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ISP, Volume 23, No. 265, 1976

Reprinted: 13-09-2001 Website:

Rapport 0427-P, 1976, Delft University of Technology, Ship Hydromechanics Laboratory, Mekelweg 2, 2628 CD Delft, The Netherlands.

Prediction of Speed and Behaviour of a Ship in a Seaway
J.M.J. Journée Delft University of Technology

Summary A computer program has been developed to calculate speed andbehaviour of a ship in a seaway. In this stage the program is suitable for seagoing vessels in head waves. In determining the speed, two factors are considered: the natural speed reduction due to added resistance caused by wind and waves and the voluntary speed reduction by the ship's captain, in order to prevent severe motions.



For almost twenty years now, a ship's captain canmake use of routing advises from weather routing departments like that from the K.N.M.I. (Royal Netherlands Meteorological Institute) at De Bilt [1]. With a known rough weather pattern in the ocean an optimum ship’s route can be found with a minimum travelling time, fuel consumption or risk of damage of the ship and its cargo. These routing advises, are based on the momentary and expected wind andwaves and the ship’s reaction to them. The forecast of wind and waves is a meteorological problem. Up to now the prediction of the behaviour of a ship in a seaway - especially the ship’s speed - is based on routing experience with the ship considered or similar ships.

When routing a ship for the first time a routing officer needs reliable speed loss graphs, to read the ship’s speed as afunction of wave height and mean wave direction. Developments in the last decade made it possible to calculate with sufficient accuracy the speed in still water and the natural speed reduction due to added resistance caused by wind and waves. At this theoretical speed dangerous motions can arise for the safety of crew, ship or cargo. Then the master will voluntarily reduce speed in order to preventsevere motions. Several criteria for this decision can be found in literature. At the Ship Hydromechanics Laboratory of the Delft University of Technology a method has been developed to calculate the natural speed, the voluntary speed reduction and the behaviour of the ship at


this speed in a seaway with head waves. This method has been worked out into an Algol’60 computer program, namedROUTE, which enables a practical use.

where the relation between Va and V is given by: Va = V ⋅ { − w(V )} 1 At a certain engine setting these two equations are solved in the program ROUTE for every wind and sea condition as shown in Figure 1.


Calculation of Speed

Apart from wind and sea conditions, the speed of a ship in a seaway mainly depends on three aspects: - dimensions and form ofthe ship's hull and superstructure, - dimensions and characteristics of the propeller and - output and characteristics of the propulsive machinery. The energy flow of a ship in operation is given in the following scheme.

The propeller behind a ship is considered as an energy transformer: torque with rpm will be transformed into thrust with a mean speed of advance. At a certain steam or fuelinlet ratio of the engine there will be equilibrium between the number of revolutions and the ship’s speed. This equilibrium is in such a way that two conditions are fulfilled: the torque needed by the propeller must be in equilibrium with the torque delivered by the engine and the thrust delivered by the propeller must be in equilibrium with the total resistance of the ship. These two conditions ofequilibrium are shown in two coupled equations as mentioned below: Qm (Q0 , n0 , c, n ) = Q(Va , n ) ηm

Figure 1 Scheme of Speed Calculation For a number of ship speeds the relations between torque needed by propeller and rpm are calculated from the torque characteristics of the propeller behind the ship and an adapted wake fraction. The relation between torque delivered by the engine to the...