This is an article from the United States Naval Institute Proceedings from December, 1972. I respectfully suggest that these concepts still apply, and should be considered by bridge and CIC watch teams.
The math makes sense from a maneuvering board perspective - think of Speed of Relative Movement (SRM) against Tactical Diameter, and then apply COLREG's. The terminology is dated, but the philosophy is still sound.
The many electronic devices developed in the last 50 years now extend the eyes and ears of the shiphandler to distances never before thought possible. Environmental conditions no longer blind the master; the limitations of the senses no longer are the determining factors as to where safety ends. Collisions still occur daily despite the most elaborate devices and advances made. There are no new ways to have a collision, only slight variations of a time-honored pattern. It is almost beyond belief that two ships, a thousand miles from the nearest land, can collide, yet such is frequently the case.
The skill and judgment necessary to maneuver a vessel safely through potentially dangerous situations is for the most part left to those officers on board who have demonstrated their abilities under proper supervision to become qualified OODs. The art and skill of ship maneuvering is constantly being passed from OOD to OOD, from the experienced officers to the newly-commissioned ensigns. Oftentimes it is impossible to pass on every trick of the trade that has been gained over the years, and, therefore, the junior officers in particular are left on their own to learn by experience the proper action required to accomplish a specific end. But the job of maneuvering a ship out of a possible collision situation should not be left entirely to trial and error experimentation. There are many skills and tricks of the trade we can still learn from the experienced masters of the past.
Unfortunately, the experience level of our OODs today is decreasing. The World War II crop of qualified shiphandlers has pretty much moved on. Retirement has taken a drastic toll. Manpower reductions have lowered the number, also. Ship movements are being reduced to those necessary to maintain readiness, thus shortening the hours of training available to the junior officers for underway watch training. Increased administrative and technical duties add to the load the junior officer must carry in addition to maintaining his qualification as an up-to-date OOD. Although there is an increased emphasis today on shiphandling and proper use of the Rules of the Road, the time available for effective training is steadily decreasing.
The Rules of the Road currently in effect are a composite of the many early ideas and general rulings of the past. Court interpretations and rulings are still trying to deal with the problems of making the Rules of the Road work more effectively in light of today's heavy ship traffic. The Rules operate upon the principle that, if they are obeyed in a timely manner, collision will not occur. The only assumption that a watchstander on board any ship can make in interpreting the Rules of the Road in a potential risk of collision situation is that the other vessel is bound to obey the Rules of the Road, and he must do likewise. No OOD can afford to attempt to outguess the other fellow when determining what maneuver he is going to make; there is just too much at stake to disregard the Rules of the Road.
The Rules of the Road, if followed as written, give the watchstander a sufficient set of instructions on how to prevent a collision. For example, Rule 18 states:
"When two power-driven vessels are meeting end on, or nearly end on, so as to involve risk of collision, each shall alter her course to starboard, so that each may pass on the port side of the other."
This, like the many other instructions contained in the Rules of the Road, is a simple and easily understood set of maneuvering instructions, telling you to turn right and avoid the other fellow. However, the records of shipping losses throughout the years give evidence of many head-on collisions despite these simple instructions. The fault in more and more collisions occurring today lies not in or with the installed equipment, but with the watchstander-human error.
The set of guidelines in the Rules of the Road gives sufficient instructions provided that the vessels act completely in accordance with the Rules. But what is the recourse, if, for some unexplained reason, the other vessel in a risk of collision situation fails in her obligation to remain clear and avoid the possibility of collision?
The watchstander of a privileged vessel operates under a most explicit set of rules requiring him in all cases to hold his course and speed until reaching the point of in extremis . In extremis is defined by Farwell:
"Whenever two moving vessels approach each other so closely that collision is inevitable unless action is taken by both vessels to prevent it, the situation is in extremis ."
Where is this mysterious point upon which the very safety of your ship may depend at some time? The Rules of the Road are not clear, never have been, and probably never will be clear concerning this much-argued point. A familiar question asked by students of the Rules of the Road and by watchstanders is, "At what point (distance) am I entering in extremis ?" The answer to that question is as unclear as the definition of the term itself. The Rules of the Road do not say where the dividing line between safety and danger is. Everyone has a different interpretation of the limits of in extremis . Nowhere is there an exact answer given in yards and feet. The watchstander is the one who is going to have to make the ultimate decision.
