Marlin Submarines S101: 1986-87

The submarine has always been a fascination to me in that, like a spaceship, it takes everything it needs into an alien environment.

They are an engineer’s delight in that they embody every branch and discipline of engineering science. The design of S101 which started around 1983 taught me, in addition to the normal mechanical engineer’s arsenal of tools, pressure vessel design and naval architecture. There were also many lessons to be learned about past submarine disasters although compared with all other means of transport these have been mercifully few. The most hazardous part of a ride in a submarine will be the trip to the dock.

S101 was built on the back of a reasonably successful engineering company but even so the budget was tight by modern standards. Although I did not analyse it too closely it probably cost around £20,000 in parts (1984 to 1986).

The design took into account where it was to be used – namely; Plymouth. Here we benefit from one of the largest natural harbours which was vastly improved by French prisoners of war with the construction of the breakwater in Napoleonic times. However, large as it may be the Queens harbourmaster will not let you dive in it anywhere. The round trip from the marina to the nearest dive point and back is 12 miles. This would require a very large battery if all electric hence the resemblance to a WW2 midget submarine in that it is fitted with a diesel engine. In this case it is a 2.5-litre, 3-cylinder Perkins engine normally found in a small tractor.

The electrical system is similar to a milk float. Those under the age of 50 or anyone living outside the UK will not know what they are, or were. So, think giant golf buggy carrying several tons of milk silently to your door in the early hours of the morning. Yes, we did that and still do but not electric sadly. The motor was a re-con unit straight out of an electric float, around 6 kW on 72V. In those days finding a decent motor control unit was hard so speed was series-parallel.

The submarine layout can be seen from the reproduction of one of my original pencil drawings. I went CAD in 1988 straight after S102 but I still enjoy drawing in pencil sometimes. The pilot lies down in the forward section which places his head in the centre of curvature of the generous window. This gives a reality to the view that few have experienced. We are so used to viewing the underwater world through a flat interface we do not appreciate how distorted it is. Just move up to an aquarium window and look sideways. The view through a spherical window is so real that when you are sat on the sea floor looking at the fish, you feel you could just reach out and scoop up a handful of sand.

I would not care for this driving position now but in youth it was fine. The co-pilot had a more comfortable seating position with their head in the acrylic conning tower. Generally, I drove the boat out to the dive point on my own which took about an hour and the days group of ‘co-pilots’ would follow in the launch.

 We first launched late summer in 1986 and operated for around 3 months before lifting out for the winter. This period was a catalogue of equipment malfunctions and failures resulting in me spending many, many hours in the cramped confines of the engine compartment. The learning curve was very steep. I carefully catalogued the failures and problems and saw an emerging theme. I was then able to summarise this into the basic principle I adhere to today whenever possible. This being; keep all the machinery on the inside and keep all of the water on the outside.

Operations in 1987 went well, having spent the winter rectifying the problems. The sub now had a much larger internal ballast tank, up from 300 litres to 550 litres. This provided more freeboard and increased surface speed.  We introduced many people to submarining and also drew the attention of the Royal Swedish Navy which is part of the S102 story.  In 1988 the sub made a guest appearance at the Oceanology ’88 show in Brighton.

Technical details

The Varta lead acid battery had a capacity of 10kWhr which proved more than adequate for our purposes due to the ease with which the vehicle went through the water. The charger was fitted on-board and the diesel had in addition to the HP air compressor a 240V alternator. In theory it could charge its own batteries but, in practise, we just plugged it into the marina 240V supply. The HP air compressor was very useful – the return trip from the dive was sufficient to bring the HP air bank back up to pressure.

Dived control was by a rudder and aft planes behind the prop. Although the rudder works the same at all speeds the aft planes reverse in function when a certain transition speed is reached. This is due to the inherent stability which must be overcome caused by the vertical separation between the centre of gravity and the centre of buoyancy or BG. The greater the BG the greater the transition speed. In our case it was 3 kn. At between 4 and 6 kn you can swoop with precision across the seabed provided there was enough viz. As you drop below 3 kn, up is down and down is up which is difficult to work with. Fortunately, we also had a motorised trim weight under the pilot’s couch which acted as a powerful low speed control. To rise, run the weight back and apply a short burst of power as the nose angles up.

The pressure vessel was designed for 200 metres although the water round Plymouth is generally less than 50m. The shell is 10mm thick stiffened with frames on 500mm centres. The overall design follows the Lloyds of London Rules for Submersibles.

In 1990 S101 was sold to a Swedish submarine pilot, who was also the pilot of S102. After a few years it was sold on to Sea Shepherds, a radical wale conservation group. Although they never used it, they gave it a great Orca paint job. It then passed to Ellis Adams, one of the founding partners of US Submarines who re-built it and used it in the Seattle area. It was then kept at Triton Submarines in Vero Beach, Florida for a while so I saw it often. In 2017 a group in California purchased the sub from Ellis and not only is it active again but has introduced me to a whole new circle of friends.