Marlin Submarines AP6 “Alicia”: 2000-2004

The AP6 Alicia was built while trading as Marlin Submarines but, thanks to the Discovery channel, Alicia became quite well known and does represent a significant step towards using more acrylic in deep diving submersibles.

P6 and AP6 were originally conceived as deep dive tourist vehicles to take 6 passengers at a time down to 300m. The client was presented with two alternatives, P6 which was a conventional steel hull with large acrylic windows or AP6 whose passenger section was mostly acrylic. Even though the AP6 design was a step into the unknown it was a far more attractive proposition.  An order was placed in 2000 and the sub was launched in 2004.

The main part of the hull consists of two intersecting acrylic spheres, 2050 OD and 100mm thick. At the intersection was a forged steel ring that became quickly known as the ‘Star-gate’. The inside diameter of this ring was 1,543mm making it easy to move between spheres. The design was based on a smaller proposal I did for US Submarines in 1995 called Discovery. Named Alicia after the client’s daughter this vehicle takes 6 people on dives to 300m providing the true ‘floating in space’ feeling characteristic of acrylic hulled submersibles.  The old style of submersible does feel very old after experiencing an acrylic hull. The large hatch inside the tower fairing made for easy entry and exit and the hull is big enough to walk to your seat. From the narrow skid below through to the acrylic deck plate above, the field of view is spectacular.

This particular vehicle was fitted with a diesel engine as well as an electric motor. This gave the boat a great deal more autonomy but a lot more complexity. This was my 4th diesel-electric boat but my view these days is that it is better to spend the money on advanced batteries and keep it simple. Submersibles are complex enough without making them worse. The need to run on the surface compromises hull form, so optimizing the shape for submerged running makes better use of installed power. 

The design of this vehicle was quite a challenge and budgets were tight. The acrylic geometry was well outside PVHO-1 so I had read just about everything Jerry Stachiw ever published about submarine windows at the start. Both conventional mathematics as well as FEA were used in the analysis which was presented to ABS for their ‘special consideration’.  ABS, though willing to co-operate, were well outside their comfort zone so I wrote to the great man himself.  Jerry was very enthusiastic about the project and offered his assistance for a very reasonable fee. Our meeting at ABS head office in Houston was memorable. Who else in the world could say ‘When I wrote the PVHO standard what I meant here was……..’  He won the day of course and set out a series of 1/10th scale model tests to validate the design. I had made a 2,000m test chamber from large hydraulic cylinder components for the series of fully strain-gauged destructive tests.  But 2,000m was not enough, the sphere survived.  The hydraulic guy assured me 250 bar would be OK so we ran the test again and the sphere failed like a 10-gauge shot gun right on 2,500 metres.

The report, endorsed by Jerry, was accepted by ABS and an order for the acrylic spheres placed with Stanley Plastics. I considered it a huge honour to have been able to work with Dr Jerry Stachiw and without him the project would not have gone forward.  Both Jerry and I would have liked to pass the new geometry through to PVHO but unfortunately, we were both contractually prevented from doing so at that time. If anyone would like to re-tread this path in the future, I am happy to help.

Alicia became well known by the general public via the Discovery Channel series which was endlessly repeated and later by the Nova version ‘Underwater Dream Machine’ which has now found its way onto You Tube. I have yet to watch it.