Most sailors have watched the same thing happen offshore.

The destination hasn’t moved. The course hasn’t changed. The wind looks roughly as forecast. The current overlay still “makes sense.”

And yet the ETA keeps slipping. Five minutes becomes twenty. Twenty becomes an hour. The number never jumps — it just quietly walks away.

This is usually blamed on “conditions changing.” That explanation is comforting, but often incomplete.

In many cases, the more accurate explanation is this: ETAs assume a world that is more stable than the ocean actually is.

What an ETA Really Is

An ETA is not a prediction of the future. It is a projection of the present.

Specifically, most ETA calculations assume that:

• Speed will remain roughly constant
• Set and drift will remain roughly constant
• Conditions will evolve slowly, if at all

These assumptions are often reasonable near shore or over short durations. Offshore, over many hours or days, they quietly fall apart.

The Hidden Assumption: Stationarity

The technical term for the assumption baked into most ETAs is stationarity.

Stationarity means that the statistical properties of the system — speed, current, wind influence — are assumed not to change in any fundamental way over the time horizon being projected.

ETAs are very good at answering the question: “If things keep behaving like this, when will I arrive?”

They are much worse at answering: “How will things behave over the next 24–72 hours?”

Why Offshore Violates This Assumption

Offshore environments are rarely stationary.

Even when conditions look calm, multiple slow-moving processes are interacting:

• Wind fields evolving in time, not just space
• Currents shifting with tidal phase and mesoscale features
• Sea state altering effective speed and steering efficiency
• Small course changes accumulating over long distances

None of these changes need to be dramatic. In fact, the most confusing ETAs drift precisely because nothing changes very much at once.

Small Errors Accumulate Quietly

A half-knot difference in effective speed is easy to dismiss. Over an hour, it barely matters.

Over 24 hours, it’s twelve miles. Over 48 hours, it’s a different landfall.

ETAs don’t fail catastrophically offshore. They fail by accumulation.

Each recalculation uses the new present as its baseline, incorporating the drift that already occurred. The number remains plausible at every step.

This is why the ETA never looks “wrong.” It just keeps moving.

Why the Drift Feels Personal

ETA drift is especially frustrating because it violates expectation.

Humans tend to interpret a changing ETA as a correction — as if the system is learning or improving.

In reality, the system is often just re-projecting a new present under the same simplifying assumptions.

The ETA isn’t becoming smarter. The horizon is just moving.

The Interface Makes This Worse

Most interfaces present ETA as a single authoritative number.

They do not show:

• How sensitive the ETA is to speed variation
• How much accumulated drift already exists
• How non-stationary the underlying conditions are

The number looks crisp. The uncertainty is invisible.

As with tides and currents, confidence outlives validity.

How Experienced Sailors Interpret ETAs Differently

Experienced offshore sailors rarely treat ETAs as promises.

Instead, they use them as:

• A rough ordering of events
• A way to compare scenarios, not predict outcomes
• A signal for when to re-evaluate assumptions

When an ETA starts to drift steadily, it’s often taken as information — not about arrival time, but about system stability.

The Practical Reframe

Offshore, an ETA is best understood as:

A continuously updated projection under fragile assumptions.

The moment you treat it that way, the frustration largely disappears.

The number becomes a trend indicator, not a commitment.

A Rule That Holds Up Offshore

If your ETA keeps drifting while conditions feel “unchanged,” it’s usually the assumptions that are moving, not the destination.

Written aboard ETSIA · Essays from lived experience at sea by Kory

This drift is the central condition explored in Dead Reckoning.