Ship Structure - Plating & Seams — Most Asked Questions
29 questions asked about ship structure - plating & seams across the Fn 3 and General and Fn 6 papers, heaviest first.
Learn the system first
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1 Coffin plate? Why called so
Also asked: “Coffin plate. Why it is called coffin plate”, “Why named as coffin plate”, “coffin plate? Why name is coffin?”
Answer
The coffin plate is the dished aft-most keel plate wrapped around the stern-frame extension — named for its coffin outline.
- It joins stern frame to flat plate keel where the frame runs two or three spaces forward for a sound connection.
- Dished to fit the frame foot, thicker than nearby shell where circulation and stress concentrate.
- Called coffin from its tapered six-sided shape on the shell-expansion drawing, not from burials.
- Survey point: wastage and cracking at the frame-to-keel junction, checked at dry dock with the stern view open.
Surveyor follow-up: point at the shell expansion and name the plate — then the strake next to the keel.
Full theory: Shipbuilding Materials, Welding and Weld Defects -
2 Cast iron Welding
Answer
Cast iron welds with nickel electrodes, never mild steel — butter the joint, weld short and cool slow, or it cracks.
- Use nickel-iron electrodes; a mild-steel rod dumps carbon into the weld and turns it brittle.
- Lay a thin buttering layer first so the main weld never drinks carbon from the casting.
- Weld short runs on clean metal, peen each pass to kill shrinkage stress, preheat heavy castings and cool slowly.
- Cylinder heads and exhaust casings earn this care; pressure parts in doubt get renewed, not welded.
Surveyor follow-up: weld cracks overnight with correct rods — preheat, peening or cooling, and which one failed?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
3 Defects in welding
Also asked: “Faults in weldings”, “Welding defects, and reasons for those”
Answer
Weld defects are undercut, slag, no penetration, porosity and cracks — each has a cause and each has a cure before the next pass.
- Undercut from too much current, long arc or overheated metal — cut current, shorten arc, steady travel.
- Slag inclusion from trapped oxides and uncleaned beads between passes — clean every bead before the next.
- Lack of penetration at the root under tension or bending load — open the groove, set root gap, prove fusion.
- Porosity, cracks and overlap from damp rods, dirty plate and rushed cooling — dry rods, clean steel, controlled heat.
Surveyor follow-up: line of slag mid-weld on a multi-pass butt — grind the cap or gouge the joint, and how deep?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
4 Safety while arc welding
Also asked: “Safety u will take while welding”
Answer
Arc welding safety is eyes, skin, air and fire: screen the arc, cover the body, ventilate the space and guard the cables.
- Insulated rubber shoes, dry clothes, full body cover, welding apron and goggles or helmet against arc flash.
- Screen others from the arc, clear combustibles, post a fire watch with an extinguisher ready.
- Ventilate for fumes, mind electric shock from damp holders and leads, isolate before changing rods.
- Hot work permit first — enclosed space and tank-top welds never start on a verbal okay.
Surveyor follow-up: welding on the main deck in rain with dry gloves — proceed or stop, and what decides?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
5 Tig mig welding?
Answer
TIG welds precise with tungsten, MIG welds fast with wire: both shield the arc in inert gas for clean marine joints.
- TIG: non-consumable tungsten electrode with argon shielding — slow, precise, best for thin and critical joints.
- MIG: consumable wire feeding through the torch under gas — fast deposition for production welding.
- Both beat open arc on quality where surveyors read radiographs; defects still read by visual, dye, ultrasonic and X-ray.
- Pick by joint: TIG for root precision and pipe, MIG for filling shell and structure at speed.
Surveyor follow-up: root pass on a fuel-tank butt — TIG, MIG or stick, and what test proves it?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
6 Enumerate at least five common faults in welding and measures to prevent. Substantiate your answer with suitable sketches
Answer
Sketch five faults with cures: undercut, slag, no penetration, porosity and cracks — current, cleaning and groove decide.
- Undercut from hot current and long arc — cut amps, shorten arc, steady travel with sketched groove edge.
