Navigation & Chart Work

Chart scale:

A large-scale chart shows a small area in great detail (e.g., 1:10,000 — a harbor chart). A small-scale chart shows a large area with less detail (e.g., 1:1,000,000 — an ocean passage chart). EXAM: when you zoom in, the scale number gets larger — but that is a "large-scale" chart.

Soundings:

Depths on a chart are given in the unit stated in the chart title — typically feet, fathoms, or meters in U.S. waters. Modern NOAA charts use feet and fathoms in most coastal areas, meters on newer editions. The datum used for soundings is Mean Lower Low Water (MLLW) — the average of the lower of the two daily low tides. This is the most conservative datum: actual water depth will usually be greater.

Heights:

Bridge clearances and feature heights are referenced to Mean High Water (MHW) — the worst case for overhead clearance (highest tide).

Contour lines (depth curves):

Lines connecting equal depth points. Bold contour lines usually mark 6-fathom (36-foot) and 12-fathom (72-foot) curves on older charts.

Chart symbols to know:

- Anchor symbol: recommended anchorage - PA: position approximate - PD: position doubtful - ED: existence doubtful - Rk or Rks: rock(s) - Wr: wreck (above water) - Wk: wreck (submerged, not dangerous) - (dots): dangerous to navigation - Yellow/green tint: shoal water - Blue: water of navigable depth - White: deep water (coastal charts) - Magenta: magnetic annotations, caution notes, light sectors - "foul" or "foul bottom": debris, old moorings, poor holding ground

DR plot notation:

- DR position: labeled with time and "DR" in pencil. Marked with a semicircle. - Fix: two or more confirmed position lines crossing — labeled with time in a triangle. - Estimated position (EP): DR position corrected for estimated set and drift — marked with a square.

Speed-time-distance triangle (60D = ST Rule):

- Distance (nm) = Speed (kts) × Time (hours) - Speed (kts) = Distance ÷ Time - Time (hours) = Distance ÷ Speed - For minutes: D = S × (T ÷ 60), or use "60D = ST" where T is in minutes

Set and drift:

- Set: the direction toward which the current is flowing (as a true bearing) - Drift: the speed of the current in knots - A current with Set 090°T and Drift 1.5 kts pushes the vessel east at 1.5 knots. - The difference between the DR position and the actual fix is due to set and drift.

Course over ground (COG) vs. heading:

- Heading: the direction the bow is pointing (compass course) - COG: the actual track made good through the water, affected by current - To maintain a desired COG, the helmsman must steer a different compass heading to compensate for current.

Tide types:

- Semidiurnal: two high and two low tides per day, roughly equal (common on the U.S. East Coast). - Diurnal: one high and one low per day (common in the Gulf of Mexico). - Mixed semidiurnal: two highs and two lows per day but with unequal heights (U.S. West Coast, Hawaii).

Tide datums:

- MLLW (Mean Lower Low Water): chart sounding datum; the zero reference for charted depths. - MLW (Mean Low Water): average of all low tides. - MHW (Mean High Water): average of all high tides; used for overhead clearance. - MHHW (Mean Higher High Water): average of the higher of each day's two high tides.

Tide tables:

Published annually by NOAA. List time and height of high and low tides for reference stations, with corrections for secondary stations.

Rule of Twelfths (estimating tidal heights between high and low):

In six hours from low to high, the tide rises by approximately: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the total range per hour. For a 6-foot range: 0.5 ft, 1 ft, 1.5 ft, 1.5 ft, 1 ft, 0.5 ft.

Tidal currents:

- Flood: water flowing toward land/into harbor (associated with rising tide, but timing differs). - Ebb: water flowing away from land (associated with falling tide, but timing differs). - Slack water: period of minimal current between flood and ebb. - Current tables (NOAA): list times of maximum flood, maximum ebb, and slack water at reference stations.

IMPORTANT:

Tide and tidal current are not the same. Current occurs before and after slack — it is not zero when the tide turns. The NOAA Current Tables and Tide Tables are separate publications.

The cardinal rule of U.S. lateral aids:

"Red Right Returning" — when returning from sea (entering harbor), the red aids are kept to starboard (right). Green aids are kept to port (left).

