Yachtmaster Ocean Theory

Ocean passage planning

Learning objectives

• Plan an ocean passage around season, route and prevailing systems rather than a straight line.
• Use pilot charts and routeing publications to choose a departure window.
• Prepare boat, crew, stores and documentation for weeks at sea.

Route, not rhumb line

Ocean routes are chosen for wind and current, not distance. The classic Atlantic crossing goes south until the butter melts, then west with the trades, because the direct great circle route sits in the variables. Pilot charts give, month by month, the percentage frequency of wind by direction and force, current sets, and the historical limits of ice and tropical storms.

Seasons and hazards

Tropical revolving storm seasons define the calendar: leaving too early or arriving too late is the single biggest planning error. Note the recommended safe latitudes and the escape options for the season you are sailing.

Preparation

• Boat: rig and steering redundancy, chafe protection everywhere, watermaker or tankage plus jerry cans, storm sails bent on and tested.
• Power: realistic energy budget with solar, wind and engine hours.
• Stores: provisioning for the passage plus a substantial reserve; a written stowage plan.
• Crew: watch system agreed before departure, medical kit and training, and a night routine everyone can sustain.
• Paperwork: clearance, insurance, ship's radio licence, crew lists.

Self-check

Plan a November Las Palmas to Saint Lucia passage: state your route rationale, expected days at sea, water and fuel budget, and the watch system.

Quick quiz

The celestial sphere & astro basics

Learning objectives

• Define the celestial sphere, geographical position, declination, GHA and LHA.
• Explain the position circle and why an intercept gives a position line.
• Set up the navigational triangle.

The model

Imagine the sky as a sphere with the earth at its centre. Every body has a geographical position (GP): the point on the earth directly beneath it. Declination is the GP's latitude; Greenwich Hour Angle is its longitude measured westward from Greenwich, 0 to 360 degrees.

If you measure a body's altitude above the horizon, you know your angular distance from its GP: zenith distance = 90 degrees minus true altitude. That places you somewhere on a circle of position centred on the GP. Two such circles, from two bodies or from the same body at different times, cross at your position.

Local hour angle

LHA = GHA minus west longitude, or GHA plus east longitude. LHA, declination and your assumed latitude form the three known parts of the navigational (PZX) triangle, from which you compute the altitude you should have seen.

The intercept

Compare calculated altitude with observed: the difference, in minutes of arc, is nautical miles. Greater observed means you are nearer the GP (Towards); smaller means Away. Plot from the assumed position along the azimuth and draw the position line at right angles.

Self-check

GHA Sun 214 18.2', your DR longitude 023 40.0' W. What is the LHA, and what does it tell you about the sun's position relative to your meridian?

Quick quiz

The sextant: use, errors & sight taking

Learning objectives

• Check for and remove sextant errors, and measure index error correctly.
• Take a clean sight of the sun, a star and a planet.
• Apply the full correction chain from sextant altitude to true altitude.

The instrument

Perpendicularity, side error and index error are the three checks. The first two are adjusted; index error is measured and applied. Set the sextant to zero, look at the horizon, bring the reflected and direct horizons into one line, and read: on the arc is subtracted, off the arc is added.

Taking the sight

Bring the body down to the horizon, rock the sextant gently to swing the body through the arc of a pendulum, and note the time at the instant of tangency — to the second. An error of four seconds of time is a mile of longitude at the equator. Use a second person on the watch if you can, and take three sights and average.

The correction chain

Sextant altitude (Hs)

• index error (+/-)
• dip (height of eye) = apparent altitude (Ha)
• main correction (refraction, semi-diameter, parallax from the almanac altitude correction tables) = true altitude (Ho)

Lower limb sights of the sun add semi-diameter; upper limb subtracts. Star and planet corrections are refraction only, plus additional corrections for Venus and Mars.

Self-check

Hs 42 16.4' lower limb sun, index error 2.0' on the arc, height of eye 2.5 m. Work Ho.

Quick quiz

Sun sights & the noon latitude

Learning objectives

• Work a sun sight from sextant reading to plotted position line.
• Obtain latitude by meridian altitude at local apparent noon.
• Run a morning sight up to noon for a running fix.

Latitude at noon

The simplest sight in celestial navigation. Track the sun as it rises to its maximum altitude — you do not need the time, only the maximum. Then:

Zenith distance = 90 degrees - Ho. Latitude = zenith distance +/- declination, combining according to whether you and the sun are on the same side of the equator.

Worked example: Ho at meridian passage 61 12.0'. ZD = 28 48.0' N of the sun's GP. Declination 14 22.0' N. Latitude = 28 48.0' + 14 22.0' = 43 10.0' N.

The intercept method for a forenoon sight

Note the time to the second, work GHA and declination from the almanac (daily page plus increments and corrections), choose an assumed position that makes LHA a whole degree, reduce the sight, and plot the intercept and azimuth.

Running fix

A single sun sight gives one position line. Take a morning sight, run it forward for the course and distance sailed, and cross it with the noon latitude to obtain a fix. The quality of the fix depends on the angle between the two lines, so pick a morning sight with a good azimuth spread from the noon line.

Self-check

Morning sight gives a position line running 070/250 at 0930. Noon latitude 43 10.0' N. You have sailed 275 T at 6 knots. Plot the fix.

Quick quiz

Star, planet & moon sights

Learning objectives

• Pre-compute stars for twilight and find them quickly with the sextant.
• Work a star sight using SHA and Aries.
• Handle planet and moon sights and their extra corrections.

