15 precise piloting

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Learning objectives:

»Know the terminology used to describe a naval ship’s tactical characteristics.

»Apply proper procedures in the use of advance and transfer to determine turn bearings during precision piloting.

»Apply correct procedures in setting up the approach plot for anchoring.

»Know the terminology associated with precision anchoring.

Lesson 13: Precise Piloting and Anchoring

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Plot a DR Position at Every Course Change

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c-125

c-090

c-080

larger the vessel = more time and distance required for either course or speed change to be effected

larger the vessel = more time and distance required for either course or speed change to be effected

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Handling Characteristics

Definition Advance Transfer Turning Circle Tactical Diameter Final Diameter

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Handling Characteristics

Rudder

»Rudder Angles Angle of Turn Tactical Data folder Turn Tables

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Ship’s Handling Characteristics:» Ships do not instantaneously adjust to a new course or speed. When operating in

open ocean, the time and distance to make a course or speed change can be regarded as negligible as the ship has ample room to maneuver and drastic changes are not common.

» Precise piloting: When plotting and directing the movements of a vessel, vessel’s turning diameterturning diameter, time to turntime to turn, and acceleration/ deceleration ratesacceleration/ deceleration rates must be taken into account. This process is known as precise piloting.

– Engaged in maneuvering of high precision I.E. DIVTACS, ANCHORING» Handling characteristics The attributes of a particular vessel relating to its

performance in making turns at various rudder angles and speeds, and in accelerating and decelerating from one speed to another. In the case of warships, these characteristics are referred to by the more specific term tactical tactical characteristics. characteristics.

– Every warship has a set of tactical characteristics peculiar to itself; even warships of the same class often differ to some extent in the manner in which they respond to a given rudder angle or engine speed change.

» Turning characteristics: The handling or tactical characteristics of pertaining to a ship’s performance during turns.

– Advance is the distance gained in the direction of the original course until the ship steadies on its final course. It is measured from the point at which the rudder is put over, and will be at a maximum for a turn of 90 degrees.

Precise Piloting and Anchoring

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– Transfer is the distance gained at right angles to the direction of the original course until the ship steadies on its final course.*

– The turning circle is the path followed by the point about which the ship seems to pivot (called the pivot point) as it executes a 360 degree turn. Every rudder angle and speed combination will normally result in a different turning circle.*.*

– Tactical diameter is a naval term referring to the distance gained at a right angle to the left or right of the original course in executing a single turn of 180 degrees. Tactical diameter can be thought of as the transfer for a turn of 180 degrees.; it will be different for each rudder angle and speed combination.

– Final diameter is the diameter of the turning circle the ship would describe if it were allowed to continue a particular turn indefinitely. For all but a few small ships, the final diameter will always be less than the tactical diameter, due to the initial “kick” of the ship away from the direction of the turn.

– Standard tactical diameter is a predetermined tactical diameter established by various tactical publications, most notably ATP 1 Volume 1, for each ship type. It is used to standardize the tactical diameters for all ships by ship type, and finds its most extensive application when maneuvering in formation.

– Standard rudder is the amount of rudder necessary to turn a ship in its standard tactical diameter at standard speed. It varies with the ship type, and also with the class of ship within a particular type.


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Advance with 90 deg change of heading

Transfer with 90 deg change of heading

SHIP TURNING CHARACTERISTICS

(Advance/transfer distance varies with Ship speed and rudder angle)

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TURNING CIRCLE – EFFECT OF CHANGE IN RPM DURING TURN

HALFAHEAD

FULLAHEAD

DEADSLOW

CHANGE OF RPM

SLOW AHEAD

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– Angle of turn is the horizontal angle through which the ship swings in executing a turn, measured from a ship’s original course to its final course.

All of the above data as well as acceleration/ deceleration rates can be found in the ship’s tactical data folder. Turn tables are contained in the ship’s tactical folder. They contain turning characteristics for the ship for

various rudder and speed changes.


Final Diameter

Tactical diameter

Transfor 45o

Transfor 90o

Rudder put over

Advance

f or

45

Advance

f or

90

(Overhead 13-1)

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M a x im u mA d v a n c e

O rig in a lH ea d in g

A d v a n c ea t 9 0 °

C h a n g e o fH ea d in g

Transfera t 9 0 °

C h a n g e o fH ea d in g

Ta ctica l D ia m eter

Tra n sfer a t 1 8 0 ° C h a n g e o f H ea d in g

M axim u m Tran sfer

Patho f C G

R udderstarts to

m o v e

'S tead y Tu rn ' R ad iu s( H a lf 'F in a l D iam eter ' )

H ea d in g

D riftA n g le

D irec tio no f M o tio n

S h ip startsto turn

T U R N IN G

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Example of Standard Tactical Diameter Table @ 15 knots requiring standard

degree rudder:

Angle of Turn Advance Transfer

15o 180 18

30o 230 30

45o 270 60

60o 310 110

Example of Acceleration/Deceleration Table:

Change in knots Minutes required Total elapsed Rate (knots/min)

