Constant bearing, decreasing range (CBDR) is a term in navigation which means that some object, usually another ship viewed from the deck or bridge of one's own ship, is getting closer but maintaining the same absolute bearing. If this continues, the objects will collide. [1] [2] [3]
Sailors, especially deck hands and those who stand watch on the bridge, are trained to watch for this situation and make it known when they detect it happening. The reasoning is subtle: non-sailors who are used to driving automobiles normally are able to detect the possible risk of a collision with implicit reference to the background (e.g., the street, the scenery, the landscape, etc.) At sea, when vessels are borne in water, the sea surface removes this vital visual clue. Unaware individuals tend to gauge the risk of two vessels colliding based on which direction each vessel is heading. In other words, a novice will think that two vessels which appear to be heading apart from each other cannot collide. This is false, as when a faster vessel is overtaking a slower one, they can in fact have a collision even though the vessels are on substantially different headings.
On a river, CBDR is less useful, as it is likely that both the observed and the observing vessel will follow curving courses along the waterway, whether or not they are on a collision course. In such situations safe navigation depends on upstream and downstream traffic keeping to separate sectors of the navigable channel. [4]
When a non-moving background (typically the shoreline) is present, CBDR situations can usually be recognized by observing that some part of the moving target vessel is not visually moving against the background. On the other hand, if the target vessel visually moves forward against the background, with relative bearing decreasing, then the target vessel will pass in front of the observing vessel and there is no risk of collision, provided both vessels maintain constant speed and course. Similarly, if the target vessel visually moves backward against the background, with relative bearing increasing, the target vessel will pass behind the observing vessel. However, this method of recognizing non-CBDR conditions is only accurate for relatively small observing vessels. With larger vessels, only an observer positioned near the bow can reliably make the first conclusion, whereas the second conclusion can only be left to an observer positioned near the stern.
Another source of confusion arises from the distinction between relative bearing and absolute bearing . One might think that another vessel which seems to be progressing from, perhaps, dead ahead to apparent movement down one's starboard side cannot result in a collision. This is not quite true. Because a vessel afloat can change its true heading (relative to north) without obvious detection (unless there are landmarks in sight) two vessels may collide even if one presents a substantially changing relative bearing.
The phenomenon also presents itself in road safety. At the crossing of two straight roads, a CBDR condition may occur and keep one vehicle in the other's blind spot. [5] Even without blind spots, the lack of optic flow makes for harder spotting. [6]
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another. The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation.
Seamanship is the art, knowledge and competence of operating a ship, boat or other craft on water. The Oxford Dictionary states that seamanship is "The skill, techniques, or practice of handling a ship or boat at sea."
In navigation, bearing or azimuth is the horizontal angle between the direction of an object and north or another object. The angle value can be specified in various angular units, such as degrees, mils, or grad. More specifically:
Piloting or pilotage is the process of navigating on water or in the air using fixed points of reference on the sea or on land, usually with reference to a nautical chart or aeronautical chart to obtain a fix of the position of the vessel or aircraft with respect to a desired course or location. Horizontal fixes of position from known reference points may be obtained by sight or by radar. Vertical position may be obtained by depth sounder to determine depth of the water body below a vessel or by altimeter to determine an aircraft's altitude, from which its distance above the ground can be deduced. Piloting a vessel is usually practiced close to shore or on inland waterways. Pilotage of an aircraft is practiced under visual meteorological conditions for flight.
The International Regulations for Preventing Collisions at Sea 1972 (COLREGs) are published by the International Maritime Organization (IMO) and set out, among other things, the "rules of the road" or navigation rules to be followed by ships and other vessels at sea to prevent collisions between two or more vessels. COLREGs can also refer to the specific political line that divides inland waterways, which are subject to their own navigation rules, and coastal waterways which are subject to international navigation rules. The COLREGs are derived from a multilateral treaty called the Convention on the International Regulations for Preventing Collisions at Sea.
This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water. Some remain current, while many date from the 17th to 19th centuries. The word nautical derives from the Latin nauticus, from Greek nautikos, from nautēs: "sailor", from naus: "ship".
In marine navigation, a pelorus is a reference tool for maintaining bearing of a vessel at sea. It is a "simplified compass" without a directive element, suitably mounted and provided with vanes to permit observation of relative bearings.
A second mate or second officer (2/O) is a licensed member of the deck department of a merchant ship holding a Second Mates Certificate of Competency, which is issued by the administration. The second mate is the third in command and a watchkeeping officer, customarily the ship's navigator. Other duties vary, but the second mate is often the medical officer and in charge of maintaining distress signaling equipment. On oil tankers, the second mate usually assists the chief mate with the cargo operations.
