Ship Structure - Keel & Bottom — Most Asked Questions
13 questions asked about ship structure - keel & bottom across the Fn 3 and General papers, heaviest first.
Learn the system first
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1 Types of keel
Answer
Keels run flat plate on modern ships, bar on old tonnage, duct where pipes must pass — garboard strakes join them all.
- Flat-plate keel: the thick middle strake of the bottom shell with a continuous centre girder above.
- Bar keel: a solid bar with garboard strakes lapped each side, drain holes and floors landed on its flange.
- Duct keel: a walk-in box girder carrying ballast, bilge and sounding lines with solid floors each side.
- Survey point: keel-to-garboard seam, duct boundaries and drain paths — the backbone shows damage first.
Surveyor follow-up: point at the bottom shell and name the keel, the garboard and the centre girder.
Full theory: Double Bottom Structure and Bottom Framing -
2 Bilge keel function
Answer
Bilge keels kill rolling without machinery: flat plates at the bilge turn stirring eddies that damp the roll.
- Direct plate-to-water drag helps weakly; the real damping comes from broken streamlines and hull pressure.
- Lengthen the roll period slightly while raising wave-making resistance a touch — cheap stability for fuel pennies.
- Sit inside side-and-bottom lines so docks and tugs never touch them, welded to strengthened structure.
- Never confuse with fins: no moving parts, no power, no control — always working in any seaway.
Surveyor follow-up: cracked bilge-keel weld with the plate still fair — grind, reweld or crop, and what survey follows?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
3 Margin line and its importance and plimsol line relation
Also asked: “Margine line and its relation with plimsoll line”
Answer
The margin line is the flooding backstop: 76 mm below the bulkhead-deck edge that damage water must never cross.
- Drawn parallel to the bulkhead deck at side, at least 76 mm below its upper surface along the length.
- Flooded ship must keep this line dry — subdivision and permeability calculations prove it for each damage case.
- Load Line freeboard marks set the intact reserve; the margin line sets the damaged limit — both guard buoyancy.
- Deck-thickness variations use least side thickness, or a mean within 50 mm of it, for the reference surface.
Surveyor follow-up: damage waterline above the margin line but below the bulkhead deck — survivable or failed, and why?
Full theory: Load Line: Marks, Letters & Conventions -
4 Duct keel entry procedure
Answer
Enter the duct keel as an enclosed space: permit, ventilate, test, light and lifeline through the pipe tunnel.
- Permit with attendant at the access hatch, comms proved and rescue gear staged before the first step down.
- Ventilate the full tunnel length with lighting proved — cables, pipes and sounding lines hide the far end.
- Test oxygen, hydrocarbons and toxics at several points along the duct, not just at the hatch.
- Log entry and exit with work scope bounded — bilge, ballast and fuel lines overhead forgive no spark.
Surveyor follow-up: O₂ proved at the hatch with the far end untested and ventilation stopped — enter or retest, and where?
Full theory: Double Bottom Structure and Bottom Framing -
5 Rise of floor
Answer
Rise of floor is the V in the bottom: keel-to-bilge height where the flat bottom meets the moulded breadth line.
- Measured up from the keel where the bottom line extended meets the moulded breadth at amidships.
- Flat-keel ships spread the flat of bottom wide for docking; V-form ships rise faster for speed and seakeeping.
- Eases drainage to bilges with limber holes clearing frames — flat floors pond, risen floors drain.
- Quote with camber, deadrise and bilge radius together — the surveyor sketches the section from your numbers.
Surveyor follow-up: flat keel claimed with bilges ponding and frames unholed — form, drainage or construction first to blame?
Full theory: Double Bottom Structure and Bottom Framing -
6 Sketch a transverse section through a duct keel labeling its component parts. Indicate the duct keels position in the ship structure
Answer
Draw the centreline box tunnel in the double bottom: inner bottom above, keel plate below, pipes and cables within.
- Box girder on the centreline inside the double bottom with keel plate forming its bottom.
- Inner-bottom plating caps the tunnel with manholes and ladders for access along its length.
- Bilge, ballast, fuel, sounding and cable runs pass through on brackets clear of the structure.
- Mark its place between floors and girders amidships — the backbone tunnel every surveyor walks in dock.
Surveyor follow-up: pipe leak inside the duct keel with bilges rising elsewhere — isolate, enter or pump first?
Full theory: Double Bottom Structure and Bottom Framing -
7 Bottock line , bilge keel and location
Answer
The buttock line is the maximum breadth girth, and bilge keels hang on it, outboard, to damp roll without fouling anything.
- The buttock line is the curve of greatest breadth around the hull girth, shown on the lines plan as the buttock lines.
- Bilge keels are fitted outboard on the turn of the bilge to upset the streamlines and damp the roll.
- They must stay inside the side and bottom lines so docks, tugs and shoals never touch them.
- They attach to a continuous longitudinal bar rather than directly to the shell, and run about half the ship’s length.
