The recent battle off the La Plata between the Admiral Graf Spee and the British cruisers has not yet been reported in that technical detail which is of more interest to the naval officer than the most broadly picturesque of descriptions. An interesting and original feature already brought out, however, is the apparent use of smoke screens by both forces.
The term “original” is used advisedly. We are familiar enough with the laying of smoke screens by planes and destroyers to cover other vessels, either in a torpedo attack or in a retirement under fire. But their use by the ships that lay them, to shield themselves in a gunnery engagement, is sufficiently novel to warrant further thought on the subject. In the following paragraphs no attempt is made to discuss the recent battle nor yet smoke screens as a whole, but merely to explore their possibilities along some particular lines which it is believed have heretofore been given less than sufficient attention.
To the casual observer on the bridge, looking aft at the narrow and unsubstantial band of smoke ordinarily streaming from his funnels, it might appear that its use to conceal his own ship would be like a large fat man trying to hide behind the frolicsome fox terrier trailing him. Such is not really the case. In the first place natural perspective makes the smoke band, especially as it gets farther away, appear much narrower than it actually is. Secondly, when smoke is laid heavily enough to persist for a substantial period, the minor fluctuations in force and direction of the wind, together with the “drag” effect, will cause the band to spread considerably as it travels down wind. Thus these fluctuations, within reasonable limits, are an aid to concealment rather than the contrary.
Granting that our stacks will emit a smoke band of suitable width and opacity, we come next to the geometrical investigation of the relationships between the direction and velocity of the wind, the bearing of the enemy, and the speeds and headings that we ourselves can take while keeping behind our own smoke. Where the expression “wind movement” or “wind track” is used, it indicates the force of the wind and the direction in which it is going rather than the direction from which it is coming.
Referring to Fig. 1, consider OE as the bearing of the enemy and OW as the true wind movement. If the ship then runs on a course and speed represented by either OS or OS1 the movements of both wind and ship will have equal components in the same direction across the enemy’s bearing, the smoke will trail directly towards the enemy along the line S1E1; and if it spreads sufficiently and lies low enough, our ship at S or S1 will be effectively concealed behind it. The course and speed OS will close the enemy while OS1 will open him. Should the wind’s movement be as shown by OW1 instead of OW, the ship’s movement OS1 will keep her screened, though OS will not. In this diagram the distance to the enemy is assumed large enough and the distance OX small enough so that the bearing will not change materially, and XE1 will be parallel to OE for a small time differential. In practice, as the bearing does change, either from the enemy’s movements or from our own, the courses of the smoke-laying ship must be changed to conform thereto.
From this elementary diagram it will be seen that there are 3 primary rules for keeping our own smoke between our own ship and the enemy:
(b) Our track must lie on that side of the wind’s track which is away from the bearing of the enemy.
(c) Our speed to close the enemy must be less than the wind’s velocity. To open him, it may be greater or less depending on its direction. In any case it must be greater than the component of the wind’s track, OX, normal to the enemy’s bearing.
Certain additional facts become apparent. A wind nearly towards the enemy, with a velocity greater than our own maximum speed, will be most favorable for chasing him. A wind of any strength nearly towards the enemy, or one with a force less than our maximum speed nearly from him towards us, will be most favorable for running away from him. Since the wind is likely to be slower than the maximum speed of a combatant vessel, the wind’s force is more likely to favor retreat than pursuit.
If the wind is neither directly towards the enemy nor away from him, but at some angle across his bearing, as OW or OW1, we cannot head either directly towards him or away from him, but must head across the bearing ourselves, as in OS or OS1. In this case XS, the component of OS towards the enemy if we are closing him, must be kept less than the wind’s component, XW, towards him, and hence will probably not be great enough for us to overtake him if he is heading radically away from us at any considerable speed. Whereas if we are retiring, the wind’s component, if towards the enemy like XW, can be of any amount at all; and if it is away from the enemy as XW1, it can be of any amount provided that our speed is still greater than the wind’s.
