The+conceptual+hull+design

The Quantum concept design consists of a ‘baby post-Panmax’ of 6,200 TEU with a slender monohull, low block coefficient and 10% increased container capacity. Alternative hull forms were studied and discarded. The design speed was set at 21 knots based on studies of the logistics and technical aspects, but the ship can sail at higher or lower speeds. The ship may be suitable for the Europe to East Coast South America service.

Print this page Save as PDF
Final hull, slender and wide with extended topsides.

“Design a baby post-Panmax container ship for the Europe to South-East America trade, carrying about 5,500 TEU” was the task we were given. To benchmark our efforts, an existing container ship of recent design and similar capacity was chosen as a reference ship.

Our initial brainstorming sessions suggested that flexibility regarding speed, draught and cargo composition would be important in the future. Although almost all current ships of this size are designed for a speed of 24–26 knots, the average roundtrip speeds may be less. The high speed is necessary in order to maintain the schedule and catch up possible delays. However, in the present market, the ability to slow steam at 21 or even 18 knots is an important feature. Lower speeds may also become a permanent feature given future emission legislation and fuel prices. We therefore made the bold decision to reduce the design speed to 21 knots. The design speed determines the hull’s fullness, shape and main dimensions. With a lower speed, we needed to find the best combination of these parameters.

Naval architects typically turn to multi-hulls when thinking of ground-breaking ship designs. DNV’s Ro-Ro concept design ‘Momentum’ successfully ended up as a trimaran, so could the same concept work for the Quantum project? Ro-Ro ships carry a volume cargo and have a moderate displacement compared to the huge volume needed. Their cargo decks are consequently stacked high, requiring a significant amount of ballast. With the trimaran side hulls taking care of stability, the need for ballast was reduced by about 8,000 tonnes.

However, container ships are different, carrying much more weight-driven cargo. Our container ship displaces 75,000 tonnes, which if applied to the Momentum would indicate a 350-metre-long hull, about a full one hundred metres longer than the reference container ship. So that alternative was excluded.
The catamaran concept was also dismissed. The Froude number is too low to benefit from the high length to beam ratio. The wetted surface is larger than on a monohull, increasing resistance. Two slender, separated cargo holds with narrow ends cannot match the efficiency of a single box hull. A catamaran may work for smaller feeder ships carrying deck cargo only, but for deep sea operation we ultimately concluded that a mono-hull was the way ahead.

Of various midship sections, the trapezoid shape has been a popular albeit seldom realised idea. The inclined sides diminish the beam at low draught, thus reducing the wetted surface and resistance. Propeller immersion is maintained, allowing a high propeller diameter. Such a ‘two ships in one’ concept is feasible for ships trading on an outbound-homebound type of route with a deep draught on the outbound leg and a lighter draught on the homeward one. The market review told us, however, that the draught is near constant on our container run. Thus a straight-sided midship was decided upon.

Even today, rough sketches, scribbles and quick hand calculations are typical of the early design process. A number of sketches were made of midship sections, lengths, container counts, profiles and details. A 5,500 TEU ship with an NPX beam of 49 metres would, not surprisingly, be short and full, with a large wetted surface, a blunt bow, a very shallow draught, a small propeller, unfavourable course-keeping and a high risk of harmonic pitch. Since none of these attributes form part of a naval architect’s dream of a liner, we came up with the ‘aircraft carrier’ solution.

The idea of this concept is that more containers can be carried above the waterline without increasing the resistance and fuel consumption, since the under-water part of the hull is unchanged. After juggling with structures, container rows, power curves and stability, the breadth at the waterline was ultimately optimised at 42.5 metres for maximum fuel efficiency and we decided on 16 containers across at amidships and a novel narrow side tank. Retaining the NPX beam (<49m) topside, the ship’s length ended up at 272,3 metres. Leaving our hydrodynamic experts with free hands to shape the best possible lines within these constraints, the final block coefficient is a very low 0.57, with very low resistance and thus high fuel efficiency. With the wider deck, the design could still carry 6,200 TEU, 700 more than the reference ship, thus offering added economy of scale.

An important project goal was to reduce or even eliminate the need for ballast. This is probably the most beneficial measure for future ship designs – due not only to the significant amount of energy required to transport this sea water across the oceans but also to the costs of treating ballast. Partly achieved by the wide beam but challenged due to the high cargo stowage topside, the freedom from ballast is dependent on sophisticated loading procedures. The need for ballast water for stability is eliminated for most loading conditions. However, the ship cannot be loaded with full flexibility in the longitudinal direction since the trim needs to be kept within limits.

A number of features were added to the core design, such as an elongated, raked bow with less flare in order to reduce speed loss in waves and slamming. A straight bow in order to maximise the waterline length was considered but found to be too wet. A large wave breaker is integrated in the foc’sle cap in order to deflect the corresponding increase in spray and a huge wind deflector is located in front of the foremost deck container stack in order to reduce wind resistance. Alternative lightweight material may be considered for these parts of the structure.

The final concept’s innovative factors are: the wider beam, which increases stability and practically eliminates the need for ballast; the wide deck enabling the carriage of more containers without increasing the fuel consumption; a slender hull with high fuel efficiency; a better bow for sea-keeping; the strength-efficient narrow double side box construction which saves space; the use of lightweight materials where possible; and the reduced wind resistance.

>>