US NAVY'S - Incat

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A Smoother, Cleaner Bottom! Captain Mike Jackson The value of having a clean ship’s bottom would be known to most people involved in maritime operations. A fouled bottom increases hull resistance and generally results in a loss of speed. Where this happens it will either involve an increase in power and therefore fuel consumption or increased passage times and the inability to maintain schedules. Or a combination of both. Recent experience with one Incat vessel where the operator reported an unexplained loss of speed resulted in much head scratching at Incat Head Office and in the end a team was sent to investigate. The craft in question was operating for long periods in tropical waters including extended periods alongside. The craft was experiencing a speed loss of five knots and was unable to attain maximum engine RPM. When the investigating team boarded the craft the first thing they noted was quite a luxuriant growth of weed on the underwater surfaces. They also determined that there was a problem with the engine control system. The engine control system was quickly resolved and once corrected allowed for an immediate two knot increase in speed. Thus it was estimated that the hull growth was slowing the craft by up to three knots. At the first opportunity divers performed a complete hull clean and the craft’s speed was reinstated. This experience demonstrates the need to keep the hull clean but perhaps also the need to use anti fouling appropriate to the nature of the operations it is undertaking. Anti fouling is anti fouling I hear you say, but is it An examination of different manufacturer’s specifications indicates a varied number of different anti fouling schemes for different applications and also different hull materials. They are all Tributyl Tin (TBT) free of course, but they include different properties from silicone to self-polishing, conventional and long life. Given that craft only have to complete in water bottom surveys every two years, then the opportunity to renew and/or repair anti fouling coatings also comes along once every two years. If the scheme is damaged or unable to last this distance for some reason then the possibility for hull growth is increased and with it the spectre of increased fuel costs. What can be done If operators are happy with their anti fouling schemes but find the craft is suffering hull growth between dockings, then hull cleaning by divers may seem to be the appropriate solution to the problem. Regular cleaning by divers may prove more effective than an expensive change of antifouling schemes, but of course that is up to the operator. 12 Incat THE Magazine Issue 36
A Smoother, Cleaner Bottom! When using divers - don’t forget to check the waterjets; you never know what you might find! Just as a conventional ship operator is interested not only in a clean hull, but smooth propellers too, so too the fast ship owner needs to look beyond the hull to the impellers. Out of sight, out of mind does not apply and just because they are housed within the jet ducts does not mean they can be overlooked. The value of having a clean ship’s bottom would be known to most people involved in maritime operations. A fouled bottom increases hull resistance and generally results in a loss of speed. Impellers are particularly vulnerable to marine fouling since it is an unpainted surface that must remain clean and shiny for proper operation. Just like a propeller, an impellor, despite its small surface area, can generate energy losses amounting to half that of the hull itself, so maintaining a clean surface is critical. A routine polish to reduce friction ensures the impeller operates at optimum efficiency. Even with routine maintenance, surface roughness can occur as a result of erosion, corrosion, or from tubeworm tracings. This roughness alone can significantly increase fuel consumption, but can be prevented simply through effective polishing by divers. Basically, a clean bottom and shiny impeller will save money. Incat THE Magazine Issue 36 13
Page 1: FUEL ECONOMY & HIGH SPEED CRAFT www
Page 4 and 5: 4 Incat THE Magazine Issue 36
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