First written in 1916 and later amended, the Spotting Rules were a formulated expression of the procedures the Royal Navy deemed most effective for spotting fire, based on battle experiences obtained through Jutland. In essence, it reduced spotting from an art to a science, and the rules effectively created an algorithm to be followed in correcting the fall of shot.
The rules were drawn up by a series of two or more committees on spotting rules, and were signed by Jellicoe on 24th September 1916. He endorsed the representative nature of the committees, voiced his endorsement of their conclusions, and directed that are to be "adopted at once in every ship of the Grand Fleet, and the control parties are to be thoroughly exercised in carrying out this procedure." In bold, he added, I must once more lay the strongest emphasis on the fact that nothing less than absolute perfection in drill and organisation and condition of materiel can be accepted if full value is to be obtained from our fire. The work was promulgated 30th November, 1916.
Background and Rationale
In the Grand Fleet Gunnery and Torpedo Orders written in September 1915, it is noted that "In reports of firings carried out during the last few months, a tendency has been noticed to use 'free' spotting corrections instead of adhering to the well established bracket system." Four further short paragraphs follow, but no clear reference to where a "well established bracket system" from this period is clearly recorded is made.
As laid forth in the document itself, the existing system of waiting to spot each salvo and incorporating a correction before firing the next was criticised as being too slow at finding the range, maintaining it, regaining it, and obtaining the fastest possible rate of fire the loading mechanisms could yield. The conclusion had been taken that the German battlecruisers at Jutland had zig-zagged independently when hit, sparking the realisation that hits would have to be obtained if a flurry of fire before such maneuvers could escape the hitting salvos. The goal was to increase the rate of fire by reducing the frequency with which the spotting and correction cycle was applied, both while hitting and while early salvos were working to find the hitting range and deflection.
The committee operated from the assumption that rates of fire and average spread varied from once per 60 seconds and 200 yards for 15-inch guns to once per 50 seconds and 400 yards for 12-inch guns (with ships firing half their battery per salvo yielding a doubled tempo of firing). It was also mentioned that in the 35 most recent ships engaging in firing practice, it was observed that the initial salvos produced 4 straddles, 16 short salvos, and 15 overs. This meant that spotting to find range was necessary around 88% of the time, and with a maneuvering enemy, this process would be continuous.
The rules for light cruisers differed from those for larger ships. By their omission, it seems the rules were not intended for smaller vessels.[Inference]
Modes of Firing
The Rules defined three modes of firing for future use.
Rapid salvos were to be "salvos of not less than three guns, fired as soon as three gun-ready lamps are burning." Its primary virtue was that it gave the director layer a free hand and by eliminating a complex circuit of command and action from the eye-hand motor skill of laying and firing. Its faults were that guns or gun-ready indicators might go out of action, and so the pragmatic liberty was given the man to "use discretion, and not withhold his fire for more than a reasonable period."
Double salvos would be paired "salvos of not less than three guns, fired as closely together as possible, but waiting to spot these two before firing again." These offered "the advantage that the method of fire is standardised under all conditions."  Its disadvantages were the requirement to wait for both salvos to fall and the increased chance that two or more guns might be ready to fire and made to wait for more to come ready.
Director Independent was to be "the system by which the director layer presses the trigger whenever his sights come on, irrespective of the number of guns that are ready." It would produce a stream of one- and two-shot salvos which would make communication and range-finding more difficult, but subject the enemy to a continuous rain of shot.
Rapid or double salvos were to be employed after hitting commenced, and were to become the means of increasing the rate of fire when hits were being produced. Note was made that these modes could be employed in director- or individual-laying, that individual independent was not to be forbidden in cases where it was the only possible method of firing, and "that the whole system must be entirely in the hands of the control officer."
Capital Ships and Armoured Cruisers
Finding deflection by double salvos
Fire was to be opened with a double salvo, with the salvos separated in deflection "by about two-thirds of the width of the target expressed in deflection", each one-third the target width to each side of the calculated deflection. The pair was to be at the calculated range, but the option was also presented that they could also vary "up 100" and "down 100" from the calculated range. If that wrinkle were applied, however, it was stressed that the 100 yard offsets were not to be customized under any condition. Double salvos would continue until the firing was on for deflection, but the separation in deflection was only mandatory on the first double salvo.
Once correct for line, the same deflection was absolutely to be used by all salvos.
Seeking the range by laddering across the target
If the salvo(s) which first fell on for line happened to straddle, its deflection was correct and its range correct as well, and firing could proceed under the rules for established hitting.
If the salvo(s) that found the line were short or long, the control officer was to fire an "up ladder" or a "down ladder", a series of double salvos with each single salvo increasing or decreasing in range by 400 yards successively until crossing the target. 400 yards was specifically chosen as a step-size that might offer a good chance of obtaining hits upon crossing a target. With a 200-yard spread in each salvo, it was noted that a pair of double salvos (four salvos) on a ladder would cover 1,400 yards.