- Slag trapped between passes from dirty beads — clean every bead before the next with sketched inclusion.
- No penetration at the root from tight groove — open root gap and prove fusion with sketched section.
- Porosity and cracks from damp rods and dirty plate — dry rods, clean steel, controlled heat with sketched pores.
- Prevent by procedure, prove by visual, dye, ultrasonic and X-ray in that order.
Surveyor follow-up: draw the undercut section — where does the notch sit, and what current change cures it?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
7 Margine plate,margine line
Also asked: “Margin line/plate”
Answer
The margin line caps flooding draught 76 mm below the bulkhead deck edge; the margin plate bounds the same limit in steel.
- Drawn 76 mm below the bulkhead-deck top at side, thinnest deck governing unless mean-plus-50 applies.
- Floodable and permissible lengths keep damaged waterlines below it — the subdivision ceiling.
- Margin plate is the structural strake carrying the same boundary in the steelwork.
- Breach it in calculation and the compartment fails subdivision — move on to the next bulkhead.
Surveyor follow-up: waterline touching the margin line in the damage case — pass or fail, and by how much?
Full theory: Load Line, Freeboard and Tonnage Measurement -
8 Thickness of plimsoll marking of dimensions
Answer
Plimsoll steel reads 300 by 25 deck line, 300 disc and 230 by 23 seasonal lines 540 from the disc centre.
- Deck line 300 mm by 25 mm on the freeboard deck edge as the datum.
- Disc 300 mm diameter 25 mm thick with summer bar through its centre.
- Seasonal lines 230 mm by 23 mm forward and aft of a vertical 540 mm from the disc.
- Punched in steel, painted for contrast — paint fades, steel remembers.
Surveyor follow-up: tape the disc and one seasonal line — dimensions and upper-edge rule first?
Full theory: Load Line: Marks, Letters & Conventions -
9 write short notes on A. Gear/Bearing puller. B. Taps and Dies C. Surface plate D. Vice and their usage on board
Answer
Four workshop basics: pullers draw, taps cut inside, dies cut outside, plates prove flat, vices hold — each with its onboard job.
- Gear and bearing pullers draw tight fits with push-bolts and jacks — even pressure, never hammers.
- Taps cut internal threads in taper, plug and bottoming cuts for blind and through holes.
- Dies cut external threads on rods and studs with cutting spray and two-turn discipline.
- Surface plates prove flatness under blue; vices hold work square with soft jaws on finished faces.
Surveyor follow-up: stud thread torn with the die — die, rod or spray, and do you cut or renew?
Full theory: Maintenance and Troubleshooting — Overhauls, Tappet Clearance and Reading a Fault -
10 Ans. The principle of arc welding process
Answer
An arc is struck between a consumable electrode and the work, and its heat melts both together into the joint.
- One electrode is the welding rod or wire, the other is the metal being welded; both connect to the supply.
- Touching the electrode to the metal and withdrawing it a short gap creates a high-temperature arc.
- The arc melts the joint edges and the consumable rod, and the rod as it burns supplies filler metal.
- The flux coating gives inert gas shielding so atmospheric gases are not absorbed into the molten metal.
Surveyor follow-up: weld with porosity through the root — is it gas, cleaning or the coating, and how would you prove which?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
11 Briefly explain the process of welding two metals plates using gas; list the equipments required and how they are used to weld effectively?
Answer
Gas welding melts both edges with a torch flame: the right flame type makes the weld, the wrong one ruins the metal.
- Equipment is oxygen and acetylene cylinders, regulators, hoses, a torch with interchangeable nozzles, and goggles.
- Acetylene cylinders carry fusible plugs that melt near 105°C to release pressure safely under heat, charged near 17 bar.
- Equal oxygen and acetylene gives the neutral flame used for steels; an oxidising flame decarburises and a carburising one embrittles.
- Weld with the torch angled about 45 degrees, keeping the molten pool in view and moving along the joint as it fuses.
Surveyor follow-up: weld brittle at the root on a steel job — is the flame oxidising or carburising, and what does the bead show?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
12 Difference between normalising and aneling which welding is difficult CI & stainless steel..and why
Answer
Normalising refines grain; annealing relieves stress. Welding cast iron and stainless reverses the risk — one cracks, the other picks up contamination.
