When do we turn? That is the question of vital importance to the commanding officer or unit commander when executing a maneuver, at the completion of which, the ship or ships will be in their proper position in relation to the guide.
Many valuable papers have appeared in the Proceedings, which give the various operations on the mooring board and explain in detail how to obtain your course and speed to go from “Where you are” to “Where you want to go.”
I have seen only one paper, however, which deals with the most important part of the whole maneuver, “When do we turn?” In the paper “Hints on Tactical Maneuvers,” by Lieutenant Commander (now Commander) Jules James, U. S. Navy, there is included a table giving the “Degrees of Bearing Loss in Turning.” This table is only approximately correct and requires considerable interpolation. In fact, it requires plotting, in order to know your distance from the guide when you execute the turn.
Take, for example, the case of a ship which is directed to change distance from the guide and retain the original bearing. Under this condition, there are no degrees of bearing lost in turning.
A ship is on the starboard bow of the guide, distant 10,000 yards and is directed to take station on the starboard bow of the guide, distant 5,000 yards, using same speed as the guide. Laying this problem out on the mooring board you find that the ship turns to the left ninety degrees and that the relative movement line is the line joining the present position of the ship and guide. There is a loss of distance (advance) but no loss of bearing on the turn up to base course, though the turn was one of ninety degrees.
With the use of the universal drafting machine, all maneuvering problems can be placed quickly and accurately upon the mooring board. In addition, the operator has before him at all times a visible record of his previous work. The use of parallel rulers is not as satisfactory nor as rapid as that of the drafting machine. The use of the mooring board is considered superior to all other apparatus such as the Battenburg indicator, etc.
Practically all maneuvering of capital ships is done with a rudder angle such that it gives the ship a tactical diameter of 1,000 yards and the following discussion assumes that such tactical diameter is used, though the data can be easily converted for use with any other tactical diameter.
A study of the tactical data of capital ships will show that the following holds true: When the signal to turn is executed, the ship continues on base course for a certain distance and then starts to turn. After a short time, the ship is turning in a nearly perfect circle whose diameter is 1,000 yards. To all intents and purposes, it may be assumed that the ship continues on the base course for a certain distance and then immediately starts turning in a perfect circle of 1,000 yards diameter, base course being tangent to the circle.
At speeds of about fifteen knots, a capital ship continues on base course for about 300 yards before turning. At lower speeds, the amount of “skate” is less and at higher speeds more than this amount. At ten knots it amounts to about 225 yards.
Let the heavy circle of Fig. 1 represent the turning circle of a ship with a tactical diameter of 1,000 yards and assume that the ship, after the completion of a turn, is at point A on course of 270 degrees. The figure shows the tactical data at 15-degree intervals for turns to the left from 15 degrees to 150 degrees. Take the case of a turn to the left of 60 degrees. The ship is on base course 330 degrees (60 degrees to right of final course). At what position should the ship execute the 60 degree turn in order to arrive at point A and straighten out on the new course 270 degrees?
Assuming the speed to be fifteen knots, the ship continues on the original course (330 degrees) for 300 yards when it hits the turning circle and then follows the circumference of the circle around for 60 degrees to point A.
At a point on the circumference of the turning circle, 60 degrees to the right of point A, draw the tangent (150 degrees, which is the reverse of original course) and lay off a distance of 300 yards. This locates point 4, the position at which to execute the turn in order to finally arrive at A. In a similar manner locate point 1 for 15 degrees, point 2 for 30 degrees, etc. The perpendicular distance from points I, 2, 3, etc., to the line “track of guide” is the distance from the track of the guide that a vessel must be when the signal to turn is executed in order to end up on the track at completion of a turn.
These distances are given in Table I:
Table i
Degrees Turn | Distance from Track of Guide. (yards) |
15 | 95 |
30 | 217 |
45 | 359 |
60 | 5io |
75 | 679 |
90 | 800 |
I OS | 919 |
120 | 1010 |
135 | 1066 |
150 | 1083 |
An inspection of this table shows that without sensible error, the following “thumb” rule holds good for turns of any amount between 25 degrees and 100 degrees.
