An experimental study of interceptors for drag reduction on high-performance sailing yachts

Interceptors have been widely used in recent years in fast ferries and small high-speed leisure and commercial craft for ride and trim control, and steering. In the context of high-performance sailing yachts, they first appeared in 2008 on the yacht Ecover 3 which was dismasted while leading the Vendee Globe Challenge race. However, in spite of their popularity in power craft, few studies have been published investigating the impact of interceptors on vessel performance, and apparently none in the case of sailing yachts. In the current study, interceptors are compared with an aerodynamic device known as a Gurney flap. It is shown that interceptors are generally substantially smaller than Gurney flaps. A comprehensive experiment programme is presented exploring the impact of interceptors on the performance of an Open 60 yacht hull. Results show a marked reduction in calm-water resistance over a wide speed range, with benefits of 10–18% in the speed range between 8 and 20 knots, accompanied by reduced sinkage and trim. The gains observed are much larger than those observed in powercraft, and also substantially greater than those achievable through trim changes by moving ballast longitudinally. The benefits appear to be largely sustained in small waves

Performance prediction for sailing dinghies

This study describes the development of an approach for performance pr ediction for a sailing dinghy . Key modelling issues addressed include sail depowering for sailing dinghies which cannot reef; effect of crew physique on sailing performance, components of hydrodynamic and aerodynamic drag , decoupling of heel angle from heeling momen t, and the importance of yaw moment equilibrium. In order to illustrate the approaches described, a customised velocity prediction program (VPP) is developed for a Laser dinghy . Results show excellent agreement with measured data for upwind sailing , and correctly predict some phenomena observed in practice . Some discrepancies are found in downwind condition s, but it is speculated that this may be related at least in part to the sailing conditions in which the measured data was gathered. The ef fect of crew weight is studied by comparing time deltas for crews of different physique relative to a baseline 80kg sailor. R esults show relatively high sensitivity of the performance around a race course to the weight of the crew, with a 10kg change contr ibuting to time deltas of more than 60 seconds relative to the baseline sailor over a race of one hour duration at the extremes of the wind speed range examined

Effects of Gas Flaring on the Behavior of Night-migrating Birds at an Artificial Oil-production Island, Arctic Alaska

