Drydocking Inspection A good place to start a Drydocking Inspection is at the head of the drydock, working down the port side and up the starboard side.
1.0 Anchor & Anchor Chain Cable Anchors and anchor chain cable if ranged should normally first be examined as follows: Anchor heads, flukes and shanks should be surface examined for cracks. If any such defects defects are found they may may be weldable, otherwise otherwise renewal will probably be necessary. In such cases welding welding may be attempted as a temporary measure pending availability of the new equipment, which may take 3 to 6 months. Anchor head crown pins and anchor shackle pins should be hammertested, hardened-up if slack, or renewed if excessively worn or bent. Swivels if fitted, should be closely examined so far as possible in way of the threaded connection, as many have been lost in service due to concealed wastage in this area. If in doubt the swivel should should be recommended to be removed. Consideration should be given to simply eliminating any questionable swivels, they are normally not essential. Patented type detachable connecting links should be opened out and slack or corroded taper locking pins renewed their holes re-reamed and new lead keeper plugs peened in. "U" type connecting shackles should be examined for excessive neck wear, slackness in the pins and for shearing of keeper pins. The pin must be a snug fit all around in these shackles, otherwise the keeper pin may shear when a strain is put on the chain. Anchor chain cable should be surface examined, hammer-tested and loose or missing studs replaced by welding at one end of the stud only, at the end of of the stud opposite opposite the link butt weld. The rest of the chain chain cable should be further examined for excessive wear and gauged if necessary to ensure continued compliance with the Rules. Verify that the number of shots of anchor chain as fitted port and starboard, equal the total length required by the Classification Rule Equipment Numeral.
2.0 Hull Plating The stem plating or the stem bar bar should next be sighted. The first few plates in the Keel, "A" and "B" strakes call for particular attention as they are vulnerable to pounding damage and also to erosion and corrosion of welded or riveted connections, particularly where chafed by the anchor chains.
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Eroded or corroded butts and seams should be cleaned or back chipped to sound metal and rewelded. rewelded. A minimum of reinforcing is desirable desirable on these welds to prevent entrapped air from the bow wave attacking the weld again. The rewelded joints should be cleaned of all slag and carefully primed and and coated. Corroded or slightly leaking leaking rivets may be caulked or ring welded if condition is not widespread and the rivets are not loose. If it is impossible to weld eroded butts, seams and rivets at time of inspection because tanks in way are not gas freed and condition does not affect the structural integrity of the vessel, consideration may be given to cleaning affected welds or rivets and epoxy coating same until the next Drydocking Inspection. Lapped seams and butts at the forward end of the vessel are relatively rare, however if present they should be examined for wear corrosion caused by by a combination combination of wash wash effect and and anchor chain rubbing. If this is excessive it may call for plate edge build-up or plate renewals, or as a preventative measure, the installation of chain chafing protection strips. The underwater bottom should be sighted frequently as the inspection progresses towards the stern for hogging, sagging, grounding damage, or distortion possibly indicative of structural weakness. A good way to sight for deformation is to bend over and look through your legs upside down at the bottom - any hills or valleys seem to stand out better from this viewpoint. Optical keel sights or checking with a tight string may be called for and it is useful to carry two small magnets and a length of string for this purpose. purpose. The bottom and and sideshell plating plating in the midship midship area, particularly below and in line with deckhouse or hatch ends, calls for close examination for the possible presence of transverse deformation between frames. It is important to look carefully for signs of deformation possibly attributable to structural weakness (e.g. wastage) as differentiated from grounding or striking indents. Unfair or set-in plating is common forward. A fair degree of deformation, say up to 75 mm (3 in.) of the underwater bottom plating forward ordinarily may be accepted without resulting in serious impairment of structural strength provided the internal framing in way is not significantly "tripped" or rendered ineffective. However for transversely transversely framed ships, severe or sharp transverse buckling of bottom plating within the amidships half-length, can significantly affect longitudinal strength of the hull girder. As might be expected, the greater greater the athwartship extent of such buckling, the greater the impairment of of hull hull strength. strength. Any appreciable buckle of sufficient athwartship extent so as to cross the keel strake and centre vertical keel, or say two strakes including an inner bottom girder, is serious. Such a buckle buckle should should be corrected by replacement of of plating and and the buckled portion portion of affected affected girders. If there is no evidence to indicate the buckle was caused by grounding or other excessive local loading, or that it is associated with excessive wastage, it may be an indication of need for providing additional internal reinforcement, i.e. a design deficiency. Buckles of of shorter athwartship
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extent may also require correction, depending upon their depth and sharpness, the number of buckles, and their respective locations. Obviously several bottom buckles within the same frame space transversely are more serious than the same number and of buckles distributed in a random manner. Localized transverse bands of accelerated corrosion or grooving may be found in association with buckling. These are indicative of advanced localized stress, which experience indicates may lead to cracking. In such cases, plating replacement may be called for even though the deterioration may be less than the allowable wastage. In such cases it may be feasible to replace less than full length plates. Any sharp or very deep indent should be scaled and examined for excessive corrosion "grooving", thickness gauged and renewals or partial renewals made if found necessary. If sharp indents or creases are in line across several bottom or sideshell strakes, they should generally be dealt with immediately. Smooth indents, say of less than three inches in maximum depth, may require no action, particularly if there are no signs of significant damage to the internal structure or of accelerated corrosion at the bottom of the indent. Bottom or "docking" plugs should be carefully examined not only for tightness but also for excessive corrosion along the edge of the weld of the bossing to the bottom plating. Directly attached bilge keels or bilge keels landing bars should be examined for fractures or corrosion grooving of the shell plating in way of any discontinuities of their attachment. While most bilge keel are attached via a landing bar, there are some vessels currently in service with the bilge keels welded direct to the shell plating with no intervening landing bar. No modifications are required to these installations, however they should be examined and if fatigue fracturing or grooving of shell plating in way of bilge keel discontinuities is encountered, consideration should be given to modification by insertion of a continuous landing bar under the full length of the bilge keel. Fractured butt welds in the bilge keel should be chipped to sound metal and rewelded and any slack rivets renewed. The decision on whether to repair or crop and remove a damaged bilge keel should be left up to the Owner. Our particular interest is to see that fractures in the bilge keel do not propagate into the hull, and that no "notches" or "hard spots" are left which might lead to cracking of the bilge strake. Overboard discharge pipes, their shell reinforcement rings and external shell plating beneath the outlets should be checked for excessive corrosion. This is particularly applicable to evaporator drain, boiler blowdown, and inert gas scrubber discharges. Sea chests should be examined for fractures particularly in way of the corners, for aerated water corrosion, and the condition of the strainers and their securing devices.
