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SUPERYACHT #9
Summer 2006

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news


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Article by
Mario Felli


Optimisation and experimentation:
new frontiers of ship design

Development of a ship design involves careful evaluation of technical solutions aimed at maximising vessel performance.

Setting out from design-related inputs, typically linked to dimensions, volume, displacement and maximum working speed, the choice of type and form of the ship's bottom - together with propulsive system and appendages - there is often a clash between aspects concerning hydrodynamic performances and requisites of manoeuvrability and safety. For example, hydrodynamic design has essentially regarded aspects linked to both reduction of resistance to progress and to wave resistance (see A. Sinisi's article "The Rounded Ship's Bottom" within this issue), employing remedies to reduce the rough surface of the quickwork (flush welds, sandblasting, low friction coefficient paints and air lubrication systems, which have resulted in a friction resistance of around 80% in comparison with vessels built in the early post-war period) and investigation into hull conformations such as to minimise energy dissipated due to the formation of wave motion.


Noise sources and levels typical of a Superyacht. The problem of vibration and noise reduction in passengers' quarters (comfort and sound fatigue) and with regard to the external environment (sound pollution) is one of the main aims of modern design approaches.

In recent years however the growing need to increase comfort aboard, reducing noise emission and the extent of induced vibrations and hull oscillation, has led to increased attention being paid to such aspects, especially with regard to the interaction with the propulsive system and the profiling of hull appendages. Incorporation of these aspects into the design specifications brings a number of advantages, mainly linked to the possibility of modulating comfort requirements with the typical vessel performance parameters (minimum resistance, propulsive efficiency, manoeuvrability), intervening with possible solutions within the optimisation process. In this way you avoid a posteriori remedies to "rehabilitate" any failures of the finished vessel in corresponding to design specifications, remedies which are wholly empirical, dictated by the designer's experience and often resulting in higher building costs, not to mention delays in delivery time.

In this scenario, design increasingly becomes a process of integration and mediation between operative needs and solutions that are often contradictory and where it is necessary to simultaneously keep in mind all the problems regarding the vessel- system. This itinerary is a critical point in the design of a vessel in which control of the said problems isn't often possible from a practical viewpoint. Structural problems cannot be separated from those of hydrodynamic seakeeping and resistance to vessel progress, just as those of operational management of the vessel cannot be separated from aspects linked to comfort and performance.


Measurement of the wave profile on the initial configuration and optimised by the bow-bulb. The optimised bottom has reduced resistance (less pressures on the bulb) and a more contained wave formation.

In the past, achievement of design requirements was based solely on the designer's experience and intuition, with an interactive "trial and error" approach which, however, with regard to negative fallout on costs and development times and analysis of the various alternatives, did not guarantee an optimal solution. This situation laid the foundations for development of a new design approach in which the support of mathematical and calculation tools meant that the problem could be tackled automatically, probing all alternative possibilities until an "optimal" solution was achieved. The design problem was thus transferred to the isolation of a grouping of variables that characterise the phenomenon, to the choice and implementation of an optimisation algorithm as well as analysis of performances aimed at verification of correspondence with design requirements. This phase is a complex and subdivided process in which the designer's experience cannot be separated from the synergic backup of Test Tank experimentation and the aid of specific numerical codes.


Seakeeping tests carried out at the INSEAN rectilinear basin. The study of the impact of wave systems on the hull is highly important for increasing safety and comfort levels on board.

Modern design approaches are based on this synergy, approaches inherited from the aerospace sector and today widespread especially in niche situations of marine engineering (racing boats, military vessels) where the optimisation process based on the use of numerical codes is integrated with the backup of advanced experimental techniques for their validation and performance verification. The optimisation algorithms used in the past were prevalently concentrated on the solution of isolated problems, without considering overall improvement of the vessel- system or any effects deriving from "conflicting" solutions. The weak point of that approach was therefore a partial vision of the actions and effects brought to bear on the vessel-system, neglecting possible interactions between "conflicting" solutions.

