The Art of Dredging

Dredging and shipping

Latitude-speed error on gyro compasses

A gyro-compass is subject to a latitude-speed error; an error increasing with latitude and speed of the ship.

Here you'll find a table, so you'll have an idea about the value of this "latitude-speed-error". 

 

This error has some implications, especially with integrated bridge systems.

 

 

Have an example: Korea, 2003, Gerardus Mercator experienced sudden hard over rudder, with the ship on auto-pilot.

 

After a few seconds  the rudder returned to midships. This happened quite a few times during that week.

Pretty dangerous stuff; should'nt give hard over rudder with engines full ahead and a loaded ship. It's not healthy for the rudders, and the ship lists too much for ease of mind.

           

It was a mystery.

           

            Eventually we found out  that:

·        The gyrocompasses switchbox gets a latitude and speed input from the GPS-receiver.

·        From this input the gyro-compass system calculates automatically the lat.-speed error,

·        and applies this error to the gyro-course.

·        The GPS experienced irregular short blinks, with large position jumps in the GPS. GPS calculated the ship's speed as 50 knots, catching up with the faulty positions.  (*)

·        This data was send to the gyrocompass,

                        which calculated the lat.-speed error instantly as 10 ° (see table)

·        the gyro applied this error, all in a matter of microseconds,

·        and the autopilot gave hard over rudder, as the ship was 10 ° off course, in an instant

 

(*) The glitches in the GPS-position may have been caused by the mountaineous terrain, maybe blocking GPS satellite signals close by land. Who knows ?

 

A gyro-error can also show up during trenching projects with a trailing suction hopper dredger.

 

An error of one degree on the gyro-course will make an error in position of the draghead of 2 meter (with ship's length 150 meter).

 

Since the error goes both ways, sailing on opposite courses while dredging a trench, you are making two trenches, besides the originally planned trench. (*)

 

Dredging high spots is another example.

If the position of the draghead errs by a few meters; high spots will prove hard to hit.

  

 

(*) To prevent this, gyro's are calibrated before starting precision dredging jobs.

 

Marc Van de Velde

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