Offshore Passage Planning Strategy - Route Selection & Weather Windows

Prerequisites

This lesson is written for candidates working towards the RYA Yachtmaster Offshore certificate of competence. You should already be comfortable with everything taught at Day Skipper and Coastal Skipper level, in particular:

• Chartwork: plotting courses to steer allowing for tidal stream and leeway, fixes, EPs and DR.
• Tidal height and stream calculations for standard and secondary ports using Admiralty tide tables, tidal diamonds and tidal stream atlases.
• Interpreting the Shipping Forecast, inshore waters forecast, Navtex and synoptic charts at a basic level.
• Pilotage plans for entering harbour by day and night, including light characteristics and clearing lines.
• Coastal passage planning for passages of up to about 60nm.

The Yachtmaster Offshore exam has no compulsory course or theory prerequisite, but the minimum sea-time is substantial: 2,500nm logged and 50 days at sea within the last 10 years, at least 5 days as skipper, and 5 passages of over 60nm, of which 2 must be overnight and 2 as skipper. At least half of the qualifying time should be in tidal waters. You must be at least 18, hold a GMDSS radio operator's certificate (SRC minimum) and a valid first aid certificate. Expect the examiner to question you closely on how you planned your own qualifying passages.

Learning Objectives

This lesson maps onto the passage planning section of the RYA Yachtmaster Scheme Syllabus (G158) for Yachtmaster Offshore: "plan a passage of up to 150nm from harbour, by day and night, in a wide range of conditions". By the end you should be able to:

• Produce a complete written passage plan for a multi-day passage, demonstrating self-sufficiency up to 150nm from a safe haven (RYA syllabus: passage planning, preparation of the yacht and crew).
• Choose between Great Circle, rhumb line and composite routes and justify the choice (RYA syllabus: route selection).
• Identify and plan around hazards: traffic separation schemes, fishing grounds, wind farms, firing ranges, cables and offshore structures (RYA syllabus: navigation hazards and IRPCS in passage planning).
• Identify bail-out ports for each leg and judge whether they are usable in the forecast conditions (RYA syllabus: ports of refuge).
• Calculate tidal gates at departure, en route and at arrival, and adjust departure time to meet them (RYA syllabus: tidal planning).
• Plan a landfall, including a night landfall, using lights, soundings and the technique of aiming off (RYA syllabus: pilotage and landfall).
• Assess a weather window and make an objective Go/No-Go decision (RYA syllabus: meteorology applied to passage planning).
• Calculate fuel, water and electrical power budgets with appropriate reserves (RYA syllabus: preparation, stowage and self-sufficiency).
• Design a watch system that fits the crew and the passage, and brief the crew (RYA syllabus: crew management and watchkeeping).
• File a passage plan with a shore contact, and prepare the documentation an examiner expects.

Why Offshore Planning Is Different

On a coastal passage, help and shelter are rarely more than two or three hours away. If the weather turns or the engine stops, you put into the nearest harbour. Offshore, that safety net disappears. Once you are 40, 80 or 120 miles from the nearest coast, the vessel and crew must be able to look after themselves for a day or more, whatever the sea throws at them.

The passage plan is therefore not just a line on a chart. It is the operational blueprint for the whole voyage and covers five areas:

The RYA asks the Yachtmaster Offshore candidate to plan as a professional would: methodically, in writing, with a reason for every choice. In the exam you may be asked to produce a passage plan verbally or on paper within minutes, so practise a standard format until it is second nature.

A standard planning sequence

1. Define the objective: departure, destination, approximate dates, crew available.
2. Gather information: charts (paper and electronic), almanac (e.g. Reeds), pilot books, tidal stream atlases, Admiralty List of Lights, notices to mariners, forecasts.
3. Draw the route and measure distance; list waypoints with lat/long and the course and distance between them.
4. Identify dangers along the route: shoals, overfalls, TSSs, wind farms, firing ranges, fishing grounds.
5. Work out tidal gates at departure, en route and arrival.
6. Identify bail-out ports for each stage of the route.
7. Plan the landfall and pilotage into the destination (and into each bail-out port).
8. Calculate fuel, water, provisions and power with reserves.
9. Design the watch system.
10. Assess the weather window and make the Go/No-Go decision.
11. Brief the crew and leave a passage plan with a responsible person ashore (and consider registering with the free RYA SafeTrx app).

