Collection: Field Notes — Old Fashioned Seamanship

Series Hub: Traditional Navigation Techniques

Subject: Dead reckoning without electronics — speed, leeway and current estimation without instruments, how to run a practical DR plot, and what the Pacific navigator's approach teaches about error

How to Run a Dead Reckoning Plot Without GPS — Course, Speed, Leeway, and Current Estimated Without Electronics

Dead reckoning is the oldest navigational method that exists, and the one every other method ultimately corrects against. You know where you started. You know which direction you have been going. You know roughly how fast. From those three things, plus a correction for leeway and current, you calculate where you are now. This is about doing it honestly, without instruments.

Frank Dye sailed a sixteen-foot Wayfarer dinghy from Scotland to Norway and on to Iceland, navigating with compass, paper charts, and dead reckoning. Tim Severin crossed the North Atlantic in a replica sixth-century leather currach, navigating mostly by dead reckoning and star observation, and made his landfalls. David Lewis spent years sailing with Carolinian and Polynesian navigators who ran DR plots of hundreds of miles without instruments of any kind and arrived at targets the size of a football pitch.

The gap between their dead reckoning and most modern sailors’ dead reckoning is not mystical. It is practice. And it starts with the embarrassing observation that in the era of GPS, DR has been quietly reclassified as a backup skill. Something you learn for the exam and hope never to actually use. The opposite is closer to the truth. DR confidence, the ability to maintain a running position estimate from course, speed, and time, corrected for leeway and current, is the navigational skill that fails last and costs nothing to maintain.

The compass heading is not the course

The compass gives you a heading. That heading is not the course over the ground. Between the compass heading and the track you are actually making good over the sea floor sits a combination of tidal current, wind-driven current, leeway, and any compass error. The heading is just the direction the bow is pointing.

This distinction is the foundation of all DR. Lewis opens the dead reckoning chapter of We, the Navigators with exactly this point. The navigator’s task is to estimate not the course steered but the course made good, the actual track of the vessel through the water and over the ground, accounting for everything that pushes it sideways. A steersman holding a compass heading in a tidal crosscurrent is not travelling in the direction the compass shows. Working out where they are actually going is the work of dead reckoning.

Speed — Hipour’s intuition against Lewis’s counting

Speed is the most apparently straightforward input and in practice the most variable. Lewis records his own attempts to measure speed on passages with Hipour by counting the seconds taken to pass a patch of foam. The method gives an approximation using the relationship between the boat’s waterline length, the time taken, and a simple calculation. His results were less reliable than Hipour’s estimates, despite Lewis using a deliberate counting technique and Hipour using none.

Lewis’s account of how Hipour and the other Pacific navigators estimated speed is worth attending to. They were not guessing. They were integrating a continuous flow of sensory inputs, the sound of water against the hull, the feel of spray, the turbulence visible in the wake, the pressure of the wind, the motion of the vessel, into an estimate that emerged as a semi-conscious output rather than a calculated result. The accuracy was confirmed at landfall. Predictions of when an island would appear on the horizon, given in exact terms, that the event then validated.

Lewis is scrupulous about not overstating this. He notes that his own estimates were sometimes forty miles adrift in a single day, but that over a 1,600-mile month-long passage, the cumulative error was only seventy-seven miles. This is the key property of dead reckoning that Lewis documents carefully. Random errors in either direction tend to cancel over time. Only systematic errors compound. A consistent overestimate of speed. A persistent compass error. A current that always runs one way.

I find Lewis’s cumulative-error point the most genuinely reassuring thing in the whole subject. The random variation in DR inputs oscillates around the truth rather than departing steadily from it. The plot is more honest than it feels while you are running it.

The practical implication is that a confident DR method, consistently applied, produces better results over a long passage than a sporadic one. The navigator who abandons their plot when conditions get difficult and restarts it when they improve has accumulated a gap in their position estimate. The navigator who maintains their plot through bad conditions, acknowledging the increased uncertainty while keeping it running, arrives at a better answer.

The chip log, which makes the speed input more objective, is in The Chip Log — Measuring Speed Over Water. For building the muscle memory of the DR calculation itself on dry land, I built a dead reckoning training app that teaches the technique step by step and also works as a quick calculator while you are learning the paper method. Worth a few sessions before any first DR-only passage.

Leeway — the angle in the wake

Any sailing vessel moving forward through the water is also being pushed sideways by the wind. On a reach in a Force 4, leeway on a moderate-displacement keelboat might be three to five degrees. On a light-displacement vessel it could be more. In strong winds close-hauled, it can be significant. Whatever the angle, it is applied consistently to one side of the heading, and if it is ignored it compounds straight into position error.

Lewis documents the leeway estimation technique used by Hipour and Tevake. Essentially identical despite their different backgrounds and waters. The navigator stands or kneels at the centreline of the vessel, looks astern, and observes the angle between the wake and the projected centreline of the hull. That angle is the leeway. No instruments. Just observation from the right position, with patience.

Lewis notes that Hipour estimated leeway reliably even on an unfamiliar vessel, Lewis’s own yacht, where he had no accumulated feel for the boat’s specific behaviour. He would contemplate the wake for a while, then give an estimate. When Lewis checked these estimates against the track made good, they were accurate. The method works.

