The Shape of the Hull

The Shape of the Hull

ll the Hunley plans I’d seen showed a smoothly faired hull as illustrated by the detail below from my own original plan. Recently published images of the laser scan data captured last fall and on display in the Warren Lasch lab for months show much less fairing. In fact, the ballast tank sides appear to be flat.

Let’s examine reasons for the discrepancy and practical reasons for the lack of fairing.

The 1996 National Park Service Site Assessment Report includes the first accurate plan, with a nicely faired shape for the ballast tanks ghosted in under the sediment. The hull description includes this text:

"Both bow and stern hull sections have a very fair shape; they narrow almost to a point at either end, but the narrowing is gradual, and the entire hull appears fair. This is only an impression because not enough of Hunley’s bow and stern sections were exposed to obtain longitudinal hull lines."

Underwater photos of the boat illustrate the murky water quality. Even after she was raised and numerous photographs published, none provided a good top or bottom view and the hull concretion obscured hull details. For my own part I think we often see what we expect to see. Looking at Chapman’s painting and drawing I now see a clear discontinuity at the aft hatch section, completely consistent with a change of surface angle at that point.

This is a profile of the same section as above but without the smooth blending.

A comparison of the two line drawings above belies the complexity of smoothly faired plates. Remember the plates are three-dimensional objects shaped to fit the semicircular hull cross-section.
In its discussion of the steel manufacturing capability of the Confederacy, the Site Assessment Report speculates the use of cold-rolled steel plates for the forward and aft hull sections. The discussion specifically addresses panel thickness, but the capability to fabricate complex shapes is also relevant.

The difference between smoothly faired and flat-profile panels is not immediately obvious because the hull has a uniform height and only tapers on the sides. The exaggerated schematic views below highlight the relative complexity.

The smoothly faired plate on the left is curved along the bottom edge in addition to the different curves on the left and right edges. The simpler shape of the other plate, with the simple flat taper, is evident in the straight upper and lower edges.

A simple flat faired plate.

It’s clear that fabrication of the faired panel with its multiple curved edges would be more difficult than the simpler panel. The bow and stern elements of the hull were cast, facilitating their more complex shapes.

What do you think?

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