ART
BEAUTÉ ET BIEN-ÊTRE
ARTISANAT
CULTURE ET HISTOIRE
DIVERTISSEMENT
ENVIRONNEMENT
NOURRITURE ET BOISSONS
INGÉNIERIE INVERSE
SCIENCES
SPORTS
TECHNOLOGIE
TECHNOLOGIE PORTABLE
Speed Through the Water: Using and Calibrating the Log
Dead reckoning needs a speed, and the word 'knot' is a fossil of how it was first measured: a wedge of wood on a line knotted at even intervals, thrown over the stern, with the knots counted as they ran out during a measured interval of a sand glass.
The chip log, already published here, is that instrument. This rung is about USING it honestly — because every log measures speed through the WATER, and the water itself is usually moving. A log reading five knots in a two-knot foul tide is a boat making three over the ground, and no log will ever tell you that.
You will build a simple log, calibrate it over a measured distance, and learn the difference between speed, distance run and distance made good — three quantities that are routinely confused and are never the same number.
Débutant
1 day
Consignes
1
1
Understand why the knot spacing is what it is
Understand why the knot spacing is what it is
A nautical mile is 1852 metres. If knots are spaced at 1/120th of that — about 15.4 metres — then the number running out in 30 seconds is the speed in nautical miles per hour directly.
Thirty seconds is 1/120th of an hour, so the two ratios cancel and the count IS the speed. That is the whole elegance of the device: no calculation at sea, in the dark, by a tired watch. Choose a different glass and you must re-space the knots to match.
Matériaux pour cette étape :
Cordeau60 metreOutils nécessaires :
Mètre ruban
Marqueur permanent2
2
Make a chip that stays put in the water
Make a chip that stays put in the water
Cut a quadrant of thin board about 200 mm on its straight edges, weight the curved edge with lead so it floats upright, and bridle it from three points so it presents its face to the pull.
The chip must stay still in the water while the line runs off the reel — it is a sea anchor, not a float. If it skids or planes, every reading is short. The traditional bridle has one leg on a tapered peg that pulls free when the line is jerked, so the chip turns edge-on and can be hauled in easily.
Matériaux pour cette étape :
Panneau de contreplaqué1 feuilleCordeau10 metreOutils nécessaires :
Scie égoïne
Papier abrasif3
3
Leave stray line before the first knot
Leave stray line before the first knot
Mark a length of 'stray line' — typically 10 to 20 metres — before the first knot, and only start the glass when the stray-line marker passes your hand.
Close astern the water is disturbed by the boat's wake and is travelling with her, so a chip in that zone is dragged along and reads slow. The stray line gets the chip out into undisturbed water before timing begins. Skipping it is the single commonest reason a hand log under-reads.
Outils nécessaires :
Marqueur permanentMètre ruban4
4
Calibrate against a measured distance, both ways
Calibrate against a measured distance, both ways
Run a known distance between two fixed marks at steady throttle, timing it, then run the SAME distance back at the same setting.
Averaging the two runs cancels the current, which otherwise adds to one and subtracts from the other. One run alone measures your speed over the ground on that day's tide and tells you nothing reliable about the log. Do it in calm conditions, and repeat at two or three speeds, because log errors are rarely a constant percentage.
Outils nécessaires :
Chronomètre5
5
Knot spacing, calibration and the three distances
Knot spacing, calibration and the three distances
Chargement du notebook Jupyter…
Outils nécessaires :
Chronomètre6
6
Record the correction and use it every time
Record the correction and use it every time
Write the correction factor for each speed on a card by the chart table, and apply it to every log reading before it goes into the plot.
A calibration in a notebook you do not consult is worth nothing. Re-check it after anything that changes the hull — a season's weed growth easily costs five per cent, which over a night passage is several miles of error accumulating steadily in one direction.
Matériaux pour cette étape :
Papier cartonné4 piècesMatériaux
3- 70 metreEspace réservé
- 1 feuilleEspace réservé
- 4 piècesEspace réservé
Outils requis
5- 1 vendor sell this, none ship to you yetEspace réservé
- Espace réservé
- Espace réservé
- Espace réservé
- Espace réservé
Blueprints liés
Ces blueprints partagent des connaissances — techniques, matériaux ou principes
CC0 Domaine public
Ce blueprint est publié sous CC0. Vous êtes libre de copier, modifier, distribuer et utiliser ce travail pour tout usage, sans demander la permission.
Soutenez le Maker en achetant des produits via son Blueprint où il perçoit une Commission Maker définie par les Vendeurs, ou créez une nouvelle itération de ce Blueprint et incluez-le comme connexion dans votre propre Blueprint pour partager les revenus.