Experimental and Numerical Investigation into the Effect of the Axe-Bow on the Drag Reduction of a Trimaran Configuration

Using the axe-bow to reduce total ship resistance on monohull ships has been well-known. This advantage has been further applied to a trimaran configuration together with its space-to-length (S/L) ratio differences. The investigation was carried out experimentally using an ITTC standard towing tank and numerically using computational fluid dynamics (CFD) analysis. The base model for the study uses an NPL 4a both for the mainhull and sidehulls of the trimaran, and later the mainhull is modified by attaching a front bulb known as an axe-bow. The resistance analysis of the trimaran was conducted with and without an axe-bow on the mainhull together with S/L ratios of S/L = 0.3 and S/L = 0.4 and at various Froude (Fr) numbers: 0.15, 0.2, 0.25, 0.3, 0.4, and 0.5. The results showed that the monohull with an axe-bow had a smaller drag than that without an axe-bow of an order up to 11.5%, whereas in the trimaran form, the reduction of drag was up to 8.4%. This indicates a positive influence of using the axe-bow on the total resistance of the trimaran configuration. Both experimental and CFD methods showed positive agreement of the order 2.7% discrepancy for the monohull form and a 3.4% discrepancy for the trimaran configuration.

Axe-Bow; CFD; Experiment; NPL; Resistance; Trimaran

        There is increased interest in trimaran vessels due to their advantages and applications (Elcin, 2003). The trimaran has sidehulls for gaining ship stability. Three hulls make the trimaran completely unsinkable. Even in the roughest weather, the ultimate hazard of capsizing is minimized. Mainhulls and sidehulls can be modified flexibly to reduce resistance (Sulistyawati and Suranto, 2020). Therefore, the decrease in trimaran resistance results in reducing fuel consumption compared to an equivalent monohull.

      In recent decades, much research has considered the advantages of the trimaran concept. When the literature on trimarans is examined in general, it is clear that the most important parameter in resistance optimization is configuring the outriggers because of the flow-interference effect between the center-hull and outriggers (Yildiz et al., 2020). Optimum placement of these will result in an interaction between the wave train produced by the center-hull and the wave trains produced by the outriggers that ideally counteract each other at primary speed(s) of interest (Chen et al., 2016).

       Preliminary research on trimarans was carried out by Gray (2003). In this study, resistance characteristics of a trimaran hull form with different arrangements were investigated to verify the theoretical prediction by comparing towing test results. The CFD method was utilized by Javanmardi et al. (2008) to analyze the hydrodynamic performance of the trimaran hull form with small-sized outriggers to determine optimum outrigger positions for minimum wave resistance performance. They also considered the wave interactions between the center-hull and outriggers to predict total wave-making resistance.

       Shahid and Huang (2011) investigated the prediction of wave resistance on trimaran hull forms using CFD software. Three different mesh sizes and two different turbulence models were used to investigate the effect of mesh structure and turbulence models on the prediction of the resistance. CFD analyses were realized corresponding to Froude number ranges from 0.14 to 0.75, and the results were compared with the experimental data. Son (2015) performed CFD computations of a systematic series of trimaran hull forms. The center-hull form of the trimaran was developed based on the National Physical Laboratory (NPL) systematic series of round bilge hulls, and the sidehulls were created by scaling the center-hull to one-third size. Poundra et al. (2017) explained that the placement of the sidehull greatly affects ship resistance, both longitudinally and transversely. Catamaran hull interference can reduce resistance, as discussed by Iqbal and Samuel (2017) and Utama et al. (2021). This interference phenomenon also occurs in a more complex form on trimaran ships. The interaction between the hulls on a trimaran ship is examined by Sun et al. (2020), who prove that the ?ow ?eld between the mainhull and sidehull of the trimaran can be captured by numerical calculations and PIV tests of the microscopic and macroscopic structures of the ?ow ?eld. Trimaran configurations with proper positions can reduce residual resistance values (Heidari et al., 2019, Yanuar et al., 2020; Yildiz et al., 2020).

       Further, the development of hull optimalization was carried out using the axe-bow. It uses straight vertical sides to dampen waves from the bow, which can result in a smooth pitching motion. Basically, the axe-bow in the extended section is empty space. The study of the axe-bow shows an increase in efficiency and a reduction in pitch acceleration because ships with axe-bows have less resistance in conventional models and reduce fuel use (Gelling, 2006).

       The axe-bow developed by Damen Shipyard has better efficiency as well as better head-sea performance, with less slamming and higher speeds (Buckley, 2010). Damen Shipyard (2012) made a delivery of the first ship with an axe-bow, the Patrol Boat. The ship exhibits effective movement behavior and significantly lower drag while sailing. This provides a 20% reduction in fuel use and, consequently, fewer emissions. Through the CFD analysis of the optimum hull, it was possible to con?rm the reduction of added resistance by the re?ected wave around the bow smoothly spread to the side (Seok et al., 2019).

       Two advantages to reducing ship resistance using the trimaran hull and the axe-bow have been mentioned in the previous literature. This paper discusses a combination of the two to obtain a better resistance reduction.

        An investigation into the effect of using the axe-bow on the resistance reduction of the trimaran configuration has been carried out numerically using the CFD approach and experimentally using an ITTC standard towing tank. The study was conducted using an NPL 4a model with and without the axe-bow on both the monohull and trimaran models with variations of S/L = 0.3 and 0.4. The use of CFD makes a very good contribution in relation to the calculation of resistance on monohull and trimaran vessels, both for NPL 4a and NPL 4a with the axe-bow. These results have been verified using experimental data with a discrepancy of about 2.7% on the monohull to 3.4% on the trimaran mode. Using the axe-bow gave a positive contribution, a reduction of up to 11.5% when compared to conventional hulls. Drag reduction of the ship is the ability of the axe-bow to reduce wave-making due to the interaction of the bow and water. In the trimaran mode with variations of S/L = 0.3 and 0.4, the mainhull with the axe-bow was able to contribute to the reduction of resistance of up to 8.4%. This reduction in resistance occurs due to the contribution of the axe-bow, which can reduce wave-making in the bow, which further reduces the hull interaction.

     The authors wish to thank the Directorate of Research and Community Services (DRPM) ITS for financing the research under a research scheme called “Postgraduate Research Grant” with contract number 920/PKS/ITS/2020. The authors also thank Mr. Langgeng Condro and Mr. Rudie Aminudin from the ITS Laboratory of Hydrodynamics for their help in the experimental test of the trimaran resistance.

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