An experimental investigation of the flow past hulls at leeway

Abstract

When a ship is sailing, the hull moves obliquely through the water at a greater angle of leeway than when under power alone. A wind tunnel is used to investigate the flow past a Mariner type hull for the range of leeway angles expected when sailing. Flow visualisation experiments are conducted, and the non-wave-making components of hydrodynamic hull forces are measured. The hull is a very low aspect ratio lifting body which develops (horizontal) lift to balance sail side force; it does this at the expense of considerable induced drag. The body is slender and fairly streamlined, but flow separation occurs at the bilges and shed vortex sheets roll up to form longitudinal vortices. These vortices, associated with much of the drag penalty, trail downstream near to the hull; they generally maintain their identities, at least for flow distances of the order of the ship's length. There is, however, some merging of vorticity between proximate vortices. A systematic series of hull-like blocks is similarly tested to investigate the influence of certain parameters such as beam, draft and trim. The flow about these simplified hulls shows the same general features as that about realistic hulls. Specific conclusions for the block hulls indicate that the hydrodynamic repercussions on sailing ship performance of various changes are as follows — An increase in beam is marginally detrimental — An increase in draft is beneficial — Allowing the vessel to heel is beneficial — Trimming the vessel by the stern is beneficial.