Fortification Friday: Plans for simple “intrenchments” – Part 2, Enclosed Redoubts

In last Friday’s installment, we discussed some of Mahan’s “simple intrenchments,” specifically the open fortifications commonly used.  The other works mentioned by Mahan were “enclosed” fortifications – in the sense that these offered no open gorge lines.  And enclosed implied that the plan of the works formed a polygon of some sort.  Also keep in mind that, just as open works, enclosed works could be “detached” from the main works, or could be part of a connected series of works.

Mahan broke the the enclosed works into two categories – those without re-entering angles; those with re-entering angles.  And even within that division, those with re-entering angles were further sub-divided based on level of complexity.  Let me hold the discussion of the re-entering angle-based works – known as star forts and bastions – for next week.  This week we will look at the redoubt, which has no re-entering angles:

Any enclosed work of a polygonal form, without re-entering angles, is denominated a redoubt.  This work is used to fortify a position which can be attacked on all sides; the works which have already been described, being unsuitable for this purpose, as their gorges are open, and therefore require to be supported by troops, or works, in their rear; except when they are so situated that an attack cannot be made at the gorge.

We might consider a redoubt to be an open work with a wall added across the gorge.  Within the logical definition, there are some geometric rules that imposed forms upon the plan for a redoubt – triangles, quadrangles, pentagons, and such.  Of these multi-sided options, the quadrangle offered less dangerous angles.  Furthermore, the least risk was found with parallel sides… you know… rectangles or squares:

The square is the most common form for a redoubt, on account of the ease with which it is constructed, and the advantage it possesses, when combined with several others, of protecting the spaces between them by a cross fire.

And we have Figure 7, showing the plan for a common square redoubt.

Plate1Figure7

Think about the layout here.  There are several ways to describe it.  Four salients.  Or four right lines.  Either way you want to turn it, the features lay directly opposite a similar feature.   Also note the entrance at the bottom, or gorge, of this redoubt.  A traverse covered the entrance in Mahan’s simple diagram.  The “saw teeth” on the upper left is part of the follow on discussion, and an attempt at remediation of the problems associated with salient angles.

Though the square redoubt served to limit dangerous angles, like all redoubts it still had flaws:

All redoubts have the same defects. The ditches are unprotected, and there is a sector without fire in front of each salient.  For the purpose of remedying the sector without fire, it has been proposed to convert a portion of each face at the salient angles, into an indented line, to procure a fire in the direction of the capitals.  This method is not of practical application; and if it could be applied would only serve the purpose of changing the position of the sectors without fire from the salients to other points.

So… there was no “fixing” the redoubt’s problems within the layout of the fortification.  Instead, redoubts needed supporting works, chiefly other redoubts, for mutual defense.

Three points I’d add to Mahan’s lesson on redoubts.  First, there was some variation between the term “redoubt” when applied to fixed fortifications. In that context, a redoubt was a work outside the main work’s scarp designed as the feature of defense in the outerworks.  For instance, a demi-lune’s main feature was usually a triangular redoubt.  Keep that in mind when reading contemporary accounts.  Redoubts, be they for fixed or field fortifications, are still enclosed works, but are employed for different purposes.

Secondly, an unmentioned flaw of the redoubt was its vulnerability to high angle fire.  With the simple interior, there was nothing to resist the bounding shell or shot which arrived on the parade ground.  To rectify this, most redoubts featured some sort of traverse across the interior.  A good example of this is seen at Fort Davis, at Petersburg.

Lastly, Mahan does not, in the passages cited above from Field Fortifications, mention some of the other methods employed to address the redoubt’s flaws.  (Though in Mahan’s defense, he went into detail of these in other lessons.)  A common method to address the unprotected ditches was to add galleries at the salients. Another option was a caponniere along the line.  For those less concerned with the exact nomenclature, these were small redans or lunettes placed at points around the redoubt.  Arguably, incorporation of such features transformed the redoubt into a bastioned fort… and thus one of those “other” enclosed works with re-entering angles.

That said… next week we’ll look at some of those enclosed works that used re-entering angles.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 12-13.)

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