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How Sighting, Boring and Alignment of Ship’s PropellerShaft Is Done?MARCH 3, 2015 BY SOUMYA CHAKRABORTY — 4 COMMENTS
It is seemingly easy to visualise a ship from the design drawings. And it is equally tough to turn
the drawings into steel in a shipyard. That too, with the same precision as it was designed and
drawn on paper, which brings us to one of the most relatable examples of this aspect.
The main engine of a ship is coupled to the propeller by means of a shaft. The translational
motion of the pistons induces a rotatory motion on the crankshaft, which is in turn, coupled to
the propulsion shaft. The shaft then passes through the stern tube. At the aft end of the shaft,
and outside the stern tube, is coupled, the propeller. The arrangement is shown in Figure 1.
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Shafting arrangement in ship (fig.1)
The centreline of the crankshaft must be along the centreline of the propulsion shaft, and the
propeller. If that fails, the propeller will wobble about its position during running condition.
Even few millimetres of wobbling can result in development of high stresses in the shafting
arrangement, leading to structural failure. Not only that, the rupture of propulsion shaft can
also lead to a major accident on ship.
As you can see above, it is very easy to visualize such an arrangement on paper or in a
computerized drawing. But consider this. Suppose, during construction of the ship, the shaft
was not positioned exactly along the crankshaft centreline. Given the fact that the shafts are
long enough, up to more than 7 to 10 meters in average ships, the offset of the shaft centreline
at the aft end would end up in order of centimetres. And that is not a design failure, but a
failure in the production method.
So how do ship builders ensure the alignment of shafts exactly as per the design? In order to
ensure that, builders follow a method called Boring and Sighting of the stern tube, which is
described in the points below:
1. Sighting and Boring of a ship’s stern tube is done to establish practically the centreline of
shafting, as accurately as per the design.
2. The stern tube consists of two bearings. One bearing at its forward end (called the forward
bush bearing) and the other at its aft end (called the aft bush bearing). It is through the
aperture of these bearings that the propulsion shaft passes. The clearings between the
bearings and the shaft are very minute, and hence the shaft centre line is to be correctly
established in line with the centres of the bearings. By maintaining this, it is ensured that the
shaft centreline matches the centreline of the bearings, and the crankshaft. Again, the bearings
are fitted within bosses (discussed in detail later). Follow Figure 2 to understand the
arrangement of bosses in a stern tube.
Stern tube fitted with aft and forward boss.(fig.2)
3. The stern frame of the ship is the aft most structure of the hull and it is forged separately,
and then attached to the remaining hull structure. The stern frame also houses the stern tube.
The stern tube, in turn, houses the aft bearing. So the shipyard orders the manufacturer of the
aft bush bearing with a machining allowance on the internal diameter. Why? Well, machining
allowance means, if the required internal diameter of the bearing was 0.5 meter, the
manufacture will order for an internal diameter with 0.49 meter. When the ship builder passes
the shaft through the bearing, it is then, that he will machine the internal diameter to 0.5 meter,
so as to match the design value.
4. Now, how do these bearings fit within the stern tube? The stern bearings are fitted within
hollow steel cylinders within the stern tube, called bosses. Therefore, the shaft is housed
within the bearings, which are housed within bosses, which again, are housed within the stern
tube, as shown in Figure 2. So the aft boss houses the aft bearing and the forward boss houses
the forward bearing. In order to be able to match the centreline of the bearing with the bosses
with that of the bearings, the bosses are ordered with a machining allowance for their internal
diameter (just for the same reason why the bearings have machining allowance in their
internal diameter.)
5. The stern frame is welded to the hull structure and the stern bosses are welded to the stern
tube.
6. Now arises a problem. Because of multiple welds on the hull structure and also because of
the cutting allowances considered for each steel plate on the hull, the geometric centreline of
the aft and forward bosses will not match the required centreline as specified in the design
drawing.
7. A telescope is placed at the required height which matches the height of the design
centreline. Multiple targets are placed at the aft and forward ends of the aft boss, forward and
aft end of the forward boss, and along the centreline of the engine output flange.
8. The arrangement is then viewed through the telescope, and the position of the targets are
aligned accordingly unless and until all the centrelines of all the targets appear to be in one
line through the telescope.