From To for change time

ACCELERATIONACCELERATION

0 10 3 3 3 1/3

10 15 2 5 2 1/2

15 20 2 7 2 1/2

Example of advance and transfer during piloting: A navigator of a medium-sized vessel has laid down an intended track on a chart to negotiate a 50 degree bend in a narrow river channel. If the ship’s rudder were not put over until it reached the intersection of the old and new track directions, the turning diameter of the ship might cause it to go aground on the left side of the channel. At the very least, it would be far to the left of the


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intended track. The navigator should recommend a point on the old track, called the “turning point”, at which the ship should put its rudder over, taking its turning circle into account, so that the ship will come out of the turn on its new track leg.*

» Step One: Calculate advance and transfer table for the speed and amount of rudder to be used. In our example, we will use a speed of 15 kts and a rudder angle of 50 degrees. Because their is no value for 50 degrees on the table we must interpolate:

Angle of turn Advance Transfer

45o 270 60

50o

60o 310 110

Advance: 5/15 * (310-270) = 13; 270+13=283 yards

Transfer: 5/15 * (110-60)= 17; 60+17=77 yards

Step Two: Plot transfer by drawing a dashed line parallel to the original track at a distance equal to the calculated table value. The intersection of the dashed transfer line and the new track indicates the point at which the turn should be completed (Point B).*

» Step Three: Plot advance by laying out its distance from Point B back down the transfer line. Point C in our diagram indicates the advance.


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» Step Four: Plot the turning point by drawing a perpendicular from the point indicating the advance to the “old track”. Point D represents the turning point.

» Step Five: To complete the plot, a suitable object to be used as a marker for a turn bearing is identified. A line will be drawn from the turning point toward this object in order to provide an indication of the time to turn. When the appropriate bearing to the object is reached, the rudder will be put over. It is desirable that this object be as close to 90 degrees from the turning point.

» It is important to note that this turn was plotted for a 15 degree rudder angle and a speed of 15 kts; a change in either speed or rudder will cause the advance, transfer and turning point to differ. Moreover, the advance and transfer tables are for zero wind and current, so the navigator must take these factors into account.

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Anchoring

Anchoring is one of the most critical evolutions that a navigator is involved with; it will draw upon all of their piloting skills and involve many members of the crew.

There are four stages of this important evolution: 1) selection, 2) plotting, 3) execution, and 4) post-anchoring procedures.

» Selection of an anchorage - An anchorage position in most cases is assigned by higher authority.

– Naval ships submit a Port Visit Request (PVST) or Logistics Requirement (LOGREQ) message requesting an anchorage assignment which is provided by a local port authority.

If ships steaming independently and are required to anchor in other than well-established anchorage areas, selection of anchorage will be made by Navigator and approved by CO

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Navigator will select an anchorage with the Commanding Officer’s approval. In any case the following conditions should always apply to an anchorage insofar as possible:

– The anchorage should be at a position sheltered from the effects of strong winds and current.

– The bottom should be good holding ground, such as mud or sand vice rocks or reef.

– The water depth should be neither too shallow, hazarding the ship, nor too deep, risking dragging the anchor.

– The position should be free from such hazards to the anchor such as fish traps, buoys and cables.

– The position should be free from such hazards to navigation as shoals and sandbars.

– There should be a suitable number of landmarks, daymarks and lighted navigation aids available for fixing the ship’s position both by day and by night.

– If boat runs ashore are to be made, the anchorage chosen should be in close proximity to the intended landing.


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Colored anchorage circles of various sizes for different ship sizes can be found on charts provided by the National Ocean Service. These circle are lettered and numbered which allows for specific anchorage assignments. Foreign charts often have anchorages marked as well. Amplifying information on possible anchorage sites can be obtained from the applicable volume of the Coast Pilots for U.S. waters, from the proper volume of the Enroute Sailing Guide for foreign waters, and from the Fleet Guide for ports in both foreign and domestic waters frequented by U.S Navy warships.

» Plotting the anchorage: The following terms apply to the anchor plot:

– Approach track - This is the track along which the ship must proceed in order to arrive at center of the anchorage. Its length will vary from 2,000 yards or more for a large ship, to 1,000 yards for a ship the size of a Navy destroyer or smaller. Under most circumstances, it should never be shorter than 1,000 yards.

– Head bearing - If at all possible, the navigator will select an approach track such that a navigational aid will lie directly on the approach track if it were extended to the navigational aid selected. This bearing should remain constant if the ship is on track during the approach.

– Letting-go circle - This is a circle drawn around the intended position of the anchor at the center of the berth with a radius equal to the horizontal difference from the hawsepipe to the pelorus.


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Anchorage Plot

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– Letting-go bearing (drop bearing) - This is a predetermined bearing drawn from the intersection of the letting-go circle with the approach track to a convenient landmark or navigational aid, generally selected near the beam.

Anchor is dropped once at this point….if not dropped until directly over anchorage point, the ship will miss anchorage point by the length of the hawspipe to pelorus

– Range circles - These are preplotted semicircles of varying radii centered on the center of the anchorage, drawn so the arcs are centered on the approach track. Each is labeled with the distance from that arc to the letting-go circle. These arcs are normally drawn at 100 yard intervals measured outward from the letting-go circle to 1,000 yards, and at ranges of 1,200, 1,500, and 2,000 yards thereafter.

