Aluminum foam in inland shipping: lightweight construction at low water
At low water, the vessel's own weight decides how much cargo can be carried at all: every metric ton of steel structure that is removed becomes available as payload or reduces the draft. Aluminum foam sandwiches replace steel components at comparable stiffness and save 20 to 30 % in weight. Havel metal foam tested this on the hull of an inland vessel in the funded ULIVES / MARTEC ERA-NET project and today produces the sandwiches in series.
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Why does lightweight construction help at low water?
A vessel's draft follows from its displacement, and that is shared between its own weight and its cargo. If the structure becomes lighter, the same displacement can either carry more cargo or be turned into a reduced draft; at low water levels both are the same advantage. In shipbuilding, aluminum foam sandwiches save 20 to 30 % in weight compared with steel construction. At Havel metal foam this design results in a reduced draft and more payload, together with vibration damping and fewer individual parts in the structure.
- Weight saving of 20 % to 30 % compared with steel construction
- Reduced draft
- More payload for the same energy consumption
- Vibration damping
- Fewer individual parts in the structure
What evidence is there from an inland vessel project?
Havel metal foam tested the design on the hull of an inland vessel in the funded ULIVES / MARTEC ERA-NET project, with built demonstrators rather than calculations. For the ship's rudder a 1:1 demonstrator was made: the rudder body, measuring 2.6 × 1.4 × 0.5 m, fell from 1,176 kg to 912 kg, a reduction of 22 %, and was simpler to manufacture. For the hull of an ice-going push barge a 1:4 demonstrator was built, in which the longitudinal and transverse girders were replaced by SAS sandwiches with 2 mm steel cover layers and a 30 mm foam core; the usual reinforcements could be omitted, and the total weight of the vessel fell by 25 % compared with the original design. A gearbox foundation with an original weight of 4.7 t became 20 % lighter.
- Ship's rudder, 1:1 demonstrator
- Rudder body from 1,176 kg to 912 kg, 22 % lighter; 2.6 × 1.4 × 0.5 m.
- Ice-going push barge, 1:4 demonstrator
- Longitudinal and transverse girders replaced by SAS with 2 mm steel cover layers and a 30 mm foam core; total weight 25 % below the original design. The hull itself remained conventional steel, so that the ice-going capability up to 70 cm above the waterline is retained.
- Gearbox foundation
- 20 % lighter with an original weight of 4.7 t. 40 of the 125 m of weld length were laser welded.
- Weight saving by type of component
- Plates and especially frames in SAS 20 to 25 %; cabins, decks, outer and inner walls in AAS 25 to 30 %; foam-filling steel tubes or segments 15 to 20 %.
How are aluminum foam sandwiches joined to a steel hull?
The joining technique depends on the cover layer. SAS sandwiches with steel cover layers are joined to the steel hull by MIG/MAG welding, the process a shipyard uses anyway. AAS sandwiches with aluminum cover layers need a material transition: they are connected to the steel via explosion-clad transition profiles, and aluminum to aluminum again with MIG/MAG. The ULIVES project also demonstrated laser welding under shipyard conditions, on the rudder for 17 of the 60 m of weld length and on the gearbox foundation for 40 of the 125 m.
- SAS to steel
- MIG/MAG welding (GMAW).
- AAS to steel
- Explosion-clad transition profiles.
- Aluminum to aluminum
- MIG/MAG welding (GMAW).
- Laser welding
- Tested under shipyard conditions: rudder 17 of 60 m, gearbox foundation 40 of 125 m.
How does the design behave in the salt spray test?
In the salt spray test to DIN EN ISO 9227 NSS, corrosion appeared on the steel cover layer, where it would also occur on a steel component. No corrosion occurred at the interface between foam core and steel cover layer; the bimetallic bond therefore held at precisely the point an inspector questions first. The material was tested for 240 hours at 35 °C at Fraunhofer ICT, an accredited test laboratory, on uncoated samples. For the design, note that aluminum foam components need corrosion protection just as steel does.
Where are the limits of the design?
An honest design takes three points into account. A sandwich gets its stiffness from its depth, so it needs space; where the structure offers no depth, steel remains the simpler solution. Where foam sits at a fastening point, preparation in manufacturing takes more work. And corrosion protection is still needed: provide it as you would for steel. Then there is the question of approval: the DNV GL certificate (Module D) needed for shipbuilding has expired and has not been renewed for now, as there is no current demand. For a project with this requirement it can be renewed at short notice; raise it early.
Low water and payload
What does lightweight construction do for an inland vessel at low water?
Also asked: Why is the vessel's own weight the problem at low water? · How are draft and the vessel's own weight related?
