GB1563797A

GB1563797A - Instruments for ships

Info

Publication number: GB1563797A
Application number: GB3587975A
Authority: GB
Original assignee: University of Glasgow
Current assignee: University of Glasgow
Priority date: 1975-09-01
Filing date: 1975-09-01
Publication date: 1980-04-02
Also published as: JPS5811007B2; IT1066438B; NL7609726A; DE2639192B2; JPS52109965A; DE2639192A1; DE2639192C3; FR2322048A1; FR2322048B1

Description

(54) IMPROVEMENTS RELATING TO NAVIGATIONAL INSTRUMENTS FOR SHIPS (71) We, THE UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW, a British corporate body, of Glasgow, W2, Scotland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to navigational instruments for ships.

It has been established that when a ship is in motion there is a change of pressure conditions below the keel of the ship which results in the vessel sinking further into the water and thus having an actual draught greater than its known static draught. This effect called for convenience "squat" occurs in all depths of water but is more pronounced in shallow water. In a large ship such as a tanker travelling at its normal operating speed the reduction in distance from the keel of the ship to the sea bed can be considerable and can be enough to cause to be dangerous a channel which would normally be considered safe enough for passage of such a ship. In large ships depressions of as much as two or three metres have been measured.

Of the variety of circumstances where the conditions described above apply the following three are most common: (a) A deep draught merchant ship (particularly a high block coefficient tanker) entering a shallow water area in a fully laden, level draught condition.

In these circumstances squat can cause an increase in forward draught.

(b) A tanker in ballast condition (trimmed by the stern) entering a shallow water area.

In this case, the ship will react by sinking deeper and increasing trim by the stern, putting the vulnerable propeller and rudder at risk of touching bottom. This reaction may be the opposite of that anticipated by the master, who may, understandably, have expected the change in trim to be by the head as in the level draught condition.

(c) High speed ships (warships or fast merchant ships) trim progressively deeper by the stren with increasing speed, normally velocity > 16 knots and or, decreasing depth of water.

It is not enough to use echo-sounding techniques to find the depth of water below the keel because this only provides an indication of the instantaneous depth. In other words before a dangerous situation is reported the ship is already in the dangerous situation. What is required is some method of determining the amount of depression of the vessel so that from the known depth of water ahead of the ship the onset of dangerous conditions can be predicted.

The inventor of the present invention has already invented a method and apparatus for making such a prediction and this method and apparatus which are the subject of the prior patent no. 1 264 516 have been found to be effective in use. The apparatus of the previous invention consists, however, in extra apparatus which must be fitted to a ship's hull and it would obviously be of advantage if existing hull equipment normally available on a wellequipped ship could be arranged to provide enough information about the conditions prevailing at the bottom of the ship to make it possible to ascertain the true draught of the ship without the necessity of installing additional hull equipment.

Many ships are fitted and are being fitted with hydrostatic pressure sensitive devices located at the bow and at the stern at or near the keel and sometimes also at intermediate positions. The object of such pressure sensitive devices is to use the hydrostatic pressure of the water at the keel to provide an indication of the draught of the ship at important points in the ship's hull.

While such pressure sensitive devices are accurate in providing an indication of the draught when the vessel is at rest nevertheless when a vessel fitted with the device is in motion a false reading of the draught of the vessel is given for the reasons of change in pressure conditions below the vessel referred to above and since it is usually a reduction of pressure which takes place below the keel the device usually indicates a draught which is less than the true draught.

The false sense of security provided may thus cause a ship's master to enter at high speed a shallow channel in the belief that there will be more water under his keel than there will in fact be. His echo-sounder is of no help here because he will only receive the indication of the dangerously small clearance below his keel when he is actually in the dangerous situation.