Route Selection

Great Circle versus rhumb line

A rhumb line crosses every meridian at the same angle, so it is a single constant course: easy to steer and easy to plot as a straight line on a Mercator chart. A Great Circle is the shortest distance between two points on a sphere, but on a Mercator chart it appears as a curve bowed towards the nearer pole, and its true course changes continually.

The diagram below compares the two tracks between the same two points.

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The saving from a Great Circle depends on distance, latitude and the direction of travel. It is greatest on long east-west passages at high latitudes and zero on a north-south track or along the equator. For passages within the Yachtmaster Offshore scope (up to about 150nm from a haven) the difference is a fraction of a mile, far less than the errors introduced by tide, leeway and wind shifts. For an Atlantic crossing of 2,500nm, however, the saving can be well over 100nm, which is why Great Circle sailing is part of the Yachtmaster Ocean syllabus.

In practice, the geometry is rarely what decides the route. The skipper weighs:

• Wind angle: a route that lets you reach rather than beat may be faster despite being longer.
• Tidal and ocean currents: a deviation to stay in a fair stream or out of a foul one.
• Hazards: keeping well off lee shores, headlands with overfalls, and rocks.
• Traffic: crossing TSSs at right angles (COLREG Rule 10) and spending as little time as possible in shipping lanes.
• Shelter: staying closer to bail-out ports when the forecast is marginal.

The result is often a composite route: a series of rhumb line legs joined at waypoints, chosen to exploit wind and current and to stay clear of danger. Routing software works on the same principles, but you must understand the inputs well enough to challenge its suggestions. Software that does not know your boat's real polars, or that is fed an optimistic GRIB file, will produce an optimistic route.

Waypoints and the route card

List every waypoint with name, lat/long, the true course and distance to the next waypoint, and notes (lights expected, depth expected, hazards, tidal gate). Check each waypoint on a paper chart at a suitable scale; a waypoint plotted on a small-scale chart can sit on a rock that only appears on the large-scale chart. Never navigate to a waypoint placed on a navigation mark: you are planning to hit it.

Passage Hazards and Rules That Shape the Route

A line drawn straight across the chart may pass through several areas where rules or hazards limit what you can do. Check each of these before you fix the waypoints.

Traffic Separation Schemes (COLREG Rule 10)

Many offshore routes cross a TSS, such as the Dover Strait, the Casquets, Off Ushant, Land's End or Off Fastnet. The rules for a yacht are:

• Join or leave a lane at the termination if you can; if joining or leaving from the side, do so at as small an angle as practicable.
• Cross a lane on a heading as nearly as practicable at right angles to the general direction of traffic flow, and by the shortest route. Your heading, not your course over the ground, is what counts: allow for the tide and steer so that your heading is at 90 degrees.
• Do not enter a separation zone except in an emergency or to fish.
• Use the inshore traffic zone only for local traffic; a yacht on passage should avoid it unless the route requires it.
• A sailing vessel or a vessel under 20m must not impede the safe passage of a power-driven vessel following a lane (Rule 10(j)).
• Plan to cross in good visibility and, if possible, in daylight, with the crew fresh and the engine ready.

On the chart, mark the lane directions and the crossing point. Radar and AIS help, but the lookout comes first.

Other hazards to mark on the plan

Writing and Filing the Passage Plan

A good plan is concise enough to use on deck and complete enough to be understood by a stranger. Many skippers use a one-page summary on a clipboard plus the detailed workings in a file. The examiner will expect:

• Overview: vessel, crew list with experience, departure, destination, ETD and ETA, distance.
• Route card: waypoints with lat and long, course (True and Magnetic), distance, hazards and notes.
• Tidal data: HW and LW times at standard and secondary ports, tidal gates and their windows, tidal stream rates and direction for each leg.
• Weather: forecast summary and sources, the decision criteria for Go/No-Go and for diversion.
• Bail-out ports: name, approach notes, entry limits, VHF channel, for each leg.
• Communications: the watch schedule for VHF and DSC; a satellite phone number if carried; the shore contact.
• Fuel, water and power: the budget figures and the reserves.
• Emergency information: liferaft, EPIRB, grab bag, who to call, the position reporting plan.

Leave a copy with a responsible person ashore. Tell them when you expect to arrive, what to do if you have not contacted them by a specific time, and how to reach the Coastguard (999 and ask for the Coastguard, or the CG66 form). The free RYA SafeTrx app can do this for you electronically. Always tell them when you arrive.

