When to Go and When to Wait - Passage Planning Risk Assesment

Collection: Field Notes — Old Fashioned Seamanship

Series: Coastal and Offshore Passage Planning

Subject: When to go and when to wait — the go/no-go decision framework; the three inputs that determine readiness; the biases that produce bad departure decisions; the ongoing nature of the decision once underway; and how the skills in this collection bear directly on the judgement

The weather killed fewer sailors before reliable forecasts existed than it does now. This seems improbable until you consider that before forecasts existed, every sailor was trained to read the sky, the sea, and the barometer, and that the decision to leave harbour in doubtful conditions was made by a navigator with those skills rather than by one who had checked an app and assumed the problem was solved.

This is not a romanticisation of pre-forecast seamanship. Plenty of sailors died before forecasts. But the specific failure mode that now kills sailors — departing on the basis of a forecast without the observational skills to verify it, monitor it, or recognise when it has diverged from reality — was less available as a failure mode when the forecast did not exist and the navigator's own judgement was the only instrument available.

The go/no-go decision is the point at which everything in this collection converges. The skills developed across Reading the Sea, Traditional Navigation Techniques, and Weather Forecasting are not academic exercises. They are instruments for making this decision well and for continuously reassessing it once the departure has been made.

The three inputs to any go/no-go decision

Jim Woodmencey's framework in Reading Weather organises the decision into three independent evaluations that must all be assessed before committing to a departure: the activity, the ability, and the risk. The weather forecast is the fourth input, but it is only useful in the context of the other three. A forecast showing Force 4 to 5 is not inherently a go or a no-go — it is a number that becomes meaningful only when set against what is being attempted, by whom, with what equipment, in what waters, with what consequences if it goes wrong.

The activity — what is actually being planned. A day sail in familiar coastal waters in company with another boat is a different activity from a four-day Biscay crossing single-handed. The same forecast applies to both; the decision is different. Key questions: how long is the passage, how flexible is the timing, what is the sea room, are there good harbour options en route, what is the commitment point beyond which turning back becomes more dangerous than continuing?

The ability — the crew's skill level and the boat's capability. Woodmencey identifies three components: experience, knowledge, and preparation. Experience means demonstrated competence in the conditions likely to be encountered, not just logged miles. Knowledge means familiarity with the specific waters, the tidal patterns, the weather tendencies — the local knowledge that Frank Dye built systematically before his North Sea and North Atlantic passages in a sixteen-foot dinghy. Preparation means that the vessel, the equipment, and the crew are ready for the forecast conditions plus the worst plausible deviation from the forecast.

The risk — the consequence of getting it wrong. Woodmencey frames this as three sub-questions. Can the crew take care of themselves if things go badly? Can they retreat to shelter, or is the commitment irreversible once made? Could they survive an unplanned overnight if the passage took twice as long or conditions prevented harbour entry? For offshore passages, honest answers to these questions constrain the weather window required for a safe departure more than the forecast alone.

The fourth input: the weather assessment

The weather input to the go/no-go decision is not the forecast. It is the navigator's own assessment of the forecast, its confidence, its trajectory, and how it aligns with current observations.

Woodmencey is direct about this in Reading Weather: if the forecast says sunny and you are getting rain, trust your observations and make your own forecast. This is not scepticism about professional meteorology — it is recognition that the gap described in Why the Forecast Is Always Wrong on Terrain-Influenced Water is real, that model resolution limits are real, and that an analysis chart that has already diverged from the previous forecast is telling you something the current forward forecast has not yet caught up with.

The weather assessment for a go/no-go decision is assembled from: the current analysis chart compared to what the previous forecast said it would show; the barometric trend over the last six to twelve hours; the cloud sequence visible from the departure point; the swell direction and period if the passage involves open water; and the specific terrain effects on the passage area covered in What Hills Do to Wind. A forecast showing Force 3 to 4 is plausible and reassuring. The same forecast combined with a barometer that has fallen two hectopascals in three hours, cirrus that was not present at this morning's observation, and a southwest swell of increasing period — that is a different situation, and it requires a different decision.

