Python-Drive:
A flexible drive-shaft
| Albatross Marine's managing director, Ian Oliver, explains some of the background and technical issues regarding the Python-Drive flexible drive shaft. |  |
Traditional Engines:
In times past boat engines were much heavier and far slower running than their modern offspring. They had large diameter, heavy flywheels to put a degree of inertia into the system, this inertia was crucial to help to eliminate the unavoidable vibration from the engine. 
In those days it was standard procedure to mount an engine on solid bearers, usually of seasoned oak or elm, and to connect the gearbox to the prop-shaft via a 'half coupling' to make a solid connection all the way through from gearbox to propeller. This could also contribute to further driveline vibrations.
On more traditional units, such as Russell Newbery and Gardner engines, this is still the case, but providing the alignment is absolutely perfect, and the length of the shaft is not too long without being supported by bearings, they cause no problems. Enthusiasts own most of this type of installation today and any vibration that may occur is usually quite acceptable and all part of the experience.
Modern Engines:
Modern engines are far more complicated. They are the exact opposite of the older engines being much lighter, running at higher RPM's. Due to this light construction they are not able to support that much needed vibration-eliminating heavy flywheel, and compromise with a smaller and much lighter in weight.
However, the Marinisers of today's engines do a relatively good job, though the diameter of their flywheel is, by and large, restricted by the manufacturers design criteria. The Marinisers ease their way by adding balanced weights to the flywheel where possible, and by fitting specifically designed engine mounts and drive plates of varying 'hardness's' that help to take out vibration - a damper plate, and special engine mounts have a cushioning effect that absorb energy, allowing the engine to move and helping to reduce sympathetic engine vibrations.
The Driveline:
Without flexibility in the driveline the propeller will bite into the water and try to screw itself back up the driveline assembly and into the boat, and is only held in position by the engine mounts [fig.1]. As you can imagine, the forces on the engine mounts are tremendous as the propellers energy pushes though the engine to the mounting blocks and on into the hull - allowing forward motion.
At his point the engine is being asked to work extremely hard and so the vibrations increase. With the vibration from the engine forces now acting in the opposite direction to the propeller the stresses on the engine blocks increases even further. However, as RPM increases centrifugal force comes into effect and the movement begins to even out and becomes less noticeable. With the prop-shaft connected directly to engine, and no flexible joint available, the whole driveline has to move with the engine as its mounts flex.
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fig.1 |
While the engine mounts are doing their job what is not apparent is the movement of the prop-shaft backwards and forwards through the stern tube.
Premature Wear:
This continual back and forth movement, along with the weight of the prop-shaft, wears the stern tube at its lowest point, gradually elongating it and causing the shaft to develop a wobble which causes it to run eccentrically. This movement wears the shaft prematurely at the point it is located within the stern tube. As the wear gradually worsens, so water works its way in, in ever increasing amounts, initiating further vibrations to occur that were not present previously.
It reasonable to expect there to be some wear at this point of contact between the shaft and stern tube because of the natural friction in the system, however the continual backwards and forwards movement of the driveline causes it to wear much more quickly than would be the case if there were some flexibility within the driveline and if the prop shaft could be prevented from sliding inside the stern tube.
Of course, all this vibration in the system not only leads to premature wear of the driveline applications but is also a significant contributor to the amount of on-board noise vibration.
The Cure:
The way negate this perennial problem is to fit a Python-Drive Constant Velocity Flexible Output Shaft [fig.2] with a CV joint at either end - one side connected to the gearbox and the other connected to a thrust bearing assembly block. This allows the engine total flexibility and permits movement in any direction, while the two constant velocity joints absorb the engine movement allowing the output shaft speed to remain constant. The thrust assembly block is rubber mounted and permits only rotational movement of the prop-shaft. This has the effect of putting the thrust forces coming back up the driveline from the propeller directly into the hull. By preventing this backwards and forwards movement in the whole driveline the Python-Drive allows the propeller forces to act on the hull and not the engine.
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The Advantages:
With this in mind, the advantages of the Python-Drive become immediately apparent. To begin with, fitting a Python-Drive makes aligning the engine to gearbox to prop shaft incredibly easy.
On an installation without a flexible output shaft the actual process of getting the engine aligned with the prop-shaft can be a fiddly, time consuming, and therefore costly job. It is done by continually adjusting all four engine mounts, and checking the gap between the gearbox output coupling and the connecting component on the prop shaft as each adjustment is made, until finally the gap is even around the whole circumference the assembly to within 1/5000th inch. The design of the Python-Drives CV bearings is on a slow spiral - this means they actually require some angle to work effectively. Fitting a Python-Drive does not eliminate the necessity of making adjustments, one still has to ensure the engine load is divided equally over its mounts, but it does mean the actual alignment is not so critical, and the time saved is considerable.
Private / Pleasure Boats:
Often when we talk to prospective boat owners they will say, "We're leaving all that to the boat builder." Conversely when we talk to boat builders they say, " I'll fit what ever the customer asks for." If you are having a boat built, our advice would be talk to your boat builder about the Python-Drive. Don't be put off by comments like, "There's not enough room," or the "Drawings will need altering," after all, the couple of inches you may lose internally by slightly enlarging the engine room will be more than compensated for by the quicker, easier installation, the savings in general driveline maintenance, and the benefits in all-round engine efficiency. The outlay incurred in fitting a Python-Drive at the build stage far outweighs the costs and the inconvenience of lifting the boat out of the water after four or five years cruising to repair or refit a worn and inefficient driveline component, or otherwise trying to remedy the situation at a later date.
The vast majority of today's boat builders are very good at what they do, they know their business, and know what makes financial and engineering sense in the long run - just like the Python-Drive does.
Hire / Hotel Boats:
All of the points raised above apply equally to this side of the business where one is dealing with the general public. The private owner is often prepared to 'rough it' a bit but this may not apply to the more discerning holidaymaker whose requirements and expectations have to match the highest standards of other self-catering options in the holiday market. The companies which are able to offer a quieter, vibration-free and trouble-free installations are the companies who not only get the same holidaymakers coming back year on year, but are also more likely generate future orders via those all important word-of-mouth recommendations.
Summary:
Python-Drive makes the alignment far easier than the traditional method, both on new and old installations, where it will greatly reduce the vibration and provide a longer life for the driveline by reducing the effects of inevitable wear. No one likes to a make change for the sake of it, but where a fully flexible Python-Drive output shaft is concerned this is definitely an important, and necessary, technical improvement to a marine driveline.
Manuals:
Click the links below to view the manufacturer's documentation, or right click and select 'Save target (link) as..' to save. You will need Adobe Acrobat ReaderTM to read these files, which can be downloaded free from www.adobe.com.
Python-Drive installation manual (pdf)
Python-Drive infomation leaflet (pdf)
Python-Drive troubleshooter (pdf)
Python-Drive
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