An attempt to solve this particular problem of when to begin collision avoidance maneuvering in a possible i n extremis type situation was proposed more than 20 years ago by Commander Davis N. Lott, U. S. Naval Reserve, in his book entitled Collision Prevention . This book, now long out of print, was written by Commander Lott as a result of his experience and observations while stationed at the Emergency Shiphandling School at Pearl Harbor in 1944. The maneuvering rules that he suggests came about from an analysis of ship's movements, collision case histories, actual observations, 2nd recorded tactical data. The guidelines he established have been proven both in ship trainers and on board ships of the Fleet under wartime and peacetime conditions. His Rules for Collision Prevention consist of a system of pre-established distances at which the watchstander ought to anticipate commencing collision avoidance maneuvers should he determine that risk of collision exists. These guidelines at least give the watchstander an idea of when to act, based upon reliable tactical data rather than initiating avoidance maneuvers based entirely upon information obtained from 'seaman's eye."
Lott's Collision Prevention Rules are based upon the tabulated and documented turning distances which have been measured for each individual vessel and incorporated into the ship's Tactical Data Folder. This collision avoidance system was acknowledged to be entirely valid by Pacific Fleet Commander Chester Nimitz in 1944, and was published and distributed to the Fleet in the form of the Emergency Shiphandling Manual . It has also been taught at Fleet training centers for the past 15 years, but it is surprising how few people have been exposed to it.
The Rules for Collision Prevention do work. Experiments in ship simulators and actual experience have shown them to be approximately 80 to 90% effective in establishing effective "miss distances," which does justify another look at them. The Tactical Data Folder is an important tool of the trade for the watchstander, and it should be put to work for the safety of the ship. As long as this information is considered sufficiently accurate for the planning and execution of alongside replenishment operations, CIC's solution to tactical problems, rescue plans, and restricted water navigation problems, why not use it for the safety of the ship? This perhaps is the most important peacetime use of this information. Only a very few figures need be extracted and computed beforehand to make this system work.
The legal aspects of Commander Lott's Rules for Collision Prevention are few. The most important consideration is this: The Rules of the Road neither deny nor acknowledge the existence of the Collision Prevention Rules. Lott's Rules are not binding or mandatory forms of action under the Rules of the Road or from higher naval authority at present, only recommendations based upon established data. They are better and more justifiable than any system or guideline now being used to prevent collision.
The Rules for Collision Prevention
The Rules for Collision Prevention have several terms which may or may not be familiar to all.
► Collision Angle . The term collision angle is described as the angle between the intersection of the projected courses of two crossing, meeting or overtaking vessels, where risk of collision is deemed to exist, measured from the projected course of the vessel from starboard to port. Collision angle by definition cannot exceed 180 degrees, as illustrated in Figure 1. Collision angle is a visual observation made by the conning officer and, while its accuracy is most desirable, it is generally agreed that it can only be made to the nearest degree or two. It must be remembered that you are dealing with the inaccuracies of the human eye and the interpretation made by people in what they see. For this reason, it is difficult to apply the Rules for Collision Prevention to the exact degree in most cases.
It should be noted that collision angle and target angle are not the same. Collision angle is determined from the true course of your ship and the observed true course of the approaching vessel as can be best determined. Relative angle does not enter into this system. The distances computed are slant range, and are compatible with radar observation.
► Full Rudder Advance . This is the distance gained in the direction of the original course from the time the ship's wheel is first moved in the direction of the turn to the time when the ship is steadied up on the new course. All turns made in accordance with these guidelines are considered to be emergency turns therefore using full rudder to gain the minimum amount of advance possible. The distance equal to the Full Rudder Advance at any particular speed is found in the Tactical Data Folder for each type of vessel. It is important to note that the use of any less rudder in compliance with these rules will result in an advance in excess of the predetermined figures. For the purpose of these rules, transfer—the distance gained at right angles to the original course—is not considered, as we are more concerned with the distance covered in the direction of the approaching vessel.