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3.0 Stern Frame & Rudder A prerequisite for carrying out a Drydocking Inspection is adequate staging and removal of the pintle aperture cover plates and rope guard so as to permit access to the upper rudder stock, palm bolts, gudgeons, pintles, top of the stern frame and stern bearing/seal assembly area. These items are of the utmost importance and they cannot be checked from the drydock floor. The stern frame should be closely examined for fractures and fissures, particularly at the forward end of the skeg connection to plating. Local eroded and corroded areas are often found in the rudder or horn just aft of the propeller blade tips. If not too severe, they may be scaled and filled with epoxy. If the condition appears to be progressing rapidly, waster plates should be fitted over the affected area. The top of the stern frame should also be examined for leakage in way of the core hole closing plates and for any suspicious bulges at the top. Water entering the hollow part of the stern frame, either from the after peak or through a leaking closing plate, may freeze in cold weather causing the frame to fracture or bulge. If water is present it should be drained out and the void space pressure-filled with an inert non-freezing filler. Zinc protection or Swedish iron plates, when fitted, should be in close metallic contact with the shell and left unpainted. Care should be exercised where these must unavoidably be welded to the stern or rudder castings so as to avoid "welding notch" effects. The practice of stress relieving stern frames after welding repairs has been seen to be different throughout the world. It is argued that in general to attempt stress relieving might do more harm than good since the stern frame itself is locked in position in the ship and heating to 600o to 650o Centigrade might cause stresses of sufficient amount to counteract the beneficial effects of stress relieving. However, it should be emphasized that the desirability of preheating up to temperatures of 200o Centigrade the actual temperature of preheating depending upon the type of actual material in use. It is not intended to convey that stress relief of welding repairs to castings is not beneficial if the casting itself can be stress relieved in a furnace , but in the ordinary course of events rarely is a stern frame removed to undertake electric welding repairs. The rudder and rudder stock should be visually checked for fractures so far as accessible, tightness of the palm bolts (or the covering cement intact), condition of external rudder stops, presence of rudder lift prevention arrangements, weardown of the carrier bearing, condition of securing arrangement and clearance of the gudgeons and pintles. On vessels fitted with clamp-type steadiment bearings, the clamp bolts should be checked for tightness. Retaining screws or securing arrangements for rudder gudgeon bushings call for close attention. These frequently work loose and must be wired or heavily punched or staked. A number of rudders have been lost due to pintle nuts backing off allowing the pintle to come out. Pintles installed with the nut and
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taper upwards are especially susceptible to this problem. Sometimes the pintle simply unscrews out of the nut leaving the nut in place, because the stopper arrangement did not include both nut-to-pintle and nut-tocasting welding. Such welding on large vessels incidentally, needs to be relatively heavy to survive the vibration and corrosion until the next drydocking, 6 mm to 8 mm is suggested. For taper-up pintles also, a stopper bar or plate (with a drain hole) welded below the pintle is suggested as additional prevention against pintle (or bushing) dropping out. Spade-type rudders are especially prone to structural failure if there is excessive clearance in the rudder stock bearing - anything over 4 mm clearance usually calls for remedial action. Slightly more clearance is permissible on the lower pintle of rudders fitted with both upper and lower rudder bearings, normally up to 6 mm, assuming the next drydocking in 2-1/2 years. Rudder carriers are usually oil or grease lubricated metal bearings and maximum clearance of the radial or guide bearing portion should be based on standards for this type bearing, usually in the range of 3 mm to 4 mm. Regarding bottom painting, it was once common practice to record the vessel's position on the keel blocks and then either "fleet" (short) the vessel so that unexposed keel or bottom plates in way of same, could also be scraped and painted, or relocate it for the same purpose at next drydocking. This is rarely done anymore but points up to a possible corrosion area that may require special attention.
3.1 Rudder Pintle Clearances For conventional-size ships (say over 61M) with lignum-vitae or laminated-phenolic-resin gudgeon bushings and for the usual semibalanced or unbalanced rudder with two or more pintles one is justified in recommending new bushings if the clearance exceeds 6 mm. More than this will frequently lead to rudder pintle problems before the next Drydocking Inspection is due. For small craft this maximum may be reduced to say, 4.5 mm. For single pintle rudders the usual limit is 4.5 mm, and for the metallic guide bearing above, 1.5 to 3.0 mm. For the rudder axle bearings of "SIMPLEX" type rudders, a 4.5 mm limit may also be applied.