The biases that produce bad go decisions

Every investigation into maritime and outdoor accidents involving weather finds the same cluster of psychological factors leading to the bad departure. Woodmencey names the most important explicitly: do not let goals cloud your thinking or change your forecast. This is a statement about goal fixation — the tendency to allow a desired outcome to distort the assessment of the evidence toward it.

Goal fixation manifests as: the passage that has been planned for months and must happen on this weekend because everyone has taken leave; the race that cannot be scratched without losing the series; the harbour that must be reached today because the berth is booked; the family commitment at the destination. All of these are legitimate pressures. None of them changes the wind speed or the wave height. What they do is change the way the navigator reads the evidence — selectively weighting confirmatory signs (the barometer has not fallen much yet, the clouds could be the edge of a high moving through) against disconfirmatory ones (the swell period has shortened noticeably since this morning, the forecast for the adjacent sea area has been upgraded).

The correction Woodmencey proposes is also simple: imagine the worst plausible weather scenario for the passage and ask what the consequences of encountering it would be. Not the catastrophic scenario — the plausible bad one. Force 6 becoming Force 7 in the southern North Sea. The cold front arriving six hours early. The Biscay swell building to three metres in the middle of the bay. If those consequences are survivable within the crew and boat's capability, proceed. If they are not, wait.

A related bias is the normalcy bias: the tendency to assume that conditions will be similar to previous passages on the same route. The North Sea crossing that was comfortable last June was comfortable because the synoptic pattern in June was favourable. The same crossing in September with a similar initial forecast may encounter a different synoptic pattern at a different stage. Each passage is a separate risk assessment, not an extrapolation from the last one.

The go/no-go scoresheet applied to sailing

Woodmencey's scoresheet assigns numerical scores to each of the three evaluation categories and the weather assessment, with a score of ten or below indicating a no-go. The precise numbers are less important than the discipline of making each assessment explicitly and honestly rather than as a single gestalt judgement that is easily corrupted by goal fixation.

Applied to an offshore sailing passage, the scoresheet translates roughly as follows.

Activity: A day coastal passage scores high. A single-handed Biscay crossing in October scores low. Anything requiring passage through tidal races with constrained timing windows scores low on the retreat question regardless of forecast conditions.

Ability: A crew that has made the passage before in comparable conditions scores high. A crew making the passage for the first time, in unfamiliar waters, with one experienced skipper and inexperienced crew, scores low on knowledge. A well-found boat with all safety equipment serviced and a navigator who has run an actual DR plot in the last six months scores high on preparation. A boat whose chart plotter is the only navigation and whose crew has never taken a compass bearing scores low.

Risk: A passage with good intermediate harbours at reasonable intervals scores high on retreat. A single-hop passage of three hundred miles with no intermediate shelter scores low. A crew that has practised man-overboard recovery in daylight scores higher on survivability than one that has not. A boat with a functioning EPIRB and liferaft scores higher than one without.

Weather: Conditions improving on a rising barometer in post-frontal Polar Maritime air scores high. Conditions steady with an uncertain five-day forecast and a secondary depression visible on the trailing cold front scores low. A falling barometer at departure, regardless of what the forecast shows, scores low.

The ongoing decision: it does not end at departure

Woodmencey's most important practical instruction in Reading Weather is that forecasting is an ongoing process. The go/no-go decision is not made once at the marina and then resolved. It is remade at every watch change, at every barometer reading, at every significant change in cloud type, wave period, or wind direction.

The signals that trigger a reassessment mid-passage are the same signals described throughout the Weather Forecasting series. A barometer that was rising at departure and has now stalled and begun to fall. A swell period shortening when it should be lengthening in post-frontal conditions. A cloud base that has dropped noticeably since the morning watch. A wind direction that has backed when the frontal sequence predicts it should be steady or veering. Any one of these is a prompt to re-examine the decision. The combination of two or more is a prompt to act.

