With the current

Flow characteristics with optimised ship design

© Hamburgische Schiffbau-Versuchsanstalt (HSVA)


The greatest possible payload and lowest possible draught accumulation is what shipping companies strive to find for when looking for their ideal ship. Of course, that can only be achieved with the optimal equipment and design, which is decisive for efficient ship operation. It influences the flow characteristics of a vessel and generates savings potential, such as with fuel.

Today's modern shipbuilding methods means the shape of a ship is pre dominantly optimised by means of computer animated flow simulations that replaces the traditional model tests in a towing tank in the long term. Using computers, it is possible to simulate complete dynamic processes, such as the roll behaviour of a ship in swell. Computers enable precise insight into the flow conditions and point out what can be changed in a vessel's shape to reduce drag.

An innovative simulation program for determining hulls of all kinds takes calculation of flow characteristics even one step further. Known as “simulation-driven design”, it is based on the familiar process of Computer Aided Design (CAD), but additionally applies the Computational Fluid Dynamics (CFD) method, which makes it possible to calculate drag and swell characteristics of vessels. A change in a single parameter alters the entire hull.

Nice nose

The bulbous bow is part of the hull shape. The teardrop shape taken from nature and imitated by shipbuilding designers ensures more advantageous flow characteristics and thus contributes to the economic efficiency of a ship. The bulbous bow design, optimised for a draught range, can reduce the water resistance of a vessel by up to 10 %, but may also increase it if the bulbous bow designed for the envisaged draught of the ship does not reach this draught.

If the flow around the hull is complicated enough without a propeller, it is even more complex when the propeller design and steering gear are taken into account. The latter is especially dependent on the water flow hitting the rudder. When the ship moves slowly, there is a correspondingly poorer flow impact on the rudder making its steering effect less than when the vessel moves at a faster speed. The rudder shape commonly used in shipbuilding at present is the streamlined semi-balanced rudder, whose front edge is connected to the hull. For large container vessels, by contrast, the future is the spade rudder. This is why it has a lower flow resistance and reduces cavitation.

Cavitation (latin: cavitare “to cave”) is the formation of partial vacuums in very rapidly flowing liquids. This also occurs at the propeller as the tips of the ship's screws move through the water at up to 45 m/sec (roughly corresponding to 160 km/h). The high partial vacuums created in this process cause air bubbles to form in the water that then abruptly collapse. The energy released can even lead to holes in the propeller surface in the long run. In the case of the container giants, a single propeller is subject to an even more unfavourable flow due to the broad hull of these vessels: there is a stronger water flow around the stern and thus triggering turbulence that additionally impairs the efficiency of the propeller. For this reason a doublepropeller propulsion system is recommended to transmit the power in the water better without consuming more fuel.

Interestingly however, an even flow under the hull is not the ideal combination for the individual propeller. Tests have shown that often an asymmetric flow would be more efficient since in the water it leads to less swirl at the tips of the blades and thus to higher efficiency. Shipbuilders also frequently work with attachments that influence the flow, such as flow nozzles or baffles, Costa bulbs (horizontal dropshaped bulge of the rudder in the propeller jet) or trailing guide wheels (driveless contrarotating propeller pro pulsion), and currently with optimised stern shapes.

Smooth coat

The optimised design of a ship thus improves the hydrodynamics and helps to save fuel thanks to low frictional drag. In addition, the right coat of paint for the vessel also improve the flow characteristics of the hull. Very smooth silicone paints provided with an antifouling agent for the hull prevent barnacles and algae from sticking to the hull and increasing the drag in the medium to long term. So called anti-ice paints for ships operating in polar regions can also enhance protection and at the same time reduce drag. Numerous vessels of the Hansa Mare fleet have already been painted with this particular coating.

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