Quantification of Frictional Drag due to Biofouling on In-Service Ships
Hunsucker, John Travis
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The purpose of this research was to improve the quantification of frictional drag due to biofouling on in-service ships by advancing the knowledge and methodology used to study the interactions between fouling communities and the surrounding environment. This research examined the influence of hydrodynamic stress on the development and frictional drag of fouling communities that developed on a fouling release coating; investigated the reliability of using ship performance monitoring to detect changes in the hull and propeller condition; and evaluated the feasibility of developing a field deployable instrument capable of measuring the local frictional resistance on a ship hull in situ. It was found that there were differences in the fouling communities that developed under different hydrodynamic stress on the fouling release coatings. The frictional drag, however, was more strongly influenced by differences in the coverage of fouling communities that developed on the individual panels rather than the condition of immersion (static vs dynamic). In these tests, the frictional drag of soft and low form fouling with different community structure were similar. It is hypothesized that that there may be a more stepwise relationship between the biofouling structure and drag. The reliability of using ship performance monitoring to detect changes in the hull and propeller condition was examined using five years of data for two cruise ships of the same class. Ship and metocean data were reduced to form a key performance indicator (KPI). The resulting KPI detected changes in the hull and propeller condition similar to what were observed in the diver inspection reports and reported by the shipping company. The KPI, however, needed to be averaged over a 60-day interval for consistent trends to be identified. The correction and filtering of ship power and speed using data from the ship and metocean models were insufficient to remove scatter in the KPI. The limiting factor in reducing the time interval to average the KPI is the quality of the ship and metocean data, with the determination of speed having the largest influence on the KPI. The feasibility of developing a field deployable instrument capable of measuring the local frictional resistance on a ship hull in situ was examined. The instrument measures the pressure gradient developed along a three-sided channel affixed to a ship hull. A pump draws ambient seawater through the instrument with a flowmeter recording the flow rate. The pressure gradient, channel height and flow rate can be used to determine the local frictional drag coefficient. The recommended final dimensions and material for a field prototype are: channel length- 90 cm; channel height- 0.95 cm; channel width- 7.62 cm; distance from entrance to first tap- 19.0 cm; tap spacing- 50.0 cm; instrument material- acetal resin (Delrin®); gasket material- high-strength oil resistant Buna-N rubber strips.