► Clearing Distance . Clearing distance is the distance at which you desire to pass clear of the other vessel at the completion of your emergency maneuver. A clearing distance is only to be used by a Burdened Vessel. It is a safety factor to allow for safe minimum maneuvering. The Privileged Vessel cannot apply a clearing distance and operate within the Rules of the Road because she is required to maintain course and speed, until within the jaws of in extremis . It would appear that the guidelines proposed as minimum distances for the Privileged Vessel might be considered those for in extremis , although they are not. The distances are but recommendations only. The Rules of the Road provide no safe maneuvering distance for the Privileged Vessel and therefore cannot apply the clearing distance when faced with imminent collision.
► Minimum Safe Range . The minimum safe range is the minimum distance at which the approaching vessel may be allowed to close the watchstander's vessel and still allow him the absolute minimum room to maneuver out of a possible collision. There are two minimum safe ranges to consider: the minimum safe range for the Privileged and the minimum safe range for the Burdened vessel.
It should be noted that the minimum safe range for vessels, whether Burdened or Privileged, will vary with the different approach situations, ship's speed, designated clearing distances, and ship types. The minimum safe range is a distance tailored for each specific ship.
Of the four definitions previously given, the minimum safe range is the most significant to the watchstander, because it is this figure that is used by the watchstander to answer the question, "How close can I come to the other vessel? How long must I maintain my position as Privileged before I must begin to take action? When am I committed to a collision?" The minimum safe range is the last possible computed moment that you can continue towards the approaching vessel and still be able to avoid collision by emergency means.
This would appear to be determining in extremis and it may be, but it is up to the watchstander from here on, as he is faced with the problem of the actual determining of in extremis . If he feels the recommendation is a realistic distance, fine. If the watchstander disagrees—and it is his responsibility to disagree—he must make the ultimate decision of when to maneuver. All that is pointed out here is that the minimum safe range is a recommended distance, computed from actual facts and tested possible actions, and its application is up to the people who must use it.
Lott's Rules for Collision Prevention are based upon strict obedience to the Rules of the Road. They may be separated into two parts: a set of guidelines for the Privileged Vessel and a set of guidelines for the Burdened Vessel.
This system utilizes only those factors readily available to the watchstander, namely his own ship's characteristics and speed. It does not use any other information concerning the approaching vessel other than a visual observation and estimation of her probable course. It would be difficult to justify knowledge of the approaching vessel's characteristics from a visual observation alone, therefore the watchstander should tend to disregard any preformed opinions as to what maneuver is expected from the other vessel. This should not be taken as sufficient reason to disregard CIC's recommendations in regard to the situation, but rather CIC should strengthen the watchstanders position as to what action to initiate.
The Rules for Collision Prevention as They Apply to the Burdened Vessel
If you are in a vessel deemed to be Burdened in an unfortunate situation involving imminent risk of collision, it is your duty to:
Comply with the Rules of the Road explicitly.
Make every attempt to maneuver early and avoid an embarrassing and possibly confusing situation. The Rules for Collision Prevention as they apply to the burdened Vessel arc but a guide in planning your actions and in estimating how long the Burdened Vessel may hold her course and speed before maneuvering is definitely necessary to avoid embarrassing the Privileged Vessel. It is emphasized that this must not be construed to mean that the Burdened Vessel may hold her course and speed until within range of these rules, but rather, she should maneuver well outside the prescribed distance that the Rules for Collision Prevention indicate and must not close under the minimum safe range.
(1) Overtaking Situation Burdened Vessel: 0-68-Degree Collision Angle :
(a) The overtaking vessel must not close the overtaken vessel closer than a distance equal to one Pull Rudder Advance at the present speed plus the specified clearing distance.
(b) Make all turns in accordance with the Rules of the Road.
(2) Low Angle Crossing Situation Burdened Vessel: Collision Angle Less than 90 Degrees but Not Considered to be Overtaking (From two points abaft abeam to almost on the beam) :
(a) Approach the Privileged Vessel no closer than a distance equal to two Full Rudder Advances at the present speed plus the specified clearing distance.
(b) If the turn is executed at the prescribed range, the Rules for Collision Prevention will work only by turning left, and with respect to good seamanship, Rudder action must be taken prior to reaching this point.