Still another point is that while we have assumed for simplicity in the basic geometry that there was no great change in the enemy’s bearing, this will not be strictly true. Should he be moving with the wind in a general direction from E towards the side of E1, then his track and our own, either OS or OS1, might together result in a fairly constant bearing. But if he were moving against the wind, as from E towards the side of E2, the situation would be different. In this latter case if we try to approach him, the only smoke-sheltered course, OS, will head us astern of him at a rapid change of bearing, since we cannot head to the left of the wind, OW, nor exceed it in speed. But if we try to retreat from him, along OS1, it will be easier. The change of bearing will slow down markedly as we open the range, and our changes of course made to conform to the changes of bearing and keep us behind the smoke will make our track a flat spiral, gradually becoming a straight course on a constant bearing with the enemy. From the foregoing it appears that approach and retirement under cover of our own smoke are Problems sufficiently different to require separate treatment.
The practical steps required of the navigator are simple. Assume that we are proceeding on a course 000°, in darkness or reduced visibility, that there is a 15-knot wind from 210°, that our speed, or the speed that we can work up to quickly, is 22 knots, and that we must know, in case of sudden contact with the enemy, what the possibilities are for immediate retreat or pursuit, under cover of our own smoke. Our present course is of importance only as affecting the most probable contact bearing; it does not affect the retirement of pursuit course itself. So proceed as follows, taking the problem of retirement first. In Fig. 2, from the center, O, of the mooring board, describe a circle of 22 knots radius to the largest convenient scale, and lay off OW to represent the direction and velocity of the wind to the same scale. Remembering then that the wind and our own retirement course must have the same component normal to the enemy’s bearing, we select the most probable bearing, 000° at E1, pass this 000° bearing through W with the parallel rules, and mark its intersection, S1, with the 22-knot circle. The radius OS1 at about 161° is then the approximate course for retiring under cover of our own smoke from an enemy sighted at E1. In a similar manner we may select possible contact bearings at intervals to right and left of E1 and plot the corresponding retirement courses. We can letter them S1, S2, S3, etc., to identify them with the contact bearings marked E1, E2, E3, etc., or we can mark them with the figures indicating the contact bearing as shown. Or if preferred, the enemy bearings and our corresponding retirement courses may be taken from the diagram and written down in tabular form. It is also possible to construct a universal table or set of curves, using (a) the ratio of ship’s speed to wind velocity, (b) the bearing of the enemy measured in degrees to right or left of the direction of the true wind, and (c) the corresponding retirement courses measured to right or left from the enemy’s bearing.
In treating the problem of an advance under cover of our own smoke, it is better to use a separate diagram and avoid confusion. In Fig. 3, from the center, O, of the mooring board, describe a circle of about 14 knots radius. We get the 14 knots from the fact that no matter what speed we are capable of making, it must be less than that of the wind, which is 15 knots in this case. Lay off OW to represent the wind as before. Through W, with the parallel rules pass the bearing 000°, intersecting the 14-knot circle at S1. The radius OS1 at about 032° is then the approximate course for approaching an enemy bearing 000° at E1, under cover of our own smoke. Similarly, if the enemy is sighted bearing 330° the approach course would be about 048°. Since these courses are so jammed together it will be best to plot in merely the wind, and a speed circle with radius about a knot less than the wind’s velocity, leaving the exact solution until the enemy’s bearing and heading are known.
It is readily apparent from the plotting sheet, as before stated, that anything like direct pursuit under cover of our own smoke will be difficult except under most favorable conditions; but the tactical necessity of it is not likely to be great. Except in the case of a destroyer attack, the very fact of our attempting to close the enemy, or even to hold our distance from him, presupposes the desire for a gunnery action. There is then no gain in blanking off his fire if we at the same time blank off our own. An exception might be in a case where we were outranged by the enemy’s guns but hoped to close to a range where our own would be the more effective.
Another possibility, suggested in the accounts of the La Plata battle, is that of holding our approximate range but dodging in and out from behind the smoke (our own or that of some other ship) to avoid the enemy’s fire while keeping up our own. It would require not only good smoke-laying conditions but perfect timing as well—a dash out of the smoke, a quick salvo or two, and a dash back again, plus a little range finding and spotting sandwiched somewhere in between—all before the enemy could do a corresponding amount. With instruments better than his for holding the hitting gun range and bearing during a blackout, we might conceivably open or close the range while behind the smoke, and still be able to straddle him as soon as we emerged, while he took several salvos to get on again. But all this is really a gunnery problem rather than a tactical one, and all gunnery officers are cordially invited to try solving it, in addition to their usual troubles!