3.2 Rudder Carriers Inspection of manual self-aligning roller bearing type rudder carrier -requirements for inspection and/or measuring of possible radial and axial (vertical) weardown: a)
Classification societies usually require that rudders, rudder pintles and gudgeons together with their respective securing
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arrangements be examined and placed in satisfactory condition. The condition of the carrier and steadiment bearings and the effectiveness of stuffing boxes are to be ascertained when the rudder (rudder stock) is lifted. Therefore opening out of subject carrier bearings would not normally be required for the completion of a Special Survey. Unusual circumstances, like obvious damage, hard running noise or sluggish rudder movement detected during steering gear operational testing would warrant opening out. b)
It appears that opening out of this type carrier bearing could be carried out since the stock could be slightly lifted and secured, allowing for the split distance piece, bearing cover and the bearing housing to be removed. After washing out the roller bearing the outer race of the bearing could be displaced up and down circumferentially and the radial and axial (vertical) weardown, as well as the condition of the inner and outer raceways, rollers, and roller cage could be ascertained.
c)
During normal drydocking surveys the vertical weardown could be detected by using a depth gauge and/or template measuring the distance from the stock shoulder (325/358 mm diameter) to the base of the bearing housing, and/or the distance between the top of the bearing housing and the underside of the tiller. The results of subsequent measurements would be compared for any indication of weardown. Excessive weardown in radial direction could be detected only by visually observing relative movement and measuring the clearance fwd/aft and port/starboard at the gap between the distance piece and the bearing cover while the stock is turned.
d)
Additionally, damage to the bearing caused by vibration, such as might result from hammering the rollers into the inner and outer raceways of the bearing, could be detected by close listening (perhaps with the aid of a stethoscope) when turning the stock/ rudder from hard over to hard over.
3.3 Conventional Rudders Inspection Notes: a)
If the wood staves wear too thin, they may be able to fall out through the space between the wood-retaining ring and the pintle.
b)
Older wood, which has lost most of its natural oil, if allowed to dry out in drydock more than three or four days, will frequently crack and fall out after returning to service. (Keep it wetted in drydock).
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c)
Ensure that the pintle tapers are firmly seated in the gudgeons with sufficient contact, say at least 65%, and particularly at the bid end. Wastage can be built-up by properly-controlled welding procedures. Alternatively, certain approved bedding compounds may be acceptable, subject to approval of the Division Headquarters office.
d)
Rapid wear of gudgeon bushings is often due to operating in rivers (sand) or distortion of the rudder. Rudder distortion can result from unsymmetrical welding stresses which have been only temporarily removed by surface heat straightening. Press straightening or corrective offset machining of the pintle seating may be required in such cases.
e)
Always make sure that wood-retainer rings are properly secured -preferably 6 to 8 mm intermittent welds, since screws - even though centre-punched - usually work out quickly.
f)
Make sure that pintle nuts and pintles are both solidly secured from working loose under the severe pounding, vibration and turning that pintles are normally subjected to (pintle-to-nut and nut-to-casting). Safety bars or closing plate (with hole) across bottom of recess are good additional protection to prevent the pintle from dropping out when the pintle is mounted taper up. Stopper welds ought to be 6 to 8 mm at least.
g)
Pintle aperture access plates are sometimes lost due to inadequate attachment welding or from panel vibration. A stiffener may be helpful in the latter case. Attachment welding should usually be at least 6 mm, however intermittent welding may be acceptable.
3.4 Rudder & Rudder Posts/Horn Defects 3.4.1 Rudder Horns The steel castings of stern frame horns have, in a number of cases, been found fractured in the area of the bossing for the lower gudgeon, see sketch. This critical area should be cleaned and examined at close range when examining the vessel in drydock. Any suspicious surface imperfections should be explored thoroughly with magnetic particle, dye penetrant or ultrasonic method. Suspicious areas may also be chipped out and ground smooth to remove minor surface imperfections. Any findings and the necessary repairs carried out should be reported upon in the report of the drydocking for future reference.
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3.4.2 Semi-balanced Rudders Loose pintle nuts and excessive bearing clearances are often encountered. Portable plates on the Rudder for access to the pintle nuts should be removed at each Special Survey for complete examination. These should also be checked during drydocking inspections where both nut, locking devices and spacers, if fitted, should be examined for efficiency. If repairs are carried out, these should be recorded for future reference. The clearance between the pintle and its bearing should be accurately determined, preferably with the aid of a dial indicator, as this is felt to be the critical point. It is difficult to state the acceptable clearance but, for general guidance, and dependent to some extent on the bushing material used, approximately 1% of the pintle diameter to a maximum of 1/4 in. would be acceptable for a further three year service. The vertical distance of the rudder proper should be determined in relation to the stern frame horn. Should this clearance be found to be less than 1/4 in., corrective measures to the carrier bearing should be taken and the carrier bearing should be examined.
