Feature 1 | GREEN SHIPS The Magnus effect, a well spun yarn


Spinning rotors produce energy that can be used to propel ships using less fuel and reducing unwanted emissions. David R Pearson, naval engineer with BMT Defence Services, explains the part that the Flettner Rotor has to play in the design of more efficient ships


throughout the 20th and 21st centuries, particularly at times of high bunker prices. Te potential benefits are obvious; with the promise of reduced fuel consumption comes the possibility of improved profit margins, a reduced freight rate and a reduction in greenhouse gas emissions. Flettner Rotors (FRs) are a form of wind


W


based propulsion that utilises the ‘Magnus effect’, a phenomenon exhibited by a spinning body in a fluid flow incident upon it. It is this effect that is responsible for the curving flight path of a ball in many sports. A FR typically comprises a cylinder with


an endplate affixed to the top, mounted vertically to the deck of a ship. Trough the action of a motor, the cylinder rotates in an air stream and a liſt force is generated that can contribute to the propulsive needs of the ship (Figure1). FRs were first installed on a ship named


Buckau in the 1920s, by a German scientist named Anton Flettner who realised their potential for ship propulsion. This installation was the proof of concept that allowed Buckau to sail across the Atlantic in 1926. Although Buckau was successful in


achieving its overall goal of saving fuel, the high capital cost alongside reduced bunker prices meant that ultimately the economics did not work and the rotors were taken out of commission. However, with the modern focus on energy efficient design and fuel saving technologies, matched with high bunker prices, the focus has once again come round to FRs and their potential to save shipowners money, as well as improve the green credentials of a ship. Tere have been a number of previous


studies that have modelled FRs and their benefits to shipping; however for the most part these do not consider ship fit factors and focus instead on potential savings


28


ind propulsion has been a popular research topic for green shipping enthusiasts


Figure 1: The Magnus effect


over set wind routes. Tis study has sought to address this by creating a model that considers the limitations and locations of rotors on a ship, and makes only general assumptions about a ship’s voyage routing for a generic assessment of suitability for a given ship type.


Model approach When planning an assessment for the potential retrofit of FRs on a ship, it must first be ensured that the candidate ship is architecturally well suited to accommodate them. A candidate ship must be of a type


that has an open area of deck, without extensive superstructure that would inhibit air flow, or deck gear/cranes whose operation may be obstructed by the FR. Te FR imparts considerable forces to the structure of the ship so the mounting sites must be carefully chosen. Certain ship types are unsuitable from


the outset; Ro-Pax and container feeder type vessels lack the clear deck space required. For container vessels the installation would require the sacrifice of some container carrying capacity, besides the requirement for clear space around the rotor.


However, vessel types such as dry bulk


carriers and tankers represent an ideal FR platform with their open deck, relatively slow steaming speeds, and favourable operating profiles which make them a more attractive proposition. The ship selected to demonstrate


the FR model is a chemical tanker of approximately 14,700tonnes deadweight.


Rotor design A ‘standard’ FR is a basic cylinder shape, with an endplate mounted at the top in order to improve the liſt/drag ratio. Te primary design parameters of a FR are shown in Figure 2. The only dynamically controllable


variable of FRs is the rotational speed, which consequently affects the velocity ratio, defined as the ratio of the cylinder surface speed relative to the air speed. Over a limited range of speeds, the coefficients of liſt and drag increase with the velocity ratio. The BMT FR model was created


within MatLab (soſtware for numerical computation,


visualisation, and


programming), and performs all of the FR related calculations for a simulation


The Naval Architect January 2015


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