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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ship construction- EKG -skeg rudders

EKG -ship construction 2016

12) With reference to twin skeg rudders

a)   Explain why a single rudder may not suitable for some vessels   (05 marks)

Twin propeller ship have a choice of installing two rudders directly behind the propeller or to install a single rudder at  the center of the ship.

Twin rudders are always more responsive than a single rudder

Single rudder will give very small response at slow speed therefore it is inevitable to have twin rudder in most ships

Some twin screw ship operate well with one screw in operation with the twin rudders operate in tandom

Currently in some twin propulsion ships rudder are used in and emergeny to act as retardant by turning the rudder at angle greater than ususal 35 dgrees this can be effectivelu used only in twin rudder ships

The  purpose of having twin propellers are to have increase power and to provide  greater safety factor to the propulsion system twin tudder alwas give a greater safty factor against a single rudder

    

b)   State the advantages of twin skreg installation in modern vessels with a large cargo carrying capacity  (05 marks)

The twin skeg concept is well proven with obvious advantages for the design of ships with full hull forms, restricted draft or highly loaded propellers fuller hul forms can be achived only when the vessels are large

In  designing for the optimum results following factors are important:

        I.            Distance between skegs

      II.            Longitudinal slope between skegs

    III.            Vertical skeg inclination

   IV.            Longitudinal skeg inclination

     V.            Skeg form

Above characteristics are difficult to achieve in small vessel with out disturbing the flow of water to the propeller therefore the benefit of twin skeg can be harnessed at appreciably lager vessels results has shown as much as 13% improvement in propeller efficiency have achieved in large vessels