Longitudinal Strength of Large-scaled Wooden Ships

Abstract

Wooden ships up to around 55 m in length have been so far designed and built with hulls of double skin planking which consists of each single layer of longitudinal planking and diagonal planking crossing with an angle of 45°. Based on successful operation of these ships, large-scaled wooden ships of around 70 m are recently under consideration adopting hulls of triple skin planking. The triple skin planking consists of the outer skin of a single layer of longitudinal planking and the inner skin of double layers of diagonal planking wihch cross the longitudinal planks with angles (θ) of ±45°.Experiments and theoretical investigation as listed in the followings have been made in order to establish design formulae for the longitudinal bending and shear strength of hulls of the triple skin planking : (1) Simplified formulae are induced for calculating the rigidities and the stresses of the triple skin planking, and the practical methods are shown on the longitudinal and shear strength of hulls.(2) The results of the structural loading tests of the box-shaped ship model sizing 6.54 × 0.9 × 1.04 m are compared with the structural analyses by the three-dimensional (3D) truss model. The structural members of the test model are in 1/2 scale of those of a prototype ship except the overall dimensions.(3) The results of the same kind of 3D truss analyses made for the whole hull of the prototype ship are compared with the calculations by simplified formulae.It is concluded as follows : ·The 3D truss model reasonably represents the behaviour of the triple skin planking.The simplified calculation methods can be applied to actual ship design.·The effectiveness of the diagonal planking on the longitudinal bending strenth is 25 % theoretically when θ= ±45°, and a practical value of 20 % is proposed.·The inner skin of two layers of diagonal planking predominantly carries shear forces. The axial stresses of diagonal planks are simply obtained by doubling the shear stresses when θ= ±45°.·Longitudinal members such as gunwale, chine and keel contribute to the shear strength of a hull by their sectional areas multiplied by a factor of 4G/E theoretically when the rotations of the sections are restrained by neighbouring members. The actual degree of contribution is subject to further investigation.