- Normalising heats above the transformation temperature and air-cools, giving a fine uniform grain and higher strength.
- Annealing heats and soaks, then cools slowly, producing a softer, ductile, stress-free structure with coarse grain.
- Cast iron welds badly because its graphite flakes act as stress raisers and it has no ductility — the crack starts in the heat-affected zone.
- Stainless welds differently: the heat-affected zone loses its chromium oxide film, so it corrodes; low hydrogen consumables and interpass control save it.
Surveyor follow-up: a stainless joint that passes the dye test and rusts six months later — is it stress, dilution or the heat-affected zone?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
13 I told him that first I will check the thickness of the gasket material, type of gasket material
Answer
Gasket checks are in that order: thickness, then material, then condition — because the wrong material fails whatever the thickness.
- Check the thickness against the flange recess first, since a thick gasket in a shallow recess is crushed or blown out.
- Then check the material: fibre, rubber, spiral-wound metal or flexible graphite, matched to the medium and the temperature.
- Check the condition for set, hardening, cracking or a damaged facing — a reused gasket is a gasket that will leak again.
- Then the flange faces themselves, because a sound gasket on a scored face is still a joint that weeps.
Surveyor follow-up: new spiral-wound gasket on a flange with a deep score — what is actually sealing, and where?
Full theory: Ship Pipelines — Seawater, Bilge, Ballast & Fuel Systems -
14 Less welding current
Also asked: “Low welding current wy”
Answer
Too little current gives a cold, under-penetrated bead; too much gives a wide, burnt one — the arc tells you which.
- Low current gives a narrow bead with poor fusion to the base metal and lack of penetration at the root.
- High current gives a wide, flat, spatter-heavy bead with excessive heat and distortion of the plate.
- Match the current to plate thickness and electrode diameter, then hold the arc length short and the travel speed steady.
- If the bead will not fuse, raise the current; if the plate distorts and spatters, lower it — before adding more heat.
Surveyor follow-up: bead wide and burnt with the plate visibly distorted — current, travel speed or fit-up first?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
15 Numbering of plates
Answer
Plates are found by letter and number: the letter gives the strake level, the number the position along it.
- Strakes are lettered from the topmost strake down the side, from A onward, on the shell expansion plan.
- Plates are numbered along each strake, normally from aft to forward, so every plate has a unique letter-and-number pair.
- Every plate also carries a shipyard mark and, where required, the grade and rolling marks for traceability.
- Cutting and renewal work is marked by that reference — cut by letter and number, never by eye.
Surveyor follow-up: renewal cut with the plate number taken from the drawing but the frame line left blank — what is missing?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
16 Purpose of baffle plate in coolers
Answer
Baffles turn the shell-side flow across the tube stack: they support the tubes and make the fluid sweep the full heating surface.
- Brass circular baffles give radial flow across the tubes and support the tube stack against its own weight.
- Guided-flow designs use radial fins between the tubes, giving flow alternately out and in from section to section.
- The result is greater surface, better heat transfer and a lower metal temperature on the tube itself.
- For oil heaters it cuts the risk of the oil cracking and fouling, which is a fouling problem before it is a heat one.
Surveyor follow-up: cooler fouling fastest in one pass with baffles fitted — is the baffle set, the flow rate or the fouling first?
Full theory: Shell & Tube Heat Exchangers: Working, Construction, Overhaul & Retubing -
17 Q8.) Arc welding give brief explanation of the principle with equipment for welding and also the measures for the personal safety of the personal carry out
Answer
Arc welding is a consumable-electrode process, and the welder’s protection comes from screens, gloves, leather and a sound earth return.
- The arc melts the joint and the consumable electrode; consumable-electrode, non-consumable and plasma are the main variants.
- Equipment is the welding set, electrode holder, work and earth return cables, electrode, chipping hammer, wire brush and screen.
- The welder wears a helmet with the correct shade filter, leather gloves, flame-resistant clothing and safety boots.
