Calculation of bottom clearance effects on Walrus submarine hydrodynamics

Due to changes in operations, the Royal Netherlands Navy (RNLN) is operating its submarines increasingly in brown water regions (i.e., waters with restriction in width and/or depth, such as littoral areas). To improve predictions for manoeuvring in restricted waterways, the RNLN has ordered the Maritime Research Institute Netherlands (MARIN) to conduct studies regarding the influence of the seabed on the behaviour of the submarine. In this work, viscous-flow calculations are used to predict the influence of bottom clearance on the hydrodynamic forces on the Walrus class submarines. The simulations representing unrestricted-water conditions have been validated with available model test results and good agreement was found. The details of the methodology and results of the viscousflow calculations are presented in this article. The bottom clearance effects on hydrodynamic forces and moments are summarised. A strong non-linear influence of the bottom clearance on the vertical force and pitch moment was found. With a trim by the bow, the vertical down force can increase up to the same order of magnitude as the vertical up force that can be generated by the sail planes.