Two engines driving one propeller ...

Kenneth, if the RPM of the 10 hp diesels is the same at the prop requirement, the easiest thing to do might be this:

Get a 10 hp and make sure the flywheel end will accept a custom adapter (which should be no problem as the flywheel itself could be drilled and tapped or the puller holes might suffice) so that a rag joint style coupler can be connected to it. The other engine then can go in front of the other, even be added later (!).
The torque load on prop-turning engines is not much, except when the prop hits something, in which case you should use a soft sacrificial key behind the engines so it will shear like a shear pin.
The forward engine has to have a centrifugal clutch, easily gotten almost anywhere. It will allow the forward engine to be added after the first (rear one) is running. A normal clutch is needed behind the rear engine because your boat will otherwise not be able to stop. A centrifugal clutch there would require too high of a boat speed (maybe 3 kts minimum when you'd prefer 1 1/2 kts in a crowded mooring).
At low speed the forward engine WILL disengage for lack of RPM to the centrifugal clutch, but all the better because then you don't need it.
This way you can choose one engine or two. The downfall is that the two firing impulses are not synchronized, and so they are always different. Sometimes it will coincidentally be very smooth, and other times the impulses will combine and shake the boat a lot (as if you had a 20 hp single lunger).

That's the one insurmountable problem, unless you always use both engines, in which case you can set them up 180 degrees apart. Doing so, however, means starting one is twice the load on the starter, and using both starters at once will be necessary. Not such a big deal I guess, just more parts wearing.

Alan

 

Running two engines into one proppeler is mutch easier then some think.

If you rev a engine and then put load on it the RPMS will drop.

If you have two engines with the same caracteristic they will naturaly share the power betwen them. It's no diference betwen running a v8 and two streight 4 mecanicaly linket together.

http://www.finnoygear.no/en/en_tomotor.html

http://www.scana.no/scana-volda/products-and-services/reduction-gearboxes-1

http://www.servogear.no/pages/product.aspx?nr=55

http://www.wartsila.com/Wartsila/gl.../twin_input_single_output_reduction_gears.pdf

 

Alan--Your rear engine would need serious crankshaft modifications .

All engines suffer from harmonic balance and were never designed to have power put into the front of it.

Not only would the normal single key arrangment be insufficient, there are critical numbers on how much power can be taken off the front of the crank shaft. Usually limited to driving alternators or fans,-- an extral load needs carefull considerations.

They were certainly not designed to accept the same HP into the front of it.

 

Moust larger marine engines can take large loads from the front, but that is because they are designed for it. I have seen some enginerom arangement for pasanger boats where two engines is for propulsion and two is Aux.

The arangement here was that all 4 engines could be used as propulsion. One end off the Aux was coupled to the Generator and the other end to the gearbox. I also remember the Nohab Polar(devils engines) rated at 3600HP on the first boat I was sailing as motorman had a propulsionsystem in one end and a firepump consuming 2000hp in the other end.

While I was working in ship design here in Taiwan I remember we had problems because the 1600hp(something like that) would not allow a higher load than xxx from the front and we had planed to runn a whole bunch off hydraulic pumps from there.

Running more power into the front off a engine from another engine would doubble the load on the crank and in theory cause the same reduced reabilety for crank as if you dubbled the hp.

Power don't change trough gearing, only torque change.
1:0,5 gearing will make twice the torque, but the same power 1:2 will make half the torque, but still the same power.

If you lose one engine it will supply half the power, but will have the same torque since the rpm is reduced by 1/2 unless the otner engine now start to be turned in the oposite direction trough the diff.

 

True regarding the power of the engine. The intent of my comment was the actual result in propelling the craft. The RPM is reduced, and so because the prop's power is relative to RPM, the power available to propel the boat has diminished in real terms. I meant propulsive power.

jack: Regarding the end-to ending of two diesels, the difference between torque generated by an engine and the sudden brake torque those engines are built to handle from the output end due to implement lock-up (hitting a stump or a rock, letting out a clutch quickly) is vast. The comment about a sacrificial shear key was to put it behind both engines, so as not to transmit shock loads through the small end of the aft engine to the rotating mass of the forward one. In a tractor, a shear pin or key would be constantly shearing, but in the water, such shock loads are rare. Their greatest torque loads are self-generated by power impulses, and absorbed by the flywheel, which is situated on the forward end. If the engines are phased 180 off, power impulses are never at the same time.

Alan

 

Regardless, every engine is different. Indian Petter diesels have flywheels at both ends and are likely as beefy from either direction. The whole discussion is really about whether running two engines makes sense in a more general sense. I don't think it does make sense, but I will also say that a good mechanic could choose the right engines and make it work well. The engines aren't designed to do this in general, and so I'd recommend in this case that a single larger engine be installed. Usually, a single cylinder engine costs more than half the cost of a twin with twice the power. The reason the 10 hp engines described are one quarter the price has more to do with their construction, which is not water cooled or made with heavy cast iron castings or coupled to a clutch, gear reduction, or reverse. A good four stroke 20 hp outboard is the way to go, I think, because the price is half of the inboard and considering it has everything needed to go, stop, reverse, start, generate voltage AND eliminate a rudder, it seems the natural choice.

A.