Bail-Out Ports

Every leg of the passage must have at least one viable port of refuge. The diagram below shows how bail-out ports are spread along the route and summarises what makes a harbour suitable.

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A harbour is only a bail-out port if it meets all of these conditions in the conditions that will make you want to use it:

1. Shelter from the forecast wind direction and sea.
2. Enough depth for your draught at the state of tide you are likely to arrive, or a safe place to wait.
3. A safe entrance in strong onshore winds: no breaking bar, no narrow rock-bound entrance, no wind-against-tide race at the mouth.
4. Usable at night: lit entrance, clear leading lights or sectored lights.
5. Ideally, facilities for repairs, fuel and medical evacuation.
6. Within a few hours' sailing under degraded conditions (reefed down, tired crew, possibly no engine).

Many superb harbours fail test 3. A river entrance with a bar may be perfectly sheltered once inside but lethal to approach in a Force 7 onshore with an ebb running out. Read the pilot book's entry restrictions for every bail-out port and prepare a brief pilotage note for each before departure, because you will not want to be doing that research while heeled at 30 degrees in the dark.

During the passage, keep a running mental (and written) list: "If it goes wrong now, where do I go, what course, how far, how long?" Update it at every watch change and every major waypoint. As you pass the halfway point, the best refuge may switch from behind you to ahead of you, and that changes the decision when the weather deteriorates.

Tidal Gates

A tidal gate is a place where the tidal stream must be favourable for the passage to be safe or efficient: a headland with a race (Portland Bill, the Lizard, St Alban's Head), a narrow channel (the Swinge, the Alderney Race), a bar, or a harbour entrance with a strong ebb. Missing the gate costs hours of delay, and worse, may put you in overfalls or wind-against-tide seas.

The diagram shows the four calculation steps and what to do if the timing does not fit.

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The calculation

1. Find the time of HW (or LW) at the relevant standard port from the tide tables, correcting for BST if necessary.
2. Apply secondary port corrections where the gate is referenced to a secondary port.
3. Use the tidal stream atlas or tidal diamonds to find when the stream through the gate turns fair, and when it is strongest.
4. Work backwards from the gate to your departure: time at gate minus (distance to gate divided by expected speed over ground) gives the latest departure time.

Worked numbers

You must round a headland 24nm from your departure port. The stream there turns fair at HW Dover minus 1 and runs fair for 6 hours, peaking at 3 knots at springs. HW Dover is 1130 BST. You expect 6 knots through the water, with roughly 1 knot foul tide for the first part of the leg.

• Gate opens: 1130 minus 1 hour = 1030 BST. It stays fair until about 1630.
• Speed over ground for the approach: about 5 knots.
• Time to gate: 24 divided by 5 = 4.8 hours, about 4 hours 50 minutes.
• To arrive as the gate opens: depart about 0540. To arrive at the peak fair stream (around HW Dover plus 2, 1330), depart around 0840.

If you leave at 0540 you arrive as the stream slackens and turns, carrying it the whole way round. Leaving after about 1130 would put you at the headland as the stream is turning foul again.

Worked example: a gate at the end of the passage

You are on a 120nm passage and must pass a tidal gate, a headland with a race, 30nm from the destination. The stream runs fair (towards your destination) from HW Dover minus 3 to HW Dover plus 3. HW Dover is 0200, so the fair window runs from 2300 to 0500. You make 6 knots through the water in the open sea and, from the atlas, the average fair stream on the approach is 1.5 knots.

• Speed over ground on the approach: 6 plus 1.5 = 7.5 knots. Time for the final 30nm: 4 hours.
• To arrive at the gate at the peak (HW Dover, 0200), be 30nm from the gate at 2200.
• If you are behind schedule and reach the 30nm point at 0100, you arrive at 0500, just as the fair stream ends. You would need to motor-sail at 8 knots or more to gain time, or accept waiting for the next fair window about 12 hours later.
• Option: if you reach the point early, say 2000, slow down or heave-to to arrive at the gate when the stream is fair, rather than arriving at slack or foul and taking the race on the turn.

The lesson is that an offshore gate is a timing problem: you manage it by adjusting speed hours beforehand, not by arriving and hoping.

When timings do not fit

If departure tide and arrival tide cannot both be met, you have four options: change the departure time, adjust speed en route (slow down to arrive at the right time, or motor-sail to make a gate), wait offshore (acceptable in settled weather, risky if it is deteriorating), or divert to an alternative port with a better tidal window. Arrival gates matter more than departure gates offshore, because by then the crew is tired and the weather may have changed.

