This layer is called the boundary layer, and the momentum imparted by the hull to the water in it is a measure of the frictional resistance. But there are important differences in the pressure distribution across the hull of a surface ship due to the surface wave disturbance created by the ship's forward motion. The residual resistance of the ship R R s, is calculated by the law of equation, Equation (10):.
This applies to the ship with the corresponding speed given by the expression. f) The ship's frictional resistance R F S is calculated with the same assumption as in note (4), using a coefficient of friction appropriate for the ship's length. g).
Section 3 Frictional Resistance
The results of the towing tests and the predictions from the model are given in Table 2. When the velocity was small, the dye remained as a straight filament parallel to the axis of the tube. Baker (1915) plotted the results of many of the available data on planks in terms of the coefficient of resistance.
Thus, the value of C will increase along a transition line of the type shown in Fig.
GRANVILLE CFO
Section 4
Wave-Making Resistance
- Ship Wave Systems. The earliest account of the way in which ship waves are formed is believed to be
- D. van Manen
- van Oosranen
- Section 1 Introduction
- The Problem. A ship differs from any other large engineering structure in that-in addition to all
- Types of Resistance. The resistance of a ship a t a given speed is the force required to tow the ship
- Submerged Bodies. A streamlined body moving in a straight horizontal line a t constant speed, deeply
- General. Dimensional analysis is essentially a means of utilizing a partial knowledge of a problem
- Section 2
The Kelvin wave pattern illustrates and explains many of the characteristics of the ship wave system. Further analysis of the resistance led to the identification of other sub-components, as discussed later. A ship moving on the surface of the sea experiences frictional resistance and eddying, separation and viscous pressure resistance in the same way as the submerged body.
The resulting additional drag corresponds to the dissipation of energy into the wave system, which spreads behind the ship and has to be constantly recreated.
Dimensional Analysis
RESISTANCE 17
As the ship's speed increases, the wave pattern must change, as the length of the waves will increase and the relative positions of their crests and troughs will change. In this process, there will be a succession of velocities when the peaks of the two systems. 12a shows the curves of the longitudinal (a) force per meter of length at all points on the hull surface; Fig.
Corresponding to the energy in the wave system, it uses the idea of an experimental technique used by Eggert. Much research on resistance to wave generation has been carried out on models of mathematical form that have sections and water lines defined by sine, cosine or parabolic functions. When the calculations are applied to actual ship shapes, the shape of the latter must be approximated using polynomials (Weinblum, 1950, Wehausen, 1973); or considering that the trunk is composed of many elementary wedges (Guilloton, 1951).
The wave pattern and the wave making resistance were then calculated from the amplitudes of the elementary waves using Havelock's concept. To a certain extent, the hull forms of relatively high-speed merchant ships have improved due to the application of the wave resistance theory. The shape of the hull and the calculated and measured wave profiles are shown in Fig.
Due to the distinct sharp corners at the bow, stern and shoulders, the four free wave systems have their outlets fixed at points along the hull. Wigley calculated the values of V/m for the minima and maxima of the wave-forming resistance coefficient C, for this form, and found them to occur at the following points:.
MEASURED
Section 5
Other Components of Resistance
Thus, the real skin friction of a ship must be greater than that of the "equivalent board". The propagation of the boundary layer continues until the velocity of the external particles at each point is equal to the potential velocity of the flow at this point (Fig. 26). He showed that the resistance to wave breaking can contribute a significant part of the total resistance of the complete forms.
A ship sailing on a smooth sea and in still air experiences a resistance due to the movement of the overwater hull through. This resistance depends on the speed of the ship and on the area and shape of the upper works. The "relative" or "apparent" wind is the vectorial summation of the ship's speeds and headings and the true wind (see Fig. 27).
An extensive study of the resistance of ships' superstructures has been made by Hughes (1930). For small wind angles from the bow or stern, the wind force in the ship's line of motion will be approx. F cos a. One of the points which the previous figures emphasize is the much greater relative effect of wind resistance on the slowest ship.
The yawing moment on the vessel due to wind depends on the position of the main superstructure. Variations in deckhouse configurations are relatively of less importance with respect to the value of the wind coefficients.