The distance in yards from the track of the guide at which to execute the turn signal is found by multiplying the number of degrees of turn by 10 and subtracting 100. For example, a turn of 45 degrees: 45X 10—100=350 yards, distance from track of guide. For turns of less than 25 degrees or more than 100 degrees we get by plotting the results from Table 1 as follows:
20? | = | 150 yards | 110? | = | 950 yards |
15? | = | 100 yards | 120? | = | 1,000 yards |
10? | = | 60 yards | 130? | = | 1,050 yards |
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| 140? | = | 1075 yards |
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| 150? | = | 1,100 |
These latter are seldom used but can be committed to memory in a few minutes, and, together with the rule given above, cover all cases of turning into the track of the guide, and if applied as explained later on, will insure turning into the track of the guide and prevent many “poorly done” signals.
To illustrate the use of the foregoing. Assume the guide to be on course 330 degrees, speed twelve, and that a ship which is 4,000 yards on the starboard beam of the guide is directed to take station 3,700 yards astern of the guide at a speed of twelve knots. What do?
Fig. 2 shows the mooring board solution. Locate the guide at the center. Then locate “Where you are” at A, 4,000 yards on the starboard beam of the guide. Each division representing 1,000 yards. Next locate B, “where you want to go,” 3,700 yards astern of the guide. Point B is on the track of the guide. Join AB, this is the line of relative movement. Lay off your speed triangle, using half scale; OC represents the course and speed of the guide. Through C draw a line CD parallel to AB, and where it intersects the speed circle 6(12 knots to half scale) draw the line DO. This represents the course and speed to proceed to the new position. Course 245 degrees, speed twelve. The ship must turn 330?—245? = 85? to the left. This is also shown by the angle COD. The line BOC is the track of the guide.
To the same scale that OC and OD represent the speeds of the guide and ship, CD represents the speed along the relative movement line. The length of CD is 8.1 divisions and as half scale was used this represents a speed of 16.2 knots or 540 yards per minute along the line AB.
If the maneuvering ship was at A when the turn was executed, then, after the turn was completed and the ship steadied on the new course, she should plot on or very near the line AB. (Note: Mooring board sheets are not absolutely accurate but are frequently distorted as much as one tenth of a division, 100 yards error where the scale is one division equals 1,000 yards. In addition there will be variations in range finder readings, and there is always a lag in the range finder report while the bearing report is instantaneous. The mooring board operator has to make due allowance for the lag in range finder readings, which may amount to as much as two or three degrees in bearing if the bearing is changing rapidly.)
In any case, as long as both ships maintain their course and speed, the relative movement line will be AB or a line parallel to AB.
Next locate the point at which to execute the turn back to base course. The amount of turn is 85 degrees and from the previous rule we have 85X10—100=750 yards as the distance from the track of the guide at which to turn. Lay off a line aa parallel to the track of the guide, BOC, and distant from it 750 yards on the side of approach. Where line aa intersects the relative movement line, AB, is the point to execute the turn. That is when the guide bears 316° and distant 3,100 yards. Where the bearing is changing rapidly as in this case, the turn should be made on bearing instead of range.
When the first turn is completed and the ship’s position plotted, if off the line, AB, it may require a further change of course, speed, or both, to reach point B.
Suppose that after the first turn was made and the ship steadied on the new course, you get a plot as follows: Bearing of guide 260 degrees, range 2,500 yards. What do?
Plot the ship’s position b. Join bB, the desired line of relative movement. Through C draw Cc parallel to bB. The intersection of Cc with the speed circle six gives the new course as 229. Therefore you have to change course to the left 16 degrees further. You would probably use 230 degrees as new course and as that requires a turn of 100 degrees back to base course, the line aa must be relocated 900 yards from the track of the guide to give your new turning point.
It frequently happens during maneuvers that you are unable to obtain the range, bearing or both at the critical time. Therefore the operator should always keep a check on the time. After the ship has completed the first turn and steadied on the new course, get an accurate plot and note exact time of plot on the mooring board. Measure the distance along the relative movement line from this “spot” to the point where you are to execute the next maneuver. Then, knowing your speed along the relative movement line, you easily find the time to run from your spot until you execute your next turn.
Suppose that in the example in Fig. 2, you get a plot as follows: Bearing of guide 260 degrees, range 3,100 yards, watch time 9h 14m 35s. At what time do you turn? Locate the spot X. Distance from there to the intersection of AB and aa (the turning point) is 2,900 yards. Relative speed was found above to be 540 yards per minute. It will therefore require 2900÷540=5.37 minutes or 5 minutes 22 seconds, and the watch time of executing the turn is 9h 19m 57s.
