Answer Construction Question 3
Question
3: Describe a method of the attachment of bilge keels; State THREE
reasons for not extending bilge keels the entire length of the vessel;
Explain TWO principles of roll damping those bilge keels exploit.
Answer: Bilge Keel
Purpose: -Bilge keels are intended to resist rolling. Their effects are complex, but may be summarised as follows:-
(a) Direct resistance between bilge keel and water has a comparatively weak effect.
(b) They slightly increase the ship's period of roll.
(c) They upset the transverse streamlines of the ship's hull and thus set up eddy-currents and increase the 'wave-making resistance'.
(d) They increase water pressure over a large area of the ship's hull and this pressure acts in such a direction as to damp the rolling.
Most ships are fitted with some form of bilge keel the prime function of which is to help damp the rolling motion of the vessel. Other relatively minor advantages of the bilge keel are protection for the bilge on grounding, and increased longitudinal strength at the bilge.
The damping action provided by the bilge keel is relatively small but effective, and virtually without cost after the construction of the ship.
Purpose: -Bilge keels are intended to resist rolling. Their effects are complex, but may be summarised as follows:-
(a) Direct resistance between bilge keel and water has a comparatively weak effect.
(b) They slightly increase the ship's period of roll.
(c) They upset the transverse streamlines of the ship's hull and thus set up eddy-currents and increase the 'wave-making resistance'.
(d) They increase water pressure over a large area of the ship's hull and this pressure acts in such a direction as to damp the rolling.
Most ships are fitted with some form of bilge keel the prime function of which is to help damp the rolling motion of the vessel. Other relatively minor advantages of the bilge keel are protection for the bilge on grounding, and increased longitudinal strength at the bilge.
The damping action provided by the bilge keel is relatively small but effective, and virtually without cost after the construction of the ship.
Position:
-For their protection, bilge keels should always be arranged to lie
within the line of the ship's side and that of the bottom of the floors.
If they were to project beyond these limits, they would be more liable
to damage. It is carefully positioned on the ship so as to avoid
excessive drag when the ship is under way; and to achieve a minimum
drag, various positions of the bilge keel may be tested on the ship
model used to predict power requirements. This bilge keel then generally
runs over the midship portion of the hull, often extending further aft
than forward of amidships and being virtually perpendicular to the turn
of the bilge. There are many forms of bilge keel construction, and some
quite elaborate arrangements have been adopted in an attempt to improve
the damping performance whilst reducing any drag.
Construction:
-Bilge keels are sometimes ripped off, however carefully they are
positioned, if the ship touches the ground. It is important to construct
them so that, if this happens, the ship's shell plating will remain
intact.
a.
Riveted bilge keels are connected to the hull by a riveted angle or
T-bar, which is strongly attached to the shell plating, but less
strongly connected to the bulb-plate. If the bilge keel is ripped off,
it will then part al ihe outer joint, leaving the hull intact.
b. In welded ships, the bilge keels are usually attached to a continuous flat bar, welded to the shell plating. The outer joint may then be riveted, or lightly welded, so that it will part before the connection to the hull.
c. Alternatively, the keel may be attached to the hull by intermittent welds, and is often scalloped throughout its length, so that the welds will part comparatively easily and will leave the hull undamaged.
d. In large ships, where the bilge keels are very deep, they may be constructed as shown in the sketch. Here, again, the outer connection is made less strong than that to the shell plating.
e. Unless they are carefully designed, the ends of bilge keels tend to produce stress concentrations which can cause the bilge plating to crack. To prevent this, the ends of the bilge keel should be tapered-off gradually and should end over a floor or tank side bracket; whilst a doubling plate should be welded to the bilge plating at this point.
f. The position of the bilge keel is shown in the figure.