3.4.3 Vessels having a Rudder Post Fractures have been found, upon examination at close range, in the rudder post casting just above or below the gudgeons. Similar fractures have been found in the rudder frame casting. In some cases the fractures in the rudder post extend from or into the core hole closing plates on after side of the rudder post. During drydocking inspection the rudder frame casting and the rudder post casting should be carefully examined at close range for possible fractures. When conditions are suspect, the rudder post casting should be test drilled about 6" above the welded joint to determine if water has entered the rudder post. Should water be found in the rudder post, the post should be tested to locate the origin of the leakage. If repairs necessitate the removal of the rudder, the pintle clearances as well as the rudder alignment should be carefully checked. Should water be found in the rudder or fractures found in the vicinity of the rudder frame gudgeons, a portion of the rudder side plating should be removed for examination of the cast rudder frame arms to which the horizontal diaphragms are attached. Particular attention should be directed to the casting at the mid-height of the rudder.
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3.4.4 Tapered Rudder Stocks There have been failures of the locking devices for rudder stock nuts on designs using tapered fit attachment between rudder and rudder stock. Such failures will permit the rudder stock nut to back off, so that the interference at the taper fit between the stock and the rudder is lost, damaging both the keys and tapers, which might lead to serious rudder damage. In order to try to prevent, insofar as practicable, this type of trouble on vessels having this arrangement in lieu of the bolted palms, the keeper and the nut should be carefully examined at regular intervals. Should there be indications of failure of the keeper, or a loose fit, it should be removed for inspection, and consideration should be given to additional or possibly more substantial arrangements.
3.4.5 Rudder Pintles Pintles are without a doubt the main known cause for loss of rudder. In most cases the loss could be attributed to a lost pintle. It is very important that the tightness of rudder pintles should be examined each time a vessel is physically drydocked. Particularly it should be confirmed that the locking arrangements are in place and will effectively prevent the nut from becoming slack and thus avoiding loss of the pintle and subsequent loss of the rudder. In connection with pintle locking devices it is to be emphasized that the arrangements should be such that the pintle cannot work free. There could be instances when the pintle backed off and left the nut securely held in place by a clip welded to the rudder frame casting. It is essential that the nut be positively secured to preclude "turning" of the pintle. In view of these casualties, it is recommended that, when a pintle is found loose or is withdrawn for any other reason, it be examined by magnetic particle or other suitable method, particularly adjacent to the sleeve and between the threads and the small end of the taper. In the course of a drydock inspection the condition of the pintle bushings, sleeves, nuts, and locking devices should be carefully examined as far as is practicable. The discovery of a loose pintle would, for example, justify its removal for further examination and verification of proper fit. When replacing pintles ensure that the pintle tapers are firmly seated in the gudgeons with sufficient contact, say at least 65%, and particularly at the big end. Wastage can be built-up by properly controlled welding procedures.
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3.4.6 Rudder Pintle Clearances (General guidance) In general initial clearances should be around 0.050" for a 6" pintle to around 0.070" for a 14" pintle. For in service acceptable clearance about 1/64" clearance per 1" of diameter of pintle up to 1/4” maximum is generally considered reasonable for another two years of service. . Pintle diameter inches 6 7 8 9 10 11 12 13 14
m/m 152.4 177.4 203.2 228.6 254 279.4 304.8 330.2 355.6
Acceptable for 2.5 - 3 years Inches 3/32 = 0.0937 7/64 = 0.109 1/8 = 0.125 9/64 = 0.140 5.32 = 0.156 11/64 = 0.171 3/16 = 0.187 13/64 = 0.203 7/32 = 0.218
m/m 2.4 2.8 3.2 3.8 4.0 4.4 4.8 5.2 5.6
The above applies to semi-balanced or balanced rudder with lignumvitae or laminated-phenolic-resin gudgeon bushings.
3.4.7 Twisted Rudder Stocks (General guidance) A.
When a rudder stock is twisted the extent of the repair will depend on the following parameter: < = angle of twist in degrees L = length of stock over which the twist appears uniform d = diameter of twisted portion of stock
B.
When < is less than or equal to L/d the stock may be accepted for further service without any form of heat treatment provided it is established by visual and magnetic particle examination that the stock is free from surface cracks or other significant defects.
C.
When < is greater than L/d but less than or equal to 5L/d the stock is to be removed and given a stress relieving heat treatment. A suitable temperature range for this treatment is 600 to 650C with a soaking period of not less than 1 hour per 25 mm of diameter.
D.
When < is greater than 5L/d the stock is to be removed and given either a full annealing or normalizing heat treatment. A suitable temperature range for either of these treatments is 860 to 900C then a soaking period of not less than 30 minutes per 25 mm of diameter. For full annealing the stock is to be cooled slowly in the furnace, while for normalizing it is to be cooled in air.
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E.
The furnace used for the heat treatments required by (C) and (D) should be large enough to take the entire stock and should be properly equipped with means for temperature measurement and control.
F.
Where necessary a new keyway is to be cut and weld repairs are to be carried out in accordance with an approved procedure and, where possible, should be completed prior to the heat treatment required by (C) and (D). In this respect either full annealing or normalizing is acceptable as a post weld stress relief heat treatment.
G.
On completion of heat treatment the surfaces of the portion of the stock affected by twisting are to be suitably cleaned and examined by magnetic particle methods.
H.
Subject to satisfactory results from magnetic particle examination and to compliance with the above requirement for heat treatment, repair of the rudder stock may be regarded as permanent.
