An introduction to propeller cavitation

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.

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Royal Huisman to build 56 metre classic ketch

An impression of what the new 56 metre classic ketch to be built by Royal Huisman will look like
An impression of what the new 56 metre classic ketch to be built by Royal Huisman will look like

A classic ketch for family oriented Caribbean cruising and more is being built for an American couple, Royal Huisman has announced. The 183’7” (56-metre) megayacht, to be christened Aquarius, is set for delivery in late 2017.

Aquarius bears styling and naval architecture by Dykstra Naval Architects. The wood clad deckhouse, teak decks, long overhangs, rich-blue hull, and oval ports give her the profile the owners wanted. Royal Huisman has yet to release full specifications on her sailplan and structural dimensions, though the yacht is intended for good performance in light airs. Aquarius may further participate in some bucket regattas.

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Fincantieri Yachts delivers one of the tenth largest mega yachts in the world

Fincanteiri Yachts has delivered one of the tenth largest mega yachts in the world and the largest to be built in Italy
Fincanteiri Yachts has delivered one of the tenth largest mega yachts in the world and the largest to be built in Italy

At 140 metres long, the “Ocean Victory” is the largest yacht ever built in Italy and, according to the sector rankings, one of the tenth largest ever built in the world. The new mega yacht is positioned on the top market segment in terms of quality and performance and is characterized by complexity, high technology content, innovation, extreme care to details in the interior and exterior design.

The new ship has been acquired also thanks to the commitment of Alex Lees Buckley, broker with more than 30 years of experience in the most prestigious projects of the internationally renowned brokerage company Camper & Nicholsons. “Ocean Victory” has been designed on the basis of a new, unique concept, brainchild of the Monaco based naval designer Espen Oeino and the Paris based interior designer Cabinet Alberto Pinto. The new unit has been developed by the team of Fincantieri Yachts – business unit of Fincantieri specifically focused on this area.

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Naming of the world’s largest containership

The world's largest containership has been named CSCL Clobe. Photo: HHI
The world’s largest containership has been named CSCL Clobe. Photo: HHI

Hyundai Heavy Industries Co, Ltd. (HHI) announced today it held a naming ceremony for the world’s largest containership, the first of five 19,000 TEU containerships ordered from China Shipping Container Lines (CSCL) in May 2013.

The naming ceremony for the world’s largest containership was attended by Xu Li Rong, chairman of China Shipping Group; Zhao Hong Zhou, managing director of CSCL; Qiu Guo Hong, Chinese ambassador to Korea; Choi Kil-seon, chairman and CEO of HHI and 150 other guests.

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Edith Maersk docks at DP World London Gateway

The Edith Maersk, the largest vessel ever to enter the River Thames, docked at DP World London Gateway
The Edith Maersk, the largest vessel ever to enter the River Thames, docked at DP World London Gateway

The Edith Maersk, the largest vessel ever to enter the River Thames, arrived at DP World London Gateway on Sunday 19 October.

The 396 metre long, 56 metre wide ship has a draught of 16 metres and can carry up to 15,500 teu.

The ship, which is just weeks shy of celebrating its first anniversary, left the terminal on Sunday night bound for Singapore.

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Mariners Alerting and Reporting Scheme (MARS)

Mariners Alerting and Reporting Scheme (MARS)
Mariners Alerting and Reporting Scheme (MARS)

What is MARS?

There is an important free resource available to the whole of the shipping industry that makes a major contribution to safety and that surveyors can help to improve. This is the Mariners Alerting and Reporting Scheme (MARS) operated by The Nautical Institute. MARS is a free resource and The Nautical Institute hopes that surveyors will help to make its existence known to the maritime world. The Nautical Institute wants as many mariners and, indeed, as many in shipping as possible, to benefit from lessoned learned from accidents and near misses. Surveyors can spread the word to let mariners and companies know the resource is there.

The background to MARS is known to all; across the major transportation modes and in many other fields, human error looms as the leading cause of both accidents and incidents. In recent years, the definition of human error has been expanded to include concepts such as unsafe supervision and organisational influences (e.g. resource management and operational processes). In the maritime industry approximately 90 percent of accidents can be traced to human error despite the promotion of regulations, training and quality management systems.

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First UK aircraft carrier centre blocks completed

Pictured are Cammell Laird's head of construction Jeff Jones, project director John Drummond and steelwork supervisor Joe Dawber.
Pictured are Cammell Laird’s head of construction Jeff Jones, project director John Drummond and steelwork supervisor Joe Dawber.

Cammell Laird has completed its first aircraft carrier centre blocks for a second ship set to become the centrepiece of Britain’s military capability.

The Birkenhead marine and engineering services company has fabricated and outfitted two aircraft carrier centre blocks for the new PRINCE OF WALES aircraft carrier, which are being transported by sea to the Rosyth shipyard in Scotland.

Construction of the blocks began in July 2013 and has involved a workforce of 250 including contractors. Cammell Laird is playing a key role in the aircraft carrier project, the biggest UK engineering project since the London Olympics.

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Revised Boat Data Book published

The Boat Data Book
The Boat Data Book

The Boat Data Book is a treasure trove of invaluable information for boat owners, designers, builders, marine surveyors and chandlers.

