Resistencia

ISP, Volume 23, No. 265, 1976

Reprinted: 13-09-2001 Website: www.shipmotions.nl

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 and behaviour 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.

1

Introduction

For almost twenty years now, a ship's captain can make 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 and waves 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 a function 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 prevent