A line shaft transmits the driving force of a motor boat through a propeller shaft. This system is not appeared in the middle of the Eighteenth century as the propeller, but is generalized at the end of the Nineteenth century.
Simplicity, robustness, compatibility with high powers, and if the tree line, remained queen of the transmissions ?
Anatomy of a transmission line tree
In such a system, the inverter, the coupling, the shaft, the bearing, the packing gland, the shaft stern, the ring hydrolube and the propeller are perfectly aligned.
This set, with at least two fixed points, transmits the rotary motion output of the transmission to the propeller.
The inverter is a gear box with three positions : reverse, forward, neutral. The diesel engines (4-stroke) installed on “small” boats (of less than 100 m!) rotate at a speed greater than that necessary for the operation of the propeller shaft. The inverter plays the role of reducer.
On the other hand, large ships are equipped with diesel engines, 2-stroke, slower, the speed of rotation is transmitted directly to the propeller shaft.
Positioned at the output of the inverter, the coupling allows to transmit the movement thereof to the propeller shaft itself.
A system of flanges and cake conical maintains the shaft in its place.
The coupling can be rigid, very solid, but pass then the vibration of the tree line to the boat, or slightly flexible, in order to amortize them, such as on engines that are less important.
The coupling must be carried out perfectly, with very low levels in order to allow the proper functioning of the engine. A motor misaligned “jumps” on its feet engine. The coupling, firmly secured to the transmission, itself fixed on an engine mounted on the boat is the first fixed point above the tree line.
Achieved in general stainless steel, its diameter is a function of the size of the propeller range. In the case of tailgating, if the propeller has only suffered little, it is possible that the tree would have flamed. On the pleasure boats, one can control his character straight with a mason’s rule or by rolling it on a table.
Finally, if one day you have your tree in hand, check the wear level of the bearing, the stuffing box and the ring hydrolube.
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An irregularity of surface on one of the last two points would lead inevitably to a leak.
A hanger a few millimeters will destroy inevitably the ring hydrolube and may cause damage to the tube bellow.
The output of the coupling, the propeller shaft is maintained on one or more other fixed points (one on pleasure vessels), landings, before entering the tube bellow.
The bearing comprises a ball bearing that should be greased regularly.
It is considered that the propeller shaft must be maintained by a bearing all of the 48 x its diameter while avoiding the closer to less than 20 x the diameter.
The convoluted bellow is pre-sealed or welded onto the hull. It allows the propeller shaft to exit the hull, put in the new one. On the fiberglass boats, the shaft stern, which can also be realized in glass fiber, is generally laminated on the hull.
Inside, the tightness is ensured by a cable gland that does not contain today more cable but the lip seals. In the past these stuffing box compresses a braid suiffée of plant fibres (tow of which are calfatait planking wooden ships) around the tree. These fixtures required a permanent monitoring of the state of the braid.
Modern systems with soft gaskets or mechanical joints which have made the packing gland to braid obsolete.
In general we encounter a cable gland to seal, the gasket should be changed every 5 years or 500 hours.
On some setups, the tube stern is not continuous and stops at the exit of the hull.
The tree is then maintained by a “chair” equipped with a ring hydrolube.
The ring hydrolube, inserted without force from the outside around the shaft in the tube stern (or the chair of tree) ensures the centering and the internal sealing of the latter, while admitting the water for lubrication of the system.
It consists of either a bronze bushing within which is set a other rubber very hard and resistant to hydrocarbons and to abrasion, is entirely elastomeric. On the internal side, the flutes provide a flow of water carrying the lubrication between the shaft and the ring.
Advantages and disadvantages of the systems to tree line
- Robustness compatible with the long distances.
- Compatible with powers of very high (mW).
- Maintenance relatively easy achievable by a well-informed amateur with a tooling limited.
- Good performance.
- Direct Transmission.
- Ease of intervention on the cable gland.
- Centering of the masses by the position relatively remote from the engine.
- Possibility to use the shaft disengaged with a shaft alternator to generate power by letting the propeller turn.
- Operations of port sensitive.
- Requires the hulls are deep enough at the back.
- Requires space inside length (motor+inverter+coupling+shaft+bearing+cable gland).
- Generates a draught.
- Saffron mandatory in comparison with transmissions off-board, hydrojet or Z-drive or pods.
Other types of systems to shafts
The V-Drive is a propulsion by two shafts, in which the motor is mounted in the opposite direction in the usual sense, and or the first tree is directed towards the front.
A mechanism of reference angle transmits the force of the first shaft towards the rear to a second shaft and to the propeller.
This system, which is obsolete, would save space inside or install engines in-board in small boats.