This updated seventh edition written by Richard Nicolson and Ian Nicolson contains new diagrams and tables of lengths, widths, weights and strengths as well as new data on a vast range of equipment from anchors to masts, propellers to gas cylinders, cleat sizes to winch bases, and hatches to piping.

This is the book you need if you want to find out:
• What size winch to fit
• The breaking strength of stainless steel rigging wire
• The recommended size for seacocks
• What length and size an anchor chain should be
The Boat Data Book is an essential reference book for boat owners, crew – both amateurs and professionals.

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Alang – the Indian graveyard of ships

Alang - the ship graveyard
Alang – the ship graveyard

If you’re planning your next vacation, you probably won’t find Alang in any travel guides. You may not even find it on the map. This desolate six-mile stretch of land was once one of the most impoverished areas in India. But, in recent years, this piece of the Indian coastline in Gurajat state has become the world’s largest shipbreaking yard.

Where is Alang?

Alang, 185 miles (298 kilometers) northwest of Bombay, serves as the final stop for about half of the world’s maritime vessels. Alang is literally a graveyard for ships — the world’s once most powerful ships come here to die. Shipbreaking is just what it sounds like. Piece by piece, workers use basic tools to dismantle ships that are too old or too costly to maintain.

But why choose this remote spot to serve as the final destination for so many of the world’s obsolete ships? For one, Alang’s beachfront location is ideal for shipbreaking. Tides are heavy there, and the natural slope of the beach makes it easy for a ship to be run on shore.

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Plastic main propellers fitted for the first time

Black Plastic Boat Propellers
Black Plastic Boat Propellers

Japanese classification society ClassNK and Nakashima Propeller Co., Ltd., have installed the world’s first carbon fibre reinforced plastic (CFRP) propellers on a main propulsion system of a merchant vessel.

The CFRP propeller was installed on the Taiko Maru, a domestic 499 GT chemical tanker owned by Sowa Kaiun YK by Marugame-based Koa industry Co., Ltd. in May 2014. The CFRP propeller installed on the vessel was developed and produced by Nakashima Propeller Co., Ltd., with support from ClassNK, which granted approval for the design and manufacturing process of the CFRP propeller, as well as provided research and funding support for the project as part of the ClassNK Joint R&D for Industry Program. Despite its ultra-lightweight composition, CFRP exhibits the same, if not superior strength, to the aluminum-bronze composite materials used in conventional propellers, according to ClassNK. Due to the light weight of the propeller, propeller shafts can be manufactured with smaller diameters, contributing to a significant reduction in weight and fuel costs.

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New Panama Canal means increased insurance risks

Panama Canal
Panama Canal

As the Panama Canal prepares to celebrate its 100th anniversary, insurers are warning of the increased risks that will arise from its plan to double the cargo carrying capacity of ships transiting one of the world’s most important waterways. Every year, over 12,000 oceangoing ships navigate the canal, a figure which could increase significantly following the anticipated opening of the new locks in 2015. It is forecast the expansion will enable between 12 and 14 larger vessels per day (approximately 4,750 additional ships per year) to pass through the canal. Significantly, many of these ships are expected to be new-Panamax class container vessels of 12,600 teu, which are far larger than the existing largest vessels able to access the canal (4,400 teu).

AGCS experts warn the increased traffic and larger vessels may challenge the Panama Canal’s improved safety record over the past decade with the risks exacerbated through the initial period of the canal opening. Captain Rahul Khanna, AGCS’s Global Head of Marine Risk Consulting, explains the potential risk management impact of this expansion: “Larger ships automatically pose greater risks. The sheer amount of cargo carried means a serious casualty has the potential to lead to a sizeable loss and greater disruption. For example, a fully-loaded new-Panamax 12,600 teu container ship is as long as four football fields with a beam of up to 160 feet and could have an insured cargo value alone of $250 million.”

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The Navigator: A magazine for every ship

Nautical Institute Chief Executive Philip Wake
Nautical Institute Chief Executive Philip Wake

The Nautical Institute appealed to the maritime industry to help it get a copy of its free publication The Navigator onboard every SOLAS vessel to encourage the professionalism of watchkeepers.

The international professional membership organisation is reacting to concerns from the industry that casualties and near misses from navigational errors are increasing, and that navigational competence needs to be continually updated.

“Finding ways to increase the competence and professionalism of officers of the watch is one of the main concerns of our members,” Chief Executive Mr Philip Wake told a gathering of senior industry figures. “One of the ways we are working towards this goal is through our free publication The Navigator, which appears three times a year, highlighting some of the most important issues for them.”

He added: “In the year and a half since we began production, the publication has become a real success, with students, cadets and young officers telling us how much they appreciate it. But the challenge, as always, is getting the message to those people – and onboard those vessels – that will benefit the most.

“Now, thanks to the generosity of IFAN, the International Foundation for Aids to Navigation, we are in a position to put a copy on the bridge of every SOLAS vessel – potentially up to 100,000 magazines. In that way, we hope that watchkeepers will sign up to the digital version.”

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