Finite Element Analysis (FEA) takes the stress out of ferry design

FBM Babcock Marine

The normal commercial pressures associated with manufacturing – the need to design and build quality products as cost-effectively as possible and with the shortest possible lead times – are supplemented, in the case of boat builders, by strict international guidelines on structural design and build quality. Desktop Engineering helped a UK-based ferry manufacturer not only to produce stronger, faster ferries at lower cost, but also to rewrite the theory of boat design.

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FBM Babcock Marine

As twin water jets, one per hull, propel the 220 tonne vessel to 45 knots across the bay from Hong Kong a first class passenger might calculate his chances of winning at the gambling tables of Macao. This run is one of the world’s most prestigious ferry crossings and the craft that carries up to 330 passengers to and from the pleasure domes of Macao was designed and built at Britain’s own offshore idyll, The Isle of Wight, by FBM Babcock Marine.

FBM Babcock Marine design and build a wide range of craft including workboats, patrol craft and fast water jet powered ferries which carry passengers worldwide.

The calculations of the playboy gambler are nothing compared to those of the naval architects who ensured that the FBM Marine Tricat advanced fast ferry was designed within the structural requirements of the Classification Society, Det Norke Veritas, Hong Kong Marine Development and The Flag Authority. One of the team of naval architects is John Kecsmar. From the FBM Babcock Marine design office in West Cowes he explained how finite element analysis software is used extensively in the development of safer, stronger, faster and more fuel-efficient ferries.

There is considerable growth in the market for fast ferries and within the design process there are a number of key aspects the naval architect must consider. First we must interpret the customer’s requirement for vessel dimensions, geometry and operational performance to arrive at a basic structural design.

“We must then use the Classification Society rules to work out the loads and moments in the structure, and analyse behaviour in the worst case scenario, to ensure the structure meets the prescribed rules. The sleek ferry exterior is made from plastic GRP, built onto the main ferry structure, and we must ensure that this, and embedded items such as windows don’t jump ship. At the same time there will be supplementary considerations, such as the operational environment which may place restrictions on engine emissions and wash.”

 

With the help of FEA, the boat is designed and analysed as one process. As John explains

“I am trying to achieve certain stress limits, which meet the rules governing boat design, while ensuring the design is as easy to manufacture as possible, and as light as possible for operational performance.”

 

A basic analysis model of the initial structural design is gradually revised and added to as detailed design work, and subsequent analyses are undertaken. For example, with FEA it has been possible to create a single design for all scantling (or main cross-sectional structure members) which meet all fatigue and stress requirements. This has helped reduce manufacturing time and cost. When the superstructure model is built, the hull stresses and strains can be used to ensure functional integrity is maintained in the superstructure components. The software is also used to analyse the performance of the transom and stern, which in turn has led to some startling revelations.

Prior to the introduction of FEA all the calculations were undertaken by hand using only normal torsional theory. The math was so complex that design iteration was a slow process. This meant that full design optimisation was unlikely. FEA has not only enabled far more complex analyses to be undertaken, but has given the designer the ability to very quickly modify and re-analyse designs to get as close an optimal design as possible. Equally it has enabled the company to gain a much better understanding of how each structure behaves.

As John explains, “The model animation not only helps to confirm whether boundary conditions and therefore the results are correct, but also gives those in the production process an instant picture of what is going on.

 

New design theory

However, it was when analysing the transom that FEA really came into its own and helped re-write the theory of boat design. The water jets used on those ferries move up to 24 tons of water per second and are subject to great stress and subsequent fatigue, particularly during manoeuvring and stopping procedures. Many similar ferry designs have experienced cracking around the transom and its junction with the hull. Traditional wisdom suggested that the solution was to add bracing material to make the structure more capable of dealing with stress.

However, the analyses John ran did not seem to produce the desired results. It almost suggested that there should be less bracing material. So John started to use FEA to assess the effect of reducing the transom thickness. As this was done, FEA showed that the structure actually became stronger.

By minimising the transom thickness and removing the strengthening, all the stress went through the (stiff) duct and away from the transom. This meant that all we had to do was arrive at a transom thickness that performed its function without excessive deflection and which didn’t attract stress from the duct. In the final design we ended up with a thin transom that acted as a diaphragm only and has no longitudinals in the bottom of the hull.”

The design has proved not only far stronger than any of the predecessors but has actually made the vessel considerably simpler and cheaper to manufacture. Manufacturing lead times have also been reduced, while product life span has been increased, further enhancing competitive advantage.

As John Kecsmar says, FEA allows him to design and test simultaneously. It has also enabled him to bring about a revolutionary design change which alters the theory of boat design. The Classification Society is supporting this new theory which will enable water jet powered boat operators to deliver passengers, gambler or otherwise, faster and more safely than every before in boats that are easier to build and assured of having an increased service life.

The design has proved not only far stronger than any of the predecessors but has actually made the vessel considerably simpler and cheaper to manufacture. Manufacturing lead times have also been reduced, while product life span has been increased, further enhancing competitive advantage.

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