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U.S. Navy (Ford Williams)
On board the USS Porter (DDG-78), Lieutenant (junior grade) Enrique Portillo (left) trains Boatswain’s Mate First Class Derrick Manzano on the use of a maneuvering board, a key tool for avoiding collisions at sea.
U.S. Navy (Ford Williams)

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Bluejacket's Manual - Collision at Sea Avoidance

Non-mariners often ask how ships on the open ocean manage to collide with each other. Most collisions occur in more confined waters, such as the shipping channels approaching busy ports, where many vessels converge on their way in or out. But ships sometimes do collide on the open sea.
By Lieutenant Commander Thomas J. Cutler, U.S. Navy (Retired)
February 2018
Naval History Magazine
Volume 32, Number 1
Article
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Avoiding collisions is complicated by the facts that the open sea has no traffic signs or signals, no designated travel lanes, and no streetlights or headlights to illuminate the way. Further, although mandated light arrays are designed to help mariners identify a ship’s aspect, they can sometimes be confused by spurious or even necessary lights (particularly on aircraft carriers or fishing vessels). The bottom line is that, unlike on land, the open sea is an undefined plane on which vessels can be variously oriented, and it is easier to get into trouble than one might imagine.

Despite the challenges mariners face, there are practices that can significantly mitigate the risk of collision. The first is knowing one’s ship. A shiphandler must know and appreciate such factors as

• How many rudders and propellers (screws) a ship has (a ship with twin rudders and screws is much more maneuverable than one with only one of each)

• What kind of propulsion she employs (ships with gas turbines are able to respond more quickly than a steam-driven ship)

• What is her displacement (the heavier the ship, the more distance required to slow or stop her)

Beyond these basics, a capable mariner also will know such things as the ship’s tactical diameter (turning radius for a 180-degree turn), her pivot point (rotating point as the ship turns), and her advance (distance gained in the direction of the original course) and transfer (distance gained perpendicular to that original course for various speeds).

Among the most important preventives to collisions at sea are the nautical rules of the road. Problems created by the lack of defined paths on the open sea are solved by universal adherence to these standard, internationally accepted practices. For example, when one ship is overtaking another, it is up to the overtaking vessel to maneuver, while the other must maintain course and speed to avoid confusion. And when two ships are approaching each other at an angle, the vessel that has the other ship on her starboard must yield. When these rules are understood and adhered to, the risk of collision is nearly nil. 

The rules of the road also establish standard lighting configurations that help mariners understand a vessel’s aspect (that is, discern whether one is looking at a ship’s starboard or port side, or know whether she is heading toward or away). Additional lights can warn that a vessel is incapable of normal maneuvering because she is trawling, is recovering aircraft, has lost steering control, etc.

Besides taking advantage of available technological advantages, such as the automatic identification system, knowledge of certain basic techniques also can greatly enhance the chances of remaining collision-free. For example, a prudent mariner will use his or her compass to shoot periodic bearings to an approaching ship and use radar to check the vessel’s range. If the bearings to a ship on the port bow are increasing, she will pass ahead; if they are decreasing, she will pass astern. If these observations reveal that the vessel is CBDR (constant bearing, decreasing range), collision is inevitable unless one ship or the other adjusts course and/or speed.

One of the most useful tools for avoiding collisions is a maneuvering board. Ship movements are plotted on the board using polar-coordinate/vector geometry, but a sailor only needs to use simple arithmetic because calculations are made graphically. Using only dividers and parallel rulers, one can use ranges and bearings to other vessels not only to predict how close they will come (known as CPA, for closest point of approach) but also to see what effects changes in course and/or speed will have. Maneuvering boards are a bit intimidating at first, but with practice, mariners can learn their almost magical qualities.

Perhaps the most important factor in avoiding collisions is crew readiness, accomplished through effective training and individual fitness. Watch teams must be well trained in the knowledge and skills required to keep ships safe at sea. And each individual must be rested adequately to ensure he or she is alert and functioning at a high level of efficiency while on watch. Describing these requirements as matters of life and death is no exaggeration. To cut corners in training or fitness is to risk serious injury and death, something that has been proven on too many occasions.

Thucydides is often quoted as having written, “A collision at sea can ruin your entire day.” While the famous Greek historian never actually penned those sardonic words, he did quote the Athenian leader Pericles as having said, “Seamanship is an art like any other; it is not something which can be picked up in one’s spare time”—a valuable lesson learned more than 2,000 years ago

Lieutenant Commander Cutler, a former gunner’s mate second class, is the Gordon England Chair of Professional Naval Literature at the U.S. Naval Institute. His many books include numerous editions of The Bluejacket’s Manual (Naval Institute Press), a copy of which every U.S. Navy enlistee receives.

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