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1. Problems of Attack.
Examination of the subject of cruisers and destroyers in attack on the enemy battle line is undertaken briefly, and with more facility after having discussed elements included in the possible defense against such an attack. Problems of attack that appear to require discussion are: When should cruisers and destroyers attack? What should be their line of approach? What formations should they use? What should be action of the covering forces?
2. The Time to Attack.
The timeliness of a destroyer attack on the enemy battle line in a general engagement is an element of prime importance. Effective coordination of a destroyer attack with the action of the battle line will probably be more important, from a viewpoint of time, than an isolated attack; for in a general engagement, torpedoes hitting when the battle line guns are also hitting will give greater accumulative momentum -- which has been described as the essence of concentration -- than a similar or somewhat greater number hitting at some other time. If the time of attack is taken as a matter of proper coordination with the battle line, the officer in most cases best fitted to determine this time is the O. T. C.
The O. T. C., in providing for the time of attack, will be influenced by the type of action he intends to fight, by the necessity of adequate battle line support for his attacking destroyers, and by the relative strength of his battle line and his cruisers and destroyers. If his battle line is superior to that of the enemy, the role of the cruisers and destroyers may be defensive primarily, in order that they may insure, so far as possible, freedom of action of the superior battle line. If his battle line is inferior, or the enemy's light forces are inferior, the O. T. C. may choose an offensive role for his cruisers and destroyers. In the former circumstance, the O. T. C. may reserve for his volition the time of the destroyer attack. In the latter case, he may direct the commander of the cruisers and destroyers to attack when adequate battle line support is seen to exist or a favorable opportunity arises. If the time of attack is left to the commander of destroyers, this officer should be influenced by the same considerations that would influence the O. T. C., ie, how best to coordinate the destroyer attack with the battle line.
In any case, the time of the attack in a general action will always be determined properly by the relations existing between the battle line and the destroyers, rather than by conditions within the destroyers themselves. For this reason, ideal approach directions, ideal firing positions, ideal destroyer formations and other theoretically best conditions for the destroyers themselves must give way to the needs of coordination with the battle line. When the order to attack arrives, the commander of destroyers must attack at once from wherever he may be.
2. The Line of Approach.
(a) Course. To attack the enemy battle line implies getting to torpedo effective range as soon as possible. It has been developed that probably for practical purposes the quickest way for cruisers and destroyers to reach a position within effective torpedo range from any given point is to close the target on a collision course. This applies equally for closing to the shortest practicable firing range, or to any firing range within effective range. The disadvantage of a collision course, in the latter case, lies in the fact that it perpetuates an initially poor target angle; but this, as a rule, cannot be improved without revering to courses that may prevent closing to the shortest possible firing range, or may increase the time under fire to a prohibitive degree. Theoretically, as illustrated in Plate 21, there are courses that permit closing to predetermined ranges and to predetermined positions quicker than a collision course, but their defect lies in the fact that enemy action probably will prevent the attainment of such theoretical positions and for practical purpose the gain in time is not great.
(b) Target Angle from which to begin Approach.
Favorable and unfavorable target angles from which to approach can be determined by consideration of the destroyers' task and the obstacles in the way. The task is considered to be that of placing the maximum number of torpedoes into the enemy battle line as soon as possible after the order to attack; the obstacles in the way are the effect of the target's course and speed and the enemy's defensive measures.
The effect of the target's course and speed and the fire action of the target itself, which is always to be assumed as present in some degree, make approaches in target angles abaft the target's beam more unfavorable than those forward of the beam. In the latter case, sufficient fire effect of the target itself tends to disappear as the bow is approached (Plate 9) and the efficacy of the target's course and speed in defense is reduced. Were these the only obstacles to consider, ie, were there no light force defensive present, an approach from dead ahead would bring the destroyers within effective firing range, or to any predetermined firing range, in the quickest time. (See Plate 21).