The present applicants have now found in full scale trials they have conducted that for any particular ship, a particular change in the draught at any specific point on the ship's hull resulting from movement of the ship is accompanied by a change in the water pressure at that point which is unique to that change in draught as a result of widely differing combustions of speed of ship and depth of water. For example, in one trial the same change of pressure of -6.0 N/ma x 103 from the pressure prevailing when the ship was at rest occured at a bow tapping when the ship was travelling at a speed of 13 knots in water depth 1000m and at a speed of 10 knots in water depth 30m. In both cases the resulting bow depression was 1.0m.It thus becomes possible to obtain an indication of the true draught of a ship in motion without requiring readings of distance from the bottom of the ship to the sea bottom.

According to the invention a navigational instrument for a ship incorporates at least one pressure sensitive device arranged when in use to be located at a particular point on a ship's hull below the unladen water line and capable of detecting the pressure of the water at said particular point and of issuing a pressure signal representative of said sensed pressure and a signal processing device having a data storage section containing stored therein data relating to a range of hydrostatic pressures measured at corresponding points in a moving ship comparable in size and type with that with which the instrument is to be used and the true draughts corresponding with these pressures as empirically measured and also any correction factors previously found as being required to adapt said stored data to the particular ship with which the instrument is to be used, said signal processing device being connected to the pressure sensitive device to receive the pressure signal therefrom and being operative to compare the stored data with each received pressure signal, apply any relevant stored correction factors, make a computation of the true draught or change of draught of the ship at said point and issue a signal representative of said true draught or change of draught.

Where the pressure sensitive device is to be located at a point on a ship's hull other than on the bottom the signal processing device is arranged to make an appropriate correction for the amount of displacement of the pressure sensitive device from any chosen point on the ship's bottom at which the amount or change of draught is required.

The instrument may incorporate a trim measuring device arranged to measure fore and aft trim and issue a signal representing the angle of trim of the ship, the signal processing device being connected to the trim measuring device so as to receive the trim signal therefrom and being arranged to issue at least one signal corrected to contain information of the actual draught of the ship at a point on the hull remote from the pressure sensitive device, e.g. the bow or the stern.

The navigational instrument may include means arranged to measure the distance from the ship to the sides of a channel in which the ship may be sailing and to issue corresponding signals representing such distance. It has been found that narrow channels also provide an effect on the draught of a ship and tend to increase the draught. The signal processing device may also be programmed to contain data relating to channel widths and to receive said signals and take the data conveyed by them into consideration when computing the draught of the ship.

The signal processing device may be fitted with visual indicating means e.g. in the form of light emitting columns movable against scales of draught. As an alternative form of trim measuring device the signal processor may be arranged to receive the signals from several pressure sensitive devices located at spaced points on the ship's hull and provide separate indications of the draught at these different points.

The signal processing device may in addition to or instead of visual indicating means contain recording means recording the draught of the ship on a chart which may be a time chart or a digital read out.

In a ship fitted with an automatic pilot device programmed to follow a predetermined course and working in conjunction with an inertial or like navigational device capable of determining the exact instantaneous position of the ship the signal processing device may be linked with these devices in such a way that a warning may be given audibly or otherwise of the approach of dangerous conditions.

The accompanying drawings serve to illustrate in a diagrammatic fashion the operation of the invention. Figs. 1, 2 and 3 are graphs showing the depresion or squat at the bow and at the stern of particular ships in various operating conditions. Fig. 1 shows the conditions actually observed in a tanker of 190,000 tonnes sailing while fully loaded at a speed of 16 knots. At rest this ship fully loaded had a level draught of 18.lem. Fig. 2 shows the conditions prevailing the same tanker sailing at the same speed but in ballast, the mean draught at rest being 9.2m, trimmed 2.8m down by the stem. Fig. 3 shows the conditions found to prevail in a tanker of 200,000 tonnes under particular loading conditions while sailing at different speeds in water of a depth of 25.6m.The draguht of this ship at rest was 18. 14m level giving a ratio of water depth to draught of 1.41. Fig. 4 shows diagrammatically the position assumed by a ship at rest in motion and the related apparatus of the invention. In the graphs of Figs. 1 and 2 the ordinate represents percentage depression while the abscissa is the ratio of the depth of water to the draught of the ship. In Fig. 3 the ordinate indicates the depression in metres and the abscissa the speed in knots. The curves in full lines are the bow depression and in interrupted lines the stern depression.