(3) Crossing Situation Burdened: Collision Angle about 90 Degrees (Approximately on the beam) :
(a) Approach no closer than a distance equal to three Full Rudder Advances at the present speed plus the specified clearing distance.
(b) The direction of the turn at the prescribed distance must be to the left for the Rules to work.
(c) To turn in accordance with the Rules of the Road, initiate the turn at a distance greater than that prescribed in (3-a) above.
(d) Do not attempt to cross the bow of the Privileged Vessel.
(4) Crossing Situation Burdened: Collision Angle Greater than 90 Degrees But Not Considered to be a Meeting Situation (From abeam to one point off the starboard bow.)
(a) Approach the Privileged Vessel no closer than a distance equal to four Full Rudder Advances at your present speed plus the desired clearing distance.
(b) Turn in accordance with the Rules of the Road.
(5) Meeting Situation Both Vessels Burdened: 169- 180-degree Collision Angle (one point on either bow)
(a) Begin to maneuver by the time the watchstander's vessel has closed the approaching vessel to a distance equal to five Full Rudder Advances at the present speed plus specified clearing distance.
(b) Turn right in accordance with the Rules of the Road.
In applying the Rules for Collision Prevention, the limits of the meeting situation have been arbitrarily set at one point on either bow. The one-point limit designation has been made for several reasons; high relative speed of approaching vessels, and court interpretations.
The limit of one point on either the bow has been designated as such partially from a liberal interpretation of court decisions. Farwell in The Rules of the Nautical Road points out that:
"Court decisions have done very little to modify the statutory description of vessels meeting end on or nearly end on."
" The writer [Farwell] was unable to find a single case of the type mentioned placing vessels in the head and head category where the courses lacked more than a point of being opposite. " A wider variance of court decisions, as he points out, came from the Inland decision wherein "I f they are approaching as much as 1½ point to 2 points, they are to be considered as head and head and bound to pass port to port. " So here we have quite a large difference of opinions, while at the same time, in contrast, Farwell notes that in the Gulf Stream (NY 1890) two vessels approached with intersecting courses varying only 1 ° and they were considered to be crossing. The courts have not been of much assistance in this area, and for purposes of clarification, it was thought that some sort of sector designation should be made to assist the watchstander compute a minimum safe range.
The definition of head and head is no better described than are the court decisions on the matter. Rule 18 describes the daylight qualifications for the ships to be meeting, as " they are end on or nearly end on; each sees the masts of the other in line or nearly in line, with her own. " This definition is subject to a great deal of interpretation by every watchstander when meeting another vessel. "Or nearly in line," does not establish very well how many degrees variance of courses is allowed and still considered to be meeting. The courts in the past have not proved any help in this matter as their decisions have been as inconsistent as are the factors pertaining to each case.
What has been done here is at least to establish limits for the use of the watchstander, limits for his use in determining the distances at which he should anticipate taking action. If a ship approaching should have a collision angle falling into a sector designated by the Collision Prevention Rules as a meeting situation, he will at least know at what distance he should anticipate taking action. The watchstander is still at liberty to call the situation meeting or crossing as he wishes and act accordingly. What action he decides upon is his decision, and all that is done here, is to advise him approximately when he should begin or at least have begun to take some evasive action.
The area described as from one point on cither bow to directly ahead represents the sectors from which the approaching vessels under most circumstances present the greatest threat to the watchstander on the basis of high relative speed. The one-point sector width represents a compromise between the many court decisions and interpretations on this subject and an analysis of relative speed and collision angle. Nowhere around the compass will the approaching vessel, in most cases, have a higher relative speed than approaching from a point directly or almost directly ahead. The bow is used as a common reference point because it is familiar to all, and the point system of measurement is easily used or at least estimated fairly accurately by the great majority of watchstanders.
