Coastal Navigation Quiz

In IALA-B (Americas), the rule is 'red right returning' — red buoys are kept to starboard when returning from sea (entering a harbor or heading upstream). Green buoys are kept to port.

The UK uses IALA-A, where red marks are kept to port when entering a harbor or heading upstream. This is the opposite of IALA-B used in the US. A US sailor must reverse their color associations when in European waters.

In IALA-B, red nuns to starboard (right) when entering = correct position in the channel. Even number 4 to starboard confirms you're in the right place and heading in the right direction (numbers increase going in).

A red-green banded buoy is a preferred channel (junction) mark. In IALA-B, a red top means the preferred channel is to starboard of the mark — keep it to your port side when following the preferred channel.

Two cones both pointing up is the North cardinal mark topmark. Safe water lies to the north — pass this mark on its north side. The topmarks point 'north' by both pointing skyward.

Nine flashes = West cardinal. Using the clock analogy: West = 9 o'clock = 9 flashes. Safe water lies to the west of this mark.

Red and white vertical stripes with a spherical topmark = safe water mark. This indicates navigable water on all sides and is typically used as a landfall buoy, mid-channel marker, or fairway approach mark.

Isolated danger marks are placed on or directly above a hazard — a lone rock or shoal — in otherwise clear water. There is navigable water all around, but you must give the mark (and the hazard beneath it) a wide berth.

In IALA-B, green cans are to port when going upstream (returning from sea). Red nuns are to starboard. 'Red Right Returning' — green is to port.

The hourglass (cones pointing toward each other, black band in the middle) is the West cardinal. Safe water lies to the west — pass on the western side. West = 9 o'clock = 9 quick flashes.

Black with red horizontal bands + two black balls topmark + Fl(2) (two flashes) = isolated danger mark. It sits on a hazard surrounded by navigable water. Give it a wide berth on any side.

Australia uses IALA-A, where red is to port when entering. A US sailor accustomed to IALA-B (red to starboard) must reverse their associations in Australian waters.

In IALA-B, odd-numbered green can buoys are to port going in. Green cans to port, numbers increasing = correctly positioned and heading inland. If you saw them decreasing or to starboard, something would be wrong.

Light blue on a NOAA chart indicates shoal water — shallower zones that require caution. White represents navigable deep water. Green or yellow marks areas that dry at low tide.

Depth contour lines (isobaths) connect all points of equal depth, similar to elevation contours on a topographic map. They give sailors a clear picture of how the bottom rises or falls without reading every individual sounding.

'Obstn' marks an obstruction of uncertain nature or position — it might be a submerged cable, debris, or an uncharted rock. The exact hazard is unknown, which makes it more dangerous, not less. Give it a wide berth.

A cross symbol with a dot indicates a rock awash at or near chart datum — it surfaces or nearly surfaces at low water and may be submerged at high water. This is one of the most dangerous hazard types because its exposure varies with the tide.

'Fl' = flashing (single flash, off longer than on). 'G' = green. '6s' = 6-second period (one flash every 6 seconds). This light characteristic tells you both what the buoy looks like at night and confirms its identity on the chart.

Colored sectors on a lighthouse define arcs of visibility. The red sector warns mariners that they are in or approaching a danger zone (shoal water, rocks, or an unsafe approach). The white sector marks the safe channel. Green sectors have similar warning meanings.

'R' indicates a rock seabed. Anchors hold poorly on rock and risk damage. Always look for 'S' (sand) or 'M' (mud) for reliable holding.

'ED' stands for Existence Doubtful. The feature was reported but not confirmed. For navigational hazards, treat ED features as real — the consequences of being wrong are more serious than navigating around a possibly non-existent feature.

Green or yellow on a NOAA chart marks areas that are exposed above water at or near chart datum — they dry at low tide. These may be navigable at high water but hazardous at low water.

'Oc' = occulting (the light is on longer than off). 'W' = white. '10s' = 10-second period total (the combined on + off time equals 10 seconds). This uniquely identifies the light among nearby aids.

'M' stands for mud — generally reliable anchor holding for most anchor types. Sand (S) is also good. Rock (R) and coral (Co) are poor holding and may damage the anchor or seabed.

A red sector on a lighthouse marks an arc where vessels are in or approaching a danger zone. The white sector marks the safe channel. Alter course until you see white, then hold that bearing through the approach.

'Rep Obstn' means a reported obstruction — it was reported but not confirmed by a survey. The consequences of hitting an uncharted obstacle far outweigh the inconvenience of altering course. Always treat reported hazards as real.