We studied movement rates and the general flight behavior of bird flocks seen on radar and recorded visually at Northstar Island, Arctic Alaska, from 13 to 27 September 2002. Most of this period (13 – 19 and 21 – 27 September) had no gas-flaring events, but a major gas-flaring event occurred on the night of 20 September. Movement rates of targets on radar and of bird flocks recorded visually in the first ~50% – 60% of the night were much lower during the non-flaring period than during the night of flaring, whereas rates in the last ~40% – 50% of the night were similar in all periods. The general flight behavior of birds also differed significantly, with higher percentages of both radar targets and bird flocks exhibiting straight-line (directional) flight behaviors during the non-flaring periods and higher percentages of radar targets and bird flocks exhibiting non-straight-line (erratic and circling) flight behaviors during the gas-flaring period. During the night of gas flaring, the bright illumination appeared to have an effect only after sunset, when flocks of birds circled the island after being drawn in from what appeared to be a substantial distance from the island. On both radar and visual sampling, the number of bird flocks approaching the island declined over the evening, and the attractiveness of the light from flaring appeared to decline. The visibility of the moon appeared to have little effect on the behavior of birds. Because illumination from extensive gas-flaring is such a strong attractant to migrating birds and because most bird flocks fly at low altitudes over the water, flaring booms on coastal and offshore oil-production platforms in Arctic Alaska should be positioned higher than the mean flight altitudes of migrating birds to reduce the chances of incineration.Nous avons étudié les taux de déplacement et le comportement de vol général des troupeaux d’oiseaux captés par radar ou consignés visuellement à l’île Northstar, dans l’Alaska de l’Arctique, du 13 au 27 septembre 2002. Pendant presque toute cette période (du 13 au 19 et du 21 au 27 septembre), il n’y a pas eu de brûlage de gaz à la torche, mais la nuit du 20 septembre, il y a eu un important brûlage de gaz à la torche. Les taux de déplacement des cibles radar et des troupeaux d’oiseaux consignés visuellement pendant la première tranche d’environ 50 % à 60 % de la nuit étaient beaucoup moins élevés pendant la période où il n’y avait pas de brûlage à la torche que pendant la nuit où il y a eu brûlage à la torche, tandis que pendant la deuxième tranche d’environ 40 % à 50 % de la nuit, les taux de déplacement étaient semblables pendant toutes les périodes. Le comportement de vol général des oiseaux a également affiché une différence considérable. De plus grands pourcentages de cibles radar et de troupeaux d’oiseaux adoptaient un comportement de vol rectiligne (direct) pendant les périodes où il n’y avait pas de brûlage à la torche, et de plus grands pourcentages de cibles radar et de troupeaux d’oiseaux affichaient un comportement de vol non rectiligne (erratique et indirect) pendant la période où il y a eu brûlage à la torche. La nuit du brûlage à la torche, la vive illumination n’a semblé avoir un effet qu’après le coucher du soleil, quand les troupeaux d’oiseaux encerclaient l’île après avoir été attirés depuis un endroit qui semblait très lointain. Tant pour l’échantillonnage prélevé par radar que par consignation visuelle, le nombre de troupeaux d’oiseaux s’approchant de l’île diminuait dans le courant de la soirée, et l’attrait de la lumière émanant du brûlage à la torche semblait également diminuer. La visibilité de la lune semblait avoir peu d’effet sur le comportement des oiseaux. Puisque l’illumination provenant du brûlage prolongé à la torche exerce une si grande force d’attraction chez les oiseaux migrateurs, et puisque la plupart des troupeaux d’oiseaux volent en basse altitude au-dessus de l’eau, le torchage effectué sur les plateformes pétrolières côtières et extracôtières dans l’Alaska de l’Arctique devrait être positionné plus haut que les altitudes moyennes de vol des oiseaux migrateurs afin de réduire les risques d’incinération

Effects of a Hazing-Light System on Migration and Collision Avoidance of Eiders at an Artificial Oil-Production Island, Arctic Alaska

During migration, Common and King Eiders (Somateria mollissima and S. spectabilis) cross the Beaufort and Chukchi Seas of Arctic Alaska. Because they may become attracted to lights, eiders are susceptible to collision with structures, including offshore oil facilities. We used ornithological radar in 2001 – 04 to characterize the behavior of eiders migrating past Northstar Island, an oil-production island near Prudhoe Bay, Alaska, and to assess the effects of a hazing-light system on migrating eiders. “Eider” radar targets exhibited pulsed, irregular periods of movement; movement rates were higher when sea ice was present, without precipitation, and during tailwinds and crosswinds but were not affected by lights. Velocities (ground speeds) were higher when ice was present and with strong tailwinds. They were lower at night when the lights were on, but higher during the day when the lights were on. Radar targets exhibited little variation in flight behavior as they passed the island; the proportion of non-directional behavior was larger when ice was present, with tailwinds, with weak winds, and near the full moon when it was not visible. Lights had no effect on flight behavior. Birds tended to exhibit more course changes as they approached the island, greater angular changes when they changed course, and larger net increases in passing distance as a result of those course changes when the lights were on; however, none of these differences were statistically significant. Overall, the hazing lights at Northstar did not disrupt the birds’ migratory movements but resulted in increased avoidance of the island.En période de migration, l’eider à duvet et l’eider à tête grise (Somateria mollissima et S. spectabilis) survolent la mer de Beaufort et la mer des Tchouktches dans l’Alaska de l’Arctique. Comme ils sont attirés par les lumières, les eiders risquent d’entrer en collision avec des structures, y compris les installations pétrolières au large. De 2001 à 2004, nous avons utilisé un radar ornithologique pour caractériser le comportement des eiders qui migrent au-delà de l’île Northstar, une île de production pétrolière près de Prudhoe Bay, en Alaska, et pour évaluer les effets d’un système d’éclairage de dispersion sur les eiders en migration. Les « eiders » ciblés par le radar présentaient des périodes de mouvement pulsées et irrégulières; les taux de mouvement étaient plus importants en présence de glace marine, en l’absence de précipitation et en présence de vent arrière et de vent latéral, mais n’étaient pas touchés par les lumières. Les vélocités (vitesses au sol) étaient plus élevées en présence de glace et de forts vents arrière. Elles étaient plus basses la nuit lorsque les lumières étaient allumées, mais plus élevées le jour lorsque les lumières étaient allumées. Nous avons observé peu de variation quant au comportement de vol des cibles atteintes par le radar pendant qu’elles survolaient l’île; la proportion de comportements de vol non directionnels était plus importante en présence de glace, de vent arrière, de vent faible et lorsque la pleine lune n’était pas visible. Les lumières n’ont pas eu d’impact sur le comportement de vol. Lorsque les lumières étaient allumées, les oiseaux avaient tendance à changer de direction plus souvent durant leur vol à l’approche de l’île et à effectuer de plus grandes variations angulaires lorsqu’ils changeaient de direction, puis présentaient une nette augmentation de la distance de passage à la suite de ces changements de direction. Toutefois, aucune de ces différences n’était statistiquement importante. En général, les lumières de dispersion de l’île Northstar n’ont pas nui aux mouvements migratoires des oiseaux, mais ces derniers ont davantage évité de voler près de l’île

