What can run you over when it’s not moving? Constant Bearing Decreasing Range (CBDR).
CBDR occurs when your vessel and another are approaching each other and the bearing to the other boat isn’t moving. It is holding constant and getting closer. This causes a collision. The Law of Gross Tonnage determines whether your boat is getting run over or it’s running someone else over. Either way, it’s a bad day for all involved.
Rule 7 of the Navigation Rules is the “Risk of Collision.” It talks about a vessel’s responsibility to determine risk of collision, whether or not you are the Give-Way Vessel (Rule 16) or Stand-On vessel (Rule 17).
Rule 7 states: “If the compass bearing of an approaching vessel does not change, that vessel must be considered to be on a collision course.” Due to the size of large ships or the long length of a tow, as they close on a constant bearing they’re going to present an even larger target for a collision. If one or both vessels are going fast, then closing on CBDR could occur quickly. A high -peed ferry doing 40 knots on CBDR with your 8 knots means there’s only a short time before, as the saying goes, “a collision at sea can ruin your day.”
Most collisions begin as a CBDR. The farther the range between vessels or the slower the relative motion, the more time you have to react. You may need every second, especially if you’ve got out 1,800' of wire and a 300' barge astern of you or have several dozen barges in front of you making both of these towing vessels effectively as long as their complete tow.
How can you maintain a situational awareness of a developing CBDR? Rule 5 “Look-Out” says to look out the windows and look at the radar. As Rule 5 also states, use all available means to properly keep a look out. Some of the coastal boats may have an Automatic Radar Plotting Aid (ARPA), an electronic aid that has alarms to alert you of a developing CBDR situation.
An ARPA doesn’t replace a good visual and radar lookout.
On rivers a CBDR situation can quickly become dire due to close quarters in a channel. If the bearing isn’t moving and you’re getting closer, one or both of you, need to do something to avoid a collision. An attempt to squeak by could be your downfall.
Rule 8, “Action to Avoid Collision” tells you to take actions that are “positive, are made in ample time and with due regard to the observance of good seamanship. Any alteration of course and/or speed to avoid collision shall … be large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations of course and/or speed should be avoided.”
Rule 17, “Action by Stand-on Vessel” covers you “in extremis.” This means that the guy who is supposed to get out of the way doesn’t do so. If you are the Stand-on vessel then you “shall take such action as will best aid to avoid collision.”
Listen to the shocking bridge audio recording of the USS Porter getting hit by a supertanker in the Strait of Hormuz. CBDR kills.
Happy New Year and sail safe in 2014.
