In the U.S., the Navy and Coast
Guard have developed hand grenade sized "depth charges" for use against hostile
scuba divers out to plant bombs on ships in harbors. The war on terror has made
navies nervous about this danger from scuba divers. Terrorists swimming
underwater can attach explosives to the side of a ship, which will punch a
large hole in the hull. This happened several times during World War II (less
than a decade after scuba gear was invented).
Even before September 11, 2001, there were
indications that Islamic terrorists were interested in carrying out these kinds
of underwater attacks. Despite the threat, there aren't too many ways to attack
the swimmers. Bullets from rifles and machine-guns tend to break up on hitting
the water. A swimmer deeper than three feet is generally immune to gunfire. The
other popular weapon is the concussion grenade. These have been available since
World War II, and are meant to be used to clear out bunkers or rooms with
blast, not fragments. So they have twice as much explosives (half a pound) as
normal grenades, and are largely made of cardboard. As an underwater weapon,
they create a shock wave in the water that can kill or disable a diver up to
about ten feet away. The problem is that these grenades have a 4-5 second
delay, so they don't usually sink more than ten feet before exploding. Another
option is special bullets for heavy machine-guns, that will not fragment when
they hit the water. These can be effective down as deep as 30 feet. You can see
where this is going. If a diver swims at 35 feet, he is only vulnerable to
other divers, or the small number of "attack dolphins" trained by a few navies.
The new weapon combines more effective detection
systems, with an "Anti-Swimmer Grenade (ASG). This is basically a concussion
grenade with three times as much explosives (1.5 pounds) and a depth setting
mechanism. In other words, the ASG can be dialed to detonate at depths from 10
to 100 feet. Combine that with the 20 foot concussion radius and you have a
deadly anti-swimmer weapon.
Meanwhile, British, U.S. and Israeli firms have
developed special sonar systems for detecting divers. These devices are about
the size of a barrel, and will detect swimmers within at least an 800 meter
radius. The diver detectors can be linked to automatic ASG dispensers, that
will electronically set the depth, then release the ASGs. Divers can't swim
deeper than 100 feet, because most ships are not that deep in the water. Even
the largest supertanker has a draft of less than 60 feet.
For all the talk about scuba equipped terrorists,
there have been very few cases of this sort of thing actually happening. Most
terrorist scuba activity have been off the Israeli coast, where the Israelis
have been able to deal with the problem without using an acoustic swimmer
denial system that makes the diver sick (using nausea inducing vibrations).
Either that, or just stay close to them until the surface, which all divers
have to do eventually.