Course Stability of a Towed Submarine Rescue Vehicle A manoeuvre-based approach

Marine vessels designed to be self-propelled are generally unstable when towed. Submarines are not an exception; holding a course while towing a surfaced submarine is a challenging operation that often requires several tug boats or special methods. The Swedish navy’s submarine rescue vehicle URF, for example, is directionally unstable when being towed at lower speeds, and this report examines methods of improving URF’s course stability under these circumstances. An experimental evaluation was conducted to assess the effect of static trim angle on URF’s course stability; by adjusting URF’s trim angle, the center of pressure can be shifted in a way that is favourable to course stability. A 1:19 scale model was developed and towed in calm water at equivalent full-scale speeds of 2 to 8 knots and at trim angles between 0 and 15 degrees. Course stability was assessed on the basis of the model’s observed behaviour during towing, including the model’s maximum angle during wandering, stable angle, tendency to dive and behaviour upon sudden release of the towline