US3296863A - Ship draft gage - Google Patents

Description

R. J. LEVINE SHIP DRAFT GAGE Jan. 10, 1967 4 Sheets-$hee t Filed April 1, 1964 FIG. 4

Fall/6e s] in} INVENTOR. ROBERT J. LEVINE ATTORNE Jan. 10, 1967 R. J. LEVINE SHIP DRAFT GAGE Filed April 1, 1964 4 Sheets-Sheet 2 INVENTOR.

ROBERT J. LEVINE ATTORNEY Jan. 10, 1967 R. J. LEVINE 3,296,863

SHIP DRAFT GAGE Filed April 1, 1964 4 SheetsSheet 5 gwih ww W i \P m v x 1 0 INVENTOR.

Q ROBERT J. LEVINE gw fl H-\ ATTORNEY R. J. LEVINE SHIP DRAFT GAGE Jan. 10, 1967 4 Sheets-Sheet 4.

Filed April 1, 1964 INVENTOR.

ROBERT J. LEVINE Y NwN ATTORNEY United States Patent 3,206,863 SHIP DRAFT GAGE Robert J. Levine, Briarcliff Manor, N.Y., assignor to Magnetic Instruments Co., Division of Polyrnetric Devices (10., Inc., .llenlkintown, Pa.

Filed Apr. 1, 1964, Ser. No. 356,388 8 Claims. (Cl. 73-304) This invention relates to a ship draft gaging system and in particular, to such a system employing a capacitive sensing element.

The draft of a ship is dependent upon the configuration of the ship and the cargo that it carries. The ships configuration is a fixed parameter so that the draft is in effect determined by the weight of the cargo.

It has been found that ships, particularly the larger ships, are subject to structural deformation as a result of uneven loading. Thus, if the ship were so loaded as to cause the fore and aft sections to sink deeper into the water than the center, a well-defined hump would result at the center placing great structural strain on the ship. Under this condition, known as humping, certain ship designs are particularly subject to breakup in a rough sea. In the reverse condition, called hogging, the center section is loaded proportionally more than the end sections which results in a lowered center section. Again, under these conditions the ship is under great structural stress. An improperly loaded vessel is not stable in heavy seas and represents a hazard to life and property.

The importance of not overloading or exceeding the allowable draft limit is emphasized by the fact that the Government has found it necessary to impose severe penalties for exceeding the permitted draft limit. In view of the penalties, there is a tendency by the ships master to embark on a trip with the vessel loaded below th maximum permitted. This results in an economic handicap for the operator, for a difference of one inch in draft of the vessel will, for a typical large tanker, mean a difference of about 100,000 gallons of cargo.

In the past, the determination of the draft was made by observing the position of the water level in relation to fixed markings on the side of the ship. However, wave action, poor visibility, and dock construction, among other factors, make visual draft readings an inexact science, particularly on large tankers.

Even in a relatively calm harbor area where a ship would ordinarily be loaded, small waves one to two feet high may cause an appreciable error in calculating the ships draft from the markings on the side of the ship. Thus, in the case of a tanker, for example, the master may, in the exercise of prudent judgment, load 4 to 6 inches less than permitted, or about 400,000 to 600,000 gallons less cargo.

The present invention reduces the error in measuring the draft of a ship to a minimum by utilizing capacitive probes which, in conjunction with various electronic circuitry and electromechanical devices provide a gage accuracy of inch over a critical range of about 12 feet with an accuracy of :1 inch (typical) for the remainder of the gage. This is accomplished, in part, by providing a probe which is divided into a plurality of sections with each section utilizing separate circuitry and meters, while keeping the number of components employed to a mini mum by using certain elements as common components.

The capacitive probe of the present invention, designed to be removed, cleaned, or replaced by the ships crew, is

placed within a well which is connected to a conduit, fitted with a sea valve that penetrates the hull below the lightload line. A plurality of such probes is utilized on each ship to enable the obtaining of true draft readings at the forward, aft, plimsol port and starboard amidships points. Thus, in addition to measuring the average draft, the operator may determine any listing of the ship due to improper loading. Because of the ships construction it is not always feasible to locate the wells at the point at which a measurement is desired. To overcome this problem, a method is provided which allows the probes to be situated at convenient locations from which the measurements taken therefrom are utilized to estimate the desired readings.

