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Appendix D: Hydrodynamics, Physical Models, and Mathematical Modeling
Pages 211-240

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From page 211...
... , and increasing knowledge from naval architects of the appropriate models for ship dynamics. Beyond ship dynamics, modeling of other operating conditions -- including shallow-water effects; restricted-water effects, including bank-suction; slowspeed maneuvering, including anchoring and normal backing evolutions; turning with tugs and thrusters; and ship-to-ship interactions -- is more complex than mathematical modeling for ships operating in deep and unrestricted shallow water.
From page 212...
... accuracy and fidelity that are needed and delivered in the replication of ship maneuvering behavior for simulation training in both computer-based and manned-model simulation are debated within the hydrodynamic modeling, marine simulation, and marine education and training communities. This appendix describes the modeling of vessel maneuvering used in marine simulation, the levels of accuracy present in the various modeling approaches, and where the different modeling approaches and levels of sophistication and accuracy may be most appropriate for specific training purposes.
From page 213...
... If these adjustments are not correct, the resulting trajectory predictions are inaccurate, regardless of the quality of the algorithms used or the apparent validity of the simulation. For these reasons, it is appropriate to validate each trajectory prediction model or perturbation in a model to determine the capabilities and limitations of the product being delivered to the trainer, the marine licensing authority, and licensing examiners and assessors.
From page 214...
... Manned models are believed by many to provide realistic representation of bank effects, shallow water, and ship-to-ship interactions. The manned-model hull forms and
From page 215...
... Except for the latter case, tug/barge units are typically modeled as rigid units, assuming that they are tightly lashed. FIDELITY AND ACCURACY For training purposes, it is important to provide the necessary level of realism and accuracy to support training objectives.
From page 216...
... Given the state of modeling practice, this approach usually results in acceptable modeling. The accuracy of the trajectory prediction models available to drive a simulation can be compared with the level of fidelity specified by the training analysis as necessary to achieve training objectives.
From page 217...
... Most simulator facilities have sought high degrees of realism to build and improve confidence in simulation capabilities among mariners, training sponsors, and marine licensing authorities. It is possible to alter the mathematical trajectory prediction models to accentuate certain vessel maneuvering behavior, for example, as an instruction technique to assist a trainee in becoming aware of a particular behavior.
From page 218...
... Turning, Checking, and Course-Keeping Abilities Turning, turn recovery, and course-keeping abilities are closely related to the level of dynamic course stability, a characteristic of hull form and rudder. In general terms, dynamic stability is the ability of a vessel to return to a steady heading (or initial turning condition)
From page 219...
... The heading of an unstable vessel, on the other hand, continues to change with time, until nonlinear hydrodynamic forces override the inherent instability of the hull form. Such a ship requires constant helm corrections to maintain a desired heading.
From page 220...
... . In the case of the very unstable ship, C, where the heading angle is divergent after the first execution of the rudder angle, recovery cannot be achieved by the use of the opposing rudder angle at the second execution.
From page 221...
... , and interactions with other ships also produce significant changes in the dynamic course stability of the hull, which in turn results in different maneuvering behaviors. Vessel draft, and especially trim, are also critical parameters affecting dynamic course stability.
From page 222...
... Mathematical models must be carefully adjusted for variations, and usually separate model tests are required to obtain proper coefficients for use at different drafts or trims of the ship to provide accurate modeling. Properly modeling the level of dynamic course stability is very important in developing a model that will provide the proper vessel response to ship control actions.
From page 223...
... Application of Physical Models to Training Sufficiently large-scale models with self-propulsion and steering can be maneuvered by an onboard crew to simulate actual vessel maneuvers in suitable lakes or other sheltered waters. Manned models offer the advantages of relatively accurate hydrodynamic representation and realistic scenarios, particularly for low-speed operations, berthing, and ship interaction forces.
From page 224...
... Regardless, experienced mariners who have participated in manned-model training generally have found it to be a useful, limited-task, shiphandling training device, particularly for understanding basic and complex maneuvers, with benefits to real-life performance. Disadvantages with manned models can include the exaggerated effects of wind and practical limitations in providing different ship hulls, channel configurations, bank effects, and currents.
From page 225...