The Collision Prevention Rule for this head-and-head area is to commence some evasive action no later than by the time the approaching vessel has closed to a distance equal to five Full Rudder Advances plus specified clearing distance. This may look like an excessive distance at first, but when it is considered in view of perhaps a 40-knot relative closure rate, it may appear very small. A relative speed of 40 knots is not an unrealistic figure these days when the cruising speed of the bulk carriers, freighters, and now the amphibious force is being steadily boosted towards 20 knots as a minimum. Two vessels approaching each other head-on generate this much relative speed easily. As an example, suppose that a ship at 20 knots has an advance of 600 yards. Five times this distance plus the clearing distance of, say, 500 yards, would place the minimum safe range out at 3,500 yards. With a 40-knot closure rate, the watchstander has a little less than three minutes of safety left in which to get out of the way of the other vessel. This hypothetical distance at least indicates that the outer distance of safety lies further away than it would first appear.
In a study of collision case histories, almost all meeting collision situations can be seen to develop at a long range previous to the collision, when defensive ship maneuvering by one vessel could easily and safely have avoided the situation. It is the waiting to see what the other vessel is going to do that has contributed more to numerous collisions and near-misses than anything else.
Rules for Collision Prevention as They Apply to the Privileged Vessel
A watch officer on the bridge of a Privileged Vessel in any type of risk of collision situation is not in an enviable position, in that he must hold his course and speed until reaching the point of extremis. International Rule 21 sets forth the basic guidelines for this situation in that:
" . . . one of the two vessels is to keep out of the way, the other shall keep her course and speed. When, from an) cause, the latter finds herself so close that collision cannot be avoided by the action of the giving-way vessel alone, she also shall take such action as will best aid to avert collision."
Rule 21 gives authority as well as a requirement for the Privileged to hold her course and speed, only giving way when she has determined that " . . . collision cannot be avoided by the action of the giving-way vessel alone. . . " Here is that ambiguous area again, Rule 21 indicates no yards-and-feet answer, when to begin evasive maneuvering, only that the watchstander must feel that collision is imminent. A prudent navigator should always have a plan of action in mind. The phrase "As will best avert collision," should in all cases not be used as authority to act irresponsibly in a dangerous situation such as this.
Rule 21 places the burden upon the watchstander as he must maintain a vigil of ever-increasing danger; he must continue on his course, always watching the approaching vessel for signs that she is going to sheer off and avoid his vessel.
The watch officer must realize fully the implications of being Privileged. Court decisions many times in the past have held the Privileged Vessel at fault in collisions resulting from early maneuvering, despite the fact that the watchstander felt that he was in imminent danger of collision.
In review, a point that should be remembered is that the Privileged Vessel is allowed to sound the danger signal. This signal serves a two-fold purpose in that it relays the fact that there is imminent danger collision with the Privileged Vessel and the Privileged Vessel is alerting the Burdened to her duty to avoid the Privileged, and she doubts that this duty is being properly carried out.
The minimum safe range as applied to the Privileged Vessel consists only of a number of specified bull Rudder Advances. There is no clearing distance included as mentioned earlier in the definition of minimum safe range.
(1) Collision Angle 0-68 Degrees—Privileged (From two points abaft abeam to port to a position two points abaft the beam to Starboard). As Privileged Vessel, you must not allow the Burdened Vessel to approach closer than a distance equal to one Full Rudder Advance at the present speed, but never less than 500 yards. The one maneuver open to you is to turn away from danger, in this case turning to starboard. A turn to starboard should be made in all situations except in the case of a vessel overtaking on the starboard quarter, in which a turn to starboard would result in collision with the overtaking vessel, therefore the turn must be to the left.
(2) Collision Angle 68-169 Degrees—Privileged (From two points abaft the beam to Port to one point on the port bow.)
The approaching vessel must not be allowed to approach closer than a distance equal to two Full Rudder Advances at the present speed, but never less than 1,000 yards. Turn away always to the right. Turning to the left in a large majority of collision case histories has resulted in collision because of the simple fact, that the approaching vessel finally turned right in accordance with her obligation under the Rules of the Road.
A visual summation of the complete set of the Rules for Collision Prevention is given in Figures 2 and 3. The two diagrams make it possible for the watchstander to easily apply the appropriate rule for each situation The visual summation in Figure 2 places the watchstander of the vessel in the center of the diagram and considers the approaching vessel from any number of Agrees of collision angle. This diagram considers all approach situations except for the situation where the Watchstander is overtaking the Privileged Vessel. Overtaking the Privileged Vessel is shown in Figure 3.
