It might seem quite strange that 171 years after William Froude commissioned the world’s first hydrodynamic test tank to examine the complexities of ship motion, clever naval architects are still discovering that there are breakthroughs to be made in the design of ship’s hulls. In both monohull and multihull designs the demand to make fuel go further and ships more economic and sustainable has encouraged a whole new generation of designs that improve seakeeping, lower resistance and help to get more “bite” from propellers.
It is at the extremities of the ship – at bow and stern – that the potential for hydrodynamic improvements have been focused. The problem has been how to fine-tune the need for a capacious hull that will carry large volumes of cargo and balance this with the underwater streamlining that will reduce the resistance of the sea through which the ship is pushing.
It is no coincidence that one of the most efficient ship shapes is the modern submarine – itself modelled on the most efficient sea creatures – but ships which float on the surface have to take into account the problems of waves and swells. The ship cannot be so “fine-lined” that it simply plunges into oncoming waves and has insufficient buoyancy to rise, or so bluff bowed that it slams into them. Every bow design is a delicate compromise.
Some new designs have taken on a “wave-piercing” shape, an underwater profile like an axe-blade which parts the seas below the waterline, but with a broader shape above that will prevent green seas sweeping the decks. The traditional bulbous bow, which has become popular in recent years, is being reshaped into different forms, while designers have found combinations of different shapes have proved beneficial in tackling both resistance and seakeeping. Some of these forms are quite remarkable, like the X-Bow (Seascapes No.85) which is proving highly successful in various offshore craft.
Around the stern, there are various interesting design trends taking place which can have a pronounced benefit to propulsive efficiency. The need to improve the waterflow over the propeller has been recognised and various fins and ducts, such as the Mewis Duct, have proved efficiency gains. The use of shaped and “active” rudders, the employment of twin skeg designs on twin screw ships and even asymmetric keels, have been declared beneficial.
Is there an “optimised” hullform emerging from all this activity? There is a certainly a great deal of improvement, but the hull of a ship will always reflect its use, as a carrier of volumetric cargoes, as a sea-kindly shape or a shape for speed. The search for better and more efficient shapes in each category of use will surely continue!
Source: BIMCO