3.4.8 The Repair of Forged or Cast Steel Rudder Stocks and Pintles by Welding The following notes are intended for the general guidance when it is proposed to carry out any form of weld repair to rudder stocks or pintles. These notes do not apply to the weld cladding of rudder stocks or pintles in way of the bearings as an alternative to the fitting of shrunk on liners. Size and location of defects Repairs, except those of an emergency nature, should only be attempted when the depth and location of the defective area is such as to provide adequate access for welding and inspection. Facilities for repair The welders employed for the repairs are to be experienced and competent to carry out this type of work. Whenever possible, the rudder stock should be removed from the ship and the repairs carried out in a properly equipped workshop under controlled conditions. Removal of defects and preparation for welding Complete removal of all defective material is essential for a successful repair. However, the material removed should be the minimum consistent with this and the excavation should be shaped so as to allow good access for welding. The complete removal of all defective material is to be verified by magnetic particle examination before welding is commenced.
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Welding consumables These should be of an approved low hydrogen type depositing weld metal with mechanical properties similar to that of the forging. Pre-heating Because of the relatively large mass and the consequent chilling effect, it is recommended that in all cases an adequate area around the repair should be pre-heated to about 100 oC. This pre-heat should be maintained until the repair is completed. Pre-heating temperature in excess of 100oC may be required. Welding As far as practicable, all welding should be done in the downhand position. Inspection after welding The surplus weld metal should be removed by machining or grinding and the surface of the repair area smoothed to a satisfactory profile. The area should then be checked for freedom from cracks and other defects by magnetic particle examination. Where extensive repairs have been carried out, ultrasonic examination may also be required for verification. Heat treatment: a.
A stress relieving heat treatment is to be carried out after completion of the repairs. A suitable temperature range is 600 to 650oC with a soaking period of 1 hour per 25 mm thickness of the repaired section.
b.
Rudder stocks and pintles should be treated in a furnace properly equipped with means for temperature measurement and control. In general, the furnace should be large enough to take the entire rudder stock. If welding is confined to a small area, the post weld heat treatment may be restricted to a suitable local area.
Final inspection After heat treatment and final machining or grinding has been completed the repaired area should be re-examined by a magnetic particle method.
3.4.9 ‘Simplex’ Type Rudders Inspection points: o
Fractures at sharp change of section on underside or rudder axle palm (top), also at top of taper on lower end of rudder axle.
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o
o
o
o
o
o
o
Rudder load being carried by conical-faced "emergency" bearing on lower end of rudder (to shoe piece). (Actually, there should be a slight clearance [1 to 3 mm] at this bearing under drydocking conditions with after peak tank empty). Excessive wastage or rudder axle adjacent to the bronze sleeves. This normally should be dealt with by cleaning, wire brushing, painting or epoxy coating. Weld build-up of the axle forging is not recommended, unless the excessive wastage extends over at least 25% the area surface, and then only with proper approved procedure and heat treatment. Excessive wastage on rudder axle lower-end taper or of the axle between the bottom taper and the lower bronze sleeve. The latter should be protected with paint or epoxy coating. The taper may need to be re-machined however, to ensure tightness of the axle in the seating. Insufficient locking arrangements or slack nut on rudder axle lower end securing. Loose or fractured palm bolts at upper end. "Notches" or stress raisers at top or bottom of rudder axle. These should be smoothed out as possible. Excessive clearances of rudder axle in rudder bearings (4.5 mm maximum recommended).
Gudgeon retaining rings (for the lignum-vitae staves) not properly secured.
o
o
With axle tight in lower taper (in shoe piece), palm at upper end of axle should bolt up without any "springing" or forcing to lineup holes.
.
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“Simplex” Type Rudder
3.5 Bilge Keel Surveys In the past bilge keels were sometimes thought of as unimportant appendages from a structural point of view. However recent experience has shown that poor design, fabrication or repair of bilge keels can have catastrophic results. In one case a weld defect in a bilge keel formed a notch and was responsible for the initiation of a brittle fracture which sped
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rapidly across the bottom of the hull and partly up each side culminating some hours later in the ship breaking in two. o
o
o
o
o
o
Bilge keels, where fitted, are to be attached to the shell by a landing bar (doubler). The bilge keel and landing bar are to be continuous throughout their respective lengths with full penetration welds at the butt joints and continuous attachment fillet welding. Scallops and cut-outs are no longer recommended for new construction. Where desired, a drilled hole at least 25 mm (1 in.) in diameter may be provided in the bilge keel butt weld as close as practicable to the doubler. The class and grade of material for bilge keels and landing bars to which they are attached are to be required as per classification rule requirements. The thickness of the landing bar should be about the same thickness as the shell plating to which it is attached. The ends of the landing bars are to be rounded or tapered. The ends of the bilge keels are to be tapered down smoothly and should terminate in line with a floor or transverse. Shell plating butt welds in way of the landing bar and landing bar butts in way of the bilge keel are to be chipped or ground flush prior to installing the bilge keel and landing bar. Proper weld sequence is to be followed in bilge keel installations. When attaching the backing bar a root of at least 6 mm should be used to allow sound weld penetration into the shell plating. Butts in landing bars and bilge keels should be staggered (offset) from each other and from the shell butts by at least 150 mm. Butts in the bilge keels and landing bars are to be welded before the adjacent fillet welds are made. Initially, fillet welds should be carried no closer than 300 mm to butt welds, i.e., fillet welds should not be carried across unwelded butts. Butts should be welded first and then fillet welds completed. Where possible, weld from amidships forward and aft or towards free ends. Butt welding in bilge keels and especially their respective landing bars are considered critical. Care should be taken to insure sound welds and representative butt welds should be non-destructive tested. Where rolled sections such as bulb angles are used for bilge keels special care should be exercised in the alignment between sections and to ensure full penetration in way of the bulb. After welding, joints should be ground smooth. During drydocking inspections, bilge keels, landing bars and attachment welding should be cleaned and carefully examined for fractures. There are, of course many vessels currently in service with the bilge keels welded directly to the shell with no intervening landing bar. Unless there is a problem with fractures, no modifications are considered necessary to these existing installations. However if fractures are found, particularly at the fillet welding wrap-around at scallops and ends, consideration
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should be given to modifications by insertion of a landing bar under the full length of the bilge keel or alternatively, to help prevent fractures at the ends, a rounded pad plate of about the same thickness as the local shell plating should be fitted under the bilge keel termination’s at each end. Damaged bilge keels must be smoothly cropped or properly repaired if there is distortion, fractures or notches that could affect or progress into the shell plating. In particular, notches or fractures in the outer edge of the bilge keel should be ground smooth or gouged out and rewelded as necessary. For repairs to bilge keels due consideration must be given to above noted remarks regarding proper welding procedures.