Landfall Planning

Landfall after a long passage is one of the most hazardous phases of any voyage. The crew is tired, the navigator may have been working from DR for hours, and there is a natural eagerness to get in. Plan it before you leave.

What to prepare

• A list of the lights you expect to raise, with characteristics, elevation, nominal range and the geographic range from your height of eye (a light's loom can often be seen well before the light itself rises above the horizon).
• Depth contours that cross your approach track, so you can use the echo sounder to confirm position.
• Radar-conspicuous targets: steep headlands, cliffs, offshore towers and wind farms.
• A clearing bearing or clearing depth that keeps you off the outlying dangers.
• The pilotage plan for the harbour entrance itself.

Aiming off

If you aim directly at a harbour and arrive at the coast with an uncertain position, you do not know whether to turn left or right. Aiming off removes the ambiguity: you deliberately steer for a point a few miles to one side, usually up-tide or up-current of the destination, so that when you reach the coast or a chosen depth contour you know which way to turn.

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How far to aim off depends on how uncertain your position is. If your EP has a 4nm circle of uncertainty, aiming off by 2nm is not enough; aim off by at least the radius of uncertainty. A well-defined depth contour (for example the 20m line) running parallel to the coast makes an excellent "handrail" to follow towards the entrance.

Night landfall

Night landfall has advantages: lights are identifiable at long range and their characteristics confirm identity unambiguously. The risks are misidentifying a light, missing unlit hazards (moorings, fishing gear, rocks) and loss of night vision. Use red light at the chart table, time each light's period with a stopwatch, and get two features to agree before you commit. If in doubt, stand off until daylight. Heaving-to 10 miles offshore is always better than feeling your way in through unlit rocks.

Weather Windows

A weather window is not simply a fine day. It is a period during which the forecast conditions stay within the limits of the vessel and the crew for the whole duration of the passage, plus a safety margin. For a 30-hour passage, you need confidence in roughly the next 36 to 48 hours.

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Assessing the window

1. Start looking at forecasts at least 72 hours before planned departure: synoptic charts, the Met Office Shipping Forecast and extended outlook, and GRIB files from more than one model.
2. Identify the main pressure systems and their expected movement. How fast is the next low approaching? Is it deepening?
3. Estimate when fronts will cross the route and what wind shifts they bring.
4. Compare the expected wind strength and direction, sea state and visibility with your limits for this crew on this boat.
5. Check consistency between successive forecasts. If each run brings the low in earlier or deeper, the forecast is unstable and your confidence should be lower.
6. Decide, and record why.

The skipper must set limits before looking at the forecast. For example: "With this crew, no departure if the forecast for any part of the passage exceeds Force 6, or if a gale warning is in force for the sea areas on our route." Setting limits after seeing the forecast invites you to stretch them.

Go/No-Go decisions

The framework below forces an objective answer.

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Schedule pressure is never a valid reason to sail. Crew flights, charter return dates and the owner's impatience have killed more sailors than any single weather system. If the forecast exceeds the limits of the crew or vessel, the professional decision is No-Go, and a good skipper explains this to the crew without apology. A barometer falling 8mb in 6 hours is a strong indicator of a gale approaching; a fall of more than 3mb in 3 hours is a widely used trigger for expecting strong winds.

Fuel Planning

Diesel consumption rises steeply with RPM because the resistance of a displacement hull rises steeply as it approaches hull speed. Pushing from 60% to 80% of maximum RPM typically doubles fuel burn for a gain of perhaps half a knot.

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Calculating the fuel requirement

1. Find (or measure) the fuel burn at your economical cruising RPM, usually 60 to 70% of maximum. Measure it on your own boat: fill the tank, motor for a timed period at constant RPM, and refill.
2. Estimate the hours of motoring expected, based on forecast calms and harbour entry/exit.
3. Apply a headwind and adverse-current penalty, typically 30%, because motoring into a sea reduces speed over ground and increases load.
4. Add a reserve of at least 20 to 25% of the total, or enough for a set number of hours of motoring to the most distant bail-out port, whichever is greater.
5. Check against usable tank capacity. The last 10% or so of a tank may be unusable when the boat is heeling or pitching, and the dregs are where water and sediment sit.

Worked example

Motoring speed at 1800 RPM is 6 knots and burn is 3.5 litres per hour. You expect about 15 hours of motoring in calms.

