An Approximate Method of Estimating the Propulsive Coefficient of Cargo Ships

Abstract

The easy estimation of the speed and power of ships is always quite necessary in the early stages of ship design, and with this purpose several methods have been already published. Among these, Dr. Yamagata's method which was indicated in his paper entitled “An Approximate Method of Estimating the Requisite Horse Power of Propelling Machineries of Cargo Ships” (Jour. of the Soc. of N.A. of Japan, Vol. LXIII.) seems to be the most successful one. But this papal is only concerned to neighbourhoods of normal speed attained at the normal horse power of propelling machinery at full load condition.Generally, speed trials and other tests on ships are carried out at light condition or 1/5 load condition, except special cases as on oil tankers, and various guarantees in building contract are chiefly appointed on such light load conditions. Therefore, when the principal dimensions of ship and principal particulars of propelling machinery are decided for a new design, it is also a matter of great importance to estimate the horse power-speed curve at such light load condition of the ship.Since effective horse power-speed curve at any lend condition of a ship can be estimated with sufficient accuracy by means of several methods by papers already published, such as Taylor's, Ayre's, Volker's, Koning's etc., the estimation of shaft horse power-speed curve for a newly designed ship at any load condition may be replaced by the estimation of suitable propulsive coefficient curve for that ship.The author analysing the results of self-propulsion tests at the Teisinsyô Ship Experiment Tank during past several years, obtained a new method of estimating the propulsive coefficient of cargo ships at full and light load conditions. In the present paper, the values of propulsive coeffi ient are presented corresponding to the adopted standards of cargo ship forms and propellers, and supplemented with correctional valúes for differences between standards and those which may be applied to a particular vessel.