3.6 Drydock Propeller Inspection 3.6.1 General Precautions o
o
o
o
o
o
o
No repair of a propeller can be performed in place in drydock as satisfactorily or as safely as it can be done under controlled conditions in a propeller repair shop. Only very minor straightening and no welding should be done while the ship is in the water or the propeller is still on the shaft in drydock. Welding repairs should never be made to the propeller unless the work is done by qualified personnel under informed supervision. Many repair yards are not equipped with either qualified personnel or proper equipment necessary for satisfactory propeller repair. Welding on the hub or near the blade root should not be attempted in the drydock even with the propeller removed from the shaft because of the difficulty of obtaining sufficient sustained heat for proper stress relief. Also, heat in the hub area, if improperly applied, can distort the bore and spoil the propeller fit on the shaft taper. Frequently, on the first few minor repairs made on the propeller, the cost can be held down by simply trimming back the blade edges to sound metal as they become ragged and torn. This obviously can be done only a few times before the vessel's performance suffers. Excessive edge trimming leads to poor manoeuvring ability, lower backing power, increased fuel consumption and cavitation erosion. It can also lead to propeller imbalance. When the diameter is trimmed, the power absorption and fuel consumption of the propeller suffer even more sharply. The more trimming done, the more costly will be the eventual repair to restore the blades to design size. It is recommended that records of all repairs for each propeller are maintained carefully. It is particularly important to record each
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blade edge trimming and diameter reduction and their later restoration. It also will be of value to note each straightening, weld repair, stress relief or other heating and the procedures followed. Records of minor repairs made "in-place" are especially useful when evaluating later major damage. o
The necessity for stress relief when required cannot be overemphasized. Not applying proper stress relief treatment to a repair presents a greater danger to the propeller than that presented by a poor weld. The poor weld merely offers the possibility of failure to the repair but the lack of proper stress relief can result in cracks forming in any part of the heat-affected zone. Such cracks can be expected to grow rapidly and can lead to condemnation of the propeller.
3.6.2 Propeller Wear The bronze propeller is an expensive and scientifically designed ship part which requires considerable care and attention. The propeller in use suffers wear like any other moving part and its life, aside from direct physical damage, is determined by its rate of wastage - a corrosive and erosive process. As far as corrosion alone is concerned, bronze propellers in still or moving sea water may lose 0.05 to 0.10 mm (0.002 - 0.004 in.) of surface metal every year. The wastage by corrosion over the blade tip area of a clean undamaged propeller working at its designed revolutions may increase by 4 to 5 times if the blade surfaces are rough. However, wastage by erosion may be much greater than wastage by corrosion. Highly loaded manganese bronze propeller blades have shown wastage up to 1.27 mm (0.050 in.) a year. These high erosion losses have lead to the development of the improved, more resistant alloys. The newer propeller alloys, though more costly, are stronger and much more resistant to wastage. Since propeller efficiency is largely dependent on drag, propeller surfaces should be polished smooth. The effect on drag of roughness on the suction face of the blade is considerably larger than the effect of a corresponding degree of roughness on the pressure face. Consequently it is extremely important that the suction faces be very smooth - not merely painted, as is sometimes done. Since the propeller material itself is much more resistant to the scrubbing action of sea water than is any paint, the right way of getting a smooth surface for highest efficiency is by machine polishing. Whereas elimination of surface roughness pays measurable dividends in fuel oil savings, care must be exercised to avoid introducing humps and hollows. For this reason only cup wheels should be used when grinding with stones (wheel type polishers which grind with the rim should be prohibited). Final polishing may be done with discs. During all polishing the machine should be kept moving to avoid making local low spots.