Greater possibilities exist in situations where there is an advantage in interrupting our own fire as the price of avoiding the enemy’s, even temporarily, without either advancing or retreating materially. Such a measure would be sound tactics, for example, when we desired to repair temporary damage without breaking off the action, or when we desired to get up ammunition, to replace casualties, or even to get a mere breathing spell (in lieu of zigzagging) if his salvos were straddling us with uncomfortable frequency.
Most feasible and useful of all, however, is the direct and unabashed flight under cover of our own smoke, when necessity dictates. By the intrinsic geometry of the problem there are more combinations of wind and enemy bearing favorable to retreat than favorable to approach. To this must be added the facts that in retreat we will be less limited in our own courses than when pursuing, that there will be no speed limit except that of our engines and boilers, that contingencies of low visibility are more apt to necessitate a quick retreat than a quick advance, and that in all probability if the enemy’s superiority is such that retreat is necessary, we will be more than willing to cancel our own gunfire for the sake of canceling his.
In both of the foregoing mooring-board solutions exactitude and detail are not strictly necessary. Should the wind happen to have changed a little after our initial plot, or should the enemy be sighted on a bearing somewhere between those plotted, we naturally will not keep plunging along as we are while fumbling over an exact geometrical solution of the pursuit or retirement course. More obviously we will throw our rudder hard over, take a quick look at the diagram as we do so, come around to the approximate heading, and adjust so as to keep behind our smoke as we get there. The really important thing is for the captain, navigator, or unit commander to be “smoke minded”—to have thought out what can be done in that line under existing conditions, and to be ready to do it instantly when the occasion arises.
Switching now to something wholly different, there is obvious attraction in the idea of a smoke screen or anything else which will hamper or nullify the enemy’s fire on us while in no way interfering with our own fire on him. In Fig. 4 we have represented two equal battle lines of four divisions each, each line spanning about 9,500 yards from van to rear, abeam of each other at about 24,000 yards. Suppose now that we are able to lay and maintain a smoke screen as shown, abreast of his last two divisions and about 2,000 yards from them. What would then prevent us from concentrating our fire approximately two to one on his leading ships until they were out of action, and then stopping the smoke and mopping up comfortably on the remainder?
Its advantages, if practicable, are too obvious for comment, so let us consider its practicability. Planes are indicated as the immediate means. Smoke projectiles fired from our own ships would be ineffectual, and destroyers could never get close enough to the enemy’s line, or survive if they did get there. To the argument that the smoke-laying plane might be shot down we can only rejoin that somebody has to get hurt in war now and then, and that our planes might do more for their cause that way and at not much greater risk, than if engaged in spotting the wrong salvos from somewhere overhead, or staying aboard their parent ships and getting knocked to tatters in the opening exchange of salvos. It is debatable whether a plane going across the line of fire at low altitude, close enough to create a very rapid rate of train, is not a harder target for the anti-aircraft guns to hit than one coming straight in to bomb us at a much higher altitude. The greatest risk of all, that of their parent ships being unable to recover them before they run out of gas, is not affected.
As to the geometry of it: The closer the smoke is laid to the enemy’s line, the more effectively will it blanket his fire. As drawn in Fig. 4 the leading ship of the enemy’s third division could probably fire on our leading division, and were the smoke screen to be laid or to drift farther out, more of the enemy’s ships would be unmasked, until at a point midway between the two lines the smoke would blanket them both equally while permitting all of them to concentrate on the opposing leaders.
It goes without saying that the problem is not as simple as shown in Fig. 4. Both fleets are moving, the smoke is traveling with the wind, and the plane cannot regulate its distances and bearings with meticulous accuracy nor yet keep station abreast a certain point in the enemy’s line. Investigation of these problems is thus in order.
To begin with, let us designate the bearing (and line of fire) from the last ship in our own column to the point, O, in the enemy’s column between the section to be obscured and the section to be fired on, as the “Blackout Bearing”—lacking any better phraseology. We are immediately concerned with the location and drift of the smoke in relation to this Blackout Hearing as well as to the enemy’s course and line of bearing. It would be ideal if the smoke after being laid would maintain the same relative position, but this is possible only when the wind’s direction and velocity happen to coincide with the enemy’s course and speed. Failing this, its movement should not be so rapid either axially (with the enemy’s course or in reverse of it) or laterally (towards or away from his line) as to unmask his line more rapidly than we can keep it effectively blanketed. Above all, its relative movement, if away from the enemy, must not be such that it can drift across to our own line and do to us the very thing that we are trying to do to the enemy.