On this matter the question of anti-roll appendages and stabilising fins may be taken as an example of how important it is to tackle all design and operational aspects of the vessel-system globally: the sizing of these appendages should be done reasonably, not only looking at optimisation of vessel performance in terms of stability but also, for example, at hydrodynamic effects (linked to increased resistance to progress) and propulsive effects (any interactions between vortexes released by the appendages and the stabilising fins with the propulsion). Another typical example of conflicting interaction between design requisites is the phenomenon known as "whipping", which refers to a bothersome resonant phenomenon determined by small but continual impacts of the stern with the water surface. The "whipping" effect is especially evident when the vessel is stationary: in this situation the active control systems are completely inefficacious and only an efficient stern design can reduce the phenomenon. Optimisation however must mainly take account of the vessel's performance under way, so the two opposite tendencies of raising the position of the transom to reduce vibrations or lowering it to increase performance must be tackled with a global design philosophy aimed at optimisation. In the same way we should point out cases in which a hydrodynamically optimised hull, with a minimised resistance to progress and vertical motion, has nonetheless demonstrated a net reduction in propulsive efficiency with a negative fallout in terms of performance.

In recent years researchers at INSEAN (National Institute for Studies and Experiences in Marine Architecture) have developed sophisticated automatic calculation instruments for hydrodynamic optimisation of vessels, by considering the vessel-system as an overall grouping of linked hydrodynamic sub-problems which can globally and efficiently model the complexity of interactions between the marine environment and the vessel.

With this approach the characteristics required for a new design, defined in terms of objectives (e.g. reduction of friction and form resistance, increase in propulsion efficiency, improvement of performance in manoeuvre and/or in a heavy sea, diminution of accelerations, reduction of broad amplitude motions) and operational restrictions of a geometrical (e.g. vessel dimensions, displacement, payload volumes) or functional (e.g. maximum height of wave profile, minimum distance between the whirling structures of the stabilising fins of the propulsion shaft) type are described through a mathematical expression known as objective function. In this way you can automatically analyse all the best configurations that satisfy design aims in respect of operational limitations and set out from an initial geometrical configuration of the hull. Of course the number of solutions to the optimisation problem will depend on the number and type of the objectives and limitations set. In some cases, in particular, the problem of optimisation will be without solution and it will be necessary to pass on down to solutions of compromise, partially remodelling the objectives or limitations which nevertheless must be formalised and evaluated in the virtual reality of the CFD simulation, and therefore with a negligible fallout on design time and costs. A more detailed overview of aspects concerning the design of a vessel and the related experimental verification will appear in the next number of "Superyacht" with a description of the main problems and methodologies for tackling development of an optimisation algorithm and of the most advanced experimental techniques.


Reduction of the total resistance of an optimised hull (progress in blue) with respect to the original design (progress in red).

GLOSSARY

Laser Doppler Anemometry. Experimental technique for measuring field of velocity in a fluid medium. The Laser Doppler anemometer is an instrument that precisely measures the velocity field in a fluid with high accuracy and without perturbing the field of motion.

CFD. (Computational Fluid Dynamics) is a discipline of applied physics that came into being around the 1960's. As the name suggests, it is a reference to the study, by computer, of the dynamics of a fluid in which there may be physical phenomena such as heat exchange, acoustic radiation, vibration transmission, cavitation etc.

Validation. Validation of CFD models is the experimental verification of results obtained, carried out through numerical simulation of an assigned engineering problem. This process of "certification" of simulation quality is generally done by comparing the values of the numerical model output variables with those measured experimentally, under the same operational conditions and at corresponding points in the field.

Whipping. Hydroelastic phenomenon due to the impact of the stern on the water surface which brings about a resonant vibration in the hull. This phenomenon causes longitudinal flexions of the hull (Figure below) which have a negative influence on comfort aboard.


Deformation of a hull following the "whipping" phenomenon (deformation has been increased to better describe the phenomenon).