Surveyor follow-up: bilge keel projecting beyond the moulded line at the turn of the bilge — fouling risk, strength, or both?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
8 Duct keel benefits
Answer
A duct keel is a faired appendage that earns its steel in thrust and cavitation: it surrounds the propeller in a nozzle.
- It increases the mass of water the propeller accelerates, which raises the generated thrust for the same shaft power.
- It smooths the flow into the propeller, delaying cavitation inception and giving a quieter propeller.
- It can raise the propeller tip speed without the cavitation penalty, allowing a larger diameter and better efficiency.
- The cost is weight, drag and the work of ducting and hull-form integration — so it suits larger, faster ships.
Surveyor follow-up: duct keel fitted with cavitation still at the same power — is the design, the prop pitch or the alignment?
Full theory: Ship Resistance, Hull Form and Propulsion -
9 Garboard strake and uses
Answer
The garboard strake is the bottom shell plate beside the keel: it takes grounding contact and sets the whole bottom’s baseline.
- It is the shell strake running along the bottom on each side of the centreline keel, between keel and bilge strakes.
- Its height is increased in small vessels so the bottom can take grounding damage without reaching the bilge or the turn of the bilge.
- It is one of the strakes shown on the shell expansion plan, lettered by level from the sheer strake down.
- Its thickness follows the bottom scantlings, and it is the first place a surveyor looks after a grounding.
Surveyor follow-up: grounding damage found on the bilge strake and not the garboard — is the ship’s draft, its grounding or the plating?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
10 Margin plate , girder, intercostal girder
Answer
The margin plate closes the double bottom at the centreline; girders carry the floors, and intercostal side girders span between frames.
- The margin plate is the vertical plate on the centreline closing the double bottom between the two tanks.
- Girders are the heavier athwartships members — a continuous centre girder and intercostal side girders under the floors.
- The centre girder is continuous along the length; the side girders are intercostal, running between frames under alternate floors.
- The margin plate is itself strengthened to take the thrust, so it is where the surveyor looks for panting in the centreline.
Surveyor follow-up: panting at the margin plate in the engine room — the thrust path, the girder spacing, or the tank top?
Full theory: Double Bottom Structure and Bottom Framing -
11 What is Garboard strake?
Also asked: “What is garbod strake”
Answer
The garboard strake is the bottom shell plate running along the keel — the strake that takes the ground and the shoals.
- It runs on each side of the centreline keel, between the keel rabbet and the bilge strake above it.
- In small craft its height is increased so grounding contact stops at the garboard instead of reaching the bilge.
- On the shell expansion plan it is one of the lettered strakes, at the bottom of the sequence from the sheer.
- It follows the bottom scantlings, and after a grounding it is the first plate to be gauged and examined.
Surveyor follow-up: grounding with the garboard intact and the bilge strake indented — is the draught, the speed or the plating?
Full theory: Ship Types, Hull Form and the Parts of a Ship -
12 What is camber, rise of floor, moulded depth, scew and rake of propeller
Answer
Four are hull definitions and one is a propeller definition — read them off the section, not off the side elevation.
- Camber or round of beam is the rise of the deck going from the side to the centreline, applied mainly to weather decks.
- Rise of floor or deadrise is the height where the extended line of bottom meets the moulded breadth line, above the keel.
- Moulded depth is the vertical distance from the moulded baseline to the moulded deck at side, measured on the midship section.
- Rake is the propeller’s fore-and-aft inclination, normally raked aft, expressed as a percentage of the propeller diameter to improve blade-tip clearance.
Surveyor follow-up: draught read against the moulded baseline with a rise of floor in the way — which datum is wrong?
Full theory: Principal Dimensions and Ship Reference Points -
13 o Less expensive than Plate type HE o Can be used in systems with higher temperatures and pressures o Pressure drop across a tube sheet is less o Tube leaks are easily to locate and plug by pressure testing o Tubular coo
Also asked: “o Less expensive than Plate type HE o Can be used in systems with higher temperatures and pressures o Pressure drop across a tube sheet is less o Tube leaks are easily to locate and plug by pressure testing o Tubular coolers in refrigeration system can act as receiver also. o Sacrificial anodes can protect the whole cooling system against corrosion o Tubular HE are preferred for lubricating oil co”
Answer
That is the tube cooler’s case: cheap, high pressure, leaks findable — and the mirror image of the plate cooler’s.
- Tube coolers are less expensive than plate type and suit systems with higher temperatures and pressures.
- The pressure drop across a tube sheet is less than the equivalent plate pack, which is why they suit high-flow oil circuits.
- Tube leaks are easily located and plugged by pressure testing, which is the one thing a plate pack cannot do.
- Against that, heat transfer efficiency is lower than a plate cooler and cleaning is harder, because the tube nest needs space at one end to be drawn out.
Surveyor follow-up: lube oil cooler required to hold 12 bar — is that a plate or a tube pack, and why?
Full theory: Plate Heat Exchangers: Working, Operation, Maintenance & Troubleshooting
Source: 123 written papers and surveyor oral banks, 24,613 entries, merged into 17,210 topics. Ticks are stored on this device only.