If a predetermined firing position, ie, a predetermined firing target angle and firing range, were sought, the best target angle (theta') from which to approach to arrive in the quickest time is that which results from use of the following formula, where theta is the target angle and r is the range at which it is desired to fire, m is the range from the target where the approach is begun, and n is the ratio of destroyer speed over target speed.
tan theta / 2 (1 - (m - nr) / (m + nr)) - - - - (1) tan theta = --------------------------------------- 1 + (m - nr) / (m + nr) tan**2 theta /2 sin (theta' - alpha) = sin alpha - - - - (2) --------- n
From this, where theta = tan-1 St / So (27/So) (which in Section II is assumed as the best value of theta in which to fire) and with a firing range of 60000 yards (r), the values of theta', where m is 20,000 yards and 30,000 yards and n is 30/20, 30/15, 25/20 and 25/15 are as follows:-
If the best firing position, ie the best target angle combined with the shortest firing range which can be attained, is desired, then the approach should begin at this target angle and it should be held by a collision course. In Section II it was illustrated that the best firing position is from the track angle is about 90º at the shortest possible range, and that this would result when the target angle is tan -1 (Torpedo speed / target speed). The following table shows the value of this angle when met in practice.
If then, there are not light forces to interfere, the ideal target angles from which to begin an approach lie between 0º and about 70º. The approaches begun at 0º may entail difficulties in obtaining a favorable firing target angle, even though desired firing ranges may be attained quicker. Favorable firing positions, which combine favorable target angle with favorable firing range, may be attained more quickly by approaches begun further aft, ie, somewhere between 20º and 70º.
When the enemy light forces come into the picture the situation changes. It will be assumed, for discussion, that they will keep interposed between their battle line and our destroyers and at a distance out from their battle line as shown by the "apple card".
Approaches of our attacking destroyers begun sharp on the enemy bow will tend to be limited in effectiveness, because of the great distance to which the destroyers may be forced prior to beginning such approaches in order to keep out of gun range until time to attack. This great distance will not only severely hamper proper coordination with own battle line, but also will probably involve some difficulty in getting out that far.
Using the War College rules as a yardstick, it was assumed, in Plates 8 and 9, that the battle line itself, using all secondary batter guns of one broadside, could defeat a destroyer squadron (DDs) before it reached a practicable firing range whenever the destroyers approached on target angles abaft 130º, and on target angles abaft 80º when using turret guns that were judged to be available. In Plate 10, with the same yardstick, it was assumed that only 3 Omahas were needed to stop 13 Oboros (DLs) without aid from the battle line, when the Oboros approached in target angles abaft 120º.
When attacking destroyers approached in target angles 0º to 120º, conditions for the approach were considered, theoretically, much more favorable for the destroyers, so far as being stopped by defending cruisers was concerned. In the arc 0º to 30º, it was assumed that the destroyers could not use both broadside fire and a collision course; nor could they use broadside fire, and any other than a collision course, without difficulty, when in the more forward portions of this arc. When approaching between 30º and 120º, it was assumed that it took the most cruisers to stop the destroyers in time. This latter situation resulted from the assumption that the destroyers, as they approached could use broadside fire more often on the cruisers. The most favorable part of this arc for the destroyers therefore, is, theoretically, from 30º to 80º, where the most opposing cruisers must come out the farthest and the enemy battle line fire tends to be ineffective.
If the foregoing assumptions are sound, and applicable, the next question is: what is the most favorable part of the arc 30º to 80º? The nearer 80º is approached, the greater the chance that broadside fire may be used by the attacking destroyers. If the battle line speed gest less than one-half the destroyer speed, broadside fire cannot be used by destroyers if the collision course is maintained; but the nearer the destroyers come to 80º the less they will need to turn off from the collision course to bring their broadsides to bear. With destroyer speeds of 25 to 30 knots and battle line speeds of 15 to 20 knots, the arc 60º to 80º gives broadside fire to the attacking destroyers for all combinations. In this arc the nearer 60º is approached the farther out the defending cruisers must come, and therefore the less chance of their being there, and the less chance of the attacking destroyers interfering with their own battle line fire. The arc 60º to 80º also tends to fore interposing cruisers into the arc between the battle lines and to within range of the attacking destroyers' battle line.