An object of the present invention, therefore, is to provide a system for accurately measuring the draft of a ship.

Another object of the present invention is to provide a system employing capacitive probes to measure the draft of a ship.

Still another object of this invention is to provide a multisection capacitive probe to determine the draft of a ship.

A further object of the present invention is to provide a system for determining the weight of the ships cargo.

A still further object of this invention is to provide for measuring the draft of a ship by a system which will not be appreciably affected by wave action.

Another object of the present invention is to provide a system for determining true draft from measured draft readings.

These and other features, objects and advantages of the invention will, in part, be pointed out with particularity and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawing, which forms an integral part thereof.

In the various figures of the drawing, like reference characters designate like parts.

In the drawing:

FIG. 1 is a pictorial view of the system which the present invention employs.

FIG. 2 is a longitudinal section of a typical probe.

FIG. 2A is a fragmentary showing an alternate method of securing the lower end of the probe.

FIG. 3 is a cross section taken along line 33 of FIG. 2.

FIG. 4 is a schematic representation of a four-bridge servo system.

FIG. 5 is a diagrammatic showing of a method of determining true draft readings.

FIG. 6 is a schematic drawing of a computing system for determining forward, aft and plimsol readings.

Referring to FIG. 1 of the drawing, reference numeral 20 indicates the deck of a ship 22 into whose hold has been inserted well 24 which is vented at the top. A conduit 26 connects well 24 to the seal through outlet 28, permitting sea water to enter into the well through sea valves 30 and attain the level of the water outside of the hull. The level of the water 32 in well 24 is sensed by capacitive probe 34. Probe 34 is connected to control unit 36 which comprises certain electronic and electromechanical equipment detailed more fully hereinafter. The control unit provides a signal to indicator 38 which displays the draft of the ship in inches and feet. One shipboard installation employs three probes mounted forward, aft and plimsol. It will be noted that the plurality of probes placed strategically throughout the ship enables the operator to determine whether the ship is evenly loaded with reference to the weight of the load.

The probe assembly 40 utilized in the system of the present invention is shown in FIG. 2. This probe is shown and claimed in my copending application entitled Improved Capacitance Probe filed concurrently herewith. Well 24, preferably constructed of galvanized steel, is fitted through deck 20 and extends downwardly into the hold of the ship. A retainer rod 47 is welded to bottom section 44 inside the well so that a line 48 may be employed to secure gland 50 of probe 34 and help keep it centered in well 24. Refer to FIG. 2A. Line 48 may be looped through retainer rod 47 and eyelets 49 to terminate in eyelet-supported spring 53. This provides a means of securing gland 50 when the probe 40 is reinstalled in well 24 after performing maintenance. As shown more clearly in FIG. 3, bolts 51 serve to keep probe 34 centered in well 24 and thereby avoid shifting of the probe clue to the motion of the ship. Cap 45, flange 45 and gasket 52 (FIG. 2), in combination with nuts and bolts 54, provide a watertight seal at the bottom of the well. Condulet 56 is secured to the top of the Well 24 by flange 58 with nuts and bolts 60 and gasket 62. Flange 58 is vented to atmosphere through vent 35. Condulet 56 may be provided with the conventional terminal strip to which the leads of the probe and the leads to the external circuits are attached. Conduit 66 provides a passage for leads 68 which connect the terminal strip to control unit 36.

Typically, a capacitive level gage has an accuracy of about 1% percent. In order to achieve the readout accuracy desired for this system, probe 34 has been effectively separated into four sections and the corresponding circuitry arranged to provide an independent measurement for each section. The critical range is covered by the upper three sections 70, 72 and 74, which are limited in length to approximately 4 feet each. The less critical bottom section employs a probe 30 feet in length. Thus, it can be seen that the /2 percent error results in a maximum expected error of only 4 inch in the critical range. The bottom section 75, 30 feet in length, will have a maximum error of i 1.8 inches. Operationally, it is not essential to monitor the draft as critically when the ship is lightly loaded; thus the less accurate readings of the bottom section are acceptable. Each of the upper three rigid sections are installed in tandem, while the lower section is of flexible construction so as to facilitate installing the probe in the well. The bottom section 75 of the probe consists simply of a Teflon coated copper-weld wire in which the coating is held to a highly accurate wall thickness and the bottom sealed in a Teflon V ring in a stainless steel gland 50. An eyelet 77 in the bottom of the bland permits the probe to be secured. By the use of this flexible lower section of the probe, it will therefore be possible to remove the upper stiff l2-foot section in one piece, then withdrawing the flexible Teflon tube. This is a highly important feature in the design of the probe since it allows easy withdrawal and maintenance, such as cleaning, should this be required.

