Dahlgren on Shrapnel, part 2: “these results are only to be considered as general terms”

As discussed earlier, while working on a system of boat howitzers to equip the US Navy, John Dahlgren conducted a detailed study of the behavior of shrapnel.  He identified three factors which governed the performance of shrapnel, from a target point of view, and thus would provide the requirements for the practice of fire for such projectiles.  Those were the range, the time of projectile flight, and height of burst above the ground. And those were the same requirements we saw in the illustration from other pre-war manuals.

OwenPlate20Fig6

What Dahlgren’s study offered was a “schooled” approach to the problem, as he felt the behavior should be scientifically defined to provide a reliable reference to the gunner.  Dahlgren determined shrapnel was most effective when exploding 50 to 130 yards in front of and 4 to 15 feet above the intended target.  So the next question – how to make a shrapnel projectile’s burst to occur with such regularity that the gunner could achieve a result within that “most effective” space.  The key to achieving such results would be accurately setting the burn time of the fuse.

But that was easier said than done, as Dahlgren observed:

The shrapnel fired from cannon may have a velocity in the different parts of its trajectory, amounting to as much as 1200 or 1500 feet per second, and hence a difference in the burning of the fuze, almost inappreciable in time, will be made very perceptible by the variations in the distances at which the explosion occurs; thus, with the 1200 feet per second, a fourth of a second will produce an error of 100 yards:  if the velocity be 600 feet per second, the difference in distance will still be fifty yards.

Keep in mind at the time of writing, most services used paper fuses.  In the use of such, the length of the paper was cut, using a rule calibrated to the burn rate of the fuse, for the desired burn time.  The Bormann fuse was just coming into use.  But in either case (and the case for most of the other types in use) the smallest time measure provided was the quarter second.  What Dahlgren was telling us is that quarter second could produce an error of between 50 to 100 yards.  In other words, this would produce an unacceptable variation which might serve to throw shrapnel completely outside the optimal window, if not the marginally effective zone.

Dalgren went on to point out another factor, which fell outside the gunner’s control, was the consistency of fuses.  Regularity in burn rate was a problem at the time.  Furthermore, the set of the fuse would often change the performance, in some cases leading to misfires and other problems.  So the bottom line this was not simply a case of selecting a fuse length and firing the shrapnel.  More thought was required.

Dahlgren did point out that British practice was to provide each shrapnel with four fuses.  These were defined by the ranges allowed based on the fuse burn time – 650 yards, 900 yards, and 1100 yards, with a fourth left to be cut based on tactical needs.  Not specifically stated, but assumed, is these allowed the gunner select a fuse based on where he wanted the shrapnel balls to hit the ground.  In other words, the 650 yard fuse would cause a burst at around 500 yards, with balls proceeding forward another 150 yards.. .give or take.

Though Dahglren spent some time describing the nature and functionality of various fuses available at the time, he shorted the discussion with a “might be too much elaborate this brief sketch” … so allow me to follow that lead at time time.  He simply noted that in US service (both land and sea) the standard was to provide shrapnel with fuses pre-configured for 1, 2, 3, 4, and 5 seconds (color coded).  It was not desirable to modify those fuses in order to shave off fractions of a second. And while he provided for some modifications or fabrications, those were considered impractical for field service.  Instead, the focus should be, he felt, on precise employment of the standard fuses (again, not only burn time but placement in the projectile to reduce the chance of misfire, failure to fire, or other derived irregularities).

What this leads us to is a somewhat fixed practice of fire.  A gunner would seek to fire shrapnel when the target was at a range which matched to the burn time of those configured fuses.  Granted, that might be inconvenient as battles are too often fought at odd ranges (you know… were four map sheets join and all).  But this plays upon that 50 to 130 yard fall of the balls after the shrapnel burst.  To be blunt, this was horseshoes and hand-grenades work.  So long as the time of burn was within that desired  50 to 130 yard, an effect could be felt.

Now the critical point fell upon the sighting arrangements of the cannon.  Given that fuses were working on a “set” time, and from that the gunner derived the zone in which the balls would fall, he had to work back to ensuring the gun tube was properly oriented to push the projectile on the desired flight path.  Azimuth… the easier part of that problem.  Elevation was the question mark.  Dahlgren explained the functioning of the fuse and elevation as such:

… the elevation is given to the piece which is required to carry the projectile to the proper distance, while the fuze adjusts the explosion to the time which the projectile occupies in traversing this space.

Given those fixed fuses, Dahlgren suggested the British method would work best for sights:

The sight in this method is graduated to the intervals of time which will carry the projectile to its desired position; and each graduation is accompanied by the two distances which include the spread of shrapnel balls.

Thus if the fuze be adjusted to 2″, and the piece elevated by the sight, raised to the line on it marked 2″, then the shrapnel will burst about 500 yards from the piece, and spread its balls from that point a considerable distance farther – effectively, at least 150 yards.

Note here the use of the symbol (“) for seconds, as opposed to referencing inches.  Select a measured fuse to fire at a measured distance and use a set elevation on the sight.  The gunner was to trust the equipment and ordnance provided to perform with regularity.  No need to know the science here.  Just basic rules of thumb – target at 650 yards, select a two second fuse, elevate gun to the two second mark on the sight.  Fire.