It is probably fair to say, from the foregoing theoretical discussion, that the most favorable target angle from which to approach for a destroyer attack in a general action, lies in the arc 50º to 80º. This takes cognizance of the difficulty which destroyers may have of gaining definite positions with reference to their own battle line. As a corollary, it is considered that the nearer their assigned positions are to their own battle line, the greater is the chance that destroyers will be in the most favorable position from which to approach for an attack.
It probably will be necessary, of course, that the highest practicable speed be used in the approach.
4. Formations for Approach.
Formations of destroyers for the approach have been a matter of much thought and experiment. This paper will not attempt to elaborate on standard publications with respect to these formations. Attention is invited, however, to the probably need for developing maximum gunfire by destroyers while making an approach, as often the destroyers' gunfire in maximum volume will be required to get the attack through. Flexibility in formations, to allow full gunfire, is an important consideration in any estimate of how to get the attacking force to the right place in time..
5. Action of Covering Forces.
Covering forces, as discussed herein, are descriptive of the cruisers and destroyers detailed to keep the way cleared for the approach and firing of the attack destroyers, and their weapons are taken as the gun, the torpedo, the ram and smoke. Their attack objective is the enemy's cruisers and destroyers which may interfere with the attack. If such covering forces take the action suggested in Section III for defending the battle line against enemy surface torpedo attack, they will, as a rule, be in an ideal initial position for covering the approach of their own attack squadrons if the latter commence their approach from the arc 50º to 80º or from near own battle line.
Once the destroyer attack is seen to go in, the covering force may have to decide between covering the battle line and covering the attack destroyers. The best position normally will be to interpose between the object to be overed and the threatening enemy. As long as the bearings of own battle line and own attack squadrons differ little from the interposed covering force, both the battle line and attack destroyers can probably be kept covered simultaneously without difficulty. This will be likely to occur when the attack squadrons begin their attack from near the battle line and approach on a collision corse, or from near their covering forces. When it is difficult to cover both the attack squadron and own battle line, the covering force must estimate the apparent intentions of the enemy and, unless specifically directed otherwise, interpose between him and the more threatened object.
If the O. T. C. orders a destroyer attack on the enemy battle line, it will be assumed that he intends it to be of maximum effectiveness. Quick effect is required, as the tendency will be for the enemy cruisers, one's own covering force and the attack destroyers to close the same area. A melée often will result in this fight for torpedo water. AS a rule, the covering forces must close the range and keep in the van of the attack destroyers. To make a destroyer attack effective, covering forces probably must close to short range in order to fight the interfering force to a decision.
Plate 22 shows two opposing battle lines at 20,000 yards. It may be difficult to provide sufficient battle line support for a destroyer attack at ranges much in excess of this. Three BLUE attack squadrons at A, B & C at zero minute are starting their approach. At this instant A & B are approaching from theoretically favorable target angles - 50º to 80º. C has been in the van with the cruisers at D. There are two BLUE cruiser divisions at D and E, interposed, just beyond the "apple", between their own battle line and enemy cruiser divisions at G and H respectively. The BLUE cruisers are assumed to have been in or near these positions ever since the battle lines have become effectively engaged, keeping interposed between own battle line and the enemy cruisers. If the BLUE attack destroyers have been near own battle line in the preliminary states, the BLUE cruisers have been covering them also. The cruiser either receive orders to cover the attack of these three squadrons, or they see them begin the approach. They also see opposing cruisers stand in to intercept the attack destroyers. BLUE cruiser commanders decide, in the absence of other orders, to cover the attack. They move to keep interposed between the enemy cruisers and their own destroyers and to close the range. At the end of 12 minutes all forces are in the positions shown. All BLUE cruisers have covered the attack destroyers, and their own battle line as well. They were able to do this by means of their initial interposing positions and by closing the range on the enemy cruisers. In a chase-tails action, as illustrated to the right, the BLUE cruisers, abaft BLUE battle line beam, performed in the same manner.
Covering operations performed in this way, will probably be successful if they are in sufficient force and adhere to gunfire requirements. If the covering force is not in sufficient strength, either the O. T. C. will not order the attack or the disparity of strength will be accepted. There may be no reason for saving cruisers in a general action for something better, when a major destroyer attack is orders.
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