National Maritime Electronics Association - NMEA SpeedLog Mesajları

Giriş

Açıklaması şöyle

This bridge equipment on a ship is used to measure the speed and the distance travelled by a ship from a set point. By calculating the same, ETA of the ship is adjusted or given to the port authority and agent.

Bu cihazın panelinde göstermek için şu mesajlarla beslenebilir.

GGA - Global Positioning System Fix Data, yani konum

VTG - Track made good and Ground speed, yani hız

ZDA - Time & Date - UTC, day, month, year and local time zone, yani takvim ve saat

1. MTW - Mean Temperature of Water

Celsius biriminden suyun sıcaklığını verir

 2. VBW - Dual Ground/Water Speed - Ground ve Suya Göre Hız

İki farklı hızı gösterir. Açıklaması şöyle

Longitudinal water speed, "-" means astern

Transverse water speed, "-" means port

Data Valid

Longitudinal ground speed, "-" means astern

Transverse ground speed, "-" means port

Data Valid

Açıklaması şöyle

- Speed over ground is the speed of the ship with respect to the ground or any other fixed object such as fixed buoy or island.

- Speed through water is the speed of the ship with respect to the water such as anything floating on water.

Speed over ground bir noktadan bir diğer noktaya varış süresi için hesaplanır. 

Speed through water ise "collision avoidance" için kullanılır.

Ground'a göre hız aynı zamanda GPS cihazının "VTG - Vector track an Speed over the Ground" mesajı ile de alınabilir. Ancak GPS her zaman her yer erişilebilir olmadığı için öncelikle VBW tercih edilmelidir.

Speed through water ölçümü zor bir iştir. Açıklaması şöyle

Measuring speed through water can be done using a speed log, which calculates the speed from a sensor mounted on the hull underwater, but it is not straightforward. There are a lot of variables to be considered. 

One of the most challenging issues is that the vessel itself creates a flow field around the hull. If speed through water is measured within this field, you can end up measuring the wrong speed. This field will vary dynamically with changes in vessel, speed, draft and trim.

A moving vessel also experiences pitch and roll – another factor that affects the readings. On top of this, flow patterns in the boundary layer around the hull created by the vessel’s movements need to be distinguished from ocean currents to ensure accurate speed measurement.

Speed logs – especially widely used electromagnetic (EM) logs – often have poor reliability and become less accurate over time. They are also difficult to recalibrate, and often provide poor data quality, logging and data transfer.

Adjusting log data to compensate for the resulting errors is both time-consuming and complex, using valuable resources that the industry could better employ elsewhere.  

3. VHW - Water speed and heading - Suya Göre Bağıl Hız

Açıklaması şöyle

degrees true

T=true

degrees Magnetic

M = Magnetic

Speed of vessel relative to water Knots/hour

N = Knots

Speed of vessel relative to waterKm/hour

K = Kilometers

4. VLW - Distance Traveled through Water

Aynı arabalardaki gibi bir yolculuğun mesafesini ölçer.