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Ships: Reshaping the underwater form

Time：2014-01-07 Browse：65 Author：RISINGSUN
Twenty years ago, it seemed that the underwater shape of any ships was a more or less settled science. Bulbous bows, (the benefits of which had been first identified by Italian naval architects in the 19th century), were widely fitted, with the most “extreme” bulbs fitted to the fastest ships. But hydrodynamics as a science has continued to develop and recent years have seen all manner of astonishing ship shapes entering service. The Ramform, the X-bow and the Axe bow, along with other dramatic ship shapes, all arrived amid claims of the superior sea-keeping qualities and operational economies they produced.


The soaring price of fuel and tough economic times have produced another wave of hydrodynamic innovation, with naval architects looking hard at all aspects of the underwater shape and its relationship to the sea. The flow of water around the stern and the efficiency of propellers, the effect of trim on performance and the operation of rudders have all been studied afresh, while the fitting of Mewis ducts and various other appendages to beef up the bite of a propeller have been appearing in newbuilds. A regular scrub to freshen up the surface of a propeller, between dry-dockings has proved and demonstrable dividends.


But what else can be done to make existing ships more economical? Obviously slowing them down is the most natural fuel-saving measure, but that brings its own problems, as a hullform that was designed for optimum efficiency at a speed of 25 knots is unlikely to work as well if the service speed is reduced by 5 or more knots. So it is interesting to note that Maersk has started to remove the bulbous bows from a number of its container ships now operating a slowed service speed regime.


Maersk has worked out that these appendages can weigh up to 140 tons and while they were doing their job perfectly at the designed high service speed, they do the opposite to what they were intended with the ship slowed down, with resistance actually increased. As might be expected with a company that looks at these matters with an analytical eye, the strategy will not work for every ship, but if a business case can be made and the economics work, then a “nose job” will follow. If the surgery is done on the right ships, then the fuel savings from the reshaped bow can amount to some 2%. Not carrying around up to 140 tons will itself produce some savings. Ten ships have been so far scheduled for bulb removal and it will be interesting to see if any of the other major operators follow the Maersk example.


Meanwhile, the science of hydrodynamics continues the search to optimise the underwater body and produce more fuel and energy savings. Computational fluid dynamics is a new weapon in the designer’s armoury, which it is hoped will produce further breakthroughs in the lowering of resistance and higher propulsive efficiency. We are given hints that exciting developments in the world of underwater coatings are soon to be revealed. Innovation flourishes when times are hard!