Influence of wake on efficency and hull induced vibration

Explain the influence of wake on:

                        (i)         propeller efficiency                                                                    

                        (ii)        hull induced vibration                                                                 

(b)(i) The working position of the propeller is at the aft end and this causes a different efficiency to that achieved if the propeller were fitted at the bow (the `open water' efficiency).  Since the propeller is fitted at the aft end, it works in wake water.  The wake speed is not constant over the propeller disc, being greatest near the hull and reducing as the distance from the hull increases.  This can cause vibration problems.  The effect of the propeller working in the wake is to acquire `wake gain' which results in the efficiency of the propeller behind the ship being greater than the open water efficiency. 

            However, there is also a propeller-hull interaction called `thrust deduction' due to the low pressure region on the forward side of the propeller causing a drag on the after end of the ship.  Hull efficiency is a combination of wake gain and thrust deduction.

  (ii)      Since the flow of water into a propeller is not uniform, the propeller blades must pass through regions where the water velocity is much different from the mean value.  As the wake speed varies, the angle of attack varies and this gives thrust and torque impulses at blade frequency (i.e. propeller rev/min x number of blades).  These vibration pulses are then passed into the hull and the problem is compounded if propeller-hull clearances in the aperture are too small. 

            Wake variation over the propeller disc should be reduced to a minimum by careful design of the stern.  For single screw ships the waterline at the aft end should not be made too steep in order to avoid eddying in front of the propeller.  The rudder post should be shaped and there should be adequate clearance in the aperture.  In twin screw ships the propeller tip clearance should be as large as possible.

propeller slip and its effects

(a)        Define propeller slip                                                                (2)

(b)        State, with reasons, FOUR conditions which will affect the propeller slip.    (8)

Since the propeller works in a fluid medium, the helical path it follows will advance less than in a solid medium in the same time.   The speed in a fluid medium, measured relative to a fixed position, is called the ship speed (V) whilst the theoretical speed (V_T) in a solid  medium would simply be pitch multiplied by revs per sec.

            Usually V_T is greater than V and one form of the slip, called the apparent slip (S_a) calculated from:         

                                                S_a        =          V_T  -  V

                                                                            V_T

            If the speed of the ship were measured relative to the surrounding water it would be called the speed of advance (V_a) then this would give a true slip (S) calculated from

                       

                                                S          =          V_T  -  V_a

                                                                              V_T

It can be seen from (a), that in both cases, if pitch and revs were constant, then theoretical speed would be constant and hence:

                                    if ship speed or speed of advance reduced, slip would increase.

            conversely,     if ship speed or speed of advance increased slip would reduce

            The following conditions could therefore affect slip (FOUR required).

●          Apparent slip is affected by currents since ship speed is measured relative to land therefore in an opposing current, speed would reduce and slip increases, whilst in a following current speed increases and slip reduces (and in fact can become negative)

●          If the hull is fouled it would give increased resistance, hence speed reduces for a given power and revs,  therefore slip increases.

●          A propeller is designed to work at optimum efficiency at service revs and speed, therefore a change of power and revs will give a speed which is greater or less than the optimum and therefore less efficiency.  Lower efficiency means less speed for a given revs and therefore more slip

●          Damaged/roughened  propeller blades will mean the propeller is working less efficiently and less speed results hence more slip.  Also, damage is generally non-uniform which can lead to severe vibration. May need to reduce speed to a region where efficiency is less, hence more slip.

●          The water can only support a limited suction from the low pressure side of the propeller before cavitation occurs.  If the propeller is cavitating, it will produce less thrust, hence less speed and more slip

ship construction -EKG dry dock repair specification

14  As achief engineer officer write a dry dock specification for the repair  for a the repair of following damage  that has occurred ,stating what factors have to be considered when costing the repairs . damage to water ballast tank  number 1 port wing  shell plating thickness 15 mm for approx  2meters square  has to be removed and replaced along with relevant damaged stiffeners

Steel  renewal in no 1 port wing ballast tank

Please quote for following scope of work during stay at dry dock

 Sea water ballast tank tank capacity 5000  m³

open manhole covers 02 nos on deck size  680mmX480mm  for the inspection and to be boxed back after the repair  with new jointings indicate repair cost for replacement of studs .

provide ventilation blowers for the tank during repair  indicate cost per day

provide lighting to the tank

provide man entry certificate and hot work permit during the period of repair

clean the ballast tank  and hose down prior to commencement of the repair

erect staging from frame no 46 to  51 at second stringer level to 2meter height and to remove on completion

erect staging from frame no 46 to  51 for the out side shell on port side to 8meter height from ships baseline and to remove on completion

crop and renew  the damage section of aprox; 02 square meters side shell plating with damaged frames in way of the plating area withclassification approved materials . the material specification andthickness is clealy marked on the shell expansion drawing provided and details of the frames are included in the drawing for preparation.

Marerials certificates welding procedure and  qualified welders are to be provided during the  repair

work to be completed to the satisfaction of the classification society surveyor

on completion of the plate renewl vaccum test to be done on the plates.

Apply paint scheme   with inside the tank as per the attached specification

Paint supplied by ship

Out side the hull apply the yard supplied paint two coats of anti corrosive and anti-fouling paint coating thickness 300 microns are to applied  paint supplied should have anti fouling approval certificate

Clean all debris generated during repair from the tank 

Tank will be pressure tested by ship staff on completion of the repairs by filling with sea water

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.

