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Feature article written by Eur. Ing. Jeffrey N. Casciani-Wood, HonFIIMS

The recent loss in mid-Atlantic of the a.s.v. CHEEKI RAFIKI and the sad loss of four lives has brought very clearly to public and professional attention the problem of fin or pendulum keel detachment and consequent capsize of many apparently perfectly good boats. This is a very serious problem from a marine surveying point of view and the author has made a detailed study of a very similar loss that took place in February 2007 in the English Channel.

To put the matter into perspective it was first necessary to look into some published historic data. Because of their concern over the number of reported keel failures, the International Sailing Federation (ISAF) developed a Keel Structure Working Party to investigate and report on the problem. As a starter they produced a data base of known failures and investigated 72 of those that had been reported since 1984 and which involved no fewer than 24 deaths by drowning. The problem with the data, however, is that it records only the worst cases in which the keel had been pulled off or otherwise lost from the hull usually resulting in a capsize and an actual total loss. The marine surveyor investigating such a casualty will frequently find that it is very difficult to establish the prime cause of failure and, in particular, to differentiate between a loss caused by a design and/or manufacturing fault and a casualty resulting from human error such as a collision or a grounding. He must be particularly careful to avoid listening and paying credence to usually ill informed bar room or internet gossip.

Continue reading “A study of a keel failure and the loss of the Cheeki Rafiki”

Feature article by Robert van Tol, Operational Director, Superyacht Builders Association (SYBAss)

The phenomenon of superyachts is known by most people and speaks to the imagination. They are often to be spotted at a tropical holiday destination or on television as back drop of Formula one races in Monaco and the Middle East. Superyachting can be seen as the ultimate leisure activity and is only accessible by the very few in the world.

The global fleet of superyachts sailing the world’s oceans totals some 4,500 with those over 40 metres in length comprising around 1,500 in total . Nowadays the average number of superyachts over 40 metres delivered worldwide per year is 80, whereas only fifteen years ago this average was 30. It is therefore a fairly young industry that has experienced significant growth in demand in a relatively short period of time. Similar growth trends have been seen in the number of Ultra High Net Worth Individuals (UHNWIs), the target audience that are able to afford commissioning these vessels.

The superyacht industry is often perceived as small and therefore squeezed between the maritime leisure and professional marine industries. Yet the output of the superyacht building industry is similar in Compensated Gross Tonnage (CGT) to the passenger shipbuilding industry, which is often considered to be much larger . (CGT) is a macro-economic indicator that provides an estimate of the amount of work involved in building a ship of a certain type and size, ultimately expressed in man-hours per gross ton (GT). Currently also a global economic impact study is being conducted to analyse the economic value of this industry.

Continue reading “The challenging and colourful world of superyachts”

Feature article written by Eur. Ing. Jeffrey Casciani-Wood HonFIIMS

As the propeller turns it absorbs the torque developed by the engine at given revolutions i.e., the delivered horsepower – and converts that to the thrust which, in turn, pushes the vessel through the water. According to Bernoulli’s law the passage of a hydrofoil (propeller blade section) through the water causes a positive pressure on the face of the blade and a negative pressure on its back. It is the resolution of the pressures that results in the torque requirement and the thrust development of the propeller. The negative pressure causes any gas in solution in the water to evolve into bubbles similar to those found when opening a bottle of lemonade or champagne. These bubbles collapse and can cause hammer like impact loads on the blades often in excess of 7 kg/cm2. It is the collapse of these bubbles that results in the observed damage to the propeller blade surfaces.

The ratio of the absorbed power or the delivered thrust to the total blade area of the propeller is called, respectively, the power and the thrust loading. If either of these exceeds a certain value which depends upon a complex relationship between the propeller type, the flow in which it works and its mean depth below the water relative to its diameter then the flow pattern of the water over the propeller blades breaks down causing a severe loss of thrust and, eventually, physical damage to the surface of the propeller blades and, also, the rudder and local steelwork of the vessel’s hull. That flow breakdown is called cavitation and is strictly analogous to the water hammer often heard in old plumbing systems. Cavitation is a highly complex phenomenon and the pitting damage it causes usually – but not necessarily – appears on the back of the blade following a clear radial pattern. It can also appear as similar damage on the driving face of the propeller in which case, almost certainly, a further factor has entered the problem in the form of an incorrect pitch distribution along the length of the blade. Most small craft propellers are usually of constant pitch over the blade length and that regime is accurate enough for 99% of boats but on high speed boats with large propeller loading factors the pitch should vary over the length of the blade i.e., the boat should be fitted with a varying pitch propeller. The effects of cavitation including loss of speed and damage to the propeller blades can be minimised by ensuring that the propeller has sufficient blade area relative to the area of the circle described by the propeller blade tips.

Continue reading “An introduction to propeller cavitation”