Exploring the impact of innovative developments to the installation process for an offshore wind farm

For offshore wind to be competitive with mature energy industries, cost efficiencies must be improved throughout the lifetime of an offshore wind farm (OWF). With expensive equipment hire spanning several years, installation is an area where large savings can potentially be made. Installation operations are subject to uncertain weather conditions, with more extreme conditions as OWF developments tend towards larger sites, further offshore in deeper waters. One approach to reduce the cost of the installation process is to evaluate advanced technologies or operational practices. However, in order to demonstrate cost savings, the impact of these advances on the installation process must be quantified in the presence of uncertain environmental conditions. To addresses this challenge a simulation tool is developed to model the logistics of the installation process and to identify the vessels and operations most sensitive to weather delays. These operations are explored to identify the impact of technological or operational advances with respect to weather delays and the resulting installation duration under different levels of weather severity. The tool identifies that loading operations contribute significantly to the overall delay of the installation process, and that a non-linear relationship exists between vessel operational limits and the duration of installation

Trim influence on Kriso Container Ship (KCS) : an experimental and numerical study

There has been a lot of interest in trim optimisation to reduce fuel consumption and emissions of ships. Many existing ships are designed for a single operational condition with the aim of producing low resistance at their design speed and draft with an even keel. Given that a ship will often sail outside this condition over its operational life and moreover some vessels such as LNG carriers return in ballast condition in one leg, the effect of trim on ships resistance will be significant. Ship trim optimization analysis has traditionally been done through towing tank testing. Computational techniques have become increasingly popular for design and optimization applications in all engineering disciplines. Computational Fluid Dynamics (CFD), is the fastest developing area in marine fluid dynamics as an alternative to model tests. High fidelity CFD methods are capable of modelling breaking waves which is especially crucial for trim optimisation studies where the bulbous bow partially emerges or the transom stern partially immerses. This paper presents a trim optimization study on the Kriso Container Ship (KCS) using computational fluid dynamics (CFD) in conjunction with towing tank tests. A series of resistance tests for various trim angles and speeds were conducted at 1:75 scale at design draft. CFD computations were carried out for the same conditions with the hull both fixed and free to sink and trim. Dynamic sinkage and trim add to the computational cost and thus slow the optimisation process. The results obtained from CFD simulations were in good agreement with the experiments. After validating the applicability of the computational model, the same mesh, boundary conditions and solution techniques were used to obtain resistance values for different trim conditions at different Froude numbers. Both the fixed and free trim/sinkage models could predict the trend of resistance with variation of trim angles; however the fixed model failed to measure the absolute values as accurately as the free model. It was concluded that a fixed CFD model, although computationally faster and cheaper, can find the optimum trim angle but cannot predict the amount of savings with very high accuracy. Results concerning the performance of the vessel at different speeds and trim angles were analysed and optimum trim is suggested

