Passage time and ETA — not the same thing
Passage time is a duration (e.g. 4 hours 20 minutes).
ETA is a clock time (e.g. today at 18:45).
They are linked — but in practice people mix them up or use wrong assumptions. This article collects the most common mistakes when sailing.
Basics are in Calculating ETA when sailing.
Mistake 1: Speed from the last sunny passage
“We had 6 kn — so we’ll use 6 kn.”
Under motor against current, in a seaway, or with poor trim, 4.5 kn is more realistic. The ETA shifts a lot:
| Distance | 6 kn | 4.5 kn |
|---|---|---|
| 30 nm | 5 h | 6 h 40 min |
| 60 nm | 10 h | 13 h 20 min |
Tip: choose a conservative SOG or note a range (best case / worst case).
Mistake 2: Confusing GPS SOG with planned speed
SOG (Speed Over Ground) from GPS is speed over the ground — current is already in it.
For the ETA formula you need a constant assumption over the whole leg. SOG “right now” can be wrong an hour later when the tide turns.
→ Current triangle · Current correction
Mistake 3: Current left out of the plan
You plan on 5 kn “from the engine”, but 1 kn against you makes 4 kn over the ground. Arrival slips by 20 % — on a long leg, several hours.
Course and current belong in course correction; ETA belongs in passage planning.
Mistake 4: ETA = “we’re there” — without harbour work
ETA hits the fairway buoy — not the berth. Allow time for:
- Waiting for bridge or lock
- Anchoring for tide or draft
- Manoeuvring in the harbour
→ Tides for HW/LW windows
Mistake 5: Mixing units
- Distance in kilometres, speed in knots — does not work
- km/h from a car nav system copied across
Nautical standard: nm and kn. Convert: Nautical conversions.
Mistake 6: Departure time “roughly”
“We’ll leave sometime after breakfast.”
Without a fixed departure there is no ETA — only a passage-time guess. For crew, harbour office, and tide windows you need a start time.
Mistake 7: Ignoring Dutchman’s log or GPS
If ETA is always wrong:
- Update Dutchman’s log or engine RPM table
- Average GPS SOG over 10–15 minutes (not a single value in waves)
→ Dutchman’s log · GPS position
Pre-departure checklist
- Distance in nm (chart)
- Realistic kn (experience, current, weather)
- Note departure time
- Calculate ETA — add buffer
- Check tides and approach
- Tell crew: ETA vs. “made fast”
Calculate ETA offline
The ETA calculator in NauticCalc runs without a network — together with course calculators and logbook.
Bottom line
Passage time vs. ETA rarely fails on the math — it fails on assumptions: optimistic kn, forgotten current, wrong units. Check those systematically and arrival stays predictable and safe.
Read next: Calculating ETA · ETA calculator · Download