Care is required in the design of the bilge keel, for although it would not be considered as a critical strength member of the hull structure, the region of its attachment is fairly highly stressed owing to its distance from the neutral axis. Cracks have originated in the bilge keel and propagated into the bilge plate causing failure of the main structure. In general bilge keels are attached to a continuous ground bar with the butt welds in the shell plating, ground bar and bilge keel staggered. Direct connection between the ground bar butt welds and the bilge plate and bilge keel butt welds and the ground bar are avoided. In ships over 65 m in length, holes are drilled in the bilge keel butt welds. The ground bar thickness is at least that of the bilge plate or 14 mm whichever is the lesser, and the material grade is the same as that of the bilge plate. Connection of the ground bar to the shell is by continuous fillet welds and the bilge keel is connected to the ground bar by light continuous or staggered intermittent weld. The latter lighter weld ensures that should the bilge keel be fouled failure occurs at this joint without the bilge plate being damaged. Bilge keels are gradually tapered (at least 3 to 1) at their ends and finish in way of an internal stiffening member.
b. In welded ships, the bilge keels are usually attached to a continuous flat bar, welded to the shell plating. The outer joint may then be riveted, or lightly welded, so that it will part before the connection to the hull.
c. Alternatively, the keel may be attached to the hull by intermittent welds, and is often scalloped throughout its length, so that the welds will part comparatively easily and will leave the hull undamaged.
d. In large ships, where the bilge keels are very deep, they may be constructed as shown in the sketch. Here, again, the outer connection is made less strong than that to the shell plating.
e. Unless they are carefully designed, the ends of bilge keels tend to produce stress concentrations which can cause the bilge plating to crack. To prevent this, the ends of the bilge keel should be tapered-off gradually and should end over a floor or tank side bracket; whilst a doubling plate should be welded to the bilge plating at this point.
f. The position of the bilge keel is shown in the figure.
Care is required in the design of the bilge keel, for although it would not be considered as a critical strength member of the hull structure, the region of its attachment is fairly highly stressed owing to its distance from the neutral axis. Cracks have originated in the bilge keel and propagated into the bilge plate causing failure of the main structure. In general bilge keels are attached to a continuous ground bar with the butt welds in the shell plating, ground bar and bilge keel staggered. Direct connection between the ground bar butt welds and the bilge plate and bilge keel butt welds and the ground bar are avoided. In ships over 65 m in length, holes are drilled in the bilge keel butt welds. The ground bar thickness is at least that of the bilge plate or 14 mm whichever is the lesser, and the material grade is the same as that of the bilge plate. Connection of the ground bar to the shell is by continuous fillet welds and the bilge keel is connected to the ground bar by light continuous or staggered intermittent weld. The latter lighter weld ensures that should the bilge keel be fouled failure occurs at this joint without the bilge plate being damaged. Bilge keels are gradually tapered (at least 3 to 1) at their ends and finish in way of an internal stiffening member.
B.
Reasons for not extending bilge keels the entire length of the vessel;
Bilge keel extends to 30% - 50% of the ship's length in the midships
region only.
i. At forward end, due to tapering of hull, the bilge keel will offer resistance to longitudinal streamlines of the hull and increase the resistance in forward motion, thus reducing speed and thereby increasing fuel consumption.
ii. At midships the breadth of the ship is maximum and damping lever is maximum from axis if rotation. At forward due to reducing breadth of the ship, k (mass moment of inertia) is maximum.
iii. During rough weather, when the ship is pounding or slamming, the forward bottom is subjected to impact loading. The bilge keel if present in this area would rip off causing damage to the hull.
i. At forward end, due to tapering of hull, the bilge keel will offer resistance to longitudinal streamlines of the hull and increase the resistance in forward motion, thus reducing speed and thereby increasing fuel consumption.
ii. At midships the breadth of the ship is maximum and damping lever is maximum from axis if rotation. At forward due to reducing breadth of the ship, k (mass moment of inertia) is maximum.
iii. During rough weather, when the ship is pounding or slamming, the forward bottom is subjected to impact loading. The bilge keel if present in this area would rip off causing damage to the hull.
C. Due to friction and eddy making, bilge keels have a dominant impact on the viscous roll damping coefficient.
Bilge keels roll damping depends on the locations of the bilge keels. The effect of bilge keels on damping motion for a rounder cross-section are much smaller compared with a conventional square cross-section.
The bilge keels act to a limited extent in reducing the amplitude of roll by producing a direct damping force. The frequency of roll is also made out of phase to some extent.
Bilge keels roll damping depends on the locations of the bilge keels. The effect of bilge keels on damping motion for a rounder cross-section are much smaller compared with a conventional square cross-section.
The bilge keels act to a limited extent in reducing the amplitude of roll by producing a direct damping force. The frequency of roll is also made out of phase to some extent.
good explation
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