The determination of which is the Burdened or Privileged Vessel is shown on Figure 2 by means of the heavy black line encompassing a sector from one point on the port bow around the port side to a position two points abaft the abeam to starboard. Any vessel approaching with a collision angle placing her in this sector results in the watchstander's vessel being considered as Privileged. The heavy black line represents the minimum safe range allowable for the Privileged Vessel, and the approaching vessel must not be flowed to cross this limiting distance.
When an approaching vessel has a collision angle that can be placed in the area from one point on the port bow, starboard to a position two points abaft abeam to starboard, the watchstander's vessel is in the position of Burdened Vessel for purposes of applying the Collision Prevention Rules.
No specific minimum safe range has been designated in the diagrams for the Burdened Vessel (Figure 2 and Figure 3) although the number of Full Rudder Advances required is set, the clearing distance for each vessel may vary as to the wishes of her Master.
It is easy to see where the most dangerous area is, namely that portion of ocean encompassing an area from directly ahead to one point on either bow. The number of Full Rudder Advances applied in this area reflects the increased range at which the watchstander must consider anticipating the commencement of an evasive maneuver to avoid the approaching vessel. A liberal interpretation should be made in regard to the determining of these distances and collision angle in regard to local conditions, type of ship approaching and estimated speed of the approaching vessel. It must be remembered that the distances precomputed are minimum distances. Should the approaching vessel have excessive speed (greater than 35 knots), this may be grounds for altering the recommended minimum safe range considerably. The minimum safe range must be increased to compensate for it, but in the greatest majority of cases, the Rules for Collision Prevention represent a sufficient distance of safety.
Figure 3 covers the situation when the watchstander's vessel is overtaking the Privileged Vessel. Determine the collision angle before automatically saying that it is an overtaking situation. Collision angle is the determining factor in classification of the situation. Once the angle is determined, apply the rules and act accordingly.
This collision prevention system is based upon the premise that for the large majority of seagoing vessels, the advantages of attempting to turn out or away from an impending collision situation outweigh the advantages of trying to reduce the headway of the vessel by means of emergency slowing, stopping or backing maneuvers. The reduction of the relative closure rate of the vessels is the important fact to be kept in mind and all maneuvers made should reflect this basic thought. If a collision is to occur, despite the best of avoidance maneuvers, collide at a slow rate of speed rather than fast.
Should there be doubt in the mind of the conning officer when observing the approach of another vessel on how to apply the Collision Prevention Rules, apply this guideline; if the observed vessel is seen to fall on or very near the boundary line of the various Full Rudder Advance sectors, it is best then to apply the rule for the next sector with the next higher number of Full Rudder Advances. This principle, if nothing else, will allow an additional safety factor in an area where there may be possible concern as to what distance action should be initiated.
The advantages of turning out or away from a collision situation are many over backing down: (a) a reduced relative closure rate; (b) better ship control is maintained because the rudder is not losing the flow of water past it, whereas in backing, rudder control is lost for the most part; (c) the advance of the large majority of vessels is a shorter distance than is the distance required to stop; (d) this maneuver is almost immediately effective, whereas an emergency backing of the engines requires that considerable time and distance which is being consumed while still approaching the danger; (e) it gives an immediately visible indication to the other vessel of what action you are taking; (f) this maneuver has been proven to work.
This policy of attempting to turn away from imminent collision follows court decisions as Farwell notes, "Any action taken in good faith to avoid collision will be upheld by the courts." In some cases the action of just slowing or stopping the engines may not be enough on the part of the watchstander. An appropriate quote from Wills in Simplified Rules of the Road says "Any action, but none." Just don't sit there and wait for the collision to come to you. Do something .
The proof of the Collision Prevention Rules is found in the Tactical Data Folder for each vessel concerned. To verify whether or not your vessel would react in accord with the statement made on her stopping in a distance greater than her advance at any speed, all that is necessary is to get the Tactical Data Folder and compare some basic figures.