Effect of barnacle fouling on ship resistance and powering

Predictions of added resistances and effective powers of ships were made for varying barnacle fouling conditions. A series of towing tests were carried out using flat plates covered with artificial barnacles. The tests were designed to allow the examination of the effects of barnacle height and percent coverage on the resistance and effective power of ships. The drag coefficients and roughness function values were evaluated for the flat plates. Roughness effects of the fouling conditions on the ship frictional resistances were predicted. Added resistance diagrams were then plotted using these predictions, and powering penalties of these ships were calculated using the generated diagrams. The results indicate that the effect of barnacle size is significant, since 10% of the coverage of barnacles, which are 5mm in height, causes a similar level of added power requirements as 50% of the coverage of barnacles, which are 1.25 mm in height

Unsteady effects during resistance tests on a ship model in a towing tank

It is known that there are oscillations in the wave resistance during the constantvelocity phase of a towing-tank resistance test on a ship model. In this work, the unsteady thin-ship resistance theory has been applied to this case. The results have been compared with experiment data obtained using a towing carriage the velocity history of which can be programmed. It is demonstrated here that generally excellent correlation exists between the theory and the experiments. In particular, one can predict the influence of Froude number, rate of acceleration, and type of smoothing of the acceleration on the characteristics of the oscillations. These characteristics include the amplitude, rate of decay, frequency, and phasing of the oscillations in the curve of wave resistance versus time

Risk factors for the evolutionary emergence of pathogens

Recent outbreaks of novel infectious diseases (e.g. SARS, influenza H1N1) have highlighted the threat of cross-species pathogen transmission. When first introduced to a population, a pathogen is often poorly adapted to its new host and must evolve in order to escape extinction. Theoretical arguments and empirical studies have suggested various factors to explain why some pathogens emerge and others do not, including host contact structure, pathogen adaptive pathways and mutation rates. Using a multi-type branching process, we model the spread of an introduced pathogen evolving through several strains. Extending previous models, we use a network-based approach to separate host contact patterns from pathogen transmissibility. We also allow for arbitrary adaptive pathways. These generalizations lead to novel predictions regarding the impact of hypothesized risk factors. Pathogen fitness depends on the host population in which it circulates, and the ‘riskiest’ contact distribution and adaptive pathway depend on initial transmissibility. Emergence probability is sensitive to mutation probabilities and number of adaptive steps required, with the possibility of large adaptive steps (e.g. simultaneous point mutations or recombination) having a dramatic effect. In most situations, increasing overall mutation probability increases the risk of emergence; however, notable exceptions arise when deleterious mutations are available

Verification and validation of numerical modelling of DTMB 5415 roll decay

The paper presents a numerical roll damping assessment of the intact DTMB 5415 naval ship at zero speed. Free model motions from four experimental roll decays with initial heeling angle of 4.0, 13.5, 19.58 and 24.50 deg, performed previously at the University of Strathclyde, Glasgow, have been analysed and the one with 19.58 deg initial heeling has been chosen for the Computational Fluid Dynamic (CFD) analysis. All calculations are performed using CD Adapco Star CCM+ software investigating the accuracy and efficiency of the numerical approach for case of high initial heeling angle of bare hull. In the numerical procedure the verification analysis of mesh refinement and time step was performed with the aim to investigate the numerical error/uncertainty. For grid refinement and time step, validation and verification procedure has been performed according to the Grid Convergence Index (GCI) method. Moreover, to verify the main source of the modelling error/uncertainty, the effect of degrees of freedom are evaluated, comparing the numerical results with the experimental results. Conclusions are identifying best practice for roll decay simulations commenting the accuracy of numerical results and required calculation time