... This reproduction has only been done in a few cases with idealized representations of confined waterways. The principal and practical limitations occur in a facility's ability to provide or construct the essential physical operating environment and to control the water levels at their facilities, and thus the underkeel clearances.
From page 226...
... General Forms of Mathematical Models All mathematical models are developed based on Newton's second law that the force acting on a ship is equal to the product of its mass and acceleration. This law applies to all three components of motion -- longitudinal, transverse, and vertical (surge, sway, and heave)
From page 227...
... This lack of knowledge is matched by limitations in knowledge about the waterway geometry and current data that are available. Therefore, high confidence is not possible in the trajectory accuracy replicated by the mathematical model in narrow channels and shallow waters with small underkeel clearances.
From page 228...
... Shallow Water, Restricted Water, and Ship-to-Ship Interactions The modeling process becomes even more complicated when the effects of shallow water, restricted water (including channel side and bank effects) , and ship-to-ship interactions are included.
From page 229...
... Model tests to study ship interactions, which are generally performed with two ships on parallel trajectories at a fixed separation distance, do not adequately represent the more-complex trajectories involved in the Texas maneuver. Thus, when simulators have been used to instruct shiphandlers in this maneuver, empirical modifications have been made in the model to achieve more-realistic results.
From page 230...
... Trajectory prediction accuracy in the simulation of vessel maneuverability could avoid creating unrealistic operating conditions that could influence an individual's performance. On the other hand, an individual's familiarity with vessel maneuverability in a particular waterway could potentially mask weaknesses in shiphandling knowledge, skills, and abilities.
From page 231...
... The result is that individuals who are self-taught modelers sometimes create and then validate the models that they have developed without the benefit of interdisciplinary perspective to ensure the overall reasonableness and accuracy of the simulated vessel's maneuvering behavior. The involvement of a large number of mariners in validating the accuracy of trajectory prediction may be useful, but such a practice should not be a substitute for including hydrodynamic expertise in the validation process.
From page 232...
... Adoption of interdisciplinary validation of adjustments to the trajectory prediction model would help ensure that marine simulation operators have as complete an understanding of the capabilities and limitations of their simulators and simulations. This understanding needs to be appropriately conveyed to trainees to minimize the potential for training-induced error.
From page 233...
... Comparative model tests may be required to verify expected vessel maneuverability. FUTURE DEVELOPMENTS Vessel maneuvering prediction modeling is a developed science that provides highly useful tools for building marine training simulators.
From page 234...
... and the navigation of ships in entrance channels of restricted depth when waves are present. Microcomputer-Based Shipboard Simulation Although physical models, shiphandling simulators, and radar simulators will continue to be the major types of simulation available for full-mission, multi-task, and limited-task training, new approaches will expand the traditional media that are available for training.
From page 235...
... Such extensions of existing methods could also be used to enhance the fidelity of land-based simulators. Physical Versus Mathematical Models Physical models provide a simple approach to imparting knowledge on the hydrodynamics of ship motions in deep water and close-in operations, including docking, coming alongside, shallow water, banks, and ship-to-ship operations.
From page 236...
... The rapidly expanding availability of differential global positioning system reference stations and systems provides great promise for obtaining accurate measures of ship behavior in restricted waterways that have heretofore been difficult and not economical to obtain. Validity Requirements From the modeler's perspective, the simulator user must specify what accuracy is needed for particular training objectives.
From page 237...
... Consideration should be given to how these models could be used in developing verified environments for training requirements. Flexibility Needs Numerous mathematical models are currently used to drive various simulators.
From page 238...
... Current mathematical models would benefit from being modularized and validated in parts so that the science of modeling can progress and accuracies can be better established. Full-scale experiments also are needed to advance the state of practice in modeling, particularly for shallow water and restricted waters with banks.
From page 239...
... As a result, it becomes necessary to compare training objectives to available simulation resources to determine the suitability of simulation as a training medium or of specific simulation resources to meet specific training objectives. Manned models offer an alternative to on-the-job training, subject to the trainees ability to adjust to the scaling factors and then to correctly translate the lessons learned back to the real world.
From page 240...
... Trondheim, Norway: International Marine Simulator Forum. SNAME (Society of Naval Architects and Marine Engineers)


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