engleski

Speed loss calculations in a seaway

ABSTRACT: The reliable prediction of attainable ship speed at actual seas is essential from economical and environmental aspects. It allows for the prediction of fuel consumption and emission of greenhouse gases from ships with more accuracy. Nowadays the pollution issue becomes very important because of the problem of global warming and increasing conscience about environmental protection. Besides the possibility of improving the fuel and propulsive performance, an important aspect for improvement in this respect lays still in hull optimization based on the ship performance in an actual sea. In this paper we propose a methodology for estimating the attainable speed in moderate and severe sea. One concern is the speed loss in higher sea states which is, besides due to environmental forces acting on hull, due to the propulsive performance in severe seas. The irregular sea is handled as a series of regular waves with different amplitudes and frequencies. The added resistance in regular waves is obtained by a direct pressure integration method in the wavelength range of ship motion. The smaller wave length cases are handled by an asymptotic small wavelength formula. In severe seas the ship is subjected to additional effects like the in-and-out-of-water-effect and ventilation of a propeller which become important issue to discuss and take into account. When ventilation does not occur, this effect is evaluated by using experimental data for propulsive coefficients at different submergences of the propeller in calm water. A quasi-steady averaging is then used to consider the effect of regular waves. The results for regular waves are used in simulating results in irregular waves. It is shown that for higher sea states this effect has much more influence on the speed loss than the added resistance in waves. The speed loss is calculated by taking into account the engine and propeller performance in actual seas. The attainable ship speed at a given sea state is obtained as time series. Correlation of speed loss with sea states allows predictions of propulsive performance in actual seas.

Added resistance; Ship speed; Seakeeping

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