Fig. 3 shows a section of the mooring and maneuvering board which is useful for turns of 90 degrees. The left half of the figure is used when the course is changed to the right and the right half of the figure for a change of course to the left. Speeds of guide and maneuvering ship are the same. When the courses of the two ships differ by 90 degrees and speeds are equal, the relative movement line makes an angle of 45 degrees with the course of the plotting vessel. (Note: Where the guide and plotting ship maintain the same speed, the relative movement line makes an angle with the course of the guide equal to 90? + ?/2; to the right of the guide’s course, if the course of the plotting ship is to the right of that of the guide and to the left of the guide’s course if the course of the plotting vessel is to the left of that of the guide. ? is the angle in degrees between the courses of the two ships.)
The diagonal dotted lines shown in the figure are relative movement lines and several have been drawn in each quadrant as reference or guide lines.
This chart will be found very useful to the officer-of-the-deck when executing 90 degree column movements as it insures turning into the track of the guide, no matter whether the ship ahead turns inside or outside.
The guide or ship plotted as guide should be more than 1,200 yards ahead of the plotting ship. At 1,200 yards distance the guide will have practically completed the turn and steadied on the new course by the time the plotting ship arrives at the position to execute the turn.
The guide is located at the middle of the “track of the guide” line and headed to the right for a right turn and to the left for a left turn.
Using a scale of 1 division equals 500 yards gives a convenient size chart and is the scale shown in Fig. 3.
As the turns are 90 degrees in all cases the place to execute the turn is 90X10—100=800 yards from the track of the guide. This is shown by the line TT.
Relative bearings are used as they hold good whatever the true course may be. This assumes that the ship plotting is on her correct course. If off the course, the actual bearing must be adjusted by the amount off the correct course.
The line separating the two quadrants represents the original course. The guide and plotting ship maintain equal speeds (except for loss of speed in turning) throughout the maneuver, and no changes of speed are contemplated.
The use of the chart is illustrated in the following examples. The ship used as guide may be any ship that has completed the turn, but preferably should be the guide of one of the divisions ahead.
Executing a column right go degree maneuver, you obtain the bearing and distance of a ship which has completed the turn. Bearing 23 degrees on starboard bow; distance 1,250 yards. How should above ship bear when you execute the order to turn in order to turn up in her track?
Locate point A, relative bearing 23 degrees, distance 1,250 yards from the guide. Draw the relative movement line parallel to the reference lines. At the point a, where this intersects the turning line TT, execute the turn. The guide then bears 48 degrees on the starboard how, distant about 1,200 yards. You will end the turn on the track of the guide and about 1,650 yards astern.
Executing a column left 90 degree maneuver. Relative bearing of guide 315 degrees (45 degrees on the port bow) distant 2,700 yards. Same data required as above.
Plot your position at B, guide bearing 315 degrees relative, distance 2,700 yards. Draw the relative movement line Bb. Point b where this line intersects the turning line TT is the point to turn. Guide bears 285 degrees relative (15 degrees forward of port beam) distance 3,150 yards. You will end the turn on the track of the guide and about 3,800 yards astern.
Two-Course Maneuver Problems
A two-course maneuver is used where to perform the maneuver with a single change of course requires a change of more than 120 degrees. In other words, maneuvering to lose distance to the van and where the new position is to be approximately astern of the old position.
The method is illustrated in the following examples and shown in Fig. 4.
Course of guide is north, speed twelve knots. A ship on the starboard beam of the guide, distant 4,000 yards, is directed to take station 30 degrees abaft the starboard beam of the guide, distant 4,000 yards. What do?
The first decision to make is what courses shall you steer, first turning away from the guide and after running for a certain distance turning and running towards the guide, finally turning up in order to be steady on the base course and the ship in the required position.
Plot your present position A, guide bearing 270 degrees, distance 4,000 yards. Next plot where you are to go, B, guide bearing 300 degrees, distance 4,000 yards.
Suppose you decide to head out at 45 degrees to right of present course and then to turn 90 degrees left and head back in at 45 degrees to the left of present course. The important decision to make is what course you shall steer as you return.