- Never weld in a container or enclosed space without ventilation, gas testing and a person outside — fume and heat build up fast.
Surveyor follow-up: hot work inside an empty tank with the aft-cover open and no gas test — who authorises it and what stops?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
18 Sheer strake and requirement
Answer
The sheer strake is the top strake at the deck edge, and it carries the freeboard, the load from the deck and the sea it takes first.
- It is the uppermost strake of the shell, and its upper edge is the line from which freeboard is measured.
- It must be adequately thick and stiffened because it takes the wave loads that the rest of the shell only sees later.
- Weep holes and freeing ports let shipped water out below it rather than ponding on the weather deck.
- Rust streaks, doublers and damaged plating at that strake are what a surveyor finds on every hull survey.
Surveyor follow-up: freeboard measured to a sheer strake with rust scale beneath it — is the marking wrong or the plating thin?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
19 Short notes on bending plates, ways of machining grooves using shapers, T slot milling on milling machineries, cutting using reciprocating saw machineries
Answer
These are the workshop operations behind the lines: roll the plate, cut the groove, and cut the plate to size — each with its own check.
- Plate is bent over a roll with the grain rolled lengthwise, and the bend allowance calculated from the radius and thickness.
- Grooves are cut on a shaper or planer, holding the depth and width to the drawing because the weld volume depends on it.
- T-slots on a milling machine are machined with the table clamping the work — never hand-hold a cut this heavy.
- Plate is cut on a reciprocating saw or flame, marked to the strake and frame reference so the piece can be found again.
Surveyor follow-up: new plate cut without the strake and frame marks — how is that piece going to be recorded in the book?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
20 Striker plate and purpose
Answer
A striker plate is a thick backing plate that takes the blow from a weight dropped down an open-ended pipe, protecting the tank bottom.
- Fitted under open-ended sounding pipes so the sounding bob cannot dent or punch the tank bottom plating.
- Sized and thick enough for the striking mass, and it is the plate that is inspected after each sounding.
- A dented striker plate is a warning: something heavy went down the pipe, and the plating below needs checking.
- Delivered by it, the soundings become trustworthy — a dented bottom would show a false level and a false tank.
Surveyor follow-up: striker plate dented with soundings taken since and no bottom inspection — whose soundings are in doubt?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
21 Striker plate on ship
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22 The defects that can cause a weld to fail is called as welding defects. The major causes for the welding defect are Hydrogen embrittlement and Residual stresses, which will reduce the withstanding capacity of the given l
Also asked: “The defects that can cause a weld to fail is called as welding defects. The major causes for the welding defect are Hydrogen embrittlement and Residual stresses, which will reduce the withstanding capacity of the given load”
Answer
Weld defects come from hydrogen, residual stress and workmanship — and they quietly cut the strength a joint was designed for.
- Hydrogen embrittlement is the worst of them: atomic hydrogen enters steel and cracks it, and it drives delayed failure.
- Residual stress from welding and restraint combines with the service load, so a joint carries far more than the drawings show.
- Workmanship defects add their own: undercut, porosity, slag inclusions, lack of penetration, overlap and excess reinforcement.
- The remedy is control, not hope — preheat and interpass temperature, dry consumables, correct sequence and a qualified procedure.
Surveyor follow-up: delayed crack in a joint welded in a cold month with wet electrodes — hydrogen, stress or both?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
23 Thickness of the ring should be less than groove depth so that the outer ring face is inside the ring groove
Also asked: “• Thickness of the ring should be less than groove depth so that the outer ring face is inside the ring groove”
Answer
Ring thickness must stay under groove depth so the ring can move axially and gas can load its face — otherwise it jams and scuffs.
- If the ring is as thick as the groove, it cannot float freely and the gas behind it cannot push it onto the liner.
- Axial clearance lets the ring expand and contract, and a recess in the groove stops the sharp edge cracking it.
- Insufficient ring-and-groove clearance makes the ring jam at working temperature, then it wears, breaks and can seize the piston.
- Measure groove depth and ring height and compare with the maker’s figures — the back clearance is the difference between them.