1

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

copper plating in refrigeration systems

Copper plating

as the term implies Copper plating is a condition in which metal parts in the compressor become coated / plated with copper. This condition is often observed in compressors which have a high moisture content in the A/C system.

One possible cause of copper plating is that as moisture (water) combines with the refrigerant, it forms an acidic solution; this chemical may then dissolve or leach copper from other components in the A/C system which are copper or contain copper-based alloys such as brass or bronze. the copper is deposited on metallic parts of the compressor (i.e. Races, Bearings, Centering ball, Fixed gear etc)   would have to be facilitated by the circulation of refrigerant, oil and moisture in the A/C system. copper plating may have detrimental effect  to the compressor durability.

There is large amount of copper in refrigerant systems in the tubings and fittings. This copper deposits on the hot parts of the compressor such as the bearings or the discharge valves.it may occur due to moisture Contamination allowed to enter and remain in the A/C System. This condition can be caused by the following:

A) System leaks.

B) Improper Vacuuming of system.

C) Contaminated system components.

D) Contaminated refrigerant and / or oil.

E) Saturated or malfunctioning drier.

EKG (GENERAL)    CLIV 16-01

1.sketch  and describe a static oily water separator
explain the sequence of events when the oil content exceeds15ppm
what is the frequency of the 15 ppm alarm test done

ship construction Q& answer thickness terms

    13  explain Each of the following with regards to thickness measurements and acceptance critiria   

   a) Gauged thickness   

b)      Reserve thickness
c)       Renewal thickness
d)      Voluntary thickness adition
e)      Substantial corrosion

      

1.       Measured thickness at the location by a certified technician with a calibrated machine under the instruction of a surveyor.
2.                wastage allowance in reserve for corrosion occurring in the two and a half years between  Intermediate and Special surveys
3.          Renewal thickness; Minimum allowable thickness, in mm, below which renewal of structural
4.                members is to be carried out  t_ren = t_as-built – t_corr – t_{own  .}Thickness of a material after deducting the maximum diminution allowed is the renewal  thick ness where the replacement of the plate has to be carried out.
5.            owner/builder specified additional wastage allowance, if applicable, in mm Owners if decides to increase the thickness of a material beyond designed thickness to achieve better safety or to reduce renewal of steel during operational life
6.      measured thickness indicates a wastage in excess of 75% of the allowable margins, but within the acceptable limits. If not renewed annual inspection has to be carried out to accertain the condtion of the plating

ship construction Q&Answer 2016 Jan

Describe possible  faults which may be found during a dry dock inspection of the following.

1. sren frame
2. sea chest
3. bilge keel
4. anodes
5. rudder

Stern frame  defect may differ according to the construction of the stern frame most of the ships currently constructed are fitted with a welded fabricated stern frame .stern frames also may take different shapes according to  type of rudder and rudder stock .

If a rudder horn is welded sharp changes in shape  at the curvature may lead to cracks

If the stern frame consist of a sole piece  that may get deformed due to loading of the aft end has tobe checked similarly bending  of the sole piece on sideways could occur 

Sea chest

Faults at sea chest

a)      Fouled sea grating- poor performance of the MGPS system or

      the anodes of the MPGS system installed in the sea chest are fully consumed

b)      Corrosion in the sea chest – zn anodes provided are wasted

c)       Steam outlet pipe section inside the sea chest corroded and wasted

d)      Pipe opening are thinned down or erroded (stub pieces are thinned down0

e)      Usually vent pipe is fiond chocked due to the small pipe diameter  the pipe get chocked

f)       In high sea sucion  upper area corrosion due to the accumulated air in the sea chest may find if the vent holes are not provided on the top

g)      Sea chest top plates from the engine room side may get thin down due to the water leakages of glands and accumulated water.

Bilge keel

Possible faults

a)      Deformation of the bilge keel due contact

b)      Intermittent weld seam detached from the doubler plate attached to the hull

c)       Damage to the welding seam of the doubler plate and the ships hull

d)      Partial or fully missing bilge keel

In the event of above the repair or renewal of the bilge keel done according to the time available

However the doubler plate and the shell plate attachment has to be ensured by carrying out a suitable crack detection test of the hull after removing the paint from the welding seams renewal of the bilge keel remains a discretion of the owner

Anodes 

Has to be divided into to categories

 Sacrificial anodes  and impress current system  

For Sacrificial anodes 

a)      Anodes are fully waisted

b)      No wastage of the anodes while corrosion appear on the paint damaged area of the hull

Impress current system

Coating damage in area around the anodes

Protective shield in the vicinity of the anodes are detached

Waitige or mechanical damage of the anodes

Rudder

a)      Excessive pintle clearance

b)      Excessive pintle clearance or missing juming bar welded to the rudder

c)       Cement sealing of the palm bolts are broken,

d)      Cracks appear at the rudder especially at radius area of the pintle  area

e)      Cracked or leakey welding seams at the slotted weld areas

f)       Wastage and thinning of the plates at the leading edge of the rudder specially propeller turbulat water area

g)      Water leaking into the pintle pin taper area

h)      Excessive clearance at the top bearing

i)        Increased rudder drop due to wear at rudder carrier bearing

j)        Misalignment due to grounding damage

k)      Leaks

 Sea chests and their gratings, sea connections and overboard discharge valves and cocks and their fastenings to the hull or sea chests are to be examined. Valves and cocks need not be opened up more than once in a special survey period unless considered necessary by the Surveyor.