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3.6.3 Protection For New Ships The life of a propeller as well as the life of the propeller shaft liner can be affected by local galvanic conditions while the ship is still berthed in the Builder's yard. When immersed in salt water, the highly polished propeller can become the cathode in the hull-propeller electrolytic cell unless there is counteracting cathodic protection. One effect of lack of counteracting cathodic protection is the formation of a thin hard film of calcium and magnesium carbonates ("cathodic chalking") which in subsequent service may wear away unevenly and cause increased wastage. Should such a film be deposited, it, along with any other fouling, should be removed by light polishing. 3.6.4 Propeller Protection During Drydocking When painting is being done in the area of the propeller, special care should be taken to protect the propeller from paint drippings. A ridge of paint on a propeller disturbs normal water flow over the blade and greatly accelerates erosion wear in that area. For the same reason, if the vessel's name is painted onto the blade surface, it should be removed before reinstalling the propeller. 3.6.5 Physical Damage Physical damage to propeller blades should receive prompt attention. For example, a bent leading edge creates a condition of disturbed flow for a considerable distance on the blade surface which can cause serious cavitation erosion damage. While minor damage may be too slight to have any observable effect on the performance of the vessel, quite small edge deformation (nicks, bends, etc.), particularly at the leading edge, can result in an important increase in the normal rate of erosion wastage of the blade surface. o
o
o
Propeller damage requiring repair falls into either or both of the following general classes: Physical damage in the form of breaks, tears or bends in the blades resulting from impact with foreign bodies in the water or from unsatisfactory repairs. Propeller damage in the form of wear, as the result of erosion and/or corrosion. Wear damage is usually the result of one or more of the following:
Normal scrubbing action of the water. Local cavitation caused by physical damage. Cavitation due to design characteristics. Hull obstructions in the slip-stream ahead of the propeller, such as poorly placed anodes or lifting eyes. These may cause local water turbulence and result in cavitation damage.
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Electro-chemical attack resulting in wastage and/or dezincification of the blade surface. (Dezincification sometimes occurs on manganese and nickel manganese bronze propellers, producing a copper colour surface). Attack by chemicals contained in harbour water. Abrasive action of sand or other bottom solids when operating in shallow water.
Water temperature may also be a significant factor influencing the rate of erosion, particularly for highly-loaded propellers.
3.6.6 ‘In-Place Repairs’ All but very minor repairs to propellers while on the shaft (afloat or in drydock) present many practical problems. Proper access to the work areas is difficult and, when afloat, control of heating and cooling rates is impossible. Therefore, propellers extensively repaired "in-place" can be expected to give trouble later on in service. No "in-place" repair in drydock should be attempted which requires heat unless all prescribed precautions are taken. It is much better and safer to do the job off the ship in a qualified shop. 1.
"In-place" straightening should be limited to areas within 100 mm (4 in.) of the edge and to thickness no greater than 32 mm (1¼ in.).
2.
Corrosion roughness can be repaired by grinding and polishing inplace, taking care not to introduce local humps and hollows in the blade surface.
3.
Occasionally surface cracks which appear to end are found to continue beneath the blade surface. When a crack-arresting hole is drilled at the visible end of the crack, a dye-penetrant examination of the drilled hole should be made to determine whether the crack extends sub-surface beyond the drilled hole. If the crack does not extend beyond the hole, the hole may be plugged and the plug peened over until permanent repair can be made.
4.
Propeller repairs in the shop are always made in the flat or horizontal position. This is never possible with an 'in-place" repair. However, the propeller should be turned to obtain a position nearest to the horizontal so that the repairer can work to best advantage. It may be necessary to turn the propeller several times before a job is finished. Proper positioning of the propeller is essential for quality repair work.
5.
Torches used in preheating or post-heating repaired areas should be of the soft flame type, such as city gas, Hauck oil torches, or similar torches with large flame and low heat. Propane or oxyacetylene torches or other sources of high, concentrated heat should not be used due to the risk of thermal gradient cracking. It
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is essential that the low-heat flame be kept moving and the temperatures carefully controlled, taking care not to overheat any area and avoiding steep temperature gradients. This should be followed by slow cooling under insulating blankets. 6.
Although good weather favours the quality of drydock repairs, it is unnecessary to postpone a minor propeller repair on drydock during inclement weather providing the stern section can be protected by canvas screening. Proper protection is particularly important during the winter months when cold winds can seriously affect the heating procedure, causing the temperature of the material to fall into a range where the material becomes brittle and cracks are almost certain to occur.
3.6.7 Propeller Removal It is of utmost importance that proper care be taken when removing the propeller from the shaft. Propellers of manganese bronze or nickel manganese bronze compositions are quite susceptible to stresscorrosion cracking. Cases are on record of manganese and nickel manganese bronze propeller hubs having cracked because of improper application of heat used to facilitate removal of the propeller from the shaft. Stress-corrosion cracking of this type is easily caused by high temperature locally applied flames, of which oxy-acetylene and oxypropane are most dangerous. If it is necessary to apply heat when removing a manganese or nickel manganese bronze propeller from the shaft, it is important to use a constantly moving low-heat flame in order to avoid setting up excessive stress. (Note: As an alternate, the use of solidified CO2 ("dry ice") held in place by layers of packing around the exposed shaft, fore and aft of, and insulated from the hub, is a safe and sometimes effective way of loosening a propeller). Heating temperatures should be evenly distributed and checked with "Tempilsticks" or a surface contact pyrometer, taking care not to exceed 200C (392F) at any time during the heating operation. Uniform heating and cooling in way of the hub is essential to avoid major bore distortion. When carried out carefully under controlled conditions stress-corrosion cracking will rarely occur. Since stress relief treatment is troublesome, time consuming and difficult to control, it should be obvious that when removing a propeller from the shaft every possible effort should be made to distribute the heat evenly and not exceed the maximum temperature of 200C, so that the need for post-heat treatment may be avoided.