• Base: 15 hours times 3.5 L = 52.5 L.
• 30% penalty: 52.5 times 1.3 = 68 L.
• Reserve for 6 hours' motoring to a bail-out port: 6 times 3.5 = 21 L.
• Requirement: about 89 L. If the tank holds 120 L with 110 L usable, you depart full and carry a 20 L jerry can lashed securely for heavy weather.

Range check: 110 L divided by 3.5 L per hour = 31 hours, times 6 knots = about 190nm in flat water, or about 145nm with the 30% penalty applied. Knowing your motoring range in miles is more useful than knowing litres.

Water and Provisions

Plan on a minimum of 3 litres per person per day for drinking and basic cooking in temperate waters. In the tropics, plan for 4 to 5 litres. Washing, unless with seawater, is extra.

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The formula is: crew times litres per person per day times days, plus an emergency reserve. For four crew on a four-day passage: 4 times 3 = 12 L per day, times 4 days = 48 L. The diagram adds a 20 L reserve to give 68 L. Note that a strict 48-hour reserve for four crew is 2 times 12 = 24 L, so 72 L would be the more conservative total. Always round up.

Good practice:

• Split water between at least two tanks or carry bottled water, so that contamination or a leak in one does not lose everything.
• Keep 5 to 10 litres of drinking water in the grab bag or in containers that can go into the liferaft.
• Define rationing triggers before departure, for example "if a tank is found contaminated, drinking and cooking only, no washing in fresh water".
• Plan one-pot meals that can be cooked heeled, and have ready-to-eat food (pre-cooked stews, sandwiches, flapjack) for the first 24 hours when crew may be seasick.

Electrical Power Budget

Modern yachts depend on electricity for navigation lights, instruments, autopilot, radio, AIS, radar, refrigeration and charging devices. Running flat 100 miles offshore means losing the autopilot, the AIS and possibly the VHF. Build a 24-hour power budget before departure.

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Building the budget

For each item, multiply the current draw in amps by the hours used per day. For example, an autopilot averaging 2.5 A for 12 hours uses 30 Ah; LED navigation lights at 1 A for 12 hours use 12 Ah. Total the consumption, then total the realistic charging input.

Charging sources are rarely as good as their rating:

• Alternator: only while the engine runs, and the standard regulator often tapers charge long before the battery is full.
• Solar: rated output only in direct sun at right angles to the panel; expect perhaps 20 to 40% of rated output averaged over a UK day, less with shading from sails and boom.
• Wind generator: good in fresh breezes, almost nothing in light airs or when running downwind (apparent wind is reduced).
• Hydro-generator: steady output above about 5 knots, at the cost of some drag.

For lead-acid batteries, plan to keep the state of charge above 50%; deeper discharge shortens their life dramatically. A 400 Ah lead-acid bank therefore gives you only about 200 Ah usable. In the example above, consumption is 120 Ah per day; with 200 Ah usable and no charging you would be flat in well under two days. If the budget shows a deficit, reduce loads (radar on standby, fridge off, hand-steer in daylight), add charging capacity, or schedule engine running purely for charging, which then feeds back into the fuel calculation.

Crew and Watch Planning

The watch system is part of the passage plan, not an afterthought once you are at sea. It must be designed around crew numbers, experience, and the expected conditions.

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Points to settle before departure:

• Who is in each watch, and is there a competent watch leader in each?
• Will the skipper stand a watch or float? Floating lets the skipper be available for navigation and decisions, but the skipper still needs protected sleep.
• What are the standing orders for calling the skipper (wind over a threshold, a ship with CPA under 1nm, barometer falling fast, any doubt)?
• How will watches overlap at handover?
• Meals: who cooks, and when?

Fatigue is covered in detail in the watchkeeping lesson. For planning purposes, assume crew will be less effective in the first 24 hours (seasickness, adjusting to the rhythm) and that performance will be worst between 0200 and 0500.

Worked Example: Falmouth to Crosshaven

The map below summarises a planned passage from Falmouth to Crosshaven in Cork Harbour with four crew.

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Route. The direct rhumb line from Falmouth to Roches Point is about 155nm. The planned route instead heads west-south-west past the Lizard towards the Isles of Scilly before turning north-west across the Celtic Sea, keeping well clear of the Land's End TSS and giving a bail-out option at St Mary's. Allowing for this routing and for tacking or wind-driven deviations, the plan uses a passage distance of up to about 220nm, as shown on the map. At 5 knots average that is up to 44 hours, so this is a two-night passage.

