Near-surface self propulsion of a generic submarine in calm water and waves

We present a study of the response of the generic Joubert BB2 geometry self-propelled near the free surface, using a dynamic overset methodology with discretized propeller and control surfaces. A grid study was performed for sail top depth 0.702 m. Even keel simulations at depths ranging from -6.9 m to 40 m were conducted in calm water and regular waves representative of sea state 2 to 7. An autopilot acting on sail and tail planes, ballast and trim tanks was used to simulate controlled self-propulsion at several depths in calm water and waves. The even keel study shows that near surface operation causes considerable vertical forces and pitching moments, and an increase in required propeller thrust to achieve self-propulsion, decreasing efficiency. The boat responds to waves mainly through the added mass forces produced by the wave-induced velocity field, causing an exponential decay of the wave influence with depth and with decreasing wave amplitude. In controlled free running simulations very near the surface the boat responds to the forces observed in even keel, requiring compensation through the trim tanks to prevent high bow down pitch. Hull/free surface interaction causes fluctuations in the wake, resulting in a broadband response of propeller thrust.