Propeller-induced pulsations can cause cracks of welding joints, damage of steering gear and increased fuel consumption. Wake equalizing ducts homogenize the wake.
Vibrations such as may be caused by the propeller of vessels where the submerged stern has unfavourable flow characteristics create[ds_preview] the risk of damage to the structural components and the associated connections. They diminish the passenger comfort in the form of sound, and the mediocre hydrodynamics from the arising cracks result, of course, in a performance decrease of both ocean-going and inland vessels. It is known that a certain improvement can be achieved by means of flow equalizing ducts. Measurement series from model tests and sea trials prove that additional spoilers considerably increase the efficiency when they are designed as an integral unit with the half shells at the hull.
The Potsdam model tank establishment is of the opinion that propeller-induced pulsations are due, amongst others, to an inhomogeneous flow into the propeller. In a non-laminar turbulent flow field the angle of attack of the propeller blades changes with each angular increment. This, in turn, means that the propeller is subjected to a permanently alternating load which, inevitably, leads to the generation of vibrations.
The pressure pulses propagate through the shell plating, and in certain circumstances even on the drive, but mainly pose a risk to the welded joints near the propeller and as such to the steering gear. Further, in the turbulent surrounding area the propeller cannot prop up itself powerfully in the true sense of the word. In the case of unfavourable wake conditions the engine has to deliver a higher performance for a certain speed than in the case of a wake homogenized across the entire propeller area.
The tough face of the fleet
Designers are therefore seeking to come up with slender and flow-favourable underwater stern sections, with the objective of precluding hydrodynamic inhomogeneity from the outset. Today, this target is achieved in the majority of cases. The face of the merchant fleet, however, is not coined by newbuildings. The majority of the current stock of bulk carriers, tankers and container vessels plough through the water with a comparatively cumbersome stern geometry and for this reason suffer losses in terms of economic efficiency (consumption and possibly also service and maintenance cost) and passenger comfort. To a limited extent, the same also applies to new ships’ bodies such as in cases where the positioning of the projecting main engine at the very rear of the stern impedes optimization of the lines.
It is, however, possible to absorb part of the losses by means of spoiler plates on
the hull, in the inflow area to the propeller and with a wake equalizing duct (WED) of
Schneekluth type. This WED is named after its designer Prof. Dr. Herbert Schneekluth. It is welded to the stern end of the underwater part of the ship at the port and starboard side.
From the physical point of view it consists of a hydrofoil section venturi bent to form two half shells, where the wider opening faces the bow and the camber of the hydrofoil the vessel’s side. From this there results a negative pressure in the cross section that draws in water, accelerates it and routes it as in channels largely homogeneously to the propeller. This channelling is assisted by matching spoiler plates.
The efficiency is demonstrated by recent studies, e. g. those of Marine Engineering Consulting in Wismar, carried out with two multi-purpose cargo ships of type »MPC Neptun 30« operated by the German shipping company of Hermann Buss, based in Leer: A hydrodynamically oriented design and arrangement of spoilers and equalizing duct in the inflow area of the three- or multi-blade propeller reduces vibrations at a rate of 50 % while achieving the same speed at a lower power input.
Results of sea trials
In the specific case the first of two identically built 31,000 t vessels (no. 551) was equipped with duct and spoilers right from the outset, while newbuilding no. 552 had initially been delivered without these attachments. During the sea trials of both vessels the nautical officers recorded travelling performance, speed and vibration behaviour. The analysis focussed, amongst others, on the vibration frequencies at various points of measurement: At the steering stand on the bridge the engineers took measurements without / with spoilers and equalizing duct of the longitudinal vibrations in the amount of 2.38 mm/s as compared to 0.93 mm/s, and vertical movements of 1.98 as compared to 1.39 mm/s. Similar differences can be seen distributed across the entire layout. Due to the multitude of influential factors, even identical constructions will not, of course, bring about a vibration pattern that is coincident in every value and location, but it is highly probable that the improved values are primarily attributable to the more homogeneous water flow in the suction run of the propeller.
»The installation costs will already have paid off after six months«
The trials described above confirm: It is the superstructure above the stern, in particular, that suffers from the propeller-induced pressure pulses, and in the case of container vessels it is the accomodation. The calls for proposals for newbuildings therefore frequently specify a vibration load of not more than 4 kPa (kilopascal) for the first-order vibrations. As stated above, shipbuilders endeavour to meet these restrictions to the maximum possible extent by way of a flow-optimized configuration of the stern section; however, this can not always be achieved on the basis of the lines alone.
Amortisation within a few months
The performance and speed increase achieved with an additional Schneekluth WED was recorded by the German tanker shipping company Karl Büttner in Bremen. They currently have a fleet of eleven tankers for oil, products and chemicals with a deadweight tonnage ranging between 13,500 and 24,000 t. The comparison between two newbuildings of the 15,000 dwt class with a propeller power of 5,400 kW was aimed at clarifying the issue of the effectiveness of the venturi half shells and their economic efficiency. The outcome in terms of figures was: the »Lemonia«, equipped with spoilers and wake equalizing duct, needed 2.6 t of heavy fuel oil per day less than the »Levana« vessel.
The measuring series were taken during builder’s sea trials. The report states: »It is noticeable that on the ›Lemonia‹ the vibration in the afterbody is considerably lower and the speed increased by 0.5 kn.« 2.6 t of fuel cost about 1,000 $. This means that the installation costs of about 70,000 € (approx. 84,000 $) will already have paid off after six months. The fineness of the underwater lines is a decisive factor for the period of amortisation. In the case of a block coefficient CB above 0.7 the power saving with convex water lines probably ranges between 3 and 8 % and with concave water lines between 3 and 5 %.
It is, however, not possible to define a standard geometry for duct and spoilers. The Potsdam model tank establishment voice the view that one would have to consider the velocity triangle between the axial and vertical flows in the stern section and would have to draw spoilers and duct accordingly. With most of the hull shapes a vector directed upward at a certain distance to the skin and a down current oriented in the opposite direction and passing directly next to the skin develop as a result of the global overall surrounding flow. This, however, does not have to be the case always. The exact conditions are line-specific and the baffles therefore have to be positioned accordingly.
Savings in maintenance costs
More than 1,500 vessels – tankers, containers, bulk carriers, LNG carriers and also inland vessels – already profit from such an installation. Shell retrofitted 14 tankers and achieved calculated fuel savings between 1 and 7 t/day. At a heavy fuel oil price of 500 $/t, a running time of 200 days per year and averaged savings of 4 t/day there results a mean value of savings in operation cost of 400,000 $ per vessel and year.
Not only the cost of energy has an impact on the financial aspects. It is also the reduction of fatigue phenomena in the form of incipient and other cracks – damage caused by the vibration load – that most probably manifests in savings in operation cost (maintenance and repair).
»Flow equalizing ducts may recover 50 or 60 % of the losses of a propeller with unsatisfactory inflow conditions, but the more intelligent approach is to avoid such losses in the first place«, Prof. Dr. Stefan Krüger from the Institute of Ship Design and Ship Safety at the Hamburg-Harburg University of Technology states. But he admits, however, that there is probably quite a large number of bulky underwater forms that would profit from an improvement of the inflow conditions for increasing the performance and reducing the vibrations.
Retrofits are the major part of orders placed at the Schneekluth-Hydrodynamik Entwicklungs- und Vertriebsgesellschaft in Dinslaken. But due to the weak shipping market the German maker of the wake equalizing duct observes a stagnation of orders at the moment – in spite of increasing fuel cost and other operating expenses.
Bernd Genath