Each section is electrically separated from the next lower one and is connected to control unit 36 by a shielded lead. In addition, the design of the present invention is such that the probe and all leads between the probe and indicator meet the intrinsically safe requirement, now under consideration by the U.S. Coast Guard, the American Petroleum Institute, and the American Bureau of Ships, for electrical equipment in explosive atmospheres.

The probes, which have no moving parts, are designed to develop electrical signals which are a linear function of the draft and transmit it to control unit 36. The electrical signals are produced by the change in the capacitance of each capacitive probe whose value changes as a function of the level of the liquid. The electrical signals are fed to one leg of a self-balancing bridge which cooperates with a servo mechanism to provide an indication of the ships draft.

In order to obtain four independent measurements from the probe, it is necessary to essentially set up four individual bridge circuits, as shown in FIG. 4, but power supply 76, bridge transformer 78, servo amplifier 80, and servo motor 82, are employed in common by each section of the probe. A feature of the invention is a rebalance potentiometer, of which there is one for each bridge circuit. Thus the design of this system provides circuits that perform four individual functions, although the number of components required for the additional three measurements is less than that required for three added separate systems.

In the self-balancing bridge system utilized in the present invention and depicted in FIG. 4, it will be seen that as an output is produced by the capacitive probe, the bridge adjusts itself so as to tend to reduce the error current prOCluced by the signal which has unbalanced the bridge. Thus, for example, as an output is produced from section 72 of probe 34, it is applied through switch arm 84, shielded lead 86, and arm 88, the bridge associated with that section becomes unbalanced which results in a current flow to amplifier 80. The amplified current actuates servomotor 82 which will then adjust potentiometer 92 in a direction to bring the bridge into balance. Servo motor 82 is connected mechanically to the pointer 98 of indicator 91 which is situated in console 38, so that as servomotor 82 is activated, dial arm 93 repositions, thereby furnishing a reading of the draft in the range of that section of the probe. While for the purposes of illustration, a standard pointer instrument is shown, digital counters or electronic-type displays may be used. Potentiometer 92 is a standard servo potentiometer driven by the servomotor 82 in tandem with potentiom- eters 90, 94 and 96, which are used in conjunction with sections 72, 74 and of the probe. The circuitry is so arranged that potentiometer 92 will balance over its entire range as the probe capacitance changes with variation of liquid level of four feet. The electrical length of winding of each of the four potentiometers is 270 degrees. Similar reference points, such as the start of each potentiometer winding, are positioned 270 degrees apart peripherally with respect to their common drive shaft 102. The drive shaft to which all the potentiometer arms are secured is capable of driving through 1080 degrees from a reduction gear train coupled to servo motor 82. An additional 1:4 reduction gear pair couples shaft 102 to the indicator. This method of gearing results in optimum system sensitivity.

Assuming the ship is partially loaded with the draft in the 32 feet-36 feet level with probe section 72 partially immersed and probe sections 74 and 75 fully immersed then, as shown in FIG. 4, switch 986 will be actuated by cam to illuminate signal lamp 112. The operator uses lever 107 to move switch arms 84 and 88 to select the proper circuit as indicated by the signal lamp. As the draft increases beyond the level of probe section 74, cam 100 will open switch 980 and close switch 98d to illuminate lamp 114. The operator would again use lever 107 to select the corresponding circuits. The legend of FIG. 4 indicates that position A of switch arms 84 and 88 is for draft under '28 feet. Positions B, C and D are for drafts of 28 feet-32 feet, 32 feet-36 feet and 36 feet-40 feet, respectively.