Weather. Planning began 72 hours out. A ridge of high pressure was forecast to build over the Celtic Sea, giving north-westerly Force 3 to 4 backing west-south-west, with the next frontal system not expected for at least three days. Limits set beforehand: no departure with a forecast above Force 6 or a gale warning for Plymouth, Sole, Fastnet or Lundy.

Tidal gate. The Lizard is the first gate. Departure was set for 2 hours before HW Falmouth to carry a fair west-going stream round the Lizard and avoid the race off the point in wind against tide. Keep at least 3nm off the Lizard in any sea.

Bail-out ports. First phase: Falmouth (return), Newlyn and St Mary's. Mid-passage, St Mary's is the nearest refuge until roughly 50nm north-west of Scilly, after which the south coast of Ireland is closer. Final phase: Crookhaven, Baltimore and Kinsale, all with lit entrances usable at night in most wind directions.

Fuel. 15 hours of expected motoring at 1800 RPM and 3.5 L per hour = 52.5 L, plus a 20 L reserve. Applying the 30% penalty gives 68 L plus reserve, still well within the 110 L usable.

Water. Four crew for four days (passage plus margin) at 3 L = 48 L, plus a 20 L reserve; tanks hold 160 L, so water is not a constraint, but the two tanks are kept isolated and 10 L of bottled water goes in the grab bag.

Power. 120 Ah per day consumption against a 400 Ah lead-acid domestic bank. Daily charging from solar plus an hour's engine run each morning keeps the bank above 50%.

Watches. Four-on, four-off with two crew in each watch; skipper floats and is called by standing orders.

Landfall. Planned for daylight on day three. Aim off to the south and west of the harbour, pick up the 20m contour, then follow it north towards Roches Point (Fl WR 3s). If arrival slips into darkness, Roches Point's sectored light and Cork Harbour's well-lit main channel make a night entry straightforward, but the crew is to be briefed on the pilotage before the final night begins.

Examiner Expectations

• You may be asked to produce a plan for a passage of about 60 to 100nm in 20 to 30 minutes using paper charts and almanac. A method works better than speed: follow your standard planning sequence out loud.
• Be ready to justify every choice: why that route, that departure time, those margins. "Because it looked right" fails; "because the stream turns fair at 0540 and the forecast gives a window of 36 hours with 12 hours spare" passes.
• Expect questions about your qualifying passages: where you went, what the weather was, what you did when it changed, and what you would do differently.
• You will be asked what you would do if the weather deteriorated, an engine failed, a crew member was ill or the electronics failed. Give a specific action, a time and a destination.
• Examiners watch for fatigue. Show that you plan rest for yourself and the crew, and that you do not make major decisions when exhausted.

Common Mistakes

• No intermediate safe havens. Every leg must have a refuge, checked for the forecast wind direction.
• Choosing refuges that cannot be entered in onshore gales. A sheltered harbour with a breaking bar is not a bail-out port in a Force 7 onshore.
• Relying on GPS for landfall. Prepare lights, depths and radar targets so you can make landfall if the plotter fails. Examiners may cover the plotter.
• Fuel calculated for flat calm only. Always apply a headwind and adverse current penalty and keep a reserve.
• Planning the weather window to the passage length exactly. Add a margin; passages take longer than planned.
• Setting weather limits after reading the forecast. Set them first.
• Ignoring the arrival tidal gate. Missing it after two nights at sea can leave a tired crew waiting offshore in deteriorating weather.
• Watch system left until departure. Design it as part of the plan and brief it before leaving.
• Not telling anyone ashore. Leave the plan, ETA and an overdue time with a responsible person.
• Crossing a TSS on the course over the ground instead of the heading. Steer so that your heading crosses the lane at right angles.
• Forgetting to tell the shore contact you have arrived. A missed arrival call can trigger a search.
• Ignoring danger areas and wind farms. Check Navtex, the almanac and Notices to Mariners before fixing waypoints.

