Railing log
The railing log (also Dutchmans or railing log ) is the simplest method for measuring the " speed " (speed) of a ship or boat in relation to the surrounding water.
You throw a piece of wood or another buoyant object overboard on the front railing - on the leeward side of the bow - and stop the time it takes to get to the stern or for a measured distance on the railing. The classic measuring section is a multiple of the " meridian tiers" (1 mtr = 0.51444 meters).
If you divide this distance or the length of the ship (expressed in meridian arcs) by the stopped seconds, you get the speed in knots (kt, 1 kt = 1.852 km / h). Is z. For example , if you drive a distance of 5.14 m (10 mtr ) in 2 seconds, the vehicle's own speed is 10: 2 = 5 knots (9.26 km / h).
The second type of voyage measurement is to estimate the number of meridians the ship travels in 1 second . It directly gives the speed in knots.
Float and accuracy
The procedure has been used in nautical science for many centuries and, despite its simplicity, gives good accuracy as long as the sea is not too rough.
The float should be about three quarters of its size under water so that the wind and waves do not affect it too much. A piece of wood is best, but never plastic or waste that is too high. Therefore, the somewhat derogatory name “ beer can log”, which some owners of modern electronic measuring devices have given the method, is not really applicable - quite apart from environmental protection .
The accuracy of this “wooden log” is - depending on the swell - at 5 to 10 percent and, together with the course, allows you to determine a distance covered or the Etmal by means of dead reckoning with 5 to 10 percent accuracy .
When a boat or ship is anchored and the anchor chain is tensioned, the railing logge provides the speed of any current .
The earlier Dutchman's log , a wooden triangle on a longer line, the so-called hand log, and the newer patent log - a towed propeller , work in a similar way . The electrical logs and those with ultrasound are somewhat more precise , but they should be checked more often. The railing logge doesn't need anything like that.
Classic application and when anchoring
In order to increase the accuracy of the railing log (even with changing winds), several markings can be made on the railing (the "railing" on the edge of the ship) at a distance of a meridian (abbreviation of the unit of measurement: mtr). After the log has been thrown onto the surface of the water, a stopwatch is used to measure the time in which the log passes a certain number of markings. If the railing is high, however, a second person is required.
At low speeds (up to about 5 knots) the method is even more accurate than the hand log. It is still used today to determine the flow speed of anchored vehicles, for which a fixed marking is attached to the railing of a suitable length (e.g. 50 meridian triangles = 25.722 m). At right angles to the keel line , the first pass of the floating object is observed through the start marker and the stopwatch is started; if you pass the eighth mark at a right angle, the clock is stopped and the time is read. The travel through water "FdW" of a vehicle can be determined from the basic physics formula for the speed "speed = distance / time":
Speed [kn] = number of meridian terties [mtr] / measured time [s]
60 arc seconds correspond to one arc minute, thus one nautical mile (1 nautical mile = 1852 m, rounded to whole meters, not a legal unit of measurement according to DIN ). One knot is the speed of 1 nautical mile per hour.
1 Meridiantertie [mtr] = 1 Sekunde [s] * 1 Knoten [kn]
= 1 Sekunde [s] * 1852 m / 3600 s
= 0.51444444 m.
1 Knoten [kn] = 1 Meridiantertie [mtr] / 1 Sekunde [s] = 0,514 m/s.
Example of flow measurement: Two observation marks were attached to the railing at a distance of 75 mtr. A throughput time of 30 seconds was measured. The current
speed of the current in the water is then 75 mtr / 30 s = 2.5 kn.
The current starts at 2.5 knots. The current course of a steady ship can be taken as the approximate direction of the current.
See also
literature
- H. Kärsten: Nautisches Taschenbuch , p. 27ff, Fachbuchverlag Leipzig 1955
- GDR author collective: Terrestrische Navigation , Verlag für Bautechnik 1968
- Handbook for Ship Control Volume 1A, page 73ff, Springer Verlag