Trim Effects. Owing to the change in pressure distribution around a ship at different speeds, it will
When the water is very deep, the wave pattern consists of the transverse and diverging waves as shown in figure. The whole energy is transferred with the wave and the wave is called a translational wave. The two straight lines themselves are the forward crests of the divergent system, and the inner crests are concave to the line of advancement rather than convex as in deep water.
Each curve is marked with the value of the ratio of the water depth h to the characteristic length of the disturbance 1, which marked the CCI for deep water. There is a further loss in velocity SV, due to increased potential flow or displacement around the hull due to area restriction from the vicinity of the bottom, giving as final velocity V, = V, - SV,. An analysis of the data suggested to Landweber an extension of Schlichting's method for predicting the resistance to shallow water in the case of lateral confinement also, i.e., in channels.
In shallow water of unlimited width, the velocity reduction is a function of Kx/h, and Landweber sought a similar parameter that would also introduce the width of the channel, b. The equivalent depth of the channel for calculating the critical wave speed is given by The effect of sweep is to increase the load near the tip of the lifting surface.
It follows that the component of lift in the direction of the undisturbed flow becomes larger with increasing backlash. According to Hoerner (1965), the induced drag increases proportionally to the sweep angle according to 1 I cos a, where a is the sweep-back (or forward) angle of the quarter-chord line on the lifting surface.
Section 6
With a yaw angle greater than about 5 degrees, the flow along the tail usually separates and the drag associated with yaw angle increases markedly. Even on yachts, with block coefficients around 0.4, this occurs because the flow on the windward side of the hull separates forward from the rudder. The submerged hull of a heeled ship will be asymmetric, with the leeside of the vessel significantly flared.
In yachts with long bow and stern overhangs, this increase in drag is compensated to some extent due to the increase in effective wave-making length of the hull as the hull heel. 56 which gives the resistance in kN for the J-class yacht Rainbow, as measured at MARIN for various angles of heel. At speeds between 6 and about 9 knots it can be seen that the increase in drag with heel angle is marginal due to the increase in wave-making length.
Due to the complicated nature of ship drag, it was natural that early resort had to be made to experiments, and it is recorded that Leonard0 da Vinci carried out tests on three models of ships with different fore and aft distributions of displacement (Tursini, 1953). From this time there was a steady growth of interest in model experiment work (Todd, 1951). Colonel Beaufoy, under the auspices of the Society for the Improvement of Naval Architecture, founded in London in 1791, conducted between 1791 and 1798 in Greenland Dock between nine and ten thousand towing experiments, with models of geometric form and flat boards. (Beaufoy, 1834).
He soon became dissatisfied with the limitations of these experiments and turned to the use of a larger tank, making proposals to the British Admiralty in 1868 which were accepted and a new tank was completed near his home in Torquay in 1871 (W .Froude, 1955). I was fitted with a mechanically powered tow truck to pull the models instead of a gravity device and because of this and its size it can be considered the forerunner of the tanks so common today.
The Uses of Models for Determining Ship Resistance
Section 7
Presenting
General. The most useful method of present- ing model resistance data depends upon the particular
Model Resistance Data
Section 8
Relation of Hull Form to Resistance
Section 9
High-speed Craft and Advanced Marine Vehicles
The range of variables and other data are given in Tables 26 and 27. The models had a heavily raked
The largest area and the largest beam were at 60 and 70 percent of the length from the front vertical, respectively. The residual drag values of the models were obtained using the ITTC friction drag coefficients of 1957. Formulas were derived for the total drag-displacement weight ratio R,/W for eleven values of volumetric displacement Froude number.
89 Mean value of the residual drag-displacement weight ratio R,/W of the methodical NPL series where L/3 equals 7.5. When using equation (68), it is essential to stay within the range of values of the independent variables used in the database. An analysis of the stagnant water values of the wetted surface of the models comprising the database resulted in the following formula, with an accuracy of .
The glider hull was developed to overcome the inherent hydrodynamic limitations associated with the high speed operation of the traditional displacement hull. Small, fast planing vessels became possible in the 1930s as a result of the development of power plants with a high power-to-weight ratio. The test results of the Series 62 and Series 65 models show that the location of the LCG has a marked effect on the R/W drag-to-displacement weight ratio.
In that case, the resistance can be calculated using the formulas of Section 8.12. When h, 5 L,/b, the arc is essentially free of the water and the resistance can be predicted from the following equation:.