Current strength is determined by tidal range and geometry. Narrow channels funnel the same volume of water through a smaller cross-section, accelerating flow significantly — similar to squeezing a hose.

During a flood tide, the rising ocean pushes water up-river (inland). The current flows inland. During the ebb, it reverses and flows toward the ocean. In some rivers, the tidal current can override the river's own downstream flow.

At most locations, maximum flood current occurs near mid-tide — roughly 3 hours before high water. The tide table and current table give different information and must be used independently.

Slack water is the brief calm period when tidal current is minimal — transitioning between flood and ebb. It is the safest time to navigate constricted passages where current creates dangerous conditions.

The difference between heading (045°) and COG (060°) is 15° to the right. This offset is caused by current (or leeway) pushing the boat to the east. To track 045°, you would need to steer 030° to compensate.

Boats at anchor or on moorings orient into the current (or wind if it's stronger). If all boats are swung the same way and the wind is light or absent, they are aligned with the current. The direction they're pointing is up-current.

Speed over ground against a head current = boat speed − current speed = 6 − 4 = 2 knots. This makes a 3-mile channel transit take 90 minutes rather than 30. Plan to ride favorable current where possible.

Current does not reverse the instant the tide turns. At most locations there is a lag — sometimes significant — between the turn of the tide and the reversal of current. Current tables list slack water times separately from tide height predictions.

COG 340° is 20° west of heading 000°. Something is pushing the boat to the west — either current or leeway. To determine which, check wind direction. If wind is from the east, leeway is possible. If wind is calm, it's current.

Slack water provides the least current and most controlled conditions for inlet transit. Maximum flood or ebb creates strong currents that can overpower maneuvering, create dangerous eddies and standing waves, and carry boats into hazards.

A buoy held in place by a downward anchor line will be pushed downstream by current, leaning in the direction the water is flowing. The angle of lean gives a rough indication of current strength. A strongly leaning buoy indicates significant current.

A fix is a confirmed position from intersecting lines of position. A DR position is an educated estimate based on sailing from a known point on a known course at a known speed. DR does not account for current, leeway, or other errors.

The most widely accepted etymology is 'ded' (deduced) reckoning — a position deduced from known course, speed, and time rather than directly observed. The term has been in use since at least the 16th century.

Chart conventions: circle = fix, semicircle = DR position (course and speed only, no current correction), square = EP (estimated position with current and leeway allowed for). These communicate the confidence level of each plotted position.

When GPS fails, a current chart plot tells you where you were and what course you were on. Without it, you have no recent position reference and must navigate blind until you can take a visual fix. Good navigators maintain both simultaneously.

With 4 hours of accumulated DR error, your position circle could be significant. Treat hazards as if they're at the closest edge of your uncertainty zone — not the center. Slow down, get a depth fix or radar fix if possible, and do not proceed toward hazards without confirmation.

Frequent fixing resets the DR error to near zero each time. Between fixes, error accumulates. A navigator who fixes every 30 minutes has small errors between fixes; one who doesn't fix for 6 hours may have miles of uncertainty — all avoidable.

A 4-mile discrepancy after 1 hour is significant and demands explanation. Possibilities: GPS datum mismatch, antenna disconnected and showing cached position, or extraordinary undocumented current. Always investigate a large DR vs. GPS discrepancy — don't blindly trust either.

Using DR to predict future position — and pre-planning what landmarks and depth contours should appear at that position — is practical contingency navigation. If reality doesn't match the prediction, the navigator knows immediately that something requires investigation.

Distance = speed × time = 5 × 3 = 15 miles. Course = 135°. DR position is 15 miles on a 135° bearing from the 1400 fix. No current correction is applied to a DR position — that would make it an EP.

Convention: fix = circle, DR = semicircle, EP = square. These symbols communicate the confidence level of each plotted position to anyone reading the chart.

With 6 hours of accumulated DR error, the vessel could be miles from the estimated position. A shoal at the edge of the uncertainty zone could easily be directly ahead. Slow down, use all available senses and instruments to get a fix, and do not proceed toward the hazard without confirmation.

Maintaining both gives you two independent position sources and a fallback if GPS fails. It also allows you to detect GPS errors — if the GPS position and DR position diverge significantly, something needs investigation.

A major discrepancy between GPS and DR demands investigation, not a choice between them. Check: Is the correct chart datum selected on the GPS? Is the antenna connected? Are there landmarks to take a visual fix from? Act on the safest assumption (reef is possible) while investigating.

