Polymath A (mostly) technical weblog for Archivale.com

December 22, 2009

Early "Seabasing" Concepts – Still Relevant

Filed under: Aeronautics,Engineering,Floating Structures,Materials,Structures — piolenc @ 6:25 pm

Recently, thanks to the efforts of a friend in the States, a report collection that was formerly available only on 35mm microfilm has been scanned into PDF files. While entering the 400 or so reports into my catalog I came across a 1934 critique, by Charles P. Burgess of the US Navy’s Bureau of Aeronautics, of a proposal by Edward R. Armstrong for a chain of floating airstrips called “seadromes.” These were to consist of an overhead deck and a submerged ballast tank, connected by a double row of vertical cylinders. If that sounds familiar, it should – it’s more or less the standard configuration for modern Very Large Floating Structures (VLFS), including the US Navy’s proposed SeaBase platforms. That was a bit of a surprise to me, because none of the articles on VLFS or sea basing that I’ve seen has acknowledged Armstrong’s much earlier work, which began during WW1 and continued until his death in 1955.

But it gets more interesting, because Burgess’ critique and alternative are just as applicable to the modern proposals as they were to Armstrong’s. Noting that a small waterplane area is the ultimate reason for the stability under wave action of Armstrong’s seadromes, Burgess proposed a more shiplike unitary hull with an anvil-shaped cross section – swollen at the bottom to accomodate ballast, spreading at the top into a wide flight deck – giving a small and very fine waterplane area and much lower resistance to forward movement than the multiple prisms of Armstrong’s concept. In the process, he created a configuration now known by the acronym SWASH – Small Waterplane Area Single Hull – about thirty years before its time. Burgess seems to have been more conscious than Armstrong of the difficulties of deep-ocean anchorage; his concept emphasizes powered station-keeping, which is facilitated by the hydrodynamically favorable hull. Burgess also anticipates modern seabasing proposals, emphasizing the value of a shiplike configuration in getting out of harm’s way if the area starts to “heat up.” I’ve uploaded Burgess’ report to the Files area of the Nation-Builders group on Yahoogroups (file name is BA157.pdf).

A good article on Armstrong and his platform proposals:
http://www.americanheritage.com/articles/magazine/it/2001/1/2001_1_32_print.shtml

The back-issue archive at Popular Science magazine’s http://www.popsci.com also has many articles and news items about Armstrong’s work.

The main difference between Armstrong’s proposal (and Burgess’ counterproposal) and what is mooted now is the current emphasis on modularity. Both Armstrong and Burgess proposed unitary platforms, while nowadays the ability to assemble large units from small, identical components is highly prized – one VLFS concept even involves dynamic assembly and disassembly in situ to suit changing conditions! Armstrong’s configuration is implicitly modular – it consists largely of identical units repeating at equal intervals – which explains its prevalence in modern proposals. Burgess the naval architect, on the other hand, gives his SWASH a beautiful continuously-curved waterline in plan, so his hull could only be built as a single unit. It turns out, though, that minor changes would make Burgess’ configuration “modularizable,” and at the same time cheapen its construction considerably, without compromising its main advantages.

The main change is redesigning the load waterline to consist of a long parallel section, tapered abruptly and symmetrically at both ends. This allows the hull to consist of a variable number of identical “center” units capped with identical “end” units at bow and stern. The end units would have identical propulsion units built in, each capable of giving the whole shebang steerage way and not much more. You end up with the SWASH equivalent of a double-ended ferry, but with only enough installed power for station-keeping. Substituting waterjets with orientable nozzles for conventional screw propellers would allow even very large assemblies to be maneuvered without tugboats. The center units, containing no machinery, could be manufactured in very summary facilities much less well-equipped than standard shipyards. It might be advantageous to make the end units in regular shipbuilders’ yards.

Taking the whole idea one step further, the individual units could be built with double hulls, providing enough reserve flotation to allow them to float, albeit with little reserve buoyancy and with decks awash, even when fully flooded. This would allow them to be assembled into complete vessels or platforms on the water. End units would even be navigable under their own power when unmated and fully flooded – the machinery spaces, located in the ballast tank area, would be sealed and connected with the deck by a narrow trunk like the conning tower of an old-style submarine. This in turn would allow end units and center units to be assembled in separate areas, the end units, mated in pairs, being driven under their own power to where their center units awaited them. The mating operation itself could be carried out in open water, with the end units connecting, independently, with center units one by one until they had enough between them; then the two half-vessels would maneuver to join up.

When newly assembled, the new platform would look like a monitor without the gun-turret, deck flush with the water, but with the hull complete it would gradually be pumped dry inside, ready for fitting-out. It might even be possible to equip the propulsion units to serve as high volume, low pressure pumps, at least in the initial stages of pumping-out.

Materials and manufacturing technology are pretty much ad lib. – steel or aluminum, riveted or welded are feasible, but my favorite is of course ferrocement, which if properly executed can be longer-lived than any other material. Joining method for mating the sections is also up in the air. If the sections are made of steel and they were intended to remain assembled, welding would be the obvious method of choice; bolts are the obvious reversible method, but they are very expensive and would have to be fitted, in our hypothetical open-water assembly method, by divers working underwater and in very poor visibility. One technique that appeals to me is adapted from a system developed for assembling buildings from prefabricated panels in earthquake-prone areas, namely lacing the structure together with steel cables. For permanent assembly, the cables can be grouted into their channels; otherwise they can be secured with cable thimbles at their ends. Post-tensioning would then be possible, which would relieve bending loads on very long assemblies.

Armstrong’s patents:
Canada:
253,140
628,310

US:
1,378,394
1,511,153
1,892,125
2,248,051
2,399,611
2,963,868

France:
532,360
572,543

Burgess’ critique: US Navy Bureau of Aeronautics, Lighter than Air Section, Design Memorandum No. 157, February 1934, “A Proposal for a Single Hulled Seadrome,” by C. P. Burgess. Available from the Files section of the Nation-Builders group on Yahoogroups (see link above).

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