In Fig. 4, 1 denotes a ship at rest and 2 denotes the hull position when the ship is moving. 3 denotes the sea bed, 4, 5 and 6 denote hydrostatic pressure sensitive devices connected to a signal processing device 7 having a data storage section containing stored therein data relating to a range of hydrostatic pressures measured at corresponding points in a moving ship comparable in size and type with that with which the instrument is to be used and the true draughts corresponding with these pressures as empirically measured and also any correction factors previously found as being required to adapt said stored data to the ship 1.The signal processing device 7 is operative to compare the stored data with each received pressure signal from the pressure sensitive devices 4, 5 and 6, apply any relevant stored correction factors, make a computation of the true draught or change of draught of the ship at the points corresponding with the pressure sensitive devices 4, 5 and 6 and issue a signal to visual indicating means 8, 9, 10, 11 which indicate the true draught at the stern, at a midship port and straboard points and at the bow respectively. 12 denotes a trim measuring device connected to the signal processing device 7.

In practice, in a ship in motion the signals issued by the pressure sensitive devices 4, 5 and 6 which are different from those issued by the pressure sensitive devices when the ship is stationary are compared by the signal processing device 7 with data stored in the signal processing device and a corresponding draught figure is selected by the signal processing device from the stored data. After any necesary correction factor is incorporated e.g. correction for displacement of the pressure sensitive devices from the point on the hull of the ship at which the draught is to be measured and/or trim angle, a signal representing true draught is produced.The true draught given as a visual signal on the visual indicating device 8 enables the navigating officer to ascertain from the course being followed when dangerous conditions are approaching so that he can act to reduce the speed of the ship in time to reduce the depressing effect on the ship and reduce the draught. Where the ship is fitted with automatic pilotage and navigational devices the system may be arranged to give an audible warning a predetermined distance in advance of reaching the position where danger could arise.

WHAT WE CLAIM IS:- 1. A navigational instrument for a ship said instrument incorporating at least one pressure sensitive device arranged when in use to be located at a particular point on a ship's hull below the unladen water line and capable of detecting the pressure of the water at said particular point and of issuing a pressure signal representative of said sensed pressure and a signal processing device having a data storage section containing stored therein data relating to a range of hydrostatic pressure measured at corresponding points in a moving ship comparable in size and type with that with which the instrument is to be used and the true draughts corresponding with these pressures as empirically measured and also any correction factors previously found as being required to adapt said stored data to the particular ship with which the instrument is to be used, said signal processing device being connected to the pressure sensitive device to receive the pressure signal therefrom and being operative to compare the stored data with each received pressure signal, apply any relevant stored correction factors, make a computation of the true draught or change of draught of the ship at said point and issue a signal representative of said true draught or change of draught.

2. A navigational instrument as claimed in claim 1 in which the signal processing device is set to make an appropriate correc tion for the situation where the pressure sensitive device is to be located at a point

