The bulbous bow seemed to be the number one measure to achieve higher hull form efficiency in the past. With requirements and operational profiles of ships changing, hull shapes are being optimized by the help of advanced CFD. At the same time, the hull surface is getting attention as new coating solutions are being developed. Hamburgische Schiffbau-Versuchsanstalt (HSVA) and Schiffbau-Versuchsanstalt (SVA) Potsdam give insight into their recent work
A multitude of variants can be investigated today at the desk before physical confirmation. Starting with basic potential flow calculations[ds_preview] to investigate the wave generation and interaction as well as the pressure distribution, up to the application of viscous flow calculations to – amongst other things – identify flow separation, alignment angles and calculating the magnitude of performance levels at an early stage. All this led to a high level in hull form design with very low residual resistance if done thoroughly.
With today’s standard on-board monitoring systems the ship owners have a very good picture about actual sailing conditions of their fleet and give specific requirements for new building projects and their designs. With this the focus of hull form design has shifted from »design point« towards a wider consideration of the operational profile (»design range«) of the vessel.
Exemplary for this change in design is the bulbous bow. Designed for one draught and one – often high – speed, bulbs were sometimes very voluminous with a poor performance at slower speeds and especially at lower draughts. Modern bulbs tend to be more slender, as such geometry is suitable for a wider draught range and less vulnerable in the lower speed range. All this is at the expense of one narrow but better performing specific design point. With the application of CFD tools the bulbous bow can be used to skew the speed power curve depending on the customer’s needs.
However, current bow designs show much more individuality nowadays with e.g. even cargo vessels featuring straight stem contours. Here as well CFD applications can determine the optimum shape – or show the customer the impact of such a design request.
The hull form optimization and the development of energy saving devices (fins, twisted rudders, ducts, rudder bulbs, propeller boss cap fins etc.) have achieved considerable progress and savings. Furthermore, ship speeds – e.g. of container vessels – have been considerably reduced by »slow steaming« and following the largest potential of reducing ship resistance, fuel consumption and exhaust gas emissions can be found in the frictional resistance of vessels amounting to 70-80% of the total ship resistance. Several research projects are addressing this topic by introducing a new hull coating and fouling protection technology, such as the European research projects eSHaRk and AIRCOAT to be shortly presented in the following. Both projects are supported by HSVA’s expertise in hydrodynamics and related testing of coating materials.
The eSHaRk project – eco-friendly Ship Hull film system with fouling Release and fuel saving properties – powered by EU funding (Grant No. 691495) is a consortium consisting of the world’s leading marine experts and engineers working together to create an innovative new fouling protection system for commercial seagoing vessels, and to accelerate its entry to market. The eSHaRk project aims to bring a greener and more efficient non-fouling solution of a surface textured foil to the market. This innovative new technology not only maintains current fouling protection standards but is superior to existing solutions in terms of eco-friendliness, ease of application, durability and drag-reduction, all of which will lead to fuel savings and a reduction in greenhouse gas (GHG) emissions.
AIRCOAT (Air Induced friction Reducing ship COATing) develops a passive air lubrication technology that utilizes the biomimetic Salvinia effect. This effect enables trapping air while submerged in water. The project (EU Grant No. 764553) technologically implements this effect on a self-adhesive foil system. Applying a ship with such an AIRCOAT foil will produce a thin permanent air layer, which reduces the overall frictional resistance significantly while acting as a physical barrier between water and hull surface. Therefore, besides substantially reducing main engine fuel oil consumption and hence exhaust gas emission, the air barrier further inhibits the attachment of fouling and the release of biocide substances (of underlying coatings) to the water and mitigates the radiation of ship noise.
Around 2010, most ship owners began to operate their ships at very low speeds because of risen crude oil prices. Many started to retrofit ships with new bulbous bows as the original bulbs had been designed for higher speeds. At low speeds the same bulb is no longer optimal.
With an optimised bulbous bow for low speeds the bow wave became significantly smoother. On the other hand, the bow wave at higher speeds is bigger with the new bulb than with the original one. Not only speed but also draught determines the optimum bulb design. The same effect can be observed if the ships draught differs significantly from the design draught.
Whether this retrofit measure is profitable or not from an economical point of view, depends to a large extent on bunker costs. Nowadays, the desired amortisation rates can often not be achieved.
The design of the bulbous bow offers a wide range of variations. In order to give the ship engineers design guidelines for the bow design, extensive series tests and parameter studies were carried out at the ship model basins. Based on these data, the bulbous bow could be adapted to the requirements in a first design loop.
However, with the development of potential theory and also viscous calculation methods, the advances in computer technology and the possibilities of parametric shape modelling, the designers have new possibilities of exploring the design space. They have not only to rely on empirical data and gain experience on a try and error basis but the bulbous bow can be thoroughly investigated and optimized to obtain the most efficient design for the given operation profile.
If a ship is designed for low speeds, a bulbous bow is no longer necessary. The wave making part of the resistance is relatively low and cannot be reduced anymore. For those ships a normal bow without bulb such as the classic curved or raked bow as well as a vertical stem is applicable. That is why full block ships such as bulk carriers or tankers are often designed with a vertical stem. A special bow design has been developed by Ulstein: the X-Bow, which is characterised by a negative rake of the stem and is beneficial in heavy seas. The added resistance due to waves is reduced as well as ship motions and accelerations. The X-Bow is mostly applied to OSVs, AHTS and similar types.
Currently at SVA Potsdam, the patented Petro Duct, named after its inventor Petromanolakis, is being investigated. It aims to reduce resistance and decrease motions due to waves. SVA is performing CFD optimization and model tests for applications on a slender and a full ship.
Henning Grashorn (HSVA), Herbert Bretschneider (HSVA), Rainer Grabert (SVA), Lars Lübke (SVA)