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3.6.8 Blade Edge Damage Blade edge damage usually takes the form of cracks, bends, or breaks and may include the loss of a small section of the blade. Minor bends or tears can sometimes be repaired without removing the propeller from the shaft. Very slight distortions along the blade edge can usually be straightened cold by hammering carefully. After straightening however, the area worked should be dye-penetrant examined for cracks which may have resulted from the hammering. A bend which may appear to the eye to be confined to the edge actually may extend a considerable distance into the blade. If there is any question as to the extent of the bend, the plan should be examined and blade checked with a pitchometer and/or gages. When this is not possible, a straightedge may be applied to the surface radially and compared either with the drawing or with the undamaged blades to assist in estimating the extent of the bend. Bends extending more than 100 mm (4 in.) in from the blade edge should be considered to be a shop job. Cracks require careful scrutiny to ascertain their extent and it must be decided if an in-place repair will suffice or if a shop job is necessary. Cracks are dangerous and are potential fatigue nuclei for a major break and possible loss of that section of the blade should that area receive a heavy blow. The permanent repair of cracks requires complete removal of the crack, followed by welding using the prescribed welding procedure for the particular metal. Edge damage which is impossible to repair with the propeller in-place, but not requiring shop repair, requires proper positioning of the blade on the drydock floor. When large areas of the blade edges or tips have been broken off, it is necessary to "burn-on" or weld-on a cast piece to replace the lost section. This procedure requires equipment and melting facilities not usually available at ship repair yards, therefore it is recommended that this type of repair be done by a propeller manufacturer.
3.6.9 Inspection propellers
of
cycloidal
propellers
and
controllable
pitch
All propellers are normally externally examined at each Drydocking. During Special Survey, the examination should include at least a functional test and oil leakage check. At Special Survey No. 2 and alternate Special Surveys thereafter, the machinery and blade assembly may be required to be dismantled, cam lobe and gear teeth
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condition checked, clearances measured, and the seal rings examined. Any worn or deteriorated parts should be repaired or renewed as necessary. Controllable pitch propellers are also to be examined externally at each drydocking, however the blade pitching mechanism is usually examined during the Tailshaft inspection. All such examinations should include at least functional test and leakage inspection. One or more blades may be required to be removed if leakage, damage, or malfunction is found or suspected, in order to check the seals, seal surfaces, and internal moving parts.
3.7 Work on Ship’s Salt Water System
The importance of careful inspection of rubber or other non-metallic expansion joints in seawater piping systems cannot be over-emphasized since any failure of these joints can result in very rapid flooding of the entire machinery space. 3.7.1 Aging Deterioration Since the principal materials in these expansion joints are usually rubber and cotton fibers, aging deterioration is inevitable. This deterioration may manifest itself as a loss of external surface resilience, then cracking; or alternatively, the arch of the bellows may become softened indicating loss of adhesion between the fabric plies. The outer cover should be visually examined for evidence of cracks. Shallow cracks in the cover which do not expose the reinforcing fabric of the expansion joint are not considered serious enough to warrant replacement of the joint, however cracks that expose the reinforcing fabric should be examined closely to determine the condition of the fabric. If the reinforcing fabric is torn, cut or otherwise degraded the expansion joint should be replaced prior to the vessel sailing or suitable temporary repairs carried out to enable vessel to proceed to a repair port. If the reinforcing fabric is merely exposed, and does not show signs of being degraded the expansion joint should be replaced at the earliest opportunity. Also examine areas in grooves where salts or other residues accumulate and also surfaces partially hidden from view such as under the rim of the bolting ring and on the underside of the expansion joint. Failure may initiate either inside or outside the joint, depending on the service conditions. Remarks noted above for the outer cover apply equally to the inside tube.
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Except as noted above, repairs to non-metallic expansion joints are not advisable and should be confined to minor repairs on blemishes on the outer cover surface.
3.7.2 Misalignment of the Circulating Piping System Misalignment or offset of the circulating piping system will materially reduce the life of the expansion joint. The maximum lateral misalignment allowable is generally 1/2" for all sizes of joints and should be checked if misalignment is suspected. Piping supports for the circulating system should be checked and renewed if found damaged or wasted. The weight of the piping should not be carried by the expansion joint. 3.7.3 Examination at Drydocking The non-metallic expansion pieces in the sea water circulating systems be examined at each drydocking inspection. In addition when vessel is placed on drydock non-metallic expansion pieces in the main sea water circulating systems are to be cleaned and examined both externally and internally. Expansion Piece Details TUBE A single piece, leakproof tube made of synthetic or natural rubber as service dictates. This is a seamless tube that extends through the bore to the outside edges of both flanges. Its purpose is to maintain the fluid tight integrity of the expansion joint and thereby protect the carcass from penetration or saturation of the material being handled. CARCASS a.
Fabric Reinforcement The fabric reinforcement is the flexible and supporting member between the tube and cover. Fabrics of high strength synthetic fibers are used depending on pressure and temperature requirements. All fabric plies are impregnated with rubber or synthetic compounds to permit flexibility between the fabric plies to reduce service strain.
b.
Metal Reinforcement
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Copper coated steel wire or solid endless rings imbedded in the carcass are used as strengthening members of the joint. This adds to the strength of the joint, permitting the rated working pressure required and supplies the rigidity to the joint for vacuum service. A hard rubber filler between the rings prevents the reinforcing ring from moving when the joint is pressurized. COVER The exterior surface of the joint is firmed from natural or synthetic rubber, depending on service requirements. The primary function of the cover is to protect the carcass from outside damage or abuse. INTEGRAL FLANGES Construction of resilient rubber, and smooth finished, the full-faced flanges form a tight seal against the pipe flange without the need of gaskets.
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