Zero adjust potentiometers 120, 122, 124 and 126 provide for adjustment of each circuit in the calibration of the unit. Variable span resistors 128, 130, 132 and 134 provide a means to adjust the unit to compensate for the different densities of water in the various seas and rivers in which the ship must operate. Thus, for example, the unit may be adjusted for readings taken in the North Atlantic in the winter, North Atlantic in the summer, fresh water, or seas heavily laden with salt, such as the Red or Black Seas.

It will -be noted that the description thus far presented accounts for only one probe, but since one probe is not sufficient to obtain complete readings, a plurality of such probes are positioned at suitable points in the ship. The output of each probe is connected to its own control unit, as described hereinabove, but the outputs of all the control units go directly to indicator console 38 from which the operator may obtain readings from all of the probes simultaneously.

As pointed out previously, it is necessary to obtain forward, aft, and plimsol readings to enable the operator to determine in which direction the ship may be listing or whether there is any hump or hog in the ship. By utilizing four probes, one forward, one aft, and two at the plimsol points, port and starboard, an immediate determination may be formulated by the operator which will indicate the direction of the ships list or whether a hump or hog has been formed because of uneven loading.

However, it is possible to determine the hump, hog, or list of a ship with only three probes, which are not all in the same line, by employing a computer which is described hereinbelow. Although the same result is obtained by using four probes, inmany installations the cost of the computer will be less than the cost of the additional probe installation.

In both systems, it is necessary to position the probes at certain points to obtain the required readings, but as explained previously it is usually impossible to do this because of prepositioned obstructions. Therefore, a method must be provided to obtain a true indication of the stem and stern readings where the measurements may actually be made 8 to feet from the stem or stern.

This is accomplished by taking the forward and aft draft measurements and then electronically drawing a straight line DD between the two measurements to thereby (a) derive the draft at the plimsol mark, and (b) compute the true draft at the stem and stern along an extension of this straight line. To illustrate this, referance is made to FIG. 5 wherein a section of the ship is shown with forward and aft probes located away from the stem and stern. The positions of the true stem and stem draft positions are spelled out in FIG. 5. The actual distance of the forward and aft probes from the plimsol mark are, respectively, A and B, with the dis tances of the stern and stern from the plimsol mark referred to as A and B, respectively.

The method of computation is as follows:

Two readings, L and L are taken by the forward and aft probes, respectively. These are multiplied by their effective lever arms A and B; the plimsol draft L may then be calculated. To find the true stern draft then, the plimsol is subtracted from the stern draft reading (L L and then multiplied by the ratio of the distance of the stern from the plimsol mark to the distance of the aft reading from the plimsol mark Bl (a)- To this is then added the plimsol reading.

It can be seen, therefore, that true stern draft the resultant equation. Similarly, the true stem draft is computed with the resultant equation, true stem draft A! L I (L. Li) A +L3 i The true plimsol reading may be found from the equation,

L L A+L B 3 A-t-B where L is the true plimsol. However, the calculated true plimsol reading would not be accurate if the ship was deformed by humping or hogging since the calculated reading is merely a weighted average of the readings taken by probes L and L Therefore, if it were desired to take into account the hump or hog of a ship, a probe would have to be positioned at the plimsol point and the direct reading of this probe inserted in the equations for the true stern and stem drafts in place of the calculated plimsol draft L Turning now to FIG. 6, there is shown a simplified schematic of a computer designed to automatically derive the true stem and stern drafts through the use of simple electronic adding and multiplying networks described hereinbelow The potentiometers 149, 151 and 145, are three-turn units connected directly to their respective shafts and servo measuring units, such as are described above and depicted in FIG. 4. A shaft of this type would correspond to shaft 102. The units 140, 142 and 144, represent the systems for each of the forward, aft and plimsol probes. The position of these potentiometers and their voltage outputs are, therefore, directly proportional to the forward, aft, and plimsol draft. Power supply 76 provides the necessary voltages for the unit through transformer 77.

