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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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Last position from which an approach can still be abandoned
The point on an approach beyond which the vessel can no longer turn back or stop safely, so the decision to continue must be taken before reaching it. It is derived from turning circle, stopping distance, channel width and available depth, and is marked by an unambiguous trigger such as a bearing, a range or a transit. If the entry criteria, for example a visual reference, a working engine or a berth confirmation, are unmet, the vessel turns away.
Read definition →The last place you can safely turn back or stop
The predetermined position beyond which the vessel is committed to the entry, chosen where there is still sea room, adequate depth and a workable escape course. It is fixed during planning by reference to a transit, a clearing bearing, a radar range or a waypoint, and paired with a contingency anchorage or bolt hole offering shelter and holding. Criteria for aborting, such as marks not identified, visibility below a stated limit or engine failure, are agreed before the approach so the decision is not made under pressure.
Read definition →How dead reckoning drifts further from the truth as time passes
Errors in heading, log calibration, leeway estimate and tidal prediction each accumulate roughly in proportion to distance run, so a DR position degrades steadily and without warning. A one degree steering bias displaces the vessel about 1 nautical mile in 60 run. Good practice therefore treats the DR as a growing circle of uncertainty rather than a point, reduces the assumed accuracy the longer it runs unfixed, and resets it at every reliable observation.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.