AIS transponders are a fantastic safety aid. They receive AIS info from equipped vessels and plot them on your plotter ot PC. They transmit your position, speed, course, vessel name and callsign to other equipped vessels. Great for extra piece of mind, and can be connected to loud collision alarms if required (Short/Single handed etc). In some countries, (eg Singapore) you cannot legally enter their territorial waters without one. Class B is the common one for private/smaller vessels, Class A for commercial ships. No more calling a ship with vague positions - call them by name and callsign, they are much more likely to answer. Instead of "can you see me off your port bow" it's "do you have us on AIS?" and the reply is often "yes, we have you. We will pass you 1nm away on your stb side in 10mins". Much more reassuring!