  Empirical data reveals a broad variety of hull shapes among the different
ship categories. We present a minimal theoretical approach to address the
problem of ship hull optimisation. We show that optimal hull aspect ratios
result -- at given load and propulsive power -- from a subtle balance between
wave drag, pressure drag and skin friction. Slender hulls are more favourable
in terms of wave drag and pressure drag, while bulky hulls have a smaller
wetted surface for a given immersed volume, by that reducing skin friction. We
confront our theoretical results to real data and discuss discrepancies in the
light of hull designer constraints, such as stability or manoeuvrability.
