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

Collection: Field Notes — Old Fashioned Seamanship

Series Hub: 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

How to Make the Go/No-Go Passage Decision — Weather Windows, Crew Ability, and Honest Reassessment of Risk

The departure decision is where the forecast meets the navigator’s actual capability. It is not a single yes or no made once at the marina gate but a continuous reassessment running through every watch change. The most useful frame I have found for it is Jim Woodmencey’s three-factor assessment in Reading Weather, set against the observation skills that let you check what the forecast is actually doing.

The weather killed fewer sailors before reliable forecasts existed than it does now. Improbable until you consider that before forecasts existed, every sailor was trained to read the sky, the sea, and the barometer, and the choice 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 romanticism about 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 aboard.

The go/no-go decision is where everything in this collection converges. The skills developed across the reading-the-sea, traditional-navigation, and weather-forecasting series are not academic exercises. They are instruments for making this decision well, and for continuously remaking it once the departure has been made.

The three inputs to any go/no-go decision

Woodmencey’s framework in Reading Weather sorts the decision into three independent evaluations that all have to be made before committing to a departure. The activity. The ability. 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 is 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. The questions to ask are how long the passage is, how flexible the timing is, what the sea room amounts to, whether there are good harbour options en route, and where the commitment point sits beyond which turning back becomes more dangerous than continuing.

The ability is the crew’s skill and the boat’s capability. Woodmencey identifies three components. 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 (though even he got caught out one time, showing off to a young lady crossing a rough bar at a harbour entrance he wasn’t used to). Preparation means the vessel, equipment, and crew are ready for the forecast conditions plus the worst plausible deviation from the forecast.

The risk is 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 does.

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 between forecast and reality 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 several inputs. 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. The specific terrain effects on the passage area. A forecast showing Force 3 to 4 is plausible and reassuring. The same forecast combined with a barometer that has fallen two dobbles in three hours, cirrus that was not present at the morning observation, and a southwest swell of increasing period is a different situation, and it requires a different decision. (Building this kind of sky-reading habit on land is what I made VAKA Skywatch for. It will not replace the watch on deck, but it will get the cloud sequences into your head before you need them.)

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 one directly. Do not let goals cloud your thinking or change your forecast. That is a statement about goal fixation. The tendency to allow a desired outcome to distort the assessment of the evidence toward it.

Goal fixation shows up as the passage planned for months that 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 direct. Imagine the worst plausible weather scenario for the passage and ask what the consequences of encountering it would be. The plausible bad scenario, not the catastrophic 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, go. If they are not, wait.

A related bias is 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 pattern at a different stage. Each passage is a separate risk assessment, not an extrapolation from the last one.

The 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 matter less than the discipline of making each assessment honestly rather than as a single gestalt judgement that is easily corrupted by goal fixation.

Applied to a sailing passage, the scoresheet sorts roughly like this. For 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.

For 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 chartplotter is the only navigation, with a crew who has never taken a compass bearing, scores low.

For 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.

For 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 does not end at departure

Woodmencey’s most useful 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 ones 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 working out the nearest suitable shelter and its distance and bearing. Working out 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. Calling it failure misses what is happening. It is the exercise of exactly the judgement that distinguishes competent passage-making from optimism.

The logbook is the instrument that makes the ongoing reassessment possible. A log that records the watch-by-watch barometer, wind, cloud, swell, and DR position supports a reassessment with actual data. A log that records the position by GPS and nothing else 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 fancy electronics looks like, matter 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 most of the instruments described in the kit post and used them consistently.

The distinction worth holding onto 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.

Woodmencey’s framework captures the failure mode. 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 in passage planning is the buffer between 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 in the current 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. Treat forecasting as an ongoing process. Build the field observation skills that let you verify the forecast against reality. Make the go/no-go decision again at every watch change rather than once and for all at the marina gate.

The whole message of this collection sits here. The point is not that electronics are dangerous or that traditional skills are superior. The habit of attention is what the go/no-go decision is actually made of. Attention to the barometer, the sky, the sea, the swell, the wind direction, the cloud sequence, the depth profile, the tidal stream. 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) is the source for the go/no-go decision framework throughout. The three-factor assessment of activity, ability, and risk. The scoresheet. The guidelines on timing, ongoing reassessment, and margin of safety. And the caution against goal fixation, which I keep coming back to as the most useful single piece of psychology in the book. Simon Rowell’s Weather at Sea (Fernhurst Books) sits behind the synoptic context within which all weather-based departure decisions are made. The observation skills that verify the forecast against reality are across the Weather Forecasting series. The navigation skills that make a departure reversible are in Traditional Navigation Techniques. The passage planning hub is here.

At VAKA I design and build boats that don’t destroy the environment. Find the plans as they are finalised at VAKA Plans and the full field notes here. VAKA. Traditional craft and natural materials. Nottingham. 2026.