Final answer:
A vessel that is required to sound a signal of one prolonged blast followed by three short blasts in a dense fog is known as the 'danger signal.' This signal is required to be sounded by any vessel that is unable to avoid a stationary vessel, a vessel being overtaken, or a vessel that is at anchor or aground.
Step-by-step explanation:
In a dense fog, a whistle signal of one prolonged blast followed by three short blasts is known as the 'danger signal.' This signal is required to be sounded by any vessel that is unable to avoid a stationary vessel, a vessel being overtaken, or a vessel that is at anchor or aground.
The purpose of the signal is to alert other vessels of the presence and potential danger posed by the stationary or constrained vessel.