US2865566A - Apparatus for the estimation of the distribution of the cargo in a ship - Google Patents

Description

Dec. 23, 1958 K SWENSON 2,865,566

APPARATUS FbR THE ESTIMATION OF THE DISTRIBUTION OF THE CARGO IN A SHIP Filed May 10, 1954 2 Sheets-Sheet 1 X A L 8 I 1 L7 L2 L5l4- L W a a "J m J & PAM

ATTORNEYS Dec. 23, 1958 K. SWENSON APPARATUS FOR THE ESTIMATION OF THE 6 2 mm t e 5 m 6 8, m 2 e DISTRIBUTION OF THE CARGO IN A SHIP Filed May 10, 1954 INVENTOR fi urf Lennarf swemso/z United States Patent APPARATUS .FOR THE ESTIMATION OF, THE DIS- "TRIBUTION OF THE CARGO IN A SHIP Kurt Lennart Swenson, Gothenburg,

Aktiebolaget Gotaverken, ,poration of Sweden ApplicationMay-10, 19,54,Seri al'No. 428,784 C ms Pri r y, pp atio S e M y.. .6,.19 2 Claims. '(Cl."235-161) Sweden, assignor to 'Gothenburg, Wedcn, a cor- In a ship for transport purposes there are different spaces for accommodation of the cargo which spaces are dimensioned in such manner that they will be completely-filled if the specific weight of the load is comparatively low. For this reason when loading the ship with a material; of a comparatively high specific weight, a-considerable part of "the load space is left empty, and in suchcase the distribution-of the cargo in said spaces may be chosen in different ways. The so-called trim indicates the distribution of the cargo in the longitudinal direction of the ship and corresponds to the difference between the drafts at the ends of the ship. Many important factors are dependent onthe trim, for instance, the propellerefficiency, the speed of the ship, the behavior or performance in-the sea and the general comfort on board. The distribution of the cargo should therefore'bechosen so as to produce a trim which is most advantageous in said respects.

-Moreover, the distribution ofthe cargo in the longitudinal direction of the ship determinesv the tensions or stresses in the longitudinal direction of the ship. The deformation of the ship owingto these tensions is not visible to such anextent as the trim 'but in certain types of ships, particularly in tankers, it is possible to, choose the distribution-of the load in so many ways that with an extremely unsuitable distribution of the cargo and under otherwise'unfavorable circumstances theship may be subjected to inadmissiblygreat tensions which may result in the formation of cracks in the material, perhaps-with the risk oftotal'rupture.

The theory of calculating the trim of a ship is generally known to the ships officers. The problem in this connection isto obtain the necessary data, for instance, the position of the center of gravity of the different load spaces and hydrostatical data. The procedure in calculating the resulting bending moment and the resulting stresses in the longitudinal ,directionof, the ship is, however, more complicated and is mostly unknown to the ships officers. It is therefore desirable to.have a possibility of controlling or checking the distribution of the load with respect to the resulting trim and the stresses caused thereby. Since said stresses reach their maximum value when the ship occupies a certain position in the waves of the sea, the problem is more complicated owingto the hydrostatical conditions being difficult to define satisfactorily.

A certain conception of the magnitude of the resulting stresses may, however, be obtained by the so-called standardized strength calculation. In this calculationit is assumed that the ship is carried by waves the distance between the wave-crests of which being equal ,to the length of the ship and the depth of which i. e. the verticalv distance between wave-crest and wave-hollow, ,being equalto one twentiethcf the length of the ship. The p t q c th h p ic is thenv ubme s in te a d whichthus actually carries theship including the cargo thereof, canbe indicated graphically'ina diagram. The displacement curve contained in this diagram, the appearlevers fore and after said plane.

ance of which varies in dependence on the trimming position of the ship anddependingon whetherthe ship is disposed with the center thereof overa-wave-crestand with the stern and the stern over two adjacent wavehollows, the hogging-pos ition, or whether the: stem and the stern are located each on a wave-crest, with a wavehollow beneath the ship atthe center'thereof,""fthe-saggingposition, confines an area, which-is as "large-as but quite different in shape from the area confinedby a curve indicating the weight ofthe ship includingthe cargo thereof as distributed in the longitudinal direction of the ship. Acombination'of these two curvesgives a loading curve, and if 'thelatter is integrated--twice,=a curve of shearing forces isfirstobtained and thence a curve relative to the bending-moment which latter gives the stresses sought in connectionwith' the section modulus calculated for the ship. The performance ofa calculation of this type requires avery long time-evenffor1a single case of loading.