While nautical charts show many types of information, their core purpose is water depth — soundings, contour lines, and hazard marking that together answer whether it is safe to navigate in a given area.

Raster charts are digital scans of paper charts — accurate but pixelated at high zoom. Vector charts store data as points and layers, allowing smooth zooming, layer toggling, and richer integration with navigation software.

Actual depth = charted depth + current tide height. 6 + 2 = 8 feet. Charted depth is the minimum expected — the tide adds on top.

Chart datum is the reference water level — in the US, typically MLLW (Mean Lower Low Water). All charted depths are measured from this baseline, meaning actual depth at any moment is charted depth + current tide height above datum.

Large-scale charts show maximum detail for a small area — exactly what you need in confined, complex waters. Small-scale charts omit many hazards visible only on the more detailed version.

1:12,000 is the largest scale of the options — it represents the smallest area but in the greatest detail. Larger scale means a larger fraction (1/12,000 is larger than 1/500,000).

The safety-critical check is minimum depth at lowest anticipated tide along every leg of the route. Many groundings happen because a skipper planned at high water without checking what the same route looks like when the tide drops.

Waterways change — channels are dredged, new hazards are discovered, buoys are relocated. An outdated chart may not show a newly discovered rock or a channel that has shoaled. Always use current charts and check for recent corrections.

Actual depth = charted depth + tide height = 4 + 3 = 7 feet. With a 5-foot draft, you have 2 feet of clearance. That said, always add a safety margin (ideally 1.5–2× draft) and consider wave action in exposed areas.

Chart datum is the baseline from which all depths are measured. In the US it is MLLW — near the lowest predictable water. Actual depth at any time = charted depth + tide height above datum.

Always use the largest scale chart available for your actual navigation area. The 1:10,000 harbor chart shows rocks, shoals, channel markers, and depth contours that are too small to appear on the 1:100,000 coastal chart.

Italic soundings on a nautical chart indicate areas that may be exposed above water at or near chart datum — they dry at low tide. These require especially careful attention when navigating near low water.

Good passage planning plots a safe route with waypoints at turning points, checks minimum depths at the lowest anticipated tide, and identifies alternates. Relying solely on GPS and reacting in real time leaves no margin for equipment failure or unexpected hazards.

A single LOP is a line, not a point. You could be anywhere along it. A second LOP crosses the first at a specific location — your fix. A third LOP confirms accuracy by how small the resulting triangle (cocked hat) is.

When three LOPs are plotted and they don't cross at exactly the same point, they form a triangle called a cocked hat. The size of the triangle reflects the combined error in the bearings — smaller is better, larger demands recheck.

Objects 60–120° apart give LOPs that cross at a favorable angle — typically 60–90°. Objects nearly ahead or astern (small angle) produce LOPs that are nearly parallel, crossing at a shallow angle with large position uncertainty.

A transit (range) is when two charted objects are directly in line. You know precisely that you're on the transit line, with no compass error. It's one of the most reliable and simple LOPs available and should be used whenever it presents itself.

If your depth sounder reads 30 feet and you're in an area where the 30-foot contour runs roughly north-south, you know you're somewhere along that contour. Combined with a visual bearing to a lighthouse to the east, this creates a two-LOP fix.

GPS is highly accurate but a single point of failure: antenna damage, dead battery, software error, or signal loss can eliminate your primary position source. Cross-checking with visual fixes and maintaining paper chart backup protects against this.

A running fix requires advancing one LOP along the assumed course and distance traveled. Any error in course (leeway, current), speed, or elapsed time propagates directly into the fix position. The longer the run between bearings, the larger the potential error.

The running fix is at the intersection of the advanced LOP #1 (moved 1.5 miles along your course) and the new LOP #2. This cross gives your estimated position at the time of the second bearing.

A large cocked hat suggests significant error in at least one bearing — perhaps a misidentified landmark, compass error, or sloppy bearing technique. Recheck each bearing and confirm you're looking at the right object before plotting again.

LOPs crossing near 90° give the smallest position uncertainty. Narrow crossing angles create long, thin cocked hats where even small bearing errors produce large position errors.

A visual bearing to the headland gives one LOP. The 40-foot depth contour (found on the chart) gives a second LOP. Where they intersect is your fix — no GPS required.

When two charted objects are in direct alignment, you know exactly which line you're on without using a compass. This eliminates compass deviation and variation errors entirely. Transits should be used whenever they're available.

Current displaces the boat from its assumed course. If you advance LOP #1 along your intended course rather than your actual track, the advanced LOP is in the wrong position. Over 30 minutes at 1 knot, the error is 0.5 miles — significant in coastal navigation.