The distances in Table 1 are for three Privileged Vessels in separate situations with a collision angle of 90 ° (Figure 4) and illustrates when they should anticipate commencement of an evasive maneuver. These are examples from actual Tactical Data Folders representing a varied cross section of vessels, a CVA, DD, and a small cutter with variable pitch propellers. Ship Z with variable pitch propellers, despite ample horsepower, could not do much better than any other ship. At speeds less than ten knots she was able to stop within her advance for this speed; none of the other vessels could. A vessel that can stop in a distance less than the advance for a specific speed is indeed a truly responsive vessel.
Captain Crenshaw's Naval Shiphandling , also mentions the advisability of making turns when in close quarters, such as formation maneuvers, and this should be applied to a potential collision situation. "Always turn away from danger." When this type of maneuver is made, at least the vessel is already aimed towards an escape route and it is far easier to add more power than it is to completely arrest the forward movement and reverse the direction of the vessel through an emergency action.
Table 1
Ship X
20 knots |
Ship Y
20 knots |
Ship Z
20 knots | |
Distance in yards required to stop
|
2,000
|
550
|
400
|
Slant range: Distance equal to 2 Full Rudder Advances
|
2,200
|
940
|
470
|
Distance required to change course 90° (Advance)
|
1,100
|
470
|
235
|
There are several arguments against attempting to reverse the direction of the vessel: (a) of primary concern is that vital time and maneuverability are lost; (b) a vessel underway responds quicker to an additional ahead engine order than it will to a backing engine order; (c) on steam vessels, less horsepower is available for backing a vessel than is available for going ahead; (d) while slowing, the vessel is still approaching the other vessel, closing the distance available for maneuvering.
Once the speed of a vessel drops because of a backing engine order, vital rudder response is lost owing to the reduced flow or complete loss of the water flow past 'he rudder just at the time when it is needed most. Captain Crenshaw points out that "If strong rudder forces are desired, the propeller must be turning ahead . . ." applies to both single and twin screw vessels. Backing the vessel results in a minimal water flow past the rudder and negligible rudder response.
The approaching vessel may or may not hear the required whistle signals indicating the engines arc backing. This point has often been questioned in collision investigations. At least the approaching vessel can see that a turn is being made in daylight or darkness.
Watchstanders quite often fail to realize that the stopping ability of vessels is proportional to their size. The bigger they are, the more it takes to slow them down. The backing power available on the majority of steam driven vessels in terms of horsepower ranges from about 25% to 50% of the horsepower available for forward propulsion.
The decision to use the turning maneuver in favor of the backing maneuver depends largely on the type of vessel and the wishes of her master. The rules for the maneuvering of a particular vessel must be made with the known characteristics of the ship in mind. Strong personal opinions on shiphandling must be tempered with the undeniable figures of performance retracted from the Tactical Data Folder.
The Rules for Collision Prevention which apply to the Burdened Vessel, require the watchstander to make a deliberate turn to the left when the collision angle falls between 68 degrees and about 90 degrees. (Figure 5). This maneuver is recommended if, for some unexplained reason, a turn to the right would involve increasing the seriousness of the situation. The much emphasized concept of reducing the relative closure rate is still very much in effect by initiating this maneuver, as at the completion of the turn, both vessels are heading in the same general direction. This maneuver in no way conflicts with the Rules of the Road since the Burdened Vessel still has the option to slow, stop, turn right or left.
When the collision angle is small and the vessel is not overtaking, it is considered more advisable to commence a turn to the left instead of attempting to make an extremely hard turn to the right to avoid collision. A turn to the right, though seemingly more correct, in this situation could be considered much more dangerous because it actually increases the closure rate of the two vessels. The watchstander on the Privileged Vessel observing, a maneuver made by the Burdened Vessel directly towards his vessel could very well become more nervous about the situation and as a result, could make an unwise decision or action that might spell disaster for all.
An old alternate method to collision avoidance used both successfully and more often unsuccessfully states, "When in danger of collision, always turn towards the other's stern." This type of maneuver has been defended many times by people who say that, "It gets the action over in a hurry," or "It gets your ship inside his turning circle, and the bow is the strongest portion of the ship, and there are fewer people in that section." This may be a valid argument for some situations, but look to the past and you will agree with Commander Lott's statement, "Blind adherence to this rule has precipitated collisions which might have otherwise been avoided." Many times this maneuver places the watchstander and his ship in direct disobedience of the Rules of the Road and wide open to a finding of fault on his part. This type of maneuver is risky to say the least. As stated previously, the watchstander must assume that the other ship is going to obey the Rules of the Road and he must do likewise. Peoples' lives depend on the knowledge of this principle.
