Vertical Accelerations and Surge on a Planing Hull

Abstract

High-speed planing craft continue to increase in popularity. These craft experience high accelerations when operating at high speeds, even in relatively minor wave conditions. In the United States Navy, planing craft often operate at high speeds to accommodate mission needs, such as insertion of Special Forces into hostile areas. Passenger safety and structural integrity are affected by the accelerations experienced. Seakeeping towing tank tests measuring accelerations are generally motivated by structural design considerations or concern for passenger safety. Traditional seakeeping tests tow the model at constant speed, regardless of oscillating slamming loads, and allow the model to move only in heave and pitch. Typically in a planing hull seakeeping test the model is fixed in surge, i.e. restrained from moving backwards and forwards relative to the towing carriage. By contrast, planing boats operating in open water are propelled by constant thrust and forward speed is known to oscillate slightly with each wave slam. A concern for the accelerations measured in planing model tests is whether the vertical accelerations measured are accurate when the freedom to move in surge is restricted. To produce more accurate acceleration data for planing models, a test was completed at the United States Naval Academy examining the effects of surge on planing hull wave slam accelerations. These tests were completed in the 380-foot USNA Hydromechanics Laboratory using a model with a 4-foot length. The model was tested at a length-based Froude number of 1.85 in regular waves with a 1.1-second period and a wave height of 2.4-inches. The testing compared a self-propelled model, which was allowed freedom in surge, to a traditional test. The self-propelled model was outfitted with a motor and propeller that ran on a manual feedback system. Velocity, drag, pitch, heave, surge, and vertical and longitudinal accelerations were recorded. Statistical analysis of the acceleration data will be presented. Acceleration time histories of the impacts with and without freedom in surge will be compared. Preliminary data shows that using a self-propelled towing apparatus produces an increase in the vertical accelerations. Longitudinal accelerations appear to be smaller using the self-propelled rig than from using the locked-in-surge towing method. Suggestions for future testing, including lessons learned, will also be presented.