9. The centres of the forward and aft boss are then marked. These centres should now match
the centrelines of the forward and aft bush bearings respectively. So according to the
obtained centres of the bosses, the internal diameter of the bush bearings are machined to
the required internal diameter so as to be able to house the propulsion shaft. (This is why, the
shipyard always orders the bearings with a machining allowance on the internal diameter.)
10. Care is taken regarding the achievement of correct internal diameter of the bush bearings
(If the internal diameter is too large, the shaft will wobble within it, and the centrelines will not
match. If the internal diameter is too less, it will not be able to house the shaft within the
bearing) This is why, the internal diameter is measured precisely by a micro-meter after
machining the internal diameter.
11. As a matter of fact, the forward and aft bush bearings are ordered with 5 mm machining
allowance on their outside diameters. The outer diameter of these bearings are machined so
that there will be an interference of about 0.01 to 0.02 mm between the internal diameter of
the bosses and external diameter of the bearings.
12. This allows the bearings, to be pressed into the bosses of the stern tube, with an
interference fit. Figure 3 shows the arrangement after the bearings are fitted within the bosses.
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Shafting arrangement after boring and sighting (Fig 3)
13. Once the centreline is achieved, the propulsion shaft is fed into the bearings, for installation.
Advanced technologies of boring and sighting also use laser technologies to ensure better
precision than the above explained method. Boring and sighting is also used to line up the
centreline of the rudder spindle with the steering gear equipment.
However, even though the shafting system is aligned such precisely during the building
process in the shipyard, the shaft may still deflect from its original alignment due to the
bending of the hull girder. Different bending scenarios occur, depending upon the loading
conditions and the sea states the ship is sailing in. Therefore, the change in shaft alignment
may occur due to bending of the propeller shaft, during this process.
Therefore, it is important for designers, to consider the effect of hogging and sagging of the
hull girder on the change in alignment of the shafting system. To give a slight peek in the
designing process of this aspect, let us understand that the underlying principle is yet very
simple, and follows the Euler’s beam bending theory. In the design of the shaft for a ship,
designers estimate the torsional, bending and shearing loads on the shaft, and thereby, the
critical points of bending are found out. Accordingly, the position of the bush bearings (aft
bearing and forward bearing) are decided so as to ensure that the deflection in the shaft is as
low as possible in the worst loading conditions.
Classification societies, being related with the development of structural safety rules for ships
on a proactive basis, have been involved in developing rules considering this effect. They have
also researched various types of ship for this aspect, and documented the obtained statistics
for future reference by ship designers, builders and dry dock personnel.
It is also very important and necessary to carry out regular checks for bearing clearances
between the bush bearings and the propulsion shaft. Due to prolonged use in various loading
conditions, the inner linings of the bearings tend to wear out, thereby increasing the
clearances between the shaft and bearing metal. This may also lead to wobbling of the shaft.
During tests for checking the shaft alignment and deflection, the observations should be noted
at lightship condition (in which case the shaft deflection will be minimum, and will exhibit the
inherent deflection in the shaft), and in the fully loaded draft condition (wherein, the deflection
will be maximum owing to the additional deflection due to the bending of the hull girder itself).
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About Soumya Chakraborty
Soumya is pursuing Naval Architecture and Ocean Engineering at IMU, Visakhapatnam,
India. Passionate about marine design, he believes in the importance of sharing
maritime technical knowhow among industry personnel and students. He is also the Co-Founder
and Editor-in-Chief of Learn Ship Design- A Student Initiative.
Comments
klied saysMARCH 3, 2015 AT 7:57 PM
Sir,
can you please make an animation regarding about this please. would appreciate it. !
thanks!
zac saysMARCH 4, 2015 AT 10:36 AM
This is a good rundown of some shipyard processes but there are even more advanced
methods than dicussed. Most large ships are aligned using strain gauges and only use
optical or lasers for very rough alignments. Find out.more at http://www.lamalotech.com
Navin singh saysMARCH 4, 2015 AT 8:36 PM
I want to join merchant navy
mehrdad saysAPRIL 21, 2015 AT 11:37 PM
Hi plese send for me proseuder of main shaft alignment with laser in big ship
best regard for you
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