Referring to Fig. 5, let OA be the enemy’s course and speed, say 18 knots, and OW1, OW2, OW3, etc., be different directions of an 18-knot wind. From W1, W2, W3, etc., lay off W1S1, W2S2, W3S3, etc., equal to the 18-knot course and speed but in reverse. Then OS1, OS2, OS3, etc. will represent the corresponding speeds and directions in which the smoke will move relative to the enemy.
OS1 and OS4 are almost the ideal cases, i.e., with the smoke keeping pace with the enemy axially and with a small drift of only about 2 knots laterally. OS1 is preferable to OS4 since a smoke screen moving away from the enemy will tend to unmask the blanketed section of his line slowly, ship by ship, whereas a screen moving towards him and finally to the other side of him will unmask all ships in the blanketed section at once. Assuming, however, that the smoke was originally laid 2,000 yards from his line, it would take about 30 minutes to cover that distance, and by that time the screen should have been renewed.
OS2 and OS5 are less favorable. The relative drift is about 5 knots axially and 12½ laterally. Finally for OS3 and OS6 it is about 32 knots axially and 10 laterally.
The following general rules become apparent. Winds from astern or from close on the quarter will be most favorable; and as the angle between wind movement and enemy’s course widens out towards 90° or 180°, the more unfavorable will conditions become. A wind from abaft the beam will be most favorable when equal to or greater than the enemy’s speed, depending on its direction relative to his course. With a wind from forward of his beam, the lighter it is the better.
As regards the danger of the smoke drifting towards us, and blanketing our own line instead of the enemy’s, let OB represent the Blackout Bearing. The wind OW2 will cause the smoke to drift just clear of us along the relative movement line OS2, while a wind along OW3 would make it clear us by a big margin. In theory the wind OW1 would cause it to cross our column, but the relative movement OS1 is so slow that it would probably be dissipated before it reached us. If we draw a line AB1 parallel to AB, then it will take a strong wind like OW7, overlapping the line AB1, to involve any great danger of the smoke being blown back onto our own column.
A little figuring on how to lay this smoke screen may now be in order. In Fig. 4 assume that the speed of the smoking plane is about 90 m.p.h., that the length O-M of the enemy’s column to be blanketed is about 5,000 yards, that the enemy’s speed is 18 knots, and that the wind is most unfavorable, say with an axial speed of 18 knots from ahead, something like OW3 in Fig. 5. The smoking plane, coming up from astern covers the distance O-M at a speed of only 54 knots, taking about 2 min. 45 sec. to do it. Arrived at M, however, it could then slow down to 36 knots (wind plus enemy’s speed), assuming that this were possible. Since it is not, the plane would presumably double back, keeping behind its own smoke if possible, and dart out ahead again, describing a series of overlapping circles each one advanced far enough to keep the leading edge of the screen about abreast of the moving point O. The point O will move past the leading edge of the smoke screen at 36 knots or about 1,217 yards per minute, so the forward circles would have to be made in rather rapid succession to keep more than one of the enemy’s ships astern of the point O from being unmasked at any time. Of course if the wind were lighter or from ahead the plane would have less work to do and could keep behind its own smoke longer.
The temptation is strong to go into all the ramifications of this proposed method of screening off part of the enemy while we concentrate on the other part, but theory must always hinge on practicability, and we have now reached the point where the discussion should be taken up by aviators, anti-aircraft battery officers, and those gunnery officers (if any) who have found time to think of destroying the enemy in battle as well as of making scores in target practice. If those best competent to judge believe that it would be impracticable as well as suicidal for a plane to attempt laying smoke so close to the enemy, then the idea need be considered no further. But the geometrical logic of it is inexorable. And should it be argued that we do not need it, that what we want is to shoot-up the fellow opposite us instead of hiding him, the dissenter might then as well protest against trying to “Tee” the enemy’s column, or to fall upon a detachment of his fleet with the whole of our own, or to do anything else disturbing to his apparent conception of battle as a sort of stately minuet between fleets which must not be interrupted by unorthodox attempts to actually win the battle.
A more realistic viewpoint, however, is that the very range and destructiveness of modern gunfire have increased the possible advantages of being able to secure even a temporary immunity from it; and that smoke, far from being an exceptional resort and a purely defensive one at that, may prove to be a decisive weapon in the hands of those who have accepted the idea of using it and are ready to do so instinctively and effectively.