Lay off your speed triangles. Guide’s course 0 degrees, your course out 45 degrees, in 315 degrees, speed twelve knots (6 on half scale). Lay off the returning relative movement line BN parallel to CD. Your object is to execute the turn at such point that, when the turn is completed and you are steadied on the new course 315 degrees, your position will be somewhere on the line BN. Except for time to complete the maneuver, you do not care where you are just so you are on the line. Of course you must be far enough to the right of B that you will not overrun on your turn up on to the base course.
A ship executing a two-course maneuver of out 45 degrees, in 45 degrees, will lose over 550 yards distance if she executes each turn as soon as she completes the previous one. Therefore do not use a 45-45 maneuver unless your new position is to be well over 600 yards astern of present position. Similarly a 30-30 maneuver should not be used unless you have over 400 yards to lose.
Next lay off the going out relative movement line AN parallel to CD.
Previous articles on this subject state that the time to execute the turn is when you reach the point of intersection N of the two relative movement lines. This would be correct, provided the ship turned instantaneously to the new course and without loss of speed.
If the turn were not executed until arrival at point N, at the completion of the turn, the ship would be considerable distance beyond BN but would be proceeding along a relative movement line parallel to BN such as the line yy and hence would be far behind her proper position when she turned up on to the track of the guide.
Strictly speaking the track of the guide is the line indicating the course of the guide and passing through the position of the guide at the center of the mooring board.
In this article the meaning is broadened to any line parallel to the course of the guide and passing through the point “Where you want to go.” At this latter point, the relative movement of the ship and guide is zero.
Line BN is, therefore, not a track of the guide line but a relative movement line.
For turning from one relative movement line to another, I have found the following “thumb” rule to be correct for capital ships:
Multiply the number of degrees of turn by six to give the distance in yards from
the second relative movement line at which to execute the turn.
Applying this rule to the above problem, we get as follows:
Number of degrees turn =90 (first course 45 degrees, second course 315 degrees) 90X6=540 yards. Lay off line xx parallel to, and 540 yards on the side of approach from the second relative movement line BN. At the point where xx intersects the first relative movement line AN, execute the 90 degree turn. At the point of intersection of these lines, the guide bears 276½ degrees, distance 5,430 yards.
Through B draw a line parallel to the guide’s course. This line is the track of the guide. As you have to turn 45 degrees back to the base course, lay off your turning line 45X10—100=350 yards from the track of the guide, and execute your turn back to the base course when the ship arrives at the point of intersection of the turning line and BN. Bearing of guide 296 degrees, distance 4,250 yards.
Take the case where the ship does not execute the turn until arrival at point N. After completion of the turn she will be somewhere on line yy parallel to, but 540 yards beyond the line BN where you wanted to be.
Suppose you get a plot as follows: Bearing of guide 288 degrees, distance 5,800 yards. What would you do? Point f is the plot. Join fB to give your desired relative movement line. From C, on your speed triangle, draw CF parallel to fB.
Line CF shows that with equal speeds, to reach B, the ship should steer a course 350 degrees, only 10 degrees to the left of the base course. This would require a long time to complete the maneuver.
You have steam for full speed, 13½ knots. What do? Your desired relative movement line is parallel to CF of the speed triangle. The intersection F of this line with the 13½ knot speed circle gives you the new course, 326 degrees. Therefore you would take up full speed and change course 11 degrees to the right. Do not forget to relocate your turning up line.
You may find on subsequent plotting that you are going to be astern of B. In this case, execute the turn at proper distance from the track of the guide and retain full speed until you regain position.
Excess Speed
Capital ships will “coast” about 100 yards for each drop of one knot in speed, so due allowance must be made for this “coast.” As you have 1 knots excess speed over the guide, resume standard speed when you are 150 yards astern of your position B.
In case you are using excess speed, and your plot shows that you are going to turn into the track of the guide ahead of your desired position, proceed as follows:
From your plotted position draw the relative movement line, then from B, lay off the relative movement line, for equal speeds. Resume the speed of the guide before your plot shows you at the intersection of the “excess speed,” “equal speed” relative movement lines. The proper distance from the equal speed line to resume guide’s speed depends on your amount of excess speed and as given above is 100 yards for each knot excess speed.