A third mate (3/M) or third officer is a licensed member of the deck department of a merchant ship. The third mate is a watchstander and customarily the ship's safety officer and fourth-in-command. The position is junior to a second mate. Other duties vary depending on the type of ship, its crewing, and other factors.
A marine radar with automatic radar plotting aid (ARPA) capability can create tracks using radar contacts. The system can calculate the tracked object's course, speed and closest point of approach (CPA), thereby knowing if there is a danger of collision with the other ship or landmass.
The end around was a submarine tactic used when the position of a submarine did not allow the submarine to mount an immediate attack. The submarine's crew estimated the enemy's bearing, heading, and speed, remaining submerged until out of the enemy's visual range. The submarine then surfaced and proceeded at maximum speed to a position in front of the target, near the target's predicted course but remaining out of view. The submarine would then submerge, approach, and attack.
Proportional navigation is a guidance law used in some form or another by most homing air target missiles. It is based on the fact that two vehicles are on a collision course when their direct line-of-sight does not change direction as the range closes. PN dictates that the missile velocity vector should rotate at a rate proportional to the rotation rate of the line of sight, and in the same direction.
The MV George Prince ferry disaster was a nautical disaster that occurred in the Mississippi River in St. Charles Parish, Louisiana, United States, on the morning of October 20, 1976. The Luling–Destrehan Ferry George Prince was struck by the Norwegian tanker SS Frosta, which was traveling upriver. The collision occurred at mile post 120.8 above Head of Passes, less than three-quarters of a mile from the construction site of the Luling Bridge which would replace the ferry seven years later. The ferry was crossing from Destrehan, Louisiana on the East Bank to Luling, Louisiana on the West Bank. Ninety-six passengers and crew were aboard the ferry when it was struck, and seventy-eight perished. This accident is the deadliest ferry disaster in United States history. It is also the deadliest peacetime nautical disaster involving a non-submersible vessel in U.S. waters since the explosion of the SS Grandcamp in 1947, which killed 581 people. In addition, it is the deadliest accident involving a single vessel in U.S waters since a fire on board the SS Morro Castle in 1934, which killed 137 people.
Motion camouflage is camouflage which provides a degree of concealment for a moving object, given that motion makes objects easy to detect however well their coloration matches their background or breaks up their outlines. The principal form of motion camouflage, and the type generally meant by the term, involves an attacker's mimicking the optic flow of the background as seen by its target. This enables the attacker to approach the target while appearing to remain stationary from the target's perspective, unlike in classical pursuit. The attacker chooses its flight path so as to remain on the line between the target and some landmark point. The target therefore does not see the attacker move from the landmark point. The only visible evidence that the attacker is moving is its looming, the change in size as the attacker approaches. Motion is also used in a variety of other camouflage strategies, including swaying to mimic plant movements in the wind or ocean currents.
Mini-automatic radar plotting aid is a maritime radar feature for target tracking and collision avoidance. Targets must be manually selected, but are then tracked automatically, including range, bearing, target speed, target direction (course), CPA, and TCPA, safe or dangerous indication, and proximity alarm. MARPA is a more basic form of ARPA.
Underwater searches are procedures to find a known or suspected target object or objects in a specified search area under water. They may be carried out underwater by divers, manned submersibles, remotely operated underwater vehicles, or autonomous underwater vehicles, or from the surface by other agents, including surface vessels, aircraft and cadaver dogs.
SS (RMS) Peveril (I) No. 76307 – the first vessel in the company's history to be so named – was a packet steamer which was operated by the Isle of Man Steam Packet Company until she sank off Douglas following a collision with SS Monarch in 1899.
In navigation, the heading of a vessel or aircraft is the compass direction in which the craft's bow or nose is pointed. Note that the heading may not necessarily be the direction that the vehicle actually travels, which is known as its course or track. Any difference between the heading and course is due to the motion of the underlying medium, the air or water, or other effects like skidding or slipping. The difference is known as the drift, and can be determined by the wind triangle. At least seven ways to measure the heading of a vehicle have been described.
SS Monarch No. 90117 was a tramp steamer which was operated by Alexander King Ltd, Belfast. She was involved in a collision with the Isle of Man Steam Packet Company steamer RMS Peveril, off Douglas, Isle of Man in the early hours of 17 September 1899, resulting in the sinking of the Peveril.
This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water. Some remain current, while many date from the 17th to 19th centuries. The word nautical derives from the Latin nauticus, from Greek nautikos, from nautēs: "sailor", from naus: "ship".
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