Today, pirates armed with automatic weapons, such as assault rifles, and machine guns, grenades and rocket propelled grenades use small motorboats to attack and board ships, a tactic that takes advantage of the small number of crew members on modern cargo vessels and transport ships. They also use larger vessels, known as "mother ships", to supply the smaller motorboats. The international community is facing many challenges in bringing modern pirates to justice, as these attacks often occur in international waters. Some nations have used their naval forces to protect private ships from pirate attacks and to pursue pirates, and some private vessels use armed security guards, high-pressure water cannons, or sound cannons to repel boarders, and use radar to avoid potential threats.