Every person who is engaged in the business of loading or unloading ships or vessels, or who is authorized or contracts to load or unload a ship or vessel, or who is in charge of a ship or vessel while it is being loaded or unloaded, and such ship or vessel has a carrying capacity of 50 tons or greater, shall employ and supply upon every ship or vessel while being loaded or unloaded, a person over the age of 18 years to act as signalman or hatch-tender whose sole duty it shall be to observe the operations of loading or unloading of each working hatch on such ship or vessel, and to warn all persons engaged in the operation of loading or unloading of any possibility of injury to any of the articles of which the cargo is composed, or of danger to any person in or about the ship or vessel while it is being loaded or unloaded.