The Jellyfish Defense System
Piracy on the open ocean is a costly and ongoing problem. With multi-million dollar cargo at risk, shipping companies might find it in their best interests to equip their ships with the Jellyfish Defense System. Unlike other deterrents, most notably providing their ships with armed guards (an expensive proposition), this new system is a buy-and-forget-about-it until-you-need-it kind of product.
This defense system invention would consist of several torpedo-like tubes that would be towed alongside the ship. They would continually monitor for the sound of propellers heading towards the ship (filtering out the ship's own propeller noise). If they heard propellers coming their way the torpedoes would detach themselves from their towline and head towards the incoming boat. Before reaching it though, the tube would split into two sections, each one veering off to the side. When the time was right they'd swing back in again, enclosing the boat in a circular net that would foul the boat's propellers if it tried to plow on through it. A warning would come from the ship right about then telling advising the boat of its predicament and suggesting it stay where it was until its identity was established. If it was a valid boarding party, the tube would retract the nets. If it wasn't, it would just keep the net around the boat while the ship sailed away.
The design could be enhanced to deal with boats that have jet drives. Jet drives require at least 3 feet of clear water beneath their intake tube. Without it the drive can get clogged and burn out. All we need to do is add grass-like strands hanging down from the net like a jellyfish's tentacles to deter boats with this type of engine.