A basic table of recommended minimum safe ranges can be prepared for the use of watchstanders on each particular vessel with very little trouble. There is no set of standard distances that can be used by all ships for establishing the minimum safe ranges because of the variances in Tactical Data for each different ship. Each vessel's Tactical Data is different as to type-configuration, propulsion, and a multitude of other things affecting performance.
For each individual vessel, it is suggested that several of the most commonly used steaming speeds be selected and the Full Rudder Advances for these speeds computed in accordance with the Rules for Collision Prevention as illustrated in Table 2. Table 2 consists of nothing more than a quick multiplication table of ship's Full Rudder Advances. The clearing distance used is an arbitrary figure established by the master, commanding officer or watch officer as the distance he feels necessary for adequate clearance.
Table 2
Proposed Table of Advances for Use by the Watchstander
Number of Full Rudder Advances
| ||||||
Speed in knots
|
1
|
2
|
3
|
4
|
5
|
Clearing distance is 500 yards
|
10
|
450
|
900
|
1,350
|
1,800
|
2,250
| |
15
|
500
|
1,000
|
1,500
|
2,000
|
2,000
| |
20
|
550
|
1,100
|
1,650
|
2,200
|
2,750
| |
25
|
625
|
1,250
|
1,875
|
2,500
|
3,125
| |
30
|
750
|
1,500
|
2,250
|
3,000
|
3,750
| |
Once the distances and procedures are made known to the watchstanders, it will take little time for them to quickly establish the minimum safe ranges for all situations when encountered. Remember, the distances that are computed are emergency distances only , and not to be used as standard approach distances. Once a vessel crosses into your minimum safe range, you must maneuver or she will get you. All maneuvers made from then on should be made with the thought in mind to reduce relative speed between the vessels. Should a steaming speed fall between the speeds for which the number of advances is precomputed, use the precomputed figures for the next higher speed and base everything on that speed unless the Tactical Data Folder is easily available for exact computations.
The suggested collision prevention rules represent a view towards preventing collision when it has been found or thought that the other vessel is not going to maneuver in accordance with her obligation under the Rules of the Road to avoid your vessel.
In conclusion, it should be noted that Admiral Nimitz, when Commander in Chief of the Pacific Fleet, issued a set of very appropriate guidelines on the subject. He said, "No officer whatever his rank and experience, should flatter himself that he is immune to the inexplicable lapses in judgment, calculation and memory or slips of the tongue in giving orders, which throughout seagoing history have so often brought disaster to men of the highest reputation and ability. The attitude towards risks should not be, 'This is war; I can afford to take chances, when it more appropriately should be, this is war; is the chance I take a military necessity?'" The same type of reasoning must be drawn for our peacetime operations even more so. Is the chance I take an absolute necessity, worth risking my ship and men?
The Rules for Collision Prevention may not be the complete answer to the question of how to prevent collision, but they are an attempt to offer to the watchstander something he does not have now, namely, a better view of where safety ends and danger begins.
Lieutenant Myers enlisted in the Naval Reserve in 1961. while a student at New Halford (Mass.) Institute of Technology. During the following two summers he participated in the Naval Reserve ROC Program. Upon graduation from college in 1964, he was commissioned an ensign in the U.S. Naval Reserve and assigned to the USS Protector , (ACR-11), patrolling the seaward extension of the Distant Early Warning line. In 1965, he was assigned to the oiler Elokomin (AO-55) in the deck department and then subsequently as navigator. The next tour of duty was spent as an instructor at Fleet Training Center, Norfolk, Virginia, where he taught shiphandling and navigation in the OOD Shiphandling and Tactics School. He was accepted into the TAR Program in 1969, and was transferred to Headquarters SIXTH Naval District, Charleston, South Carolina, assigned to the Naval Reserve Supplement as Assistant Surface Program Officer. He is currently the Commanding Officer, Naval and Marine Corps Reserve Training Center. Gulfport, Mississippi.
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