Dahlgren admitted that the factors necessary (range to burst, elevations, etc.) for this arrangement needed refinement, agreeing his department needed to work out those details.  Still, he concluded:

Even when obtained, these results are only to be considered as general terms that are to guide the intelligent officer to a proper application of shrapnel or of shells, when used upon uncovered troops; there being left in the fractions of seconds, a wide margin for the tact and discretion that are to make his fire more or less effectual.

A bureaucratic way of saying, “We’ll get the gunner in the ballpark, but someone still needs to observe fires in order to hit the target.”

We can actually trace Dahlgren’s practice of fire directly into the manuals.  From The Ordnance Instructions for the United States Navy of 1860, we have this paragraph regarding shrapnel from boat howitzers:

Similar terms are used in marking the sight and the fuze.  Thus, if the fuze be adjusted to 2″, and the piece elevated by the sight raised to the line on it marked 2″, then the shrapnel will burst about 500 yards from the piece and spread its balls from that point to a considerable distance farther – effectively at least 150 yards.

So by 1860, the observation became the rule. A “practice of fire” described in one paragraph.

However.. you know there was going to be a “but… ” here…. There is a problem with this practice of fire.  Aside from that paragraph in the Instructions, there is no table in that manual detailing the particulars for three second, four second, or five second settings. Now there was a separate set of manuals detailing how the Navy wanted sailors to use boat howitzers.  And there we find this table (The Naval Howitzer Ashore, 1865):

BoatHowitzerTable

This is “good stuff” but not the raw inputs needed for the “practice of fire” for shrapnel.  We don’t find table increments for seconds as defined on the sight.  Instead we have degrees of elevation, with ranges and time of flight provided.  But notice this is broken down, were available, for shrapnel and shell.  While not directly supporting the “practice of fire” we can see how the figures given in the practice play out.

Turning again to the two second time of flight, we see a close approximation at 1.9 seconds associated with a range of 500 yards for shrapnel from a 12-pdr smoothbore howitzer (the caliber and type Dahglren cited in his tests).  That occurs with a 1º elevation.

Notice if we go down a line to the particulars for shells, the two second time of flight would fall somewhere between 516 yards and 730 yards, likely around the midpoint between those two.  With an elevation between 1º and 2º.This requires some bending of the mind around the ballistics, but I submit the ranges given for shrapnel coincide with the range where the burst occurs, not where the balls would fall.  However, that given for the shell is where the projectile hits ground (and the gunner wants it to explode).  Thus, I’d submit we are reading different definitions for “range” on those lines accounting for the different uses of projectile types.

That “but….” presented within this table lingers over the “practice of fire” like a cloud in battle obscuring the target.  Yes, Dahlgren defined the practice in clear terms before the war.  And yes, those practices were incorporated in the pre-war (1860) manual.  However, at least by 1865, the tables provided failed to give the gunner such “tolerable approximations” that Dahlgren sought in 1852.  One gets the impression that shrapnel was just not that important after all!

(Citations from John A. Dahlgren, A System of Boat Armament in the United States Navy, Philadelphia: A. Hart, 1852, pages 56, 69-70; Ordnance Instructions for the United States Navy, Washington: George W. Bowman, 1860, page 109.)

 

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2 thoughts on “Dahlgren on Shrapnel, part 2: “these results are only to be considered as general terms”

  1. In response to my cross-posts of your excellent articles on technicalities of the use of case shot on the Authentic Civil War Artillery FB page, a reader mentioned a period artillery manual that I hadn’t been aware of previously. It is “A Course of Instruction in Ordnance and Gunnery” by Cpt. J. G. Benton, Ordnance Dept, 1862. A digital copy is available here:

    https://archive.org/details/courseofinstruct00bentrich

    Pages 465-468 (original pagination) cover the use of shrapnel. While outlining the previously mentioned problems of precise application, this manual, as the others you have mentioned, seems to offer little in the way of technique to the gunner in conquering the difficulties presented.
    I was, however, delighted to find yet another manual from the period to explore further for tidbits of technique that made 1860’s gunnery the art that it truly was!

    • Thanks, John. I edited your comment to provide a direct link to the book at Archive.Org. The link you used was throwing a browser security error.

      Benton is a favorite of mine. it was written, as the title implies, for the instruction of cadets. Good information, none-the-less. I didn’t reference it earlier, to avoid repetition. I think the important passage in Benton to consider, in regard to the discussion about case-shot/shrapnel, is:

      “The principal difficulty experienced in firing a spherical case-shot is, to burst it at the proper distance in front of the object. This arises from the difficulty of estimating the correct distance of the object, the rapid flight of the projectile, and the difficulty of observing the effect of a shot in order that correction may be made for the succeeding one, if necessary. To overcome these difficulties requires skill and judgement on the part of the gunner, and great accuracy and delicacy in the operation of the fuze.”

      So… in short, Benton says the gunner has to develop the skills and judgment required to be successful with shrapnel… .and how does one develop such skills? I’d submit that would only come from practice. So…. how much live fire practice did the gunners get?

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