Pyrotechnics.
By pyrotechnics is meant the use of flares of different colours as a means of conveying certain meanings. For example, if a vessel fires a red flare it means internationally that it is in trouble and requires assistance. The use of various coloured flares is mainly used by the military. Its use is very limited and therefore will not be discussed any further.

Amplified voice.
Each vessel is equipped with a fixed and usually, a portable loud hailer. Again the vessels have to be very close to each other for inter-ship use. Again because of the limited use it will not be discussed any further.

Ship’s siren.
The ship’s siren is used almost exclusively for rule of the road purposes. In other words, it is used to convey to all vessels in the vicinity, the manoeuvres the vessel intends carrying out. In thick fog or restricted visibility the siren is also used to identify a particular type of vessel and its presence.

Underwater telephone.
This method uses underwater sound to communicate with submerged vessels, ie submarines or other submersibles. Again it is used mainly by the military and scientific research vessels. The communication can be either in voice mode or data, ie computer to computer. It has a limited use and is very susceptible to water conditions, ie layers of water could have different temperatures or densities. This would tend to block or reflect sound waves and anything lying beneath the upper layer would not be able to intercept the message. This means is not used by merchant vessels.