For this case Havel metal foam supplies aluminum foam sandwiches that replace steel components at comparable stiffness and save 20 to 30 % in weight. The relationship behind it is simple: a vessel's draft follows from its displacement, and that is shared between its own weight and its cargo. If the structure becomes lighter, the same displacement can either carry more cargo or be turned into a reduced draft. At low water levels this is the same advantage, because the limiting factor is then the depth of water, not the cargo volume. Havel metal foam names further effects of this design: a reduced draft, more payload, vibration damping and fewer individual parts in the structure.
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What can the weight saving be used for, payload or draft?
Also asked: Can I turn the saving into cargo? · Does the design help at normal water levels too?
Both are possible, and the decision lies with the operator, not with the material. Havel metal foam documents a weight saving of 20 to 30 % compared with steel construction; whether that tonnage is carried as additional cargo or left as a reduced draft is a question of voyage planning. At sufficient water levels it pays off as cargo; at low water it decides whether the vessel can sail at all. There is no separate percentage for payload: it depends on the vessel type, on the share of components replaced and on the weight of the cargo, and cannot be determined without these three details. What is reliable is the weight saving, and the rest follows from it by calculation.
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How much lighter does a whole vessel become in this design?
Also asked: Is there a figure for a complete vessel? · What did the push barge demonstrator show?
For an ice-going push barge, Havel metal foam achieves a total weight 25 % lower than the original design. This was measured on a demonstrator at a scale of 1:4 in the funded ULIVES project under MARTEC ERA-NET, in which Fraunhofer took part as a partner. The longitudinal and transverse girders were replaced by SAS sandwiches with 2 mm steel cover layers and a 30 mm foam core; this allowed the usual reinforcements to be omitted, which explains part of the saving. Important for context: the hull itself remained conventional steel, so that the ice-going capability up to 70 cm above the waterline is retained. The 25 % is therefore the result of a mixed structure, not that of a vessel built entirely in sandwich construction.
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Manufacturing and joining
Who produces aluminum foam sandwiches for ship components in series?
Also asked: Is this a research material or a series product? · In which formats are the sandwiches available?
Havel metal foam produces four aluminum foam technologies in series and is therefore a manufacturer, not a research partner. Two build-ups are relevant for ship components: SAS with steel cover layers, 8 to 40 mm total thickness and a maximum of 2,950 by 1,450 mm, and AAS with aluminum cover layers, 6 to 60 mm total thickness and a maximum of 2,800 by 1,400 mm. The foam core is foamed between the cover layers and bonds to them metallically, without adhesive; the result is a single-material product and therefore fully recyclable. The available core density is 0.5 to 0.8 g/cm³. Larger components are made by joining several plates, not by larger formats.
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Can the sandwich be joined to an existing steel hull?
Also asked: Can a shipyard process it with its own methods? · How is aluminum joined to steel?
Yes, and the joining technique depends on the cover layer. SAS sandwiches from Havel metal foam have steel cover layers and are joined to the steel hull by MIG/MAG welding, the process a shipyard uses anyway. AAS sandwiches have aluminum cover layers and need a material transition: they are connected to the steel via explosion-clad transition profiles, and aluminum to aluminum again with MIG/MAG. For a conversion this is the decisive point, because it determines which build-up fits into an existing steel structure without additional transition parts. The cover layers themselves can be welded, bent, drilled and coated with standard methods.
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Limits and approval
When does steel remain the simpler solution?
Also asked: What are the drawbacks of sandwich construction? · Does aluminum foam need corrosion protection?
At Havel metal foam, an honest design takes three points into account. A sandwich gets its stiffness from its depth and therefore needs space: where the structure offers no depth, steel remains the simpler solution. Where foam sits at a fastening point, preparation in manufacturing takes more work, which is noted explicitly for the drive foundation in the funded project. And corrosion protection is still needed: an aluminum foam component needs it just as a steel component does. In addition, fastening points, rails or nuts can be foamed or welded in only with SAS; AAS allows neither, which helps decide the choice of build-up.
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What should be clarified early when such a component needs approval?
Also asked: Is there a classification certificate? · What is the status of Module D?
The certificate status is the point to clarify early. The Module D certificate needed for shipbuilding has expired and has not been renewed by Havel metal foam for now, as there is no current demand; it can be renewed at short notice if needed. For a project that requires classification, this is the usable information: the approval can be obtained, but it is not currently held, and the time needed for renewal belongs in the project plan rather than in the construction phase. On the material side, material testing of AAS and SAS to DIN EN 45545-2 resulted in classification as HL3; no certificate was applied for.
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