Explosive Hydroforming of ship hulls
Today, curiously absent
As a kid, I loved to watch Beyond 2000. It’s a TV show from Australia that started airing in 1985 and lasted until 1999. It was full of wonderful future tech some panned out, others did not but it has such a positive view of the future that it was infectious. Watching the show you could not help but feel positive about the future and what it could bring.
I wanted to recapture that feeling after doing an article about AI and discussion around it. What I found is a myriad of projects that had more or less success but what struck me is that we don’t look back anymore and today we are constantly trying to reinvent the wheel. One such wheel is hydroforming, explosive hydroforming to be exact.
Explosive Hydroforming
Hydroforming is not a new technology per se, it has been around since the 1950s but has not seen a mass adaptation as a manufacturing process and saw a noticeable decrease in the 1980s as newer metal forming technologies were introduced that did not require elaborate safety measures.
Explosive hydroforming, or HERF (High Energy Rate Forming) unlike the older method, which shapes metal using pressurized hydraulic fluid pumped into a forming chamber, HERF utilizes a small explosive charge to create the necessary pressure.
The explosive charge is typically positioned at a specific distance from the workpiece, and both are immersed in a fluid, usually hydraulic fluid or simple water. Water is the most commonly used medium as it is the least expensive, excellent for creating uniform peak pressure, and readily available.
The most popular explosive to use was RDX or Cyclotrimethylene trinitramine. RDX comes in the form of pressed granules, and has a detonation velocity of 8,380 m/s, 1,270 kJ/kg of energy, and delivers a maximum pressure of 23.4 GPa.
Explosive hydroforming has a number of benefits with only one major drawback being that it uses explosives.
The smoothness of the contours of the molded metal is easier to control
It can produce large components, thus eliminating the need for costly welds that introduce additional points of failure
The process is less expensive than most other forming processes, especially for larger pieces
Capable of maintaining precision
Tooling costs are lower because only one-half of a die is required. In some cases, like in the case of steel ball, no tooling is required.
Die can be made out of concrete making it extremely cheap compared to traditional steel dies. This is particularly the case for larger pieces
The die is simply a hole in the ground with concrete walls lined with plastic which equates to waxing a mold. Material that is used in such production of a sailing ship hull is a 5 mm 5083 N321 aluminum sheet.
The story from Beyond 2000 that caught my eye was the use of explosive hydroforming in the production of sailboat hulls. Aluminum as a building material for sailboats has a lot of Pros
Good strength to weight ratio
10 times stronger than fiberglass
Doesn't rust
But has one major Con and that is that it is susceptible to Electrolysis and galvanic corrosion if more than one material is used in manufacturing. Galvanic corrosion is when two different metals physically connect in seawater, an electrical current flows between them like a battery.
Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal (that's more reactive) acts as anode and the other (that's less reactive) as cathode. The electropotential difference between the reactions at the two electrodes is the driving force for an accelerated attack on the anode metal, which dissolves into the electrolyte. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited. The presence of an electrolyte and an electrical conducting path between the metals is essential for galvanic corrosion to occur.
This same problem plagued the USS Independence (LCS-2). Unlike such massive ships, smaller ships like sailboats could be made without mixing metals and instead be a combination of an aluminum body with wood or fiberglass trimmings.
The sailing ship in Beyond 2000 was built in Australia by Don Richardson. He changed the usual aluminum hull design as he refused to weld to the hull and instead used adhesive. This is not as crazy as it sounds as the adhesives are heavily used in car production with Lotus Elise being the forerunner of the trend.
Don Richardson used VHB (Very High Bond) adhesive tape to connect the shell to the rest of the boat. Such tape has high adhesion to metal: the connection is quite strong and elastic, does not lose its properties under the influence of oil products, seawater, and temperature drops. VHB tape is already widely used in aviation and space technology, and the maritime industry.
The use of VHB meant that the hull was stronger as welding aluminum can introduce fractures and also removed the corrosion problem as the entire ship is made from aluminum.
I tried to find what happened to the Australian shipbuilding company that used hydroforming but find nothing. I did find that the Gelignite sailboat is still around but now has different owners. As for the process of explosive hydroforming of ship hulls, it’s like it fell thru the shipbuilding memory hole. I could not find any additional information about it or if any shipbuilders use it.