New insights into dead water

Dead water refers to a mysterious increased resistance experienced by boats in certain waters. Ekman's 1904 PhD thesis concluded this drag stemmed from internal waves in density-stratified seas, but few further investigations have followed.
Analytical approaches assume potential flow, neglecting important vortex dynamics, which experiments have also failed to visualise. We use the pseudospectral Dedalus code to simulate the problem, modelling the fluid with Boussinesq hydrodynamics and incorporating the boat using an Immersed Boundary method. We reproduce the effect and demonstrate that it is most pronounced in strongly nonlinear regimes, coinciding with a previously overlooked trailing vortex.