Help! Newbie daft mercruiser outdrive questions

I have been thinking (still!) 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 is technology I do understand!).

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 for my application 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 different 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...

 

Long shafts between the engine and the drive are not uncommon with the Merc Bravo drives; many speedboat engines are too wide to sit side-by-side and so must be staggered. I've never seen an Alpha like this but have no reason to suspect it wouldn't work.

All of the propulsive and steering loads should be transmitted through the drive's transom assembly, and the transom must be suitably reinforced.

For your motorbike engine application, I would think your idea of supporting the sprocket and driveshaft on bearings mounted on a sturdy bracket (which is affixed to the drive's transom assembly) should be doable. I'm not sure what bike you're getting the motor from; make sure you do your propeller size calculations BEFORE you weld any sprockets on, as you'll probably need some reduction in the chain drive. (Actually, come to think of it, you might want to key or spline the sprockets, rather than welding them.)

 

Thanks Matt,
Re the propeller calculations, it's going to retain its 6 speed sequential gearbox, and I'll work out alll the sums for 3rd gear, so if its a bit out, Ive got lots of scope either side to play with.
When you say Bravos can be connected to a remote engine by long shaft, does that mean Alphas can too? not sure how the bravos are different other than beefier.

 

That last remark is the wise part of your post.

For your project you need the leg and also the outer and inner transom housings.

The drive shaft with its universal joints goes through the transom assembly which contains a large gimbal bearing. It keeps the shaft centered but allows deviations of several degrees by means of a curved outer ring. Between the shaft and the inner ring there is no tight fit, just two O-rings to keep it quiet.

The engine's flywheel has an engine coupler: a disc with a rubber mounted long splined bushing that transmits the torque. The end of the shaft slides in or out when the drive is lowered or raised.
None is the rotating parts is designed to take any radial or axial load, just torque. The thrust is taken up by 2 swivel pins and the trim cylinders attached to the transom housing and ultimately to the transom.

For a chain driven construction the sprocket must have its own bearing and supporting frame. It also needs to have the splined bushing from the Merc. engine coupler, so the shaft can move axially without moving the sprocket.

Any engine with a flywheel can be adapted to fit the drive. In fact Mercruiser has never had its own engines but uses old GM blocks. There are also drives with Steyr and Cummins diesel engines.

 

Thanks for an informative reply. exactly the sort of thing I need. I now know twice as much as I did yesterday about outdrives.

Right, so it's possible but a pain in the ...
I've bought a whole boat for this so the engine is there with it (it's scrap, seized, but complete, but the drive has been overhauled and kept inside) I'm assuming therefore I have this 'drive coupler'. How do I check? What does it look like and can I tell if its there looking at the engine in the boat? The boat has the motor in it but the drive is off as it has been overhauled. The boat has its full inner and outer transom gubbins, steering system etc. as well as all electrics, controls trim pump etc.

I'm trying to work out if I can do this with the parts I have, with just addding a few bearings, mounts, shafts. Remember the engine is a scrapper so I can grind into bell housings??? etc. to rig this up. Even use the back end of the engine as a mount for this drive coupler if I have to, though want to keep things reasonably light. After all, I know the crank of the engine can't be shunted fore and aft, so I just need to understand how the fore and aft movement of the Alpha1 driveshaft when tilting is taken up by this drive coupler. Is the drive coupler simply a drive mechanism with a greased female splined hole inside it for the male splined leg driveshaft to simply slide in and out of as the leg is lifted up and down?

Thanks for any help on this. I've paid a deposit on the boat you see, so want to ensure I can make the project happen before I pay the balance! I can still get out of this with no great financial loss but some good lessons learnt.

I want the trim function of an outdrive, and want the easy experimentation of gearing afforded by easily available different props, and I don't really like jet drives, BUT I need to weigh up the pain of making this outdrive work by chain drive as compared with a jet unit. Of course a jet unit wouldn't just be a case of sticking the chain sprocket on the end of the driveshaft either, but it would be easier than this. Budget is small too, so I cant stretch to a jet drive with trim and tilt.

Using rather simple language, I see that the job here is basically fooling the drive into THINKING it is attached to a static rotating object like an engine crankshaft rather than a chain driven sprocket. Any loads outside of its specific set of operating conditions will wreck the drive in pretty short order?

