Before we get to the shell of a historic mini-sub that made a brief appearance on eBay last week, we should take a look at the history — always noting that We Are Not Frogs and, just as importantly, We Were Not There, so all this is subject to revision by those who do have inside information — if they ever feel like talking, which they haven’t, much, to date. (Contrary to popular opinion, not every SEAL gets a book agent’s contact info engraved on the back of his shiny new Trident. Most of them clam up as well as their fellow quiet professionals in other branches).
It all started when World War II ended, and the contracting Royal Navy shared its underwater technology with its American cousins. While the US had developed excellent combat swimmer units in the Navy’s UDTs, and early SCUBA gear of several kinds in the highly compartmented OSS Maritime Unit program, the UK had a capability the US couldn’t touch: machines that could deliver a swimmer, and more to the point, a very large high-explosive charge, over considerable distances — underwater. So the US accepted the gift of “chariots” (the British improvement on the Italian Siluro a Lento Corsa [SLC], “low speed torpedo” and Siluro San Bartolomeo), and of a quantity of “X-Craft,” miniature submarines. The chariots proved to be highly limited, and not very popular; but the frogmen loved the X-Craft.
Until Big Haze Gray officiated at a turf battle between what was then the UDT community and what was, and still is, the Submarine Service. The Sub Service was massive, full of admirals, had been critical to WWII victory and had a vision of an all-nuclear fleet that would put the Navy back in the strategic-warfare game. Those sub admirals also had a profound jealousy of anything else that dipped under the waves, in what they considered King Neptune’s — and their own — personal territory. The UDT community was tiny and could maybe latch on to one Captain. An agreement was hammered out — that is to say, dictated to the nascent special warfare community — that limited the UDT (and their offshoot, the SEALs) forevermore to wet subs like the Chariot. The Navy promised to support the frogmen, but the submarine service would take charge of that.
One look at the Not-Invented-Here X-Craft, and the Submarine Service sent them to scrap. They also took over a minisub the frogmen had been developing — and scrapped it, too. The choices were: wait for the Submarine Service to support you with a dry sub, which was never going to happen, or develop your own free-flooding wet sub.
So UDTs spent significant time in the 1950s trying to develop a better wet sub. (Indeed, the SEALs are still trying to develop a better wet sub). This remains a major unforced limitation on SEAL capabilities — the problem is, any wet sub can either  operate in the tropics or  deliver hypothermic SEALcicles in the temperate zones, arctic or in areas of cold currents. Meanwhile, the subs built for general sub service have gotten ever bigger and more coastal-shy over the decades, meaning the frogs are looking at a longer ride in the wet sub at best, or leaving missions on the table for lack of clandestine infiltration capability.
The early Swimmer Delivery Vehicles (SDVs) were… well, we can charitably chalk them up as learning tools. Orr Kelly wrote in his SEAL/UDT history, Brave MenDark Waters:
Then, in the mid 50s, came the Mark 2, built by Aerojet General according to a design from General Electric. From the outside, the Mark 2 looked like a little airplane, with the two-man crew sitting side-by-side. Inside, it would very much like a 1956 Ford pick up. [Naval Coastal Systems Center ocean-engineering head WT “Tom”] Odum says the designers reasoned that the crew members would find the little craft extremely claustrophobic but that they would be more comfortable in familiar surroundings, so they modeled it after the interior of a popular truck.
The Mark 2 was the Navy’s first effort at a sophisticated swimmer delivery vehicle. It was powered by silver zinc batteries, used a gyroscopic compass, and was equipped both with a hovering system and with thrusters that permitted it to maneuver left, right, up, and down.
Unfortunately, it was, as Odum says, “a hydrodynamic nightmare – it just didn’t have any stability.” The little craft never got beyond the experimental stage. It was so unstable that it could not even be towed through the water until it had been pulled up onto a large sheet of plywood.
At that point, Kelly drops the SDV program and picks it up in a paragraph or three with the Vietnam-era experimental Mark 6. Assuming the original dry sub that was sub-napped and sent to bureaucratic Davy Jones’s Locker by the Submarine Service was the Mark 1, the Marks 3, 4, and 5 were probably intermediate submersibles. This period photo shows numerous early SDVs including the Mark 2… some of the other experiments here may have been some of those little known intermediate numbers. The SDV Mark 2 is third from the left. The small pod on the far right may be a Mark 1 Swimmer Propulsion Unit, another experimental device that was not fielded in quantity. The three machines between the Mark 2 and the possible Mark 1 SPU may all three be a single kind of machine with different transparencies fitted.
It is unknown how many SDV Mark 2s were manufactured — given its poor performance under test, there may have been only one. But, almost miraculously, one of these early efforts survived. It showed up on sale on eBay with the following blurb:
interesting 2 man wet submarine by defense contractor AeroJet General for Navy Seal use. It has been salvage non operating with missing parts. for years and it probably more of a collector or decorator item needing at least refurbishment or cosmetics. It is about 20 feet long and is pretty heavy Local pickup in central Indiana. sold in “as is where is” condition. Questions call xxx-yyy-zzzz. The black and white photos are from a rare book about seals.
One gets the impression that no naval architect or hydrodynamicist was let within many miles of this design effort. The sources of the instability seem obvious. It looks like the interesting interior fitments are gone, and it looks like there may have been some kind of thrusters on fins or dive planes forward of the doors.
It’s pickup heritage doesn’t seem too occult in these pictures. How it got to Indiana is an interesting question — the SDV program remains today very sensitive, very close hold, and the Navy has not been above doing research in the desert, hundreds of miles from the sea. Maybe it passed through NAVSEA Crane?
If some Dr Evil has visions of deploying the SDV Mark 2 in the sea again, all we can say is, good luck with that. The US Navy never got this thing going, and they have more money than God. You can’t spend your way around faulty design.
Of course, as this guy in Indiana shows, you can display it on your lawn.