

Worryingly I can't find any boats with mercruiser drives with non mercruiser engines attached to them, and certainly can't find any boats with mercruiser drives with long driveshafts coming out of them leading to engines which are further towards the bow or offset (ie., not close coupled).

Methinks I should have looked into this BEFORE buying an outdrive leg...
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Old 07 December 2009, 17:05   #2
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You usually mount the engine directly to the splined shaft, this isn't supported on any additional mounts.
Usuing a chain drive with the engine offset will upset the balance of the boat. It probably will be possible but could be alot of hard work.
I can't remember the name of the drives but the engine is mounted with the shaft facing forwards then onto a chain which goes onto a propshaft.
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Old 07 December 2009, 17:16   #3
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I have been thinking about how or whether I can connect a Mercruiser Alpha 1 leg by chain drive to a motorcycle engine.
Are you having a laugh?

No, a motorcycle engine will never be any good, it would never generate enough torque if HP. The smallest engine Mercruiser fit to that leg is a 3.7 ltr straight 6.

Also, direct alignment is critical.
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Old 07 December 2009, 17:26   #4
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Are you having a laugh?

No, a motorcycle engine will never be any good, it would never generate enough torque if HP. The smallest engine Mercruiser fit to that leg is a 3.7 ltr straight 6.

Also, direct alignment is critical.
That should do.
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Old 07 December 2009, 18:05   #5
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Give over .. I've had girlfriends that have had sex toys with bigger motors .. they were northerners right enough .. can I say that on here ? .. northerners I mean ?
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Old 07 December 2009, 18:27   #6
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Give over .. I've had girlfriends that have had sex toys with bigger motors .. they were northerners right enough .. can I say that on here ? .. northerners I mean ?
Say what you want mate... everyone else does.
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Old 08 December 2009, 03:34   #7
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mollers is half right, a motorbyke engine ain't got the torque or a flywheel, me and my brother have been down this road years back and although with a remote mounted engine the lineup isn't as important as what drive coupling you use you'll never get it to rev
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Old 08 December 2009, 03:56   #8
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mollers is half right, a motorbyke engine ain't got the torque or a flywheel, me and my brother have been down this road years back and although with a remote mounted engine the lineup isn't as important as what drive coupling you use you'll never get it to rev
Perhaps I should've said 'straight' alignment.
I had a 5.7ltr V8 on an Alhpa One leg. A tit dropped the lump in slightly out of alignment, it took out the all of the top end transmission bearings, 800 worth.
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Old 08 December 2009, 04:15   #9
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that would be better, have a look at pics the gearbox is behind that thing with the drain pipe out the top and that is a belt drive coupled to a very small propshaft with uj.s at both ends, the outdrive has no gearbox as such. think of a lorry the axle goes up and down so they can't be in line but they don't blow up, you just got to mount things solid and let the ujs in the prop do the work
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Old 08 December 2009, 04:32   #10
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that would be better, have a look at pics the gearbox is behind that thing with the drain pipe out the top and that is a belt drive coupled to a very small propshaft with uj.s at both ends, the outdrive has no gearbox as such. think of a lorry the axle goes up and down so they can't be in line but they don't blow up, you just got to mount things solid and let the ujs in the prop do the work
I don't know where you got those photos from, but I haven't got a clue what you're talking about!

The engine doesn't appear to be connected to anything at all - or am I being thick here?

Where is the belt drive?

Where is the very small propshaft?
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When a boat looks that good who needs tubes!!!
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