Patent GB-731428-A (GB)
Title: Means for the estimation of longitudinal bending moments in the hull of a ship as caused by the distribution of the cargo in the ship
IPC: B63B9/00
Filed: 1951-08-31
Abstract: 731,428. Electric analogue calculating systems. AKTIEBOLAGET GOTAVERKEN. Aug. 31, 1951 [April 9, 1951], No. 20647/51. Class 37. Apparatus for the estimation of longitudinal bending moments in the hull of a ship as represented by the difference between positive and negative moments and caused by the distribution of the cargo in different holds or similar spaces within the ship, comprises an electric balancing bridge having a resistor for each hold or space adjustable to a magnitude representing the moment of the cargo therein with respect to a reference plane, the resistors corresponding to holds or spaces giving respectively positive and negative moments being arranged in series in opposite branches of the bridge, and potentiometer means connected between the two branches for zero-setting of an instrument which in its zero-position indicates bridge balance. As indicated in Fig. 1, a ship is divided by a plane A-A into two parts in which moments are taken about reference planes B-B, C-C respectively. The moments due to cargoes between planes B-B, C-C are taken as negative, and moments for cargoes outside these limits are taken as positive. It is stated to have been found that, for different distributions of a cargo with an approximately equal displacement, the sum of the bending moments for the hogging position (in which the centre of the ship is on a wave-crest with the stem and stern over adjacent wave-hollows), and the " sagging " position (in which the stem and stern are on wavecrests) is substantially constant. The planes B-B, C-C are so chosen that the difference between the positive and negative moments about these planes is equivalent to the amount by which the greater of the hogging and sagging moments exceeds the average. In the Wheatstone bridge of Fig. 2, positive and negative moments are represented by resistors M<1>, M3 ... Mn, and M<2>, M' ... Mm respectively. Since the distances of the corresponding holds from the reference planes remain constant the resistor scales may be graduated in units of weight, equal cargo weights preferably being represented by equal displacements of the adjustment members in all resistors. The scale associated with the potentiometer P, graduated, e.g., in ton-meters, is adjusted so that the distance between the zero and midpoint of the potentiometer resistor corresponds to the moment (called " residual moment ") due to the hull and the buoyancy of the displacement (assumed constant within a certain displacement and trimming range). In operation, the potentiometer arm is moved to produce zero current in galvanometer I. Since resistances R<1>, R2 are equal, the distance of this arm from the mid-point corresponds to the difference between the resistances in DE and FG, and the distance from zero corresponds to the " excess moment." Marks +T, - T on the scale indicate the limits between which the arm should lie. A plurality of differently adjusted scales for different ranges of load may be used. Alternatively, a potentiometer scale having its zero at the mid-point may be used, and the different residual moments represented by additional adjustable resistors in one of the branches of the bridge.