Acting mid-passage means: identifying the nearest suitable shelter and its distance and bearing; establishing whether the current conditions and projected development allow that shelter to be reached before conditions exceed the crew's capability; making a specific go/no-go decision for the new immediate objective rather than the original destination. This is not failure. It is the exercise of exactly the judgement that distinguishes competent passage-making from optimism.

The navigator's logbook is the instrument that makes this ongoing reassessment possible. A log that records barometric pressure every watch, wind direction and force every watch, cloud type and base, swell period and direction, and the DR position is a log that supports a weather reassessment with actual data. A log that records the position by GPS and nothing else is a log that has recorded the track and missed everything that matters for the decision.

Frank Dye and the standard of preparation

Frank Dye's passages to Norway and Iceland in a sixteen-foot Wayfarer dinghy — described throughout this collection as the inspirational benchmark for what capable seamanship without electronics looks like — are relevant here not for their audacity but for their preparation. Dye was not reckless. He was meticulously prepared, technically capable, and deeply knowledgeable about the North Sea's weather and tidal patterns before he made those passages. He had practised the skills he needed in familiar waters to the point where they were reliable. He carried the instruments described in The Kit and used them consistently.

The relevant distinction is between Dye's level of preparation and the modern sailor who departs without having built the observational skills to monitor conditions, without a DR plot running, without a lead line aboard, without any instrument other than the chartplotter that is about to lose signal. Dye's passages were challenging because the North Sea in a sixteen-foot dinghy is inherently challenging. They were not reckless because the navigator making them had the capability to monitor developing conditions and respond appropriately.

Woodmencey's framework captures this: overestimating ability and underestimating the weather is the bad combination. The skills in this collection are not theoretical additions to a sailor's knowledge. They are practical instruments for not making that combination — for knowing what the weather is actually doing rather than what the forecast said it would do, and for having the navigational capability to manage the passage if the two diverge.

The margin of safety

Woodmencey's final principle is the one that most directly addresses the gap between a successful passage and an incident report: predicting the weather will always be an inexact science, so leave yourself a wide margin of safety.

The margin of safety in passage planning is the buffer between the predicted conditions and the limit of the crew and vessel's capability. If the crew and boat can comfortably handle Force 6, the decision to depart should require a forecast that makes Force 7 genuinely unlikely for the duration of the passage — not merely absent from the thirty-six hour model run. If the passage crosses the Biscay swell and the crew begins to be compromised at two-metre sea states, the decision to depart should require a forecast in which three metres is genuinely improbable — not merely not forecast.

The margin is eroded by optimistic weather reading, by goal fixation, by underestimating the consequences of the worst plausible scenario, and by the specific failure mode of treating the forecast as a guarantee rather than a probability distribution. It is maintained by the habit Woodmencey describes throughout Reading Weather of treating forecasting as an ongoing process, of building the field observation skills that let you verify the forecast against reality, and of making the go/no-go decision again at every watch change rather than once and for all at the marina gate.

This is the complete message of the Old Fashioned Seamanship collection: not that electronics are dangerous or that traditional skills are superior, but that the habit of attention — to the barometer, the sky, the sea, the swell, the wind direction, the cloud sequence, the depth profile, the tidal stream — is what the go/no-go decision is actually made of. The forecast provides the background. The observation provides the foreground. The judgement that combines them is the navigator's own, and it cannot be outsourced to any instrument, old or new.

Jim Woodmencey's Reading Weather (FalconGuides) provides the go/no-go decision framework throughout this post: the three-factor assessment of activity, ability, and risk; the go/no-go scoresheet; the guidelines on timing, ongoing reassessment, and margin of safety; and the specific caution against goal fixation. Simon Rowell's Weather at Sea (Fernhurst Books) provides the synoptic context within which all weather-based departure decisions are made. The observational skills that verify the forecast against reality are across the Weather Forecasting series. The navigation skills that make the departure reversible are in the Traditional Navigation Techniques series.

Passage planning hub: Coastal and Offshore Passage Planning. The full collection: Old Fashioned Seamanship.