In cases where a control officer were certain that the range was very incorrect, he was allowed to give a large initial correction before proceeding to the ladder. If line were to be lost while firing ladders, the process was to fall back to that spreading salvos for deflection, but it is not clear whether 400 yard laddering was to continue in combination with this process.
Example: upon first bringing the fire on for line, the salvos are short, a double salvo of the ladder would have the control officer command "Up 400, Shoot," and again say "Up 400, Shoot," as soon as he heard the first salvo fire.
A ladder was to continue until salvos crossed the target or one was seen to straddle the target. If one or more salvos in a ladder is not observed and the next salvo is seen to fall on the other side of the target, the ladder was to be reversed.
If the target is crossed but not straddled, a 200 yard correction in the reverse direction would be applied to the first salvo to cross. Example: when both salvos of an up ladder fall over, as soon as the first one falls, the orders would be "down 600, rapid (or double) salvos, shoot." The 600 figure used here is because a correction of "up 400" has already been applied after firing the first salvo. If the first salvo had fallen short and the second one had fallen over, the officer would have commanded, "down 200, rapid (or double) salvos, shoot." If neither of the first two salvos after falling out of a ladder straddles, the rules call upon the rate officer to adjust his rate.
Of course, if a salvo within a ladder actually straddles the target, the control officer would correct backward 400 yards (if the first salvo of a double straddled) and then fall into rapid or double salvos upon that range.
When hitting is established
Fire upon a target which is being hit or straddled was to be at rapid or double salvos, with 100 yard corrections used if the straddles appear to be threatening to slip long or short. 100 yard corrections were not to be used other than in correcting fire on a target that was being straddled or hit.
A previous practice of intentionally spreading shots within a salvo for range was to be entirely discontinued as a failure, and the previously considered notion that the same could be done for deflection was also condemned as such practices were seen as needless complications that might result in a fire control team outthinking itself.
Deflection lost after hitting established
A firing ship most often lost deflection when its target altered course or considerably altered its speed.
If a salvo missed for line and no change in the target's inclination could be observed, a spotting correction for line alone would be used to correct the discrepancy.
However, "(i)f the enemy alters course, and by some means (such as Rangefinder method of measuring angle subtended and thus obtaining alterations of course), this is detected" but the direction of the alteration not established, a spotting correction was again to be used to address it along with a double salvo fired with plus and minus 200 yards to that presently on the sights. For example, if a salvo seems to fall 6 knots to the left of line and the enemy's inclination is seen to change, the control officer would be "Double salvos. Right 6. Up 200, Shoot. [boom] Down 400, Shoot." The result of spotting these salvos should then tell the direction of alternation and prompt a rate correction. If needed, a ladder could also be started.
If a salvo missed for line and a definite direction of course change of the enemy determined, a spotting correction should bring fire back on for line, the Transmitting Station informed of the new inclination, and double salvos on an up (or down, as indicated) ladder on 200 yard increments started. For example, if a salvo misses 6 knots to left and the enemy is clearly seen to have turned away, the control officer would order, "Double salvos. Right 6. Up 200. Shoot. [boom] Up 200. Shoot. Enemy altering course to starboard, inclination 20 to the left."
The Rules caution against mistaking errors for line which are actually due to factors such as trunnion tilt for errors in shooting deflection, as these should not be corrected. The rule of thumb given for these errors is that:
MinutesOfDeflectionTowardLowTrunnionImpartedByTilt = (SlopeOfTrunnionsInDegrees * ElevationInDegrees)
In the case of Lion's 13.5-in Mark V (L) gun firing at 16,000 yards (about 10.5 degrees elevation), a 1 degree tilt of the trunnion would equate to 10.5 minutes or a little over 4 knots of deflection.
Range (alone) lost after hitting established
A ladder of double salvos was to be started, normally at 200 yard increments, but at 400 yards if the enemy was observed to alter course or a significant time has passed since the last spotted salvo. In the original 1916 edition of the Rules, it was said that a "rate correction and readjustment of the Dumaresq" should also occur as the ladder is commanded. In a later amendment, the nature of these rate corrections was further specified as part of the Rules. Additionally, the Rules spelled out the need to keep the Dreyer range calculator updated to reflect the present range and rate, and its indications applied as spotting corrections when suitable.
Variant for Light Cruisers
Light cruisers used a modified version of the above system for larger cruisers and capital ships. The changes were principally due to the more rapid rate of fire of such ships.
In finding the deflection, the double salvos offset in deflection were used, but these never employed the optional plus-or-minus 100 yards range offset permitted the larger ships. Upon finding the deflection, the ladder was conducted with rapid (rather than double) salvos at 400 yard intervals until a salvo was observed to have crossed the target. Since there may be several such salvos in flight at the time one is observed to cross or straddle, fire would then be checked for a little more thinking. An opposite correction of 400 yards would be applied for each salvo that had been in flight at the time of crossing or straddling, and if that salvo crossed rather than straddled, an additional 200 yards would be applied. If it is uncertain how many salvos are in flight, a simple expedient was offered: maintain the check fire and apply 400 yards each time one falls.
