High speed catamarans need to be able to carry cars, trucks or passengers in order to be viable from a commercial perspective. Ultimately, they need to maximise the amount of cargo they can take, while travelling at a high speed. This means that catamarans must be light enough to travel at high speeds, but not so light that they cannot withstand the conditions in the ocean, in particular the slam loads experienced when waves crash into them.
Professor Giles Thomas, UCL Mechanical Engineering
In order to improve the understanding of the loads and impact catamarans can take – as well as to find out if design improvements could be made – Professor Giles Thomas from UCL Mechanical Engineering led a research project to interrogate this area further. The research was done in collaboration with Incat Tasmania, Revolution Designs and the University of Tasmania, as well as the National Research Council of Italy, Institute of Marine Engineering (CNR-INSEAN) and the University of New South Wales. It was funded by a series of Australian Research Council Linkage grants.Focusing on slam loads
“The slam load is when the boat goes into a wave and you get a big bang,” Professor Thomas explains. “It’s the largest kind of wave load, it happens really quickly and it’s very dynamic.” Slam load is a common cause of structural problems on boats. On catamarans, the effect can be more pronounced, due to the specific design with two hulls (demihulls) and the archway between them that fills up with water. Modern catamarans have a centre bow within this archway, which acts as a shock absorber against the waves.
“A key thing we’ve been trying to understand is how big the loads are, and the influence of whole form changes on the loads,” Professor Thomas says. The team realised that the shape and design of the centre bow may play a role on the impact felt by catamarans from slam loads. “In particular, we wanted to see if changing the geometry at the bow of the ship could reduce the size of slam loads.”
The team gathered data for the research in multiple ways. As well as gaining measurements from real boats while they were in operation, they did model experiments to measure the motions and loads in controlled conditions. They undertook numerical modelling using computational fluid dynamics and finite element analysis, and also combined the two. In addition, the team did some drop tests, where a model catamaran was dropped directly into tanks of water.
Interrupting the flow
After gathering and analysing the extensive data from the various experiments and real life scenarios, the team discovered the key to how catamarans can best cope with the impact of the waves. “In terms of trying to reduce the magnitude of the slam load, you want to try and interrupt the flow as it goes around the centre bowl,” explains Professor Thomas. “Interrupting the flow is our biggest take home from this project. Getting the size of the centre bow correct is crucial.”
The research shows that designing the centre bow to exactly the right specifications, both in terms of shape and size, is fundamental to helping catamarans withstand the slam load. Prior to the research, “classification societies didn’t really know how big these loads were, and hence the designers didn’t know this either,” Professor Thomas says. “Knowing how big the loads are means you can design your structure more effectively both against these big loads, but also against what we call fatigue effects.”
This new understanding of the loads catamarans need to be able to withstand, and how centre bow design can help with this, will help to increase the lifespan and commercial viability of these vessels. It means catamarans will receive less damage through the course of their life too, which will take them out of service less for repairs. Professor Thomas is also working on continuous monitoring systems that can go onboard catamarans, to continuously monitor data such as the motions and loads the vessels are coming up against. This will all help to improve the performance and lifespan of catamarans on an ongoing basis.