Summary

• An offshore passage plan covers navigation, weather, logistics, people and contingencies, and must demonstrate self-sufficiency.
• Within 150nm the Great Circle saving is negligible; route choice is driven by wind, tide, hazards, traffic and shelter.
• Every leg needs a bail-out port that is safe to enter in the conditions that would make you use it.
• Tidal gates are worked from standard port HW, through secondary port corrections and the stream atlas, back to a departure time.
• Aim off on landfall by at least your position uncertainty, and use depth contours as handrails.
• A weather window must cover the whole passage plus a margin; limits are set before reading the forecast and schedule pressure is never a reason to go.
• Fuel: base burn plus 30% penalty plus at least 20% reserve. Water: 3 L per person per day plus 48 hours' reserve. Power: consumption must be matched by realistic charging, with lead-acid batteries kept above 50%.
• The watch system is designed before departure.

Check Your Understanding

1. On a 120nm passage at 50 degrees north, why would you normally sail rhumb line legs rather than a Great Circle?

Answer: Over that distance the Great Circle saving is a fraction of a mile, much smaller than the errors from tide, leeway and wind shifts. Rhumb lines give a constant course that is easier to steer and plot, and the route is chosen around wind angle, tide, hazards and shelter instead.

2. Name four tests a harbour must pass to be a useful bail-out port.

Answer: Shelter from the forecast wind and sea; adequate depth for your draught at the likely state of tide; a safe entrance in strong onshore winds (no breaking bar or narrow rocky entrance); and usable at night with a lit entrance. Facilities for repair and medical evacuation and being within a few hours' sailing are also desirable.

3. A headland's stream turns fair at HW Dover minus 2 and runs for 6 hours. HW Dover is 1400. The headland is 30nm away and you expect 6 knots over the ground. What is the latest sensible departure to arrive while the stream is still fair with at least 2 hours to spare?

Answer: The fair stream runs from 1200 to 1800. To arrive by 1600 (2 hours before it turns), with 5 hours' passage time (30 divided by 6), depart no later than 1100. To arrive as it turns fair at 1200, depart at 0700.

4. Your EP has a 3nm circle of uncertainty. How far should you aim off on landfall, and why?

Answer: At least 3nm, so that even at the worst end of your uncertainty you are certain which side of the harbour you are on and therefore which way to turn when you reach the coast or chosen depth contour.

5. The forecast for your 36-hour passage shows Force 4 to 5 for 30 hours, then a cold front with Force 7 arriving at hour 34. Is this a weather window?

Answer: No. The window must cover the whole passage plus a safety margin. Passages often take longer than planned, so the front would very likely arrive before you are safely in. Delay departure or choose a shorter passage.

6. You expect 20 hours of motoring at 4 L per hour. Using a 30% penalty and a 20% reserve on top, how much fuel do you need?

Answer: Base 80 L, times 1.3 = 104 L, plus 20% reserve (about 21 L) = about 125 L.

7. How much water would you plan for five crew on a three-day passage in temperate waters, including a 48-hour reserve?

Answer: 5 times 3 L = 15 L per day. Three days = 45 L. Reserve 2 days = 30 L. Total 75 L, split between separate tanks or containers.

8. Your daily consumption is 140 Ah and your lead-acid domestic bank is 300 Ah. With no charging, how long before you reach 50% state of charge?

Answer: Usable capacity is 150 Ah (50% of 300). 150 divided by 140 is just over one day, about 26 hours. You must plan engine charging or reduce loads.

9. Why is "the crew need to be back for work on Monday" not an acceptable input to a Go/No-Go decision?

Answer: Schedule pressure does not change the weather or the vessel's or crew's capability. Allowing it to influence the decision leads to departing into conditions beyond the crew's limits, a factor in many offshore accidents. The decision must be based only on forecast conditions against pre-set limits.

10. At what angle should a yacht cross a traffic separation lane, and does the angle refer to heading or course over the ground?

Answer: As nearly as practicable at right angles to the general direction of traffic flow (COLREG Rule 10(c)), and the angle refers to the vessel's heading, not the course made good. Allow for tidal stream by pointing up-tide so that the heading stays at 90 degrees.

11. List the information that a passage plan left with a shore contact should contain.

Answer: Vessel description and registration, crew list, route and waypoints, ETD and ETA, bail-out ports, communications (VHF, satellite phone), liferaft and EPIRB details, and an overdue time with instructions on whom to call (Coastguard) if you have not made contact.

12. A tidal gate is open from 1000 to 1600. You are 36nm from it and make 6 knots over the ground. When must you be at the starting point to arrive at the middle of the window?

Answer: The middle of the window is 1300. Time to the gate is 36 / 6 = 6 hours, so you must be at the starting point at 0700.