What I really need to do is see one of these things in the company of somebody who knows a bit about them. I reckon I could work out in 5 minutes if I have, or can develop the tools and technical ability to complete this project. I can't do this on the actual boat as its a long way away and If I go there I'm kind of committed to buy it.

Anybody in the South of England got such an outdrive set up similar to an Alpha1 they can show me and spend a few minutes helping me understand what functions each of the components performs? I'm near Southampton. There's a pub lunch in it for you if you can! Theres all sorts of other things I need to get my head round like the height above the boat floor that the driveshaft comes in at.

 

Theres also the fact that I see for sale shaft centering tools for Mercruiser driveshafts. If it's a splined shaft into a splined hole, I don't understand how anything needs centering. How sensitive are they to this? Trying to connect it to a sprocket and chain drive sounds exactly the sort of thing which would play havoc with any kind of centering.

 

...And the question of power steering. Do Alpha 1s typically have hydraulically or ellectrically assisted steering or are they just a tiller inside and a direct push pull cable to the steering wheel? I don't mind heavy steering, and adding a PAS pump to the driveline is further work and power loss.

 

Alphas and Bravos both stock have a tiller steering arm that comes through the transom assembly; attached to this is a power steering "assist" rack. The push/pull mechanical cable from the helm pushes or pulls the tiller directly, but at the same time pushes or pulls on a valve which directs the power steering pressure to push or pull on the tiller by means of a hydraulic cylinder which is mounted parallel to the mechanical push/pull cable. If the power steering fails, you have some (hard) steering via the push/pull cable directly. But normally it's the hydraulic cylinder that is doing the work via the valve.

 

This 'assist rack'. To clarify, is there a power steering pump attached by drive belt to the engine to create pressure for this fluid, like on a car with PAS? Or does it get its pressure from the drive in the leg somehow?

 

That is the engine alignment tool. If the engine is out of alignment or the mounts allow it to drop over time, the transom assembly gimbal bearing or rubber engine coupler will fail over time. Imagine the extra stress you put on the rubber engine coupler if it's no longer parallel with the outdrive shaft that docks inside it (with the other point "fixed" by the gimbal bearing in the transom assembly.) Likewise if the outdrive shaft becomes out of parallel or not-quite-centered with the gimbal bearing it passes through.

 

Power steering pump is almost always mounted to the front of the engine turned by belt pulley. Two hydraulic hoses connect the small power steering pump to the steering rack mounted to the transom assembly.

 

This is interesting. When I hear about a boat being converted for outboards, part of the process is always to add knees to strengthen the transom and transmit the new loads to the stringers better. I always thought that a sterndrive would provide some structural support for the transom through the two rear engine mount bolts that connect the engine block to the transom assembly and then transmit this force through the block to the stringers through the block's two forward engine mounts. Now that I write that here, it comes to mind that the forward mounts aren't really great to transmit fore/aft forces.) And as I think about it, maybe the outboard conversion addition of knees is because the leverage is increased from the outboard bracket being higher up than the sterndrive mounting??? Do the rear engine mounts on the sterndrive transom assembly contribute anything to the transom strength?

 

I wouldn't count on a sterndrive's engine mounts to take any thrust. They're probably too soft- the transom will have already flexed half an inch before the engine mounts would be taking a substantial portion of the thrust.

A sterndrive's transom assembly is low in the hull, with a large flange at a point on the transom that is right between the heavily reinforced engine stringers. An outboard is hanging off the top of the transom by a few bolts, applying a large moment arm on an area that is comparatively weak unless knees of some form are added.

 

The inside transom housing is just a bracket on which the engine rests, secured with two long bolts, rubber pads and very short coil springs. The 4-cyl. engines have a single front mount of a similar construction, the V-8's have legs on each side. Thrust is absorbed by the outer transom housing, secured with 6 bolts to the transom from the inside. The transom is supposed to have approx 2" wall thickness.

Jakeeeef, the engine coupler is a steel disc attached to the flywheel with 8 small screws around the circumference. In the center is the splined aluminum tube, bonded to a large rubber grommet.

 

All well thought out advice!

Has anyone of you a idea if such a motorbike engine stands boating for 4 or for 5 minutes?
My bet tends more towards 4! The average bike engine with 150 - 200hp / Ltr. will not live much longer at WOT.

Regards
Richard