The object of the present invention isto providean apparatus by means of which the suitability of a certain load distribution both with the respect to the trim and the stresses in the bulk owing to the length-wise bending moments may be ascertained. {The invention is characterized in that the apparatus is constituted by anelectric balancing device including ,a group of series-coupled resistances which are adjustable to valuesproportional to the products of the weights of the loads on one side of an imaginary plane dividing the ship into one'forebody and one after-body, and the distances of saidiloads from said plane, asecond group of series-connected resistances which are adjustable in a similar manner with respect to the loads on the-opposite side of saidplane, a first additional resistance which is M proportional to the moment of the bulk weight and the displacement with respect to said plane and a second additionalresistance which is proportional to the hydrostatic effect of the forebody and the after-body and of the moment of'the bulk weight, a switching means being provided for alternately inserting the resistances in one of two electric balances in'such manner that the one branch of the one balance is constituted by one of said groups of resistances and the other branch is constituted by the other group/of resistances in series with said first additional resistance, while the one branch of the second balance is constituted by the series connection of said both resistance groups, the' branches of said balances each being interconnected through the resistance part of a potentiometer, the movable tap of which in a ,balancedcondition of said'baltively, said forces being balanced by a system of upwardly directed displacement lifting forces. The effect of the last-mentioned forces may besubstituted -by-a single upwardly directed force equal to the magnitude of the displacement and located in its centre of gravity AA, Fig. 1. The trimming moment is then equal to the algebraic sum of the moments of the loads and of the bulk weight with respect to the plane AA, signs being used for If instead of the plane AA another arbitrary plane X-.-X is chosen the same trimming moment may be calculated by adding;to..the algebraic sum of the moments of theloads .andthe bulk weight with respect to,the newplaneX-TX,themQment of the displacement lifting force with respect to jthesame plane while using the same rule for thesigns as men-- tioned above, the force being introduced with a negative sign. The formula for the trimming moment T may generally be written as follows:

1 in which I=2'vLx+f '01) Y v=the weight of each partial load,

Lx=the length of the lever to the planeX-X for each partial load,

-h=a hydrostatic characteristic an d in which f'(h) includes, on the one hand, the moment fof the bulk weight and, on the other hand, the moment ofthe displacement with respect to the plane XX and is a constant value at a given displacement condition. For .the levers the above-mentioned sign-rule shall be applied. The trirnof the ship to be measured is not directly propo rtional to the trimming moment but is also dependent. on. the hydrostatic conditions, particularly the draft. These hydrostatic conditions are represented by the factor f"(h) and its value is equal to thereciprocal value of the trimming moment required at a certain draft to trim the ship one trimming unit, for example 1 inch. The function of f"(h) is contained in the scale of potentiometer unit 21-22 later described and a different scale for that potentiometer is required for each value of mean draft used with the apparatus. The trim t may therefor be written as follows:

-When calculating the trimming moment of the ship according to the aforesaid, the ship and its load may at first be assumed to be balanced on a support in the cen Qtr'e of gravity of the displacement. For calculating the lengthwise bending moment in a section YY, Fig. 2, dividing the ship into a fore-body and an after-body, a corresponding simplification as to the hydrostatic forces may be made.' In such case one must consider each part of the ship separately and concentrate the two parts of the displacement to the respective centres of gravity thereof. Thus, the ship may be considered to be lying on two supports in the planes BB and CC. That this is in agreement with real facts, will appear from the following: If a weight is placed at the centre of the ship, the ship will be subjected to increased sagging stresses (negative moment). If, however, the weight is placed at one of the ends of the ship the hogging stresses will be increased (positive moment). Therefore, a plane must ex- ,ist in an intermediate position between the centre and the respective ends of the ship in which plan'e an increase of the weight does not result in any positive or negative increase of the moment. In each part of the ship the resulting moment is the algebraic sum of the moments of the load and the bulk with respect to the plane BB and .CC respectively, and the total lengthwise bending moment in the plane Y-Y is the sum of the two resulting moments from the fore-body and the after-body of the ship.