A signal, dependent upon the position of arm a (which is controlled, in turn, by amplifier 144a and servo motor 14412) taken across a portion of potentiometer 145, is coupled through one-to-one transformer 14 7 to resistors and 166. The signal taken from potentiometer 145 is also applied to resistors 162 and 164, These signals therefore represent a function of the output of the plimsol probe. However, the signal applied to resistors 160 and 166 is negative with respect to the signal applied to resistors 162 and 164; thus the signals at these points represent, respectively, the minus L and plus L factors in the equations described hereinabove. The output of the forward probe is coupled through amplifier 140a and servo motor 14Gb which drives potentiometer arrn 149. The signal taken from potentiometer 149 is applied to resistors 186 and 158 and represents the factor L Similarly, the output of potentiometer 151, a function of the output of the aft probe coupled through amplifier 142a, servo motor 14217 and arm 151a, represents the value of L and is applied to resistors 156 and 184.

Potentiometers 168 and 170 represent the values B/B and A/A, respectively. These potentiometers are adjusted when the probes are positioned in the ship and the distances A, B, A and B are accurately determined at that time by conventional measuring instruments. The potentiometers may be adjusted for each ship to correspond to those distances enumerated above or on any single ship if it is found necessary to reposition the probes.

It can be seen that the signals L and L are added together and then multiplied by B'/B to be developed as across resistor 172. That signal is, in turn, added to +L giving the resultant B/ (L -Layne which is fed to amplifier 174. This amplifier along with amplifier 176 are used to convert the calculated A.C. signal to the DC. signal which drives the indicator meters 17 8 and 180, respectively, meter 17S giving the true stern draft and meter 180 the true stern draft.

7 Similarly L is added to L and multiplied by A/A and the resultant signal developed across resistor 182. This signal is, in turn, added to +L and the resultant signal fed to amplifier 176.

It is desired to know the exact draft at the stern, stern and plimsol mark on the ship even though it is not possible to make direct measurements at these points. In the practical sense it will generally be impossible to locate a probe where the desired measurement is required. For that reason, the method described hereinabove has been employed. Basically it allows the probes to be placed in the forward and aft parts of the ship at convenient locations. The measurements are made at these points and since they are known with relation to the center line of the ship, it is then possible to effectively draw a straight line through the two measurements and forecast the equivalent draft along this line at the stem, stern and plimsol points. It is, however, necessary to know exactly where the points are with relation not only to the center line of the ship, but also with relation to the keel of the ship. Care must be taken in the design and specification of the pipes in which the probes are mounted to make sure that the mounting flange on which the probe is secured is exactly as specified with relation to the keel,

Thus, it can be appreciated that a method is provided for automatically determining whether the stem or the stern rests deeper in the water and to what depth, simply by comparing the true stem and true stern readings. However, in order to determine whether the ship is listing to port or starboard and to what extent, an additional plimsol probe must be employed, as pointed out previously.

Although the various methods described up to this point will enable an operator to determine the forward or aft cant of the ship and its port or starboard list, there has not been provided a means to determine whether the ship has been deformed into a hump or hog condition and the extent of the aforesaid condition. A value for L without hump or hog condition is calculated. This value is then compared with the reading taken by the plimsol probe. If the plimsol probe reading is larger than the calculated plimsol reading, then it can be seen that the ship is in a hog condition and the diflerence indicates to What extent. Conversely, if the calculated plimsol reading is larger than the plimsol probe reading, then a hump condition is indicated.

The computer shown in FIG. 6 provides for estimating the hump or hog of the ship. Thus the calculated plimsol measurement is made by taking a weighted average of the output of potentiometers 149 and 151 through weighting resistors 184, 186, and 188, representing the values A, B and C, respectively, where C=A+B. The resultant signal is fed to amplifier 190 which converts the calculated A.C. signal to direct current which drives indicator 192. Thus, indicator 192 furnishes the operator with a calculated plimsol reading based on the readings taken at the forward and aft probes. The operator then merely compares this reading with the true reading given by the plimsol probe to determine the hump or hog as described hereinabove.

