Semaphore

Semaphore Flags are a system for conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags; it is read when the flag is in a fixed position. It is still used during underway replenishment at sea and is acceptable for emergency communication in daylight or, using lighted wands instead of flags, at night.

The newer flag semaphore system uses two short poles with square flags, which a signalman holds in different positions to signal letters of the alphabet and numbers. The signalman holds one pole in each hand, and extends each arm in one of eight possible directions. Except for in the rest position, the flags cannot overlap. The flags are coloured differently based on whether the signals are sent by sea or by land. At sea, the flags are coloured red and yellow (the Oscar flag), while on land, they are white and blue (the Papa flag). 

Displacement of a ship

A ship's displacement is its mass at any given time, generally expressed in metric tons or long tons. The term is often used to mean the ship's mass when it is loaded to its maximum capacity. A number of synonymous terms exist for this maximum mass, such as loaded displacement, full load displacement and designated displacement. Displacement is a measurement of mass, and should not be confused with similarly named measurements of volume or capacity such as net tonnage, gross tonnage, or deadweight tonnage.

The word displacement refers to the mass of the water that the ship displaces while floating. Another way of thinking about displacement is the amount of water that would spill out of a completely filled container, were the ship to be placed into it. A floating ship always displaces an amount of water of the same mass as the ship.

The density (mass per unit of volume) of water can vary. For example, the average density of seawater at the surface of the ocean is 1025 kg/m³ (10.25 lb/gallon, 8.55 lb/US gallon), fresh water on the other hand has a density of about 1000 kg/m³ (10.00 lb/ga, 8.35 lb/US gallon). Consider a 100-ton ship passing from a saltwater sea into a freshwater river. It always displaces exactly 100 tons of water, but it has to displace a greater volume of fresh water to amount to 100 tons. Therefore it would sit slightly lower in the water in the freshwater river than it would in the saltwater sea.

It can be useful to know a ship's displacement when it is unloaded or partially loaded. Terms for these measurements include light displacement, standard displacement, and normal displacement. These terms are defined fully below.

Mediterranean Mooring

In some harbours where there is sufficient room to berth many ships alongside, ships may be obliged to berth at right-angles to a jetty, with their sterns secured to it by berthing lines and their anchors(s) laid out ahead.

This type of mooring can only be employed where there is a negligible range of tide, and is commonly used in Mediterranean ports; for this reason it is often called the Mediterranean (or Med) moor. 

The major considerations are to veer sufficient cable so that the ship can swing clear of other ships at the berth when leaving, to lay the anchors sufficiently far apart to make the ship more secure in wind, and to ensure the anchors do not foul those of other ships. 

In a shallow harbour it is recommended that a frigate veers about 4 shackles of cable on each anchor, and that cables are spanned with and included angle of about 50 degrees, so as to make the ship more secure in a wind. It is obvious, however, that to berth will not be safe if a gale blows from abeam.

If such weather is forecast it is advisable to put to sea or seek a sheltered anchor berth. Ships fitter with a single anchor can, in theory, carry out a Med moor; however, the single anchor will hold the bows steady only in benign weather conditions.