As mentioned in connection with the calculation of the trimming moments, the resulting moment in each part of the ship may, however, be calculated with respect to an arbitrary plane, while paying due regard to the effect of the moments of the hydrostatic forces with respect to this new plane. It is therefore suitable to calculate the moments with respect to the common plane YY for the two parts of the ship. The total lengthwise bending moment M in the plane YY may therefore be written in which v=the weight of the partial loads Ly=the length of the lever to the plane YY for each partial load h=a hydrostatic characteristic 7 ';f"(h) includes the hydrostatic effect of both the fore body and the after-body and, moreover, the moments from the parts of the bulk, since these are constant values. designates in case the numerical sum.

When measuring by means of a single apparatus the trim and the lengthwise bending moment, it is suitable that the plane X-X be identical with the plane YY, which is chosen so as to lie where the lengthwise bending moment has its maximum .value or elsewhere where it is desired to know its magnitude.

The Formulas 1 and 2 may then be written as follows:

in which 2 is the algebraic sum and 2" is the numerical sum of the moments of the'ditferent loads.

The invention. will hereinafter benmore fully described with reference to Figs. 3 and 4 of the drawing. Fig. 3 shows a circuit diagram of the apparatus and Fig. 4 shows one embodiment of an apparatus according to the invention;

The electric balancing means according to Fig. 3 includes a current source 9, a switch 10, two known, constant resistances R and a group of series-connected resistances 11, 12, 13, and 14, which are adjusted to values proportional to the products of the weights v -v of the loads on one side of the plane XX (Fig. 1) and their distances L -L from the plane XX. The corresponding load spaces are designated 1-4 in Fig. 1. The balancing means comprises further a second group of series connected resistances 15, 16, 17 and 18 which are adjustable in a similar manner with respect to the loads in the after-body of the ship, the corresponding load spaces being numbered as 5-8. A resistance 19 is adjustable to a value proportional to f'(h) and a resistance 20-corresponds to f(h). The resistance portion of a potentiometer is designated 21, the movable tap of the potentiometer being designated 22. A second potentiometer has a resistance portion 23 and a movable tap 24. The potentiometers may alternately via a galvanometer 25 be connected to a point between the two resistances R The potentiometer 21, 22 is provided with a plurality of scales 26, 27 for different drafts corresponding to the factor f"(h). Switches by means of which two different electric balances may be obtained are designated 0 O and 0 If the switches are in the positions indicated with full lines in Fig. 3, the two resistance groups 11-14 and 15-18 are connected in series to the one side of the potentiometer 23, 24, the other side of which being connected to the resistance 19. The potentiometer 21, 22 and the resistance 20 are then switched out of circuit. The operating parts constitute in this case the one electric balance, the one branch of which being constituted by 2"vLy and the other branch of which being constituted by f"'(l1). In this setting of the apparatus the lengthwise bending moment M of the ship may be determined. The balance consists in the embodiment shown of a Wheatstone bridge which in a manner known per se is operated in such manner that the two branches are balanced by means .of the potentiometer 23, 24, and the current through the galvanometer is zero. The potentiometer scale may advantageously be such that in this case the lengthwise bending moment M or, if desired, the corresponding bending stress is indicated. The scale may be provided with marks indicating the limits of the permissible range of variations of the moment or stress.

It the switches 0 O and 0 are set in the positions indicated by broken lines in Fig. 3, a bridge is obtained the one branch of which being constituted by the resistance group 11-14 and the second branch being constituted by the resistance group 15-18 in series with the resistance 19. In this case the values E'vLy and j'(h) are balanced over one another via the potentiometer 21, 22, the state of balance being reached when by ad'- justment of the potentiometer arm 22 the galvanometer 25 reads zero. The trim t is then obtained in an analogous manner and can be read on the scale 26 or 2'1 for thedf'a'ft'iri question; "As'the'r'h'ovabl armor 22 will usually have to be adjusted in one direction or another. the zero point on the trim scale can be located at a middle point on the scale. Depending upon load distribution, the trim will be either positive or negative (by stem or by stem, respectively) from zero and will be properly indicated dependent upon the direction of movement of the arm 22 required to obtain bridge balance. Should the trim and/or the lengthwise bending moment not lie within the permissible limits a different load distribution must be tried by changing the resistance values of one or both of the resistance groups 11-14 and 15-18 until a load distribution is reached which is acceptable both with respect to the trim and to the lengthwise bending moment.