Buoy colors and shapes:

- Red buoys: nun (conical) or lighted (with even number). Keep to starboard entering. - Green buoys: can (cylindrical) or lighted (with odd number). Keep to port entering. - Red-and-green horizontally banded: preferred channel. Top color indicates which side has the preferred channel. If top is red, treat as red (keep to starboard); if top is green, treat as green. - Safe water (mid-channel): red-and-white vertical stripes, spherical shape, white light. - Isolated danger: black-and-red horizontal bands, with two black balls topmark. - Special purpose: yellow, X topmark. Marks hazards, military exercise areas, fish trap areas. - Information/regulatory buoys: white with orange markings. Open-faced diamond = danger.

Fixed aids:

- Lighthouses: major fixed aids. Light characteristics describe their identity: F (fixed), Fl (flashing), Iso (isophase), Oc (occulting), Q (quick flashing), VQ (very quick). - Dayboards: fixed markers on structures. Lateral markers follow same color rules as buoys. - Ranges: two fixed markers on a leading line. When aligned, the vessel is on the range bearing.

Light characteristics:

A light is identified by its characteristic (color, phase, period). Light List published by USCG identifies all aids. Key: a Fl R 4s light flashes RED with a period of 4 seconds (one flash every 4 seconds). A Fl (2) R 10s shows 2 red flashes then darkness in a 10-second period.

Variation:

- East variation: Magnetic North is east of True North; to convert from Magnetic to True, ADD easterly variation. - West variation: Magnetic North is west of True North; to convert from Magnetic to True, SUBTRACT westerly variation. - Memory aid: "Variation East, Compass Least (subtract from True to get Magnetic). Variation West, Compass Best (add to True to get Magnetic)." OR use "TVMDC" method.

Deviation:

- The magnetic compass is also affected by ferrous metal in the vessel, electrical equipment, and cargo. - Deviation is the angle between Magnetic North and the direction the compass needle actually points. - Deviation varies with the vessel's heading; it is recorded in a deviation table or card. - Like variation, deviation can be East or West.

TVMDC correction method:

True → Variation → Magnetic → Deviation → Compass To convert True to Compass: subtract East error, add West error at each step. To convert Compass to True: add East error, subtract West error at each step.

Key concepts:

- GHA (Greenwich Hour Angle): the angular distance west of the Greenwich meridian, in degrees. - LHA (Local Hour Angle): GHA adjusted for the vessel's longitude (LHA = GHA + East longitude, or GHA − West longitude). - Declination: celestial equivalent of latitude — how far north or south of the celestial equator the body is. - Altitude: the angle of a celestial body above the horizon, measured with a sextant. - Zenith angle: 90° minus the altitude; used in sight reduction.

Sight reduction:

After taking a sextant observation and correcting for index error, dip, refraction, and semi-diameter, the navigator computes a calculated altitude (Hc) using tables or a calculator, then plots a Line of Position (LOP) at right angles to the azimuth (direction to the body).

Navigation publications for celestial:

- Nautical Almanac (USNO / HM Nautical Almanac Office): GHA and declination for Sun, Moon, planets, 57 selected stars for each hour and minute of the year. - HO 229 (Sight Reduction Tables for Marine Navigation): tabular Hc values by LHA, declination, and assumed latitude. - HO 249 (Sight Reduction Tables for Air Navigation): used for star identification and as an alternative to HO 229. - The American Practical Navigator (Bowditch, NIMA Pub No. 9): the comprehensive navigation reference.

Radar as a navigation aid:

Radar provides range and bearing to charted objects. A radar fix is obtained by taking two or more bearings or ranges to positively identified charted objects and plotting them on the chart.

Parallel indexing:

A technique for monitoring a vessel's track in coastal navigation. A line is drawn on the radar display parallel to the intended track at a specific distance from a fixed radar target. As long as the target maintains that distance from the track line, the vessel is on course.

Electronic chart systems:

- ECS (Electronic Chart System): an aid but not a legal replacement for paper charts. - ECDIS (Electronic Chart Display and Information System): certified to IMO standard; may replace paper charts on vessels required to carry them (SOLAS vessels). - Raster Navigational Charts (RNC): digital scans of paper charts. - Electronic Navigational Charts (ENC): vector-based, allows course/speed plotting, collision alarm.

GPS:

Global Positioning System provides continuous latitude/longitude. Accuracy in open ocean: approximately 10–15 meters. Not immune to error — always cross-check against visual or radar fixes when near hazards.

Loran-C:

Discontinued in U.S. waters as of 2010. No longer operational.

AIS (Automatic Identification System):

Required on SOLAS vessels ≥300 GRT international, ≥500 GRT non-international, and all passenger vessels. Transmits vessel ID, position, COG, SOG, heading, and status. Not a substitute for radar watch or a proper lookout.