For a modern sailor this is easy to practise and easy to verify. On any beam-reach or close-hauled passage with identifiable landmarks, compare your compass course with the bearing you are actually tracking over the ground. The difference is your leeway plus any current component. In pure wind with no cross-current, it is primarily leeway. Attending to this regularly builds the same accumulated familiarity with a specific boat’s behaviour that Hipour had with his canoe. The semi-conscious integration that produces reliable estimates.

Current — Lewis’s balloon

Lewis opens the current section of his dead reckoning chapter with a comparison that stays with you. A navigator in a tidal current is in exactly the same position as a passenger in a free balloon being swept along by the wind. From inside the system, there is no sensation of movement. The water beneath the boat moves at the same speed as the boat itself. There is nothing to feel.

This is why current is the hardest variable in dead reckoning, and why Lewis documents the variety of methods Pacific navigators used to address it. The variety itself, he observes, is a reliable indicator that no single method was fully satisfactory.

Their approaches included back-bearings on landmarks at departure to assess the direction and rate of set, which is in What Moving Water Tells You. Reading changes in wave character that indicated current boundaries. Encoding known current allowances into traditional star courses so that the correct track was built into the sailing direction itself. And treating the current as a known quantity during the middle of a passage, having established its character from the departure back-bearings.

The last of these is worth dwelling on. Lewis documents how the traditional sailing directions of the Carolines incorporated current allowances verified over generations of passages. The star course from one island to another was not the geographical bearing between them. It was the bearing that, when sailed, compensated for the expected current and produced the correct landfall. Dead reckoning not as a calculation performed each voyage but as accumulated traditional knowledge encoded in the sailing direction itself.

For the contemporary sailor without traditional knowledge of a specific sea area, the tidal atlas and pilot book current tables provide the starting point. The back-transit method provides real-time verification and correction. The combination is at least as good as the traditional knowledge for familiar waters, and probably less reliable for unfamiliar ones, which is why the first passage into new tidal water should be treated with appropriate caution.

Running the plot

Dead reckoning on paper requires a chart, a pencil, dividers, and a note of starting position, time, compass course, and estimated speed for each period. The mechanics are the same whether the passage is fifty miles or five hundred. From the last known position, lay off the course steered. Mark the distance made good in the elapsed time. Apply a correction for estimated leeway. Apply a correction for known or estimated current set. The result is the estimated position at that time.

The plot accumulates uncertainty over time. After two hours from a confirmed position, the error circle is modest if conditions have been stable and the inputs reliable. After twelve hours without a position check, the error circle may be significant. The DR navigator manages this uncertainty by treating the estimated position honestly, as an estimate not a fact, and by actively seeking confirmation whenever it becomes available. A depth that matches the chart. A landmark bearing. A buoy that appears where it should. A tidal stream that turns when the atlas said it would.

The dead reckoning training app lets you practise this calculation off the water, working through example passages on dry land until the arithmetic and the corrections become automatic. Useful for a winter evening before the sailing season starts.

What Dye showed

Frank Dye crossed the North Sea to Norway and later to Iceland in a sixteen-foot Wayfarer dinghy, navigating with compass, charts, dead reckoning, and the shipping forecast. His book Ocean Crossing Wayfarer, written with his wife Margaret, describes the navigation in functional terms. He was not performing a demonstration. He was finding his way. The accounts confirm what Lewis documents from the Pacific. A consistent DR plot, applied without dramatising the uncertainty, produces reliable results. Dye made his landfalls.

The Wayfarer is a production plywood racing dinghy. It has no keel, it makes more leeway than a keelboat, it is driven by currents and blown sideways by wind in ways a deep-keeled yacht is partly protected from. Dye’s crossings demonstrate that the inputs to dead reckoning are manageable in a small open boat in serious offshore conditions, provided they are attended to continuously rather than measured occasionally.

Building the skill

The practical entry point is any passage in familiar waters where GPS is available for comparison. Run a paper DR plot alongside the GPS track, making honest estimates of speed and leeway rather than reading them from instruments, and applying the tidal atlas current at intervals. Compare DR position with GPS position at the end of each hour. The gap between them tells you which inputs you are estimating poorly. After two or three passages of this kind the main sources of error become clear, and the habit of attending to them becomes natural.

Lewis notes that the navigators he worked with had accumulated their skills through years of practical voyaging during which every landfall was a test of the dead reckoning. The test was not optional. There was no GPS to fall back on. The quality of the work was confirmed or questioned by what appeared, or failed to appear, on the horizon. This is the best possible training regime, and it is available to any sailor willing to treat the GPS track as the assessment rather than the navigation tool.

If you want a piece of sheltered water to practise running a plot in low-stakes conditions, the Hithe Finder is a community register of slipways, hards, and beaches for small boats. A tidal river or sheltered estuary, with the GPS in your pocket for ground truth, is exactly where this skill is built.

The chip log, which provides an objective speed input for dead reckoning, is covered in full in The Chip Log — Measuring Speed Over Water. The back-transit method for current detection on departure is in What Moving Water Tells You. The full series index is at Traditional Navigation Techniques.

References

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific. University of Hawai'i Press

Dye, F. and Dye, M. (1977). Ocean Crossing Wayfarer. David and Charles.

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. If you are looking for a launching spot, the Hithe Finder is a community register of slipways, hards, and beaches for small boats.

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