Surveyor follow-up: ring jammed in the groove with the ring and groove depths within limits — is it the recess, the material or the temperature?
Full theory: Maintenance and Troubleshooting — Overhauls, Tappet Clearance and Reading a Fault -
24 What is called edge preparation in welding plates and illustrate different types of edge preparation, how they are put to use on board?
Answer
Edge preparation is the cut edge given to plate so filler can reach the root: square, single vee, double vee and U-butt are the four.
- Square preparation suits light plate under 6 mm where full penetration is reached without a vee.
- Single vee around 60 degrees is the standard for medium plate, giving filler a path to the root.
- Double vee cuts both faces for thick plate, cutting the weld metal and the number of passes needed.
- U-butt has a narrow root and parallel sides, for heavy plate where a wide vee would waste metal and distort the edges.
Surveyor follow-up: 40 mm plate welded with a single vee on both sides — is that double vee, U-butt or a defect?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
25 What is min. And max. Thickness of feeler gauge.?
Answer
Feeler blades run from about 0.02 mm to 1.0 mm in a standard set, and that is the range most clearance work is judged in.
- Blades start around 0.02 mm and the set is normally complete to 1 mm, with thicker leaves available in separate sets.
- Used for shaft, bearing, tappet and thrust clearances — thrust bearings, for example, run 0.5 to 1.0 mm new and under 2 mm in service.
- Keep blades clean, unbent and unstacked, because a stack reads falsely against a rough journal.
- If a blade enters when the calculated gap says it cannot, the joint is outside limits — not the gauge in error.
Surveyor follow-up: thrust clearance reading 2.3 mm on a set two years in service — measure again, or condemn the bearing?
Full theory: Crankshaft Alignment, Web Deflections & Bearing Clearances -
26 What is striker plate? Use?
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27 What precautions u take for arc welding (earthing
Answer
Arc welding safety is the earth, the clothing and the fume: a sound return close to the job, full PPE, and extraction.
- Terminals on the welding machine must be tight and clean, with no wear on cables and lugs — replace anything damaged.
- Make the earth as close to the job as possible at a good conductive point, never far away on the hull.
- A distant earth lets current circulate through the hull, which is the catastrophic case the precaution exists to prevent.
- Wear insulated rubber shoes, full body protection, goggles and a helmet with the right shade, and keep clothing dry.
Surveyor follow-up: welding aloft with the earth clamped on the masthead instead of the deck — what is the current doing?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
28 Write short notes on the following with materials/tools used for same- (a) Riveting (4) (b) How to remove a rivet (4) (c) Soldering (4) (d) De-soldering; (4
Answer
Four trades, four sets of tools: riveting and punching, soldering with flux and iron, and de-soldering with a wick and heat.
- Riveting: snap-head and shop-head rivets driven with a riveting gun and snap, on a strong-back or buck bar.
- Removing a rivet: drill or punch off the head, then punch the shank out from the underside with a drift.
- Soldering: tin-lead or silver solder with matching flux, heated on an iron or brazing lamp, and the joint cleaned and tinned first.
- De-soldering: braid and flux with heat draw the solder into the braid, or the whole joint taken off with a blowtorch and a new one made.
Surveyor follow-up: steam pipe joint leaking after de-soldering with a torch — is the new joint sound or the parent metal thinned?
Full theory: Shipbuilding Materials, Welding and Weld Defects -
29 the solution are in contact with the plate
Answer
A solution only works where it wets the metal: the flux must break the oxide film and make the solder or brazing alloy spread, not bead.
- A beading solution sits on the surface and never wets; the joint is weak, porous and will part under vibration.
- Flux dissolves the oxide and lets the molten alloy run into the joint by capillary action — that contact is the joint.
- A beading solution has enough surface tension to resist capillary entry, so the joint is made on top of the weld instead of in it.
- So the answer is wetting: the joint exists where the solution reached the base metal, and nowhere else.
Surveyor follow-up: brazed joint where the filler beaded on top and never entered the gap — what is the joint actually carrying?
Full theory: Shipbuilding Materials, Welding and Weld Defects
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