The submarine operates at three levels.
On the surface she is like a surface ship, easy to see and
very vulnerable to shot, shell and missile.
At periscope depth she is much more discreet and difficult to
see except at very short range. She can still run her diesels,
using the snort mast.
S99 underway on the surface.
Deep dived she is in her element, moving silently below the
temperature layers and almost impossible to detect. She can
remain dived on battery power for up to five days at a time.
Using her passive sonars she is able to hear, track and to
sink ships 10 kilometres away by using torpedoes.
S99 alongside, the forward passive sonar housing bulge clearly
External Torpedo Tubes
There are four External Torpedo Tubes situated
aft of the fin, outside the pressure hull. The Midships group
are offset to 8.5 degrees to the centre-line of the submarine.
Three of the submarine's external Torpedo Tubes clearly visible as
the ferry approaches to drop off visitors.
Robert A (BOB) Johnston)
Inside diameter 553mm