Twilight is the window

Stars need a visible horizon, so the working window is nautical twilight, morning and evening — often only twenty minutes. Preparation decides whether you get three good sights or none.

Pre-compute: choose three or more stars with azimuths spread around 120 degrees apart, calculate their expected altitude and azimuth for the mid-twilight time, pre-set the sextant to the expected altitude and look along the azimuth. The star appears in the field.

The Aries method

For stars the almanac gives GHA Aries and each star's sidereal hour angle: GHA star = GHA Aries + SHA star. Then LHA = GHA star -/+ longitude, and reduce as usual.

Planets and the moon

Planets: use the almanac's own GHA and declination columns, plus the small additional correction for Venus and Mars. Planets are bright enough to be shot before the horizon fades, which makes them valuable.

Moon: has its own altitude correction table in two parts, plus an HP (horizontal parallax) correction — parallax is significant because the moon is close. Moon and sun together in daylight give an excellent two-body fix.

Self-check

GHA Aries 128 44.3', SHA Sirius 258 33.1', longitude 032 15.0' W. Find the LHA of Sirius.

Quick quiz

Time, almanacs & sight reduction

Learning objectives

• Understand UT, zone time, chronometer error and rate.
• Extract GHA and declination from the Nautical Almanac with increments and corrections.
• Reduce a sight with tables (NP 303 / HO 249) and plot the result.

Time

All almanac data is tabulated against Universal Time. Convert your watch reading to UT via zone description and the chronometer's known error and rate. Four seconds of time equals one minute of arc of longitude: keep the watch checked against a time signal or GNSS and record the error every day.

Using the almanac

Daily pages give GHA and declination at whole hours. The increments and corrections pages ("yellow pages") give the amount to add for minutes and seconds, plus the d correction for declination change and v for the irregular bodies. Work down a printed sight form every time — the discipline is what prevents errors.

Sight reduction — the full sequence

1. Correct the sextant altitude to Ho.
2. Choose an assumed position: latitude a whole degree, longitude such that LHA is a whole degree.
3. Enter the tables with LHA, declination and assumed latitude; read Hc and Zn.
4. Intercept = Ho - Hc. Positive is Towards the body's azimuth, negative is Away.
5. Plot from the assumed position: along Zn by the intercept, then the position line at right angles.

Self-check

Ho 41 58.3', Hc 41 44.0', Zn 138. State the intercept and describe exactly how you plot the position line.

Quick quiz

Ocean meteorology & routeing

Learning objectives

• Describe the global circulation: trades, doldrums, horse latitudes and westerlies.
• Recognise the formation, structure and avoidance rules for tropical revolving storms.
• Use grib files, weather routeing software and long-range forecasts sensibly.

Global circulation

Air rises at the equator, giving the doldrums (ITCZ) with light and squally conditions, descends around 30 degrees in the subtropical highs (the horse latitudes), and flows back towards the equator as the trade winds — NE in the northern hemisphere, SE in the southern. Poleward of the highs lie the westerlies and their procession of depressions.

The ITCZ moves seasonally, which is why the point at which you turn west matters as much as the date you leave.

Tropical revolving storms

Season, sea temperature above about 26.5 degrees and distance from the equator are the ingredients. Signs: a long swell from an unexpected direction, an unusually high then falling barometer, and cirrus banding.

Avoidance uses the dangerous and navigable semicircles. Northern hemisphere: the right-hand semicircle relative to the storm's track is dangerous; put the wind on the starboard quarter and run to increase distance. Southern hemisphere: mirrored.

Routeing tools

Grib files give a model's wind and pressure, not a forecaster's judgement — treat them as one input. Routeing software optimises against a polar diagram; sanity-check its answer against pilot chart climatology and your own tolerance for heavy weather.

Self-check

Mid-Atlantic, barometer 1004 and falling, long swell from the east, wind NE 6. Assess the situation and state your action.

Quick quiz

Communications, welfare & emergencies offshore

Learning objectives

• Plan communications beyond VHF range: HF/SSB, satellite, EPIRB and AIS.
• Manage crew welfare, watchkeeping, fatigue, hygiene and medical emergencies with no outside help.
• Prepare for abandonment and for the emergencies that cannot be outsourced offshore.

Communications

Beyond about 30 miles VHF is gone. Options are HF/SSB with a Pactor modem for email and weather, satellite messengers and phones for data and voice, and NAVTEX close to shore. Register an EPIRB to the vessel and carry PLBs for crew. Establish a shore contact with a schedule and a written plan for what they do if you miss it.

Welfare

Watch systems must be sustainable for weeks: three or four hours on at night with a longer rest by day works for most crews. Feed people properly, enforce sleep, and treat dehydration and seasickness early — a dehydrated crew member is a casualty, not a nuisance.

Hygiene, saltwater sores and hand injuries cause most offshore medical problems. Carry a serious medical kit, know how to get telemedical advice, and have at least one crew member trained beyond basic first aid.

Emergencies far from help

Rescue may be days away, so self-sufficiency is the plan: damage control materials, a rig repair kit, a spare steering solution, and a grab bag with water, EPIRB, handheld VHF, documents and medication. Abandon to the liferaft only when you are stepping up into it.

Self-check

Serious hand injury on day 9 of a 21-day passage, 800 miles from land. Describe your communications, treatment and passage decisions.

Quick quiz