– Swing circle - This is a circle centered at the position of the anchor, with a radius equal to the sum of the ship’s length plus the length of the chain let out.

– Drag circle - This is a circle centered at the final calculated position of the anchor, with a radius equal to the sum of the hawsepipe to pelorus distance and the final length of chain let out. All subsequent fixes should fall within the limits of the drag circle.

– Example: Suppose that a ship having 300 ft (100yds) of total length and a hawsepipe to pelorus distance of 150 ft (50yds) has been directed to anchor.


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Drag / Swing Circle

Swing Circle

Drag Circle

300 yards

130

yds

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Anchorage Plot

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Step One: First, an estimated swing circle is plotted to ensure the area around the anchorage is safe.

Step Two: Draw letting-go circle with radius equal to horizontal distance b/w anchor hawspipe and pelorus from which bearings observed (50 yards).

Step Three: The approach track is selected and plotted based on the available objects for a head bearing, taking into account the wind and currents in the area. Key is to point directly into strongest force.

Intended track leading up to the final approach is laid down, with care being taken to allow for the proper length of the approach track. Draw arcs at 100 yard intervals outward from letting-go circle, 1,000yds, 1,200 yds, 1,500yds, and 2,000yds.

Step Five: To complete the plot, construct a letting-go bearing to an object if available.


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Anchorage Plot

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Executing the anchorage: When executing the anchorage, the navigator’s objectives are to keep the ship as close to the approach track as possible, and to have all of the headway off the ship when the hawsepipe is directly over the center of the anchorage. The navigator will take constant fixes and make course and speed recommendations throughout the evolution.

» Step One: With 1,000 yards to go, most ships are usually slowed to a speed of 5 to 7 knots.

» Step Two: Depending upon wind and current, the engines should be stopped when 300 yards from the letting-go circle, and the anchor detail should be instructed to “stand by”. As the vessel draws near the drop circle, engines are normally reversed so as to have all remaining headway off the ship as it passes over the letting-go circle.

» Step Three: When the pelorus is at the letting-go bearing, the word “Let go the anchor” is passed to the anchor detail, and the anchor is dropped.

» Step Four: As the anchor is let go, the navigator calls for an immediate round of bearings and marks the ship’s head. After the resulting fix is plotted, a line is extended from it in the direction of the ship’s head, and the hawsepipe to pelorus distance is laid off along the line, thus plotting the position of the anchor at the moment it was let go. If all goes well, the anchor will be placed within 50 yards of the center of the anchorage.


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Post-anchoring procedures:

» Step One: After the anchor has been let go, chain is let out or “veered” until a length or “scope” of chain five to seven times the water depth is reached. At this point, the chain is secured and the engines are backed, causing the flukes of the anchor to dig into the bottom, thereby “setting” the anchor.

» Step Two: The navigator will take another round of bearings, record ship’s head, and note the direction the chain is tending. With this information, the navigator plots another fix and recomputes the position of the anchor by laying off the distance plus the scope of chain in the direction the chain is tending. This second calculation of the position of the anchor chain is necessary because the chain may have been dragged during the process of “setting” the anchor.

» Step Three: After the anchor’s final position has been determined, the the swing circle is drawn using the anchor’s computed position as the center and the sum of the ship’s length plus the scope of the chain let out as the radius.

» Step Four: If no obstructions are found to be within the swing circle, the navigator will then draw the drag circle using a radius equal to the sum of the hawsepipe to the pelorus plus the scope of the anchor chain let out. All subsequent fixes should fall within the drag circle; if they do not, the anchor should be considered to be dragging.


(Overhead 13-5)

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Drag / Swing Circle

Swing Circle

Drag Circle

300 yards

130

yds

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» Step Five: After plotting the drag circle, the navigator selects several lighted aids to navigation that are suitable for use in obtaining fixes by day or night, and he enters them in the Bearing Book for use by the anchor bearing watch. This watch is charged with obtaining and recording in the Bearing Book a round of bearings to the objects designated by the navigator at least once every 15 minutes, and plotting the resulting fix on the chart (the area of the chart where the fixes are plotted will be covered by a piece of clear plastic in order to save the chart from excessive wear).

If a fix falls outside the drag circle, another is taken immediately If the second fix falls outside the drag circle, the anchor is considered to be

dragging and all appropriate personnel are notified

» The importance of the anchor watch cannot be overemphasized as failure to recognize when the anchor is dragging or changes in navigation considerations (such as wind, currents, or other ships) can cause the ship to go aground or to strike a hazard.

– Increase in weather= increase in watchstanders


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Homework

Workbook Cpt. 14

» Problems 3-6, 8 Precise piloting and anchoring practical Tuesday and Thursday Read Chapter 15 – Voyage Planning and Time 23 OCT – Read Chapters 28-30 25 OCT – Celestial videos and Exam Review (Read Chapters 16-19) 30 OCT – Exam #2

15 precise piloting

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