Light cruisers with mixed calibre armaments firing at extreme ranges were advised to range with one type only to avoid confusion introduced by varying times of flight.
The Rules anticipated that double salvo firing by concentrating pairs would be as effective as single salvo firing, but that Director Independent should not be used when concentrating, as it would be too confusing.
Fog and night firing at short ranges should be done with regular salvos while finding the range, and then double or rapid salvos when hitting.
Destroyer attack when there was no range clock available was to be done by firing a fixed barrage ahead of the destroyer in Rapid Independent, with the barrage shifting when the target passed through it. Six-inch guns and smaller were encouraged to use Individual or Director Independent rather than double salvos.
November 1916 Amendment for Rate Correction
On 20th November, 1916, a Memorandum H.F. 242/86 added "Rules for Rate Correction". This refinement was intended to tie automatic rate corrections to the procedure of recovering from a lost range after hitting. This is rationalised by an example of a light cruiser firing 4 salvos per minute who loses the range, as a 400 yard ladder to reacquire the range would be equivalent to a rate change of 1,600 yards per minute. As errors in rate were likely attributable to changes in enemy inclination or speed, large initial corrections upon first losing a hitting range were seen as clumsy over-reactions. A more gradual reaction based on paired range and rate corrections was counseled for ships of all sizes who lose the hitting range.
The range ladder increment to be used to regain a lost range was to normally be 200 yards unless the enemy had definitely been seen to be altering course or a delay in spotting recent salvos encountered, in which case 400 yards would be used unless the firing ship were able to fire very rapid salvos which should warrant 200 yards. However, when a 200 yard ladder seemed too timorous, the spotter was authorised to switch to 400 yards.
If the enemy had not been seen to alter course, each range correction in the ladder would be accompanied by a 100 yard per minute rate correction in the same direction, and this rate copied to the Dumaresq. If, during this process the enemy was seen to alter course, the automatic rate corrections would cease and the Dumaresq's indication of enemy heading or inclination used in lieu, and its resultant range rate set on the clock.
June 1917 Addenda for Light Cruisers
This separate pamphlet was to supersede the sub-parts of the Spotting Rules pertaining to light cruisers, and was based upon practice firings in the preceding six months. It replaced the use of rapid salvos in laddering with triple salvos, as this would remove the possibility of confusion as to how many salvos were in flight at the moment one was observed to cross or straddle the target.
The first salvos to be fired were to be a double salvo fired at the same range and spread for deflection of two thirds the width of the target but never less than 8 knots. Rather than split the salvos on either side of the target, the first was now to be fired on the calculated deflection and the other by the offset toward the target's bow. Double salvos would be used until the deflection were found, but only the first required the spread for deflection.
The calculated deflection was to be obtained by use of a "deflection calculator" (likely one converting speed-across to gun deflection) in addition to the Dumaresq, and "(a)n arbitrary correction may be used for starboard guns (illegible) found necessary; the amount recommended is 1 knot for each 1,000 yards above 5,000."  The range was to be found by a series of triple salvos with a 400 yard increment size. When the target is crossed or straddled, the range would be corrected and rapid salvo fire undertaken. The control officer was permitted in exceptional circumstances such as when the ladder could not be completed or it was clearly seen to be well short of the target, to issue a single large correction.
The previous rules' admonition to range with a single type of gun at long ranges was given a definite threshold: 10,000 yards was the maximum range at which ships with 6- and 4-inch guns could employ both in ranging.
Rapid salvos was the preferred firing mode while hitting, unless this caused a significant reduction in fire over that which independent fire could deliver, in which case it could be used, especially at ranges under 6,000 yards.
Ships were permitted to fire at targets beyond maximum gun range using double salvos spread for deflection until a salvo on for line went over, at which time a ladder would be initiated.
If the Transmitting Station had good range data available to it on a target beyond maximum range, the range clock would be set and tuned from time to time with rate data from the Dumaresq and range cuts. Data from Dreyer calculator, adjustments for lag in recording range cuts and such would be applied to the rangefinder range. Gunsights would be set to maximum range or a few hundred yards shy of it and fire opened when the clock range comes within 2,000 yards of the range on the sights. At such time as the clock range decreases below the range on the sights, the sights were to start tracking the clock range.
Alternatively, if the T.S. has no reliable range data, a guess range and rate could be set on the clock when well outside gun range. If a good range were ever obtained, it could be immediately set on the clock with the same corrections listed in the paragraph above.
It seems clear from the emphasis given to firing at the very earliest opportunity, even at extreme range, that the Royal Navy was more interested in doing damage as early as possible than it was in husbanding ammunition should a battle prove long-lived.
- ↑ Grand Fleet Gunnery and Torpedo Orders. No. 36.
- ↑ I do not understand this, as this document could clearly establish any standard whatsoever
- ↑ I have no idea what this means
- Gunnery Branch, Admiralty. (1916). Spotting Rules, 1916. C.B. 272. Possibly at The National Archives. ADM 137/2028.