beam. Here DSNS and MARIN carried out a systematic series of tests with different aft and fore body types and a varied midship length. “We used RAPID to optimise the for- ward part of the ship and hull optimisation. Then for ship resistance we deployed MARIN’s computer program DESP to make our own calculations prior to the testing phase.”


For the 3D hull form parameters Rhinoceros is utilised, together with the Orca3D plug- in. This involves a lot of manual work, he stresses. Once the 3D shape is created, RAPID is applied to calculate the pressure distribution over the hull. RAPID undoubtedly lives up to its name and can make calcula- tions within a few minutes, compared to CFD, which could take a day for one calcu- lation. However, RAPID does not have all the answers, he says. Although RAPID is used for hull form optimisation, it doesn’t really give the answer about the actual resistance itself. Therefore, DSNS uses RAPID first but then once it has a good optimised design it turns to CFD.


Actual resistance calculations He ex- plains: “For hull optimisation we can use CFD for the resistance calculation itself. CFD includes all the basic phenomena to calcu- late actual resistance. At DSNS we would essentially like to use it as a ‘numerical towing tank’ in the future.”


Additionally, when a hull form does not have a sharp waterline entry, RAPID is limited, he adds. “Definitely for a sharp bow optimising the hull form is a lot quicker with RAPID. However, it is not built for the blunter form so we hope CFD can help us there. They can run alongside each other;


Anti-Roll Tank model tests on a hexapod including ReFRESCO calculations


RAPID calculations for optimisation then CFD for the final result.” There will be a stage when the blunt bow forms are likely to go directly to CFD, but at this stage this is still not there yet, he adds.


Currently, DSNS has two more SIGMA-Class frigates for the Indonesian Navy under con- struction. They will both be the larger 105 m version with a 14 m beam. This time around, the most advanced CFD tools are being used. Part of the calculation programme is being carried out by MARIN using its viscous-flow CFD code ReFRESCO. “Here we look at the propeller wake, wake field cal- culation and shaft strut alignment. CFD was being used to align the struts and then the scale model was built and tested.”


Strut alignment Just a few years ago, the struts would have been aligned by paint smear tests, he points out. So here, MARIN is involved in many aspects of the project from the ReFRESCO calculations for the append- age optimisation, resistance and propulsion to the model scale tests. The propeller design/ wake field and final propeller geometry is also being tested at MARIN.


DSNS is working on future SIGMA designs, but for smaller vessels with a high-speed requirement. Again, here Mr van Straten expects to deploy ReFRESCO calculations for the strut alignment and propeller wake field, as well as using it for input for the propeller design. But he points out that although many different problems are being tackled using ReFRESCO, validation is still ongoing for some applications. However, MARIN’s timeline for the validation of ReFRESCO concurs with that of DSNS - the


bare hull, appendages, propeller/ship configuration.


CFD validation DSNS joined the Damen Shipyards Group in 2000 and recently, a company-wide R&D project has been launched looking at CFD validation. Different CFD tools are being considered for various vessel types.


Additionally, Damen recently joined MARIN’s ReFRESCO Operation as its launching customer and thereby, has the licence for ReFRESCO, alongside other codes. “One of the advantages of using ReFRESCO is that it is not a general CFD tool but it is tailored for ship hydrodynamic problems. Furthermore we get the same version MARIN is using for its daily work and we can use their best practice guidelines.


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