From the foregoing, it can be seen that a practical, convenient and accurate means and method have been provided in the present invention to determine the draft of a ship at the strategic positions necessary for properly loading and trimming the ship. In addition, it is possible, utilizing the present invention, to determine the hump or hog of the ship. It is therefore evident that this invention enables a ships operator to increase the profit of running the ship by providing a means to load the ship to its maximum indicated limit and to do so with maximum possible safety,

There has been disclosed heretofore the best embodiments of the invention presently contemplated and it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A draft gaging system for a sea borne ship comprising:

(a) a vertical well positioned aboard the ship, said well having means communicating with the surrounding sea whereby sea water floods said well to the mean level of the surrounding sea;

(b) a sensing probe having a plurality of isolated sections, said probe being mounted in said well for sensing the level of the sea water in said well;

(c) dis lay means connected to said sensing probe and responsive thereto for indicating the sea water level sensed by said probe, said display means including a self-balancing probe circuit connected to each section of each of said plurality of isolated probe sections;

((1) an amplifier to which all of said probe circuit outputs are connected;

(e) a servo motor electrically connected to and controlled by said amplifier, said servo motor being arranged to rebalance said probe circuits;

(f) cam means rotated by said servo motor;

(g) a plurality of switches actuated by said cam means;

and

(h) a plurality of annunciators each connected to a respective one of said switches whereby said annunciators are responsive to the condition of said switches.

2. A ship borne gaging system comprising:

(a) a plurality of vertical wells positioned aboard a ship, said wells having means communicating with the surrounding sea whereby sea water floods said wells to the mean level of the surrounding sea;

(b) capacitive probes mounted in said wells so that the capacitance thereof varies in response to the variation in level of the sea water; and

(c) display means selectively connected to said probes and responsive to the capacitance thereof for indicating the sea water levels sensed by said capacitive probes, said display means comprising:

(1) a self-balancing bridge circuit adapted to be sequentially connected to each succeeding section of said plurality of capacitive probes as the sea water level covers a preceding probe;

(2) a single amplifier to which the output of said bridge circuits are selectively connected;

(3) an indicator; and

(4) a servo motor electrically connected to and controlled by said amplifier, said servo motor being coupled to said indicator for actuation thereof.

3. The apparatus of claim 2 including cam means rotated by said servo motor, a plurality of switches actuated by said cam means to place them in a given operating condition and a plurality of annunciators, said annunciators being responsive to the condition of said switches.

4. The apparatus of claim 2 wherein said wells include stern and aft wells located away from the center line of the ship and compensating circuits in series with the said display means for modifying the effect of the capacitance of said probes so that said display means indicates true stern and aft levels.

9 10 5. The apparatus of claim 4 wherein said cornpensat- References Cited by the Examiner ing circuits include high and low limit adjustment means. UNITED STATES PATENTS 6. The apparatus of claim 4 including means to ad- 2,868,015 1/1959 Haropulos 73304 iiuesltltsisgiecl compensating circuits for sea water of different 5 2,942,467 6/1960 Campani 2,986,613 5/1961 Figueira 73304 X 7. The apparatus of claim 2 wherein said indicating 3 010 320 11/1961 sonectito means include a circular scale divided int0 a plurality 3,128,375 4/1965 Grimnes 73 65 X of angular segments, each said segment comprising a scale indicative of the draft range of each of the plurality of FOREIGN PATENTS capacitive probe sections comprising a said capacitive 10 1,021,744 12/1957 Germany. probe.

8. The apparatus of claim 7 including means for in- LOUIS PRINCE Pnmary Emmmer' dicating the said segment to be read. S. CLEMENT SWISHER, Assistant Examiner.

Claims (1)

1. A DRAFT GAGING SYSTEM FOR A SEA BORNE SHIP COMPRISING: (A) A VERTICAL WELL POSITIONED ABOARD THE SHIP, SAID WELL HAVING MEANS COMMUNICATING WITH THE SURROUNDING SEA WHEREBY SEA WATER FLOODS SAID WELL TO THE MEAN LEVEL OF THE SURROUNDING SEA; (B) A SENSING PROBE HAVING A PLURALITY OF ISOLATED SECTIONS, SAID PROBE BEING MOUNTED IN SAID WELL FOR SENSING THE LEVEL OF THE SEA WATER IN SAID WELL; (C) DISPLAY MEANS CONNECTED TO SAID SENSING PROBE AND RESPONSIVE THERETO FOR INDICATING THE SEA WATER LEVEL SENSED BY SAID PROBE, SAID DISPLAY MEANS INCLUDING A SELF-BALANCING PROBE CIRCUIT CONNECTED TO EACH SECTION OF SAID OF SAID PLURALITY OF ISOLATED PROBE SECTIONS;

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