Knot So Fast
Today's focus in the shipping world is increasingly on fast ships. However, the trend is also towards higher speeds for conventional tonnage — vessels such as containerships, RoRos and bulkers. And while the speed increases are only up to five knots, this has important implications and is a challenge for the ship designer.
To ensure stability, safety and performance criteria are met, it is important to identify and solve problems during the design process. Fuel costs, of course, is a very important factor which directly affects the speed of ships, but the focus on fuel cost varies.
For example, the general thought in the 1960s and early 1970s was to increase ship speeds, with the consequent higher power demands and higher fuel consumption. This trend was abruptly broken with the oil crisis in 1973, and as a result, speeds were reduced, sometimes drastically, to decrease fuel costs.
The next trend to increase speeds started in the late 80s.
The Need For Speed Towards the end of the 1980s, a change was noted, as several projects were initiated, specifying fast transport. While the projects mainly involved ferries, it was notable that plans called for ships of 1,000 dwt and larger at speeds of 35 knots and above. At the same time, a demand for higher speeds in the RoRo and containership segments was discovered, and the typical speed increase raised services speeds to around 25 knots.
Speed Affects Design The development of null lines is mainly an evolutionary process where the requirements of the cargo and cargo handling are important. The design of the hull lines has to be, in general, completed within a short time frame, and is concerned with the actual ship. In depth optimizations are rarely made, but rather refinements and extension of existing designers. A speed increase from 20 to 25 knots has implications not only on fuel consumption, but also on vibration and noise levels, sea loads and to some extent on maneuvering properties. This implies that a more careful and systematic design procedure is needed.
Speed-power relation and the choice of the optimal dimensions should be considered at the outset of the project. While experience and data bases exist to help make correct choices, these resources need to be complemented, as modern ship forms imply dimensional relations outside earlier experiences.
Another area of concern with increasing speeds is that of noise and vibration. With ship speeds increasing, vibration problems have seemed to reappear, despite improved methods to predict vibration and noise. It should also be noted that the common hull shapes with rather flat aftbodies appear to be more susceptible to pressure pulses.
Desian Tools In the past, the main tools for the hydrodynamic design of ships were model tests and experience. The model tests were, in general, quite time consuming and focused on the speed-power relation. The limited time for the design also meant that the design goal generally was to provide a satisfying solution rather than an optimized design.
The availability of computational methods as Computational Fluid Dynamics (CFD) for resistance and propulsion, time simulation of ship's motions, finite element analyses of vibrations and noises, etc., provide possibilities to investigate the relative merits of different design alternatives within a limited time frame. In this way, different design concepts can be analyzed with regard to speed-power, vibrations, sea loads and other aspects almost simultaneously. However, to obtain a high assurance of the project, model tests are recommended to verify the computational results.
The Future The demands on sea transport will focus more and more on competitiveness, safety and environmental aspects. Competitiveness means not only low fuel consumption, but also low sea loads, good maneuverability and low vibration and noise levels. Ships which do not comply with basic regulations and requirements in regard to complying with safety and environmental demands will not be accepted, and it can be anticipated that such demands will only increase. Future ships will require larger design efforts and will consequently demand efficient and reliable design procedures. In regard to ship hydrodynamics and related fields, the development of computational tools will continue, and such methods will be used more. Most aspects of ship hydrodynamics which earlier relied on model test results — can now be treated computationally, primarily for comparisons and optimization of the designs. With more extensive use of computational methods, improvements in performance seem to be possible. As an example, CFD calculations have been used in an optimization test on resistance, and the results indicate reductions in the order of 10 percent. Model tests will still be required to verify and validate computational results. Design work will thus include both computational work and model tests.
