Example small section
Web about 4 – 5 mm, height about 45 – 60 mm: deck and superstructure stiffeners on small craft.
Marine structural profiles
Bulb Flat: Aluminium Profiles Compared with Steel HP
A bulb flat is a flat bar with a rounded thickening, the bulb, along one edge. Welded to a plate by its flat edge, it works as a stiffener with much more bending stiffness than a plain flat bar of similar weight, which is why bulb flats are the standard longitudinal stiffener in ship hulls, decks and bulkheads. Steel bulb flats follow the HP series of EN 10067; aluminium bulb flats are extruded, usually in 6082, 6005A or 5083, for fast craft, superstructures and offshore modules. Metline supplies the aluminium sections; steel HP data is included for comparison.
- Steel standard
- EN 10067 (HP sections)
- Aluminium alloys
- 6082, 6005A, 6061, 5083
- Typical lengths
- 6, 9 and 12 m
- Main use
- Hull, deck and bulkhead stiffeners
What a bulb flat is and why it is used
Plate panels in a ship are thin compared with their span, so they need stiffeners to stop them buckling under water pressure, cargo loads and hull bending. The simplest stiffener is a flat bar, but a flat bar is inefficient: most of its material sits close to the plate where it contributes little to stiffness. Moving metal to the free edge, as a bulb, raises the section modulus and the resistance to tripping (sideways buckling) for little extra weight.
- Efficiency: more section modulus per kilogram than a flat bar, and close to an unequal angle of similar weight
- Easy welding: the plain web is fillet welded to the plate from both sides, ideal for automated welding lines
- Good coatability: rounded bulb and open shape are easier to paint and inspect than angles or T-sections, with fewer corrosion traps
- Simple fabrication: easy to cut, cope and bend to follow hull curvature
- Symmetry for slotting: passes through slots in frames and web frames with simple cut-outs
Bulb flats appear in almost every steel ship, from bulk carriers and tankers to ferries, and in aluminium vessels such as fast ferries, patrol boats, workboats and the superstructures of larger ships. They are also used in offshore platforms, bridge decks and other stiffened-plate structures.
How bulb flat sizes are described
Sizes are written as height × web thickness, for example HP 200 × 10: a 200 mm high section with a 10 mm web. The bulb dimensions follow from the height in the standard series, so each height has a fixed bulb width and radius, and several web thicknesses.
Key dimensions
- b
- Overall height of the section, including the bulb
- t
- Web thickness
- c
- Bulb width (projection beyond the web)
- r
- Bulb radius
- A
- Cross-sectional area, used for weight and strength
- I, W
- Moment of inertia and section modulus, used for stiffener design with attached plate
Section properties for design should be taken from the standard or the profile drawing, normally combined with an effective breadth of attached plating.
Steel HP bulb flat sizes and weights (EN 10067)
The table lists a selection of common HP sections with their nominal cross-section. Mass per metre is calculated from the area using 7.85 g/cm³ for steel; the last column shows what the same geometry would weigh in aluminium (2.66 g/cm³), which illustrates the weight saving, although aluminium sections are normally redesigned rather than copied.
| Section | b (mm) | t (mm) | Area (cm²) | Steel mass (kg/m) | Same shape in aluminium (kg/m) |
|---|---|---|---|---|---|
| HP 80 × 6 | 80 | 6 | 6.21 | 4.9 | 1.7 |
| HP 100 × 8 | 100 | 8 | 9.74 | 7.6 | 2.6 |
| HP 120 × 8 | 120 | 8 | 11.72 | 9.2 | 3.1 |
| HP 140 × 10 | 140 | 10 | 16.63 | 13.1 | 4.4 |
| HP 160 × 9 | 160 | 9 | 17.80 | 14.0 | 4.7 |
| HP 180 × 10 | 180 | 10 | 22.46 | 17.6 | 6.0 |
| HP 200 × 10 | 200 | 10 | 25.66 | 20.1 | 6.8 |
| HP 220 × 11 | 220 | 11 | 31.20 | 24.5 | 8.3 |
| HP 240 × 12 | 240 | 12 | 37.29 | 29.3 | 9.9 |
| HP 260 × 12 | 260 | 12 | 41.31 | 32.4 | 11.0 |
| HP 280 × 12 | 280 | 12 | 45.48 | 35.7 | 12.1 |
| HP 300 × 12 | 300 | 12 | 49.79 | 39.1 | 13.2 |
| HP 320 × 13 | 320 | 13 | 57.45 | 45.1 | 15.3 |
| HP 340 × 14 | 340 | 14 | 65.54 | 51.4 | 17.4 |
| HP 370 × 15 | 370 | 15 | 77.10 | 60.5 | 20.5 |
| HP 400 × 16 | 400 | 16 | 89.48 | 70.2 | 23.8 |
Areas are nominal values for the HP series; masses are calculated. Check EN 10067 or the producer's table for full dimensions and section properties.
Steel bulb flats are hot rolled in ship-structure steels such as normal-strength grade A and higher-strength grades AH32, AH36 and DH36, certified by classification societies for each order. EN 10067 controls the shape, dimensions, mass, straightness and twist tolerances.