The apparatus according to the invention may for example be built in the manner indicated in Fig. 4, according to which the casing of the apparatus consists of two boxes 30 and 31 the latter being pivotally connected to the box 30 at the lower end thereof so that it can be moved from a vertical to a horizontal position and vice versa. The circuit elements in the boxes are interconnected by means of conductors 32 and 33. The panel of the box 30 is subdivided into different sections or fields. The section 34 indicates the name of the ship and a schematic illustration of the positions of the different load spaces in the ship. It is assumed that the ship comprises 25 load and storage spaces each being represented by a potentiometer r corresponding to the resistances of the two groups of resistances shown in Fig. 3. These resistances are grouped together in a surveyable manner in difierent fields 35, 36, 37, 38 and 39.

The box 30 accommodates the resistance 20, the movable part 22 of the first potentiometer cooperating with a plurality of scales, the part 24 of the second potentiometer and the resistance 19. The galvanometer is designated 25 and a handle for actuating the switches shown in Fig. 3 simultaneously is designated 48. It has three different positions, namely two end positions for switching in the two different balances and an intermediate position in which the apparatus is switched off. Lamps 41 and 42 indicate which of the two balances is connected into circuit. A space 43 contains directions for the use of the apparatus, tables showing the hydraulic characteristics of the ship at different degrees of loading, the capacity of the load and storage spaces and suitable corrections for obtaining a better load distribution if the distribution first chosen should prove to be unsuitable.

The apparatus is set or adjusted by means of the values obtained upon a standardized calculation of strength. When using the apparatus a certain distribution of the load may be chosen, which according to prior experience is likely to be suitable. In this connection the liberty of choosing the distribution may be limited in certain respects. So, for instance, the load may consist of oils of different types and quantities in which case it may be necessary to accommodate a certain type of oil in a certain space due to its quantity, while another type of oil cannot be accommodated in a given load space due to the composition of this type of oil.

When this preliminary load distribution has been chosen the resistances 11-18 are adjusted to the corresponding values and the resistances 19 and 20 are then adjusted according to the existing hydrostatic conditions at the total load in question.

For judging the trim the handle 40 is thereafter turned to the one end position corresponding to the trim and the member 22 is then turned until the pointer of the galvanometer 25 occupies the zero position. The value of the trim may thereafter be read on the scale of the member 22. When the handle 40 has been turned to its other end position the member 24 is turned until the galvanometer again shows the zero value, whereafter the sought moment or the corresponding bending stress may be read on the scale of the member 24. If the chosen load distribution should prove to be unsuitable, the procedure has to be repeated after a suitable redistribution of the load as determined by the resistances representing the partial loads, which may be effected with the guidance of a table specially composed for this purpose, so that both the trim and the stresses at least show acceptable values.

What is claimed is:

1. An apparatus for estimating the influence upon a ship of the distribution of a certain quantity of cargo therein, comprising a first group of series-connected resistances, means to adjust said resistances to represent the moments with respect to an imaginary plane dividing the ship into a fore-body and an after-body and caused by parts of said cargo when placed in various holds in the fore-body of the ship, a second group of series-connected resistances, means to adjust said resistances of said second group to represent the moments with respect to said plane and caused by parts of said cargo when placed in various holds in the after-body of the ship, a first additional resistance representing the trimming mo ment of the Weight of the hull and the hydrostatic displacement lifting force with respect to said plane, a second additional resistance representing the lengthwise bending moment of the Weight of the hull and the hydrostatic displacement lifting force, a dual purpose electric balancing device containing a supply of electric current, two reference resistances and a galvanometer, a first potentiometer, a second potentiometer, a switching device for alternatively arranging the various resistances in relation to the balancing device in such a manner, that for one purpose a first branch is constituted by one of said groups of resistances and a second branch is constituted by the second group of resistances in series with said first additional resistance, said first and second branches being connected by said first potentiometer, whereas for the other purpose a third branch is constituted by the series connection of both groups of resistances and a fourth branch is constituted by said second additional resistance, said third and fourth branches being interconnected by said second potentiometer.

2. An apparatus as claimed in claim 1 arranged for the estimation of the influence of various quantities of cargo, wherein said first potentiometer is provided with different scales for different values of the draft of the ship.

References Cited in the file of this patent UNITED STATES PATENTS 1,826,762 Franklin Oct. 13, 1931 2,123,142 McMaster July 5, 1938 2,443,098 Dean June 8, 1948 2,520,428 Nilakantan Aug. 29, 1950 OTHER REFERENCES Laws Electrical Measurements, McGraw-Hill Book Co., Inc., 1917, Fig. 96 on page 171.

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