Page 25: of Marine News Magazine (February 2006)

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COATINGS & CORROSION CONTROL

February, 2006 • MarineNews 25

During the past few years the marine coatings industry has engaged in responding to the concerns of marine biologists to rid the world's shipping fleet of Tributyltin

Self-Polishing Co-Polymers (TBT SPCs) coatings. In doing so, the industry has produced two systems that meet those concerns, and have, in addition, retained or improved the means of controlling hull fouling. By 2008, it is expected that these systems will encompass 80 percent of the anti-fouling market.

The first of the coatings, tin free SPCs, works in a sim- ilar way to TBT SPCs. The surface coating, slowly dis- solving as the vessel moves in the sea, releases biocides that mitigate the targeted marine organisms that invade the hull. With the tin having been replaced by copper, zinc or silyl, the biocides do not have the same effect on the wider marine life at sea. Second is a system that seeks to prevent the attachment of marine organisms to the hull. Foul release coatings use silicone to give it an element of flexibility; a flexibility that makes it difficult for organisms to attach to the hull, and where successful, causes them to drop off at the faster speeds of motion through the water. While useful, sometimes additional efforts are required to keep a hull clean and efficient. For the ship owner, hull efficiency can only be quantified through fully normalized performance data. Incremental days lost at sea due to fouling can be plotted, and then, by taking appropriate action to return the hull to good condition, whether that requires underwater brushing, dry-docking for more extensive maintenance or a modi- fied coating formulation at next drydock. CASPER (Computerized Analysis of Ship PERformance) uses an advanced hydrodynamic method for determining the true speed of the vessel hull through the water. Developed in Scandinavia by Torben Munk, MSc., it was brought to the U.S. and is offered here through Propulsion Dynam- ics. Daniel Kane, VP Business

Development, said that through these hydrodynamic methods, all variables are removed with excep- tion of the added resistance of the hull and propeller. As a result of this, a clear picture emerges: per- formance losses represented as either a speed loss or increase in fuel consumption are depicted in an 'Actual Obtainable Speed and

Fuel Consumption Curve', and the increase in ship resistance is plot- ted in real time. To plot the devel- opment of hull and propeller resis- tance, the owner will need to know the true speed of its ship and the fuel it consumes. Although most ship owners regularly undertake fundamental calculations, the data obtained onboard suffers from sev- eral unknowns and uncertainties.

Corrections will have to be made for wind, waves, and the vessel's draft; corrections which require technical knowledge and trained personnel. More diffi- cult still, is how to determine the speed through the water; the speed log is of insufficient accuracy for this purpose, and the U.S. Coast Guard's maritime Differen- tial Global Positioning Service, while providing great accuracy, has to be corrected for the sea current, which is almost impossible to ascertain from the vessel.

Using statistical hydrodynamics, CASPER is designed to offer the solution. Observations from aboard the ves- sel are transmitted by the Internet to Propulsion Dynam- ics once a week. Most of the parameters are already part of the normal noon reporting, and there is no need for additional instruments, computer software, crewmem- bers, or specialist training, to gather the information. "The results that CASPER has achieved for Propul- sion Dynamics clients have been significant," said Kane. "On many of the vessels we have analyzed, we have found that even for well-maintained ships, the hull and propeller resistance has increased at a faster rate than would be expected, due to marine growth, in some cases by more than 40 percent of the smooth hull resistance.

In addition, our clients have also discovered ships of sim- ilar age with a difference in added resistance of more than two times, therefore having accurate figures for total ship resistance give the shipowner a tool to maximize propulsion efficiency."

Figure II indicates the added resistance of two vessels.

Prior to dry-docking, Ship B shows a higher resistance in the water than Ship A, due to different hull treatments and maintenance programs during the previous five years. Leaving dry dock, the baseline resistance for Ship

A was 15 percent, and for Ship B it was three percent higher. The owner of the two vessels put the difference in the baseline resistance down to different treatments being applied to the hulls.

Two hundred days after leaving dry dock, both vessels are showing signs of increased hull and propeller resis- tance; but in Ship A, it has increased at a slightly faster rate, furnishing the ship owner with cost-benefit infor- mation for the different treatments applied to the hulls.

The monthly report that Propulsion Dynamics pro- duces for the ship owner evaluates the performance of the hull and / or propeller coating, benchmarks hull treat- ments between ships enrolled, and ensures that the ther- mal load of the main engine is not exceeded.

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DIVERSIFIED

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ENGINE AND

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ENGINEERING
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CASPER Service For Hull Coating Evaluation

Hull Efficiency Degradation Pattern of Two Tankers.

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