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. devices in such a way that a warning may be given audibly or otherwise of the approach of dangerous conditions. The accompanying drawings serve to illustrate in a diagrammatic fashion the operation of the invention. Figs. 1, 2 and 3 are graphs showing the depresion or squat at the bow and at the stern of particular ships in various operating conditions. Fig. 1 shows the conditions actually observed in a tanker of 190,000 tonnes sailing while fully loaded at a speed of 16 knots. At rest this ship fully loaded had a level draught of 18.lem. Fig. 2 shows the conditions prevailing the same tanker sailing at the same speed but in ballast, the mean draught at rest being 9.2m, trimmed 2.8m down by the stem. Fig. 3 shows the conditions found to prevail in a tanker of 200,000 tonnes under particular loading conditions while sailing at different speeds in water of a depth of 25.6m.The draguht of this ship at rest was 18. 14m level giving a ratio of water depth to draught of 1.41. Fig. 4 shows diagrammatically the position assumed by a ship at rest in motion and the related apparatus of the invention. In the graphs of Figs. 1 and 2 the ordinate represents percentage depression while the abscissa is the ratio of the depth of water to the draught of the ship. In Fig. 3 the ordinate indicates the depression in metres and the abscissa the speed in knots. The curves in full lines are the bow depression and in interrupted lines the stern depression. In Fig. 4, 1 denotes a ship at rest and 2 denotes the hull position when the ship is moving. 3 denotes the sea bed, 4, 5 and 6 denote hydrostatic pressure sensitive devices connected to a signal processing device 7 having a data storage section containing stored therein data relating to a range of hydrostatic pressures measured at corresponding points in a moving ship comparable in size and type with that with which the instrument is to be used and the true draughts corresponding with these pressures as empirically measured and also any correction factors previously found as being required to adapt said stored data to the ship 1.The signal processing device 7 is operative to compare the stored data with each received pressure signal from the pressure sensitive devices 4, 5 and 6, apply any relevant stored correction factors, make a computation of the true draught or change of draught of the ship at the points corresponding with the pressure sensitive devices 4, 5 and 6 and issue a signal to visual indicating means 8, 9, 10, 11 which indicate the true draught at the stern, at a midship port and straboard points and at the bow respectively. 12 denotes a trim measuring device connected to the signal processing device 7. In practice, in a ship in motion the signals issued by the pressure sensitive devices 4, 5 and 6 which are different from those issued by the pressure sensitive devices when the ship is stationary are compared by the signal processing device 7 with data stored in the signal processing device and a corresponding draught figure is selected by the signal processing device from the stored data. After any necesary correction factor is incorporated e.g. correction for displacement of the pressure sensitive devices from the point on the hull of the ship at which the draught is to be measured and/or trim angle, a signal representing true draught is produced.The true draught given as a visual signal on the visual indicating device 8 enables the navigating officer to ascertain from the course being followed when dangerous conditions are approaching so that he can act to reduce the speed of the ship in time to reduce the depressing effect on the ship and reduce the draught. Where the ship is fitted with automatic pilotage and navigational devices the system may be arranged to give an audible warning a predetermined distance in advance of reaching the position where danger could arise. WHAT WE CLAIM IS:-

1. A navigational instrument for a ship said instrument incorporating at least one pressure sensitive device arranged when in use to be located at a particular point on a ship's hull below the unladen water line and capable of detecting the pressure of the water at said particular point and of issuing a pressure signal representative of said sensed pressure and a signal processing device having a data storage section containing stored therein data relating to a range of hydrostatic pressure measured at corresponding points in a moving ship comparable in size and type with that with which the instrument is to be used and the true draughts corresponding with these pressures as empirically measured and also any correction factors previously found as being required to adapt said stored data to the particular ship with which the instrument is to be used, said signal processing device being connected to the pressure sensitive device to receive the pressure signal therefrom and being operative to compare the stored data with each received pressure signal, apply any relevant stored correction factors, make a computation of the true draught or change of draught of the ship at said point and issue a signal representative of said true draught or change of draught.

on the ship's hull displaced from that at which the amount or change of draught is to be measured.

3. A navigational instrument as claimed in claim 1 in which the instrument incorporates a trim measuring device arranged to measure fore and aft trim of a ship and issue a signal representing the angle of trim of the ship, the signal processing device being connected to the trim measuring device so as to receive the signal therefrom and being set to issue at least one signal corrected to contain information of the actual draught of a ship at a chosen point on the hull remote from the pressure sensitive device.

4. A navigational instrument as claimed in claim 1 including means capable of measuring the distance from a ship fitted with the instrument to the sides of a channel in which the ship may be sailing and issuing signals representative of said distance to the signal processing device which is additionally programmed with data relating to channel widths, the signal processing device being set to receive said signals and take the data conveyed by them into consideration when computing the draught of the ship.

5. A navigational instrument as claimed in claim 1 in which the signal processing device incorporates visual indicating means connected to the signal processing device and arranged to receive the output signal therefrom and provide a visual indication of the true draught of the ship.

6. A navigational instrument substantially as described with refrence to Fig. 4 of the accompanying drawings.