|Length:||11.2 m (36 ft 9 in)|
|Propulsion:||98 kW diesel engine (surfaced)
15 kW electric motor (submerged)
|Endurance:||72 hours max. time submerged|
|Test depth:||410 m (1,350 ft)|
Alicia (Marlin Submarines AP6) is a 6-seater submarine designed and built by Marlin Submarines of Plymouth, England. Designed for research and tourism to depths of up to 305 m (1,001 ft), Alicia offers passengers unparalleled views by having a forward section constructed of transparent acrylic spherical sections made by Stanley Plastics.
The submarine is owned by Peter M. Robbins, who founded the Sierra Madre Express. Marlin Submarines is headed by Paul Moorhouse.
Alicia was pressure tested down to 410 m (1,350 ft) (required is 375 m (1,230 ft)). The budget was US$1.5 million and the final cost ended at US$2 million. The dive angle is at least partially accomplished with the battery holder of 1,750 kg (3,860 lb) sliding back and forward on rails. 
The passenger section of the pressure hull consists of two intersecting transparent acrylic spheres, 1.85 m (6 ft 1 in) inside diameter and 100 mm (3.9 in) thick. The passengers and crew are treated to a spectacular all round view previously enjoyed only from the most advanced research submersibles.
The vessel is the first ever to employ this twin sphere geometry. The craft is a 30% larger version of the US Submarines Discovery design concept, also created by Marlin Submarines in 1995, prior to the Alicia's development. Discovery is just 1.44 m (4 ft 9 in) inside diameter to reduce the crane weight. The passenger section of the Alicia, at 1.85 m (6 ft 1 in) inside diameter, has just over twice the volume.
The acrylic sphere, (the viewing area) was produced by Stanley Plastics Ltd, West Sussex.
The diesel-electric configuration of the Alicia allows the submarine to be placed on station at the dive site relatively rapidly, without the need for a large towing vessel and without eating into battery reserves. In addition it is possible to charge the air banks (used to "blow" the main ballast tanks) when under way on the surface, saving on maintenance time at the dock. Charging the main battery using the diesel engine to drive the main motor as a generator is also possible. The next generation will use the same rare-earth motor technology as S201. The motor will take the full power from the diesel generator set and like S201, drives the prop without the need for a reduction gear.
Commanded by Richard Dawson, one of the design team, the Alicia, AP6 prototype successfully completed sea trials in 2004. These started in No. 2 Basin in Devonport Dockyard and then continued in open water South of Plymouth Breakwater. 25 dives were successfully completed. The test dives reached 180 feet with the surveyor on board. The vessel proved very easy to control and could be hovered a few inches above the seabed. At slow speed a great variety of sea life could be observed. A final dive took a World War 2 U-Boat veteran to visit the last ship he had sunk, the James Egan Lane. The submarine is now in the US.
The new P7
Future Marlin Submarines vessels in the series will offer improvements over the original design, whilst retaining the advantages of the Alicia (Marlin Submarines AP6 concept). The new P7 design has 25% more cabin volume without increasing the overall displacement. The spheres are 50 mm (2.0 in) larger inside diameter and the included angle of the acrylic has been increased. The result is 50 mm (2.0 in) more headroom and a cabin 200 mm (7.9 in) longer. Subtle changes have also been made to the structure to improve the view even further.
- Dream submarine - National Geographic Channel