ship construction-EKG crack propagation in 'A' frames

13 a)

State reasons why crack propagation may occur in propeller shaft ‘A’ Brackets or spectacle frames

It may cause due to following

Overloading of shafts – while operating intentionally or due to  bad weather shaft may get temporarily overloaded the stresses generated  due to the rotating mass are  heavy at the propeller end. As a result of this ‘A’ brackets  get over loaded. Vibration associated with over loading and bad weather largely contribute to the propagation of the cracks

Vibration  of stern area is usual for many ships  A Brackets are slender and vibration of the ship in stern is common due propeller induced and hull induced vibration also can lead to  crack propagation at the ‘A’ brackets and spectacle frames

It is quite difficult to achieve the required welding length at the roots of the A’ brackes.excessive  weldlead to distortions and the awkward edges meeting the hull girders demand large heat input result in large (HAZ)heat affected Area poor sequence of welding or poor technique may lead to failure of the weld 

At the design stage  sufficient safety factor is maintained  giving due consideration to the  above said conditions. therefore failure  cannot be attributed to the design

b) explain why full power availability  for stern running of the propeller is inconsequential in arresting headway of a ship executing crash stop

about 80%t of the power generated from the propeller is done from the back of the propeller blade when turning in ahaead direction. it is not available when turning in reverse direction
therefore 100% power available in in astern direction not lead to similar output at the propeller as in ahead direction. therefore full power availability in astern direction is inconsequential in arresting headway.  

when a crash stop is requested to  a ship moving ahead in full  power  propeller shaft cannot be  be moved in reverse direction  until the shaft  RPM is reduced to  a predetermined reasonable level due a  to the forward momentum generated by the propeller trying to  apply  a greater torque may result in damage to the shaft.  however power is applied in reverse direction by way of air kicks until vessel can be started in reverse direction. at theat time full power is not required to bring the vessel to stop

c) explain the effectiveness of using the rudders as breaking devices in arraesting the headway of large vessels

The rudders are most effective when the are operating at the maximum speed t as the ruder angle increases lift force also increases and turning is more effective which makes easier to have a zig-zag movement at very high frequency and produce the stopping distance of the vessel

Large rudder angles always increase the drag force  component at a rate faster than the lift component which resist the forward movement of the vessel therefore lift and drag both forces generated by the rudder is contributory factors in reducing the stopping distance  of a ship.

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