Autonomous High Speed Sea Surface Vehicles for Fast Rescue and Research Applications
The use of unmanned small planing crafts is currently becoming of considerable importance in many civil, scientific and military applications. The seaworthiness of these crafts present a significant challenge, particularly in regard to the high accelerations and pitch/roll motions these develop in seaway which pose a hazard to the payload and the craft's structure, While in manned operation the operator has some capability to reduce the accelerations by naturally selecting the craft's course in waves, in an unmanned operation this course selection presents an inherent challenge. To promote the development and operation of unmanned small planing crafts, the capability to predict in real time the acceleration and motions expected to develop on the craft based on the knowledge of the incoming waves is much required. Predicting the motion of a planing hull in waves in the time domain is a complicated task and involves several phenomena including planing, hydrodynamic impact, surface waves and hydrostatics. Dynamic pressure dominance, changes in the wetted length, drag, considerable changes in the rising of the vessel and its trim angle are all among the factors that should be considered in accurate modeling of planing crafts in waves. Access to a fast, accurate technique for predicting the motion of these hulls plays a significant role in future development of unmanned planing crafts.
A nonlinear mathematical model for the simulation of motions and accelerations of planing monohulls in seaway, having a constant or variable deadrise angle, in head or following waves has been formulated. The model is based on the 2-dimensional strip theory and provides the acceleration, velocities and expected motions of a planing craft in the vertical plane and in the time domain. The simulations can be carried out for a planing craft sailing in regular and irregular head or following seas, at a constant forward speed or a constant thrust. The sectional hydromechanic forces are determined by the theory of a wedge penetrating a water surface. The wave excitation in the vertical direction caused by the vertical orbital velocity of the wave and the wave elevation, altering the total wetted length, the sectional wetted breadth and immersion is directly integrated in the expressions for the hydromechanic forces. A near transom pressure corrections affecting both the hydrostatic and the hydrodynamic terms of the load distribution are introduced. The dynamic drag force is calculated based semi empirical model. An existing JetSki platform is made remotely operable and is instrumented with an IMU to log the linear and angular accelerations and the angular attitude of the craft, The incoming waves are measured by a separately deployed buoy. The wave induced heave, pitch and vertical accelerations has been validated with existing experimental data and with the real time experimental data. Comparison of the obtained vertical acceleration with various classification societies semi empirical relations is also presented. Visual sensors assisted algorithm for selection of optimal navigation path in rough seas with a control method will be integrated into the existing platform to improve seakeeping of the craft.
Keywords - Hydrodynamics, Unmanned, Mathematical model, Planing craft, Seakeeping, Time domain simulation.