Set 270° means the current flows toward 270° (west). Drift is 2 knots. If you're standing still in this current, you will be carried westward at 2 knots. Current direction conventions: 'set' describes where current is going, just as wind direction describes where wind is coming from — the key difference from meteorological convention.

Drift is simply the speed of the current — how many knots the water mass is moving in the direction of set. Set + drift fully describe the current's effect: direction and magnitude.

Your actual position is 1.6 miles west of your DR position. Draw a line from DR to actual fix: it points west = 270°. Set = 270° (toward west). Drift = 1.6 miles ÷ 2 hours = 0.8 knots.

Drift = distance from DR to fix ÷ elapsed time = 2.4 ÷ 3 = 0.8 knots.

If current pushes east, you must steer west of your intended track to compensate. The current will carry the boat east while you're heading west of north, and the combined effect should track you due north. Steer west of 000° — roughly 345–350° depending on exact current magnitude.

When correctly compensating for current, you deliberately steer an angled heading. Your heading (where the bow points) and COG (where you're actually tracking) are different — but if the correction is right, the COG matches the intended track perfectly.

Set 180° means the current is flowing toward 180° — due south. It is pushing the boat southward at 1.2 knots.

Actual fix is 1.0 mile north of DR position. Draw line from DR to fix: direction = 000° (north). Set = 000°. Drift = 1.0 mile ÷ 2 hours = 0.5 knots. The current has been setting north at 0.5 knots.

The current pushes south. To track east, steer north of east so the southward current carries you back onto the easterly track. Heading north of east + southward current = easterly COG.

The most reliable current measurement is the one derived from your own navigation — comparing DR with actual fix gives the real current experienced by your vessel at your specific time and location. Published tables give predictions; observations give reality.

Heading 035°, COG 045° — you're steering 10° west of your actual track. A current setting east is carrying you eastward while you steer west of track, resulting in easterly COG. This is correct current compensation in action.

Because the Moon orbits Earth roughly once every 29.5 days, it moves relative to any fixed point on Earth by about 12.2° per day. It takes the Earth an extra ~50 minutes of rotation to 'catch up' to the Moon's new position, causing each successive high tide to arrive about 50 minutes later.

A diurnal tidal pattern produces one high and one low tide per day. Semidiurnal produces two of each. Parts of the Gulf of Mexico experience diurnal tides.

Spring tides occur at new and full moon when the Sun, Moon, and Earth align. The combined gravitational pull produces greater tidal range — higher high tides and lower low tides. This happens twice a lunar month.

Spring tides produce the lowest low waters of the month. On a bar with barely enough depth, spring low water (possibly a negative tide below chart datum) is the most dangerous time — actual depth may be less than the charted sounding.

A negative tide height means the water level falls below chart datum — shallower than the charted depth at that location. This is most common during spring tides and is significant for vessels navigating shallow areas.

The rule of twelfths follows the pattern 1, 2, 3, 3, 2, 1 twelfths per hour. Hours 3 and 4 each see 3/12 of the range rise — the fastest period of the flood or ebb tide.

Required depth = draft + safety margin = 6 + 1.5 = 7.5 feet. Charted depth = 4 feet. Minimum tide height needed = 7.5 − 4 = 3.5 feet above chart datum.

Speed over ground against current = boat speed − current speed = 5 − 2 = 3 knots. This means a 6-mile passage takes 2 hours against the flood vs. about 50 minutes with it — a significant difference worth planning around.

By 0800 (2 hours after low): rise = (1/12 + 2/12) × 6 = 3/12 × 6 = 1.5 feet above low water level.

Spring tides occur at new and full moon when gravity combines — producing maximum tidal range. Neap tides occur at quarter moon when forces partially cancel — minimum range. Neither is related to the seasons.

Actual depth = charted depth + tide height = 3.0 + (−0.8) = 2.2 feet. A negative tide reduces depth below the charted minimum — this bar has only 2.2 feet of water at this low water prediction.

Actual depth = 3 + 2.5 = 5.5 feet. Required = 5-foot draft + 1-foot margin = 6 feet. 5.5 < 6. No — not safe with this margin requirement. You would need at least 3 feet of tide above datum to enter safely.

As the Moon orbits Earth (taking ~29.5 days to complete a circuit), it moves about 12° relative to any fixed point on Earth each day. The Earth needs approximately 50 extra minutes of rotation each day to bring that location back under the Moon — hence the ~50-minute daily shift in tidal timing.