Extruded aluminium bulb flats
Aluminium bulb flats are extruded rather than rolled, which means the shape can be optimised freely: the bulb can be made rounder or flatter, the web tapered, and a weld-prep edge or locating lip added. Typical aluminium bulb flats are smaller than steel HP sections, with web thicknesses of about 4 – 8 mm and heights of about 45 – 150 mm, because aluminium hulls use thinner plating and closer stiffener spacing.
| Alloy and temper | Minimum tensile (MPa) | Minimum 0.2 % proof (MPa) | Characteristics |
|---|---|---|---|
| 6082-T6 (wall 5 – 25 mm) | 310 | 260 | Highest strength of the common marine extrusion alloys; good corrosion resistance |
| 6005A-T6 (wall up to 5 mm) | 270 | 225 | Excellent extrudability for thin, complex marine profiles |
| 6061-T6 | 260 | 240 | Widely available; common in North American practice |
| 5083-H112 | 270 | 110 | Best seawater resistance and strength retained after welding; harder to extrude |
Minimum values for extruded profiles summarised from EN 755-2 and class rules; limits vary with wall thickness and are confirmed on the mill certificate.
The choice is a trade-off. 6xxx alloys are stronger in the delivered T6 temper and extrude easily, but welding reduces strength in the heat-affected zone to roughly half the parent value, which class rules account for in design. 5083 is weaker before welding but loses much less, and it is the more durable choice for members permanently immersed in seawater.
Example medium section
Web about 5 – 6 mm, height about 75 – 100 mm: bottom and side shell longitudinals on workboats and patrol craft.
Example large section
Web about 6 mm, height about 125 – 130 mm: main hull longitudinals on larger fast ferries.
Steel versus aluminium bulb flats
| Factor | Steel HP bulb flat | Aluminium bulb flat |
|---|---|---|
| Density | 7.85 g/cm³ | 2.66 – 2.71 g/cm³ |
| Elastic modulus | About 210 GPa | About 70 GPa, so deeper sections are needed for equal stiffness |
| Production | Hot rolled to the standard HP series | Extruded to a standard or custom shape |
| Corrosion | Needs coating systems and cathodic protection | Marine alloys resist seawater well; often left unpainted inside hulls |
| Welding | Routine, little strength loss | MIG welding; heat-affected zone strength reduction in 6xxx alloys |
| Typical structures | Large ships, tankers, offshore platforms, bridges | Fast craft, superstructures, helidecks, offshore accommodation modules |
Where an aluminium superstructure sits on a steel hull, the two are usually joined with explosion-bonded structural transition joints, which avoid direct galvanic contact and allow each side to be welded with its own process.
Tolerances, testing and class approval
| Item | Steel (EN 10067) | Aluminium extrusions |
|---|---|---|
| Dimensions and shape | Tolerances on height, web thickness and bulb | EN 755-9 or the agreed drawing tolerances |
| Straightness | Limits on bow and camber as a fraction of length | EN 755-9 straightness limits per metre and over total length |
| Twist | Limited per metre | Limited per metre, depending on section size |
| Surface | EN 10163-3 surface condition | Free of defects harmful to use; anodising or paint quality on request |
| Material approval | Classification society certificate (for example DNV, ABS, LR, BV) | Class-approved alloy and producer, with mechanical tests and, for 5xxx, corrosion testing as required |
For classed vessels, confirm early which society and rule set apply, because they decide the approved alloys, testing frequency and certificate type. Material supplied for classed structures normally carries an EN 10204 3.1 or 3.2 certificate.
Fabrication tips
Cutting and coping
Saw or plasma cut ends; cope or snipe ends where stiffeners land on brackets or pass through frames to avoid hard spots.
Bending
Bulb flats can be bent in the plane of the web to follow hull shape. Aluminium 6xxx in T6 has limited ductility, so use generous radii or bend before final temper where possible.
Welding to plate
Double continuous or intermittent fillet welds per the rules. For aluminium, use ER5183 or ER5356 filler and keep heat input controlled.
Slots and collars
Standardised slot shapes in web frames let bulb flats pass through; collar plates restore strength where needed.
References and standards
- EN 10067 – Hot rolled bulb flats: dimensions and tolerances on shape, dimensions and mass
- EN 755-2 – Aluminium and aluminium alloys: extruded rod/bar, tube and profiles, mechanical properties
- EN 755-9 – Extruded profiles: tolerances on dimensions and form
- DNV Rules for Classification: Ships, Part 2 – Materials and welding (aluminium alloys)
- ASTM B221 – Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles and Tubes
Bulb flats: frequently asked questions
What is a bulb flat used for?
It is a stiffener welded to plating in ships, offshore structures and bridges. The bulb on the free edge increases stiffness and buckling resistance compared with a plain flat bar of similar weight.
What does HP mean in HP 200 × 10?
HP refers to the Holland Profile bulb flat series standardised in EN 10067. HP 200 × 10 is a section 200 mm high with a 10 mm web.
Are aluminum bulb flats available?
Yes. Aluminium bulb flats are extruded, commonly in 6082-T6, 6005A-T6, 6061-T6 or 5083-H112, for aluminium hulls, superstructures and offshore modules. Shapes can follow a standard profile or a customer drawing.
Which aluminium alloy is best for bulb flats?
6082-T6 offers the highest strength and easy extrusion; 5083 gives the best seawater resistance and keeps more strength after welding. The class society rules and the location in the hull usually decide.
What lengths are bulb flats supplied in?
Common lengths are 6, 9 and 12 m, with cut lengths to suit the fabrication plan. Longer aluminium extrusions are possible within press and transport limits.
How much does a bulb flat weigh?
Multiply the cross-sectional area by density: steel HP 200 × 10, with about 25.7 cm², weighs about 20.1 kg/m. The same shape in aluminium would weigh about 6.8 kg/m.
Need aluminium bulb flats for a marine project?
Send section size or drawing, alloy, length, quantity and class requirements to sales@metline.in, and our team will reply with availability and certification.
