The effect of proximity to free surface on the optimum length/diameter ratio for a submarine

Conventional submarines regularly operate near the surface to recharge batteries and to meet operational requirements. During these operations, a submarine will generate surface waves which can significantly increase its resistance and thus the required propulsive power. In addition, forces and moments are induced in the vertical plane that can adversely affect the trajectory of the submarine.

A number of design and operational factors influence the wavemaking resistance and the vertical forces and moments. These include the proximity to the free surface, forward speed, size and configuration of the appendages, and the length to diameter (L/D) ratio. These effects are dependent on the interaction of the various hull-borne wave systems, which in turn are highly dependent on the Froude number.

In order to quantify these effects, a series of captive model experiments on a 1.556m model of the DARPA SUBOFF submarine was conducted in the Australian Maritime College towing tank supplemented by Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamic (CFD) simulations of the model under varying conditions and configurations. This paper describes the RANS CFD modelling of the SUBOFF geometry at different L/D ratios and Froude numbers to determine the influence of these parameters on resistance.