International Shipbuilding Progress

A ship-propeller interaction theory is presented in the form of computational procedure for evaluation of the nominal and effective wake fractions and of propeller thrust deduction, taking into account the contributions of potential flow, viscous boundary layer and ship wave-making. This method will give the distribution of the effective wake, and thereby will permit a more precise design of wake adapted propellers, the action of which heretofore had to be inferred from the experimental data on the distribution of nominal wake.

Calculations are based on representing the propeller race by an array of cylindrical vortex sheets, thus accounting correctly for mean axial and radial flows, but neglecting the time fluctuations and rotation of the propeller race. Any form of the radial thrust distribution can be specified. The hull is represented by distribution of sources and sinks, in the evaluation of which the velocity field of the propeller and the ship wave flow are taken into account. The ratios of computed effective and nominal wakes can be used then as correction factors to observed nominal wakes. For submerged bodies of revolution, calculations can be made using desk machines. For a surface ship the use of punched card computing machines is visualized.

With the aid of the results of many model experiments which have been carried out by the Netherlands Ship Model Basin for upwards of twenty years, attempts have been made at devising a method to determine ship resistance. To this end, investigations have been made with a view to finding out what parameters are of importance for characterizing the ship form and next to ascertaining what influence each of these parameters has on ship resistance. Diagrams are provided for determining the residuary resistance as a function of speed. The construction of these diagrams is described. Moreover, some conclusions are mentioned which could be drawn during the investigations. Finally, the total efficiency of propulsion is given as a function of the number of revolutions of the propeller and the ship’s length. By these means it is possible to derive the ship’s pow er under service conditions from the resistance of the smooth ship, after various allowances have been taken into account.

For the calculation of the strength of welded joints, use has been made of a critical surface of the ultimately allowable values for the average stresses p either in the smallest longitudinal section (throat section) o r cross section of the weld.

In statically loaded structures the shape of the critical limit curve for loads perpendicular to the longitudinal axis of the weld can be deduced from the experiments of the Netherlands Foundation for Applied Scientific Research (T.N.O.). For the shape of the surface of limits, a surface of revolution (pearoid) has been assumed, of which the meridian section forms the above-mentioned limit curve.

Starting from the average stress in the throat section of the weld no theory of rupture or plastic flow has, up to now, given a satisfactory explanation of all phenomena of rupture observed in end fillet welds at the moment of fracture. This is to a great extent caused by the uneven distribution of stresses.

In comparison with Kist’s method of calculation in use at present in the Netherlands, the principles here presented allow not only some simplification but above all economy in the case of weld sections predominantly loaded in shearing or compression.

In dynamically loaded structures a symmetrical surface of revolution has been adopted as the shape of the critical surface of allowable effective stresses. This effective stress is to be obtained by multiplying the numerically greatest value of the oscillating stress by a coefficient γ, which is consistent with the fatigue diagram of the material and the type of weld. The values of γ are given both for butt and fillet welds for two frequency ranges of load oscillations. A full explanation based on both theory and experimental work is given.