Autopilot on a steel boat

Installing an autopilot with an electronic or fluxgate compass on a steel boat can mean a lot of trouble. The reason for this is that the compass measures not only the earth magnetic field but all magnetic fields around it. Since steel is a magnetic material the whole boat can create a magnetic field around it. Sometimes it can work quite good, we have many customers that has managed to do it. But in some boats it simply does not work. The problem is not that it is difficult to get the compass to measure exactly correct heading. One can live with a small difference. The problem can be that for some headings the compass will go wild and suddenly change the heading some 90 degrees getting the boat completely of course. So what can you do if you have problems.

Try first of all to move around the autopilot compass to see if there are some places where the interference with the boats own magnetic field is weaker. You can try this with a hand held analogue compass if that is more convenient.

If you can’t find a good place within the boat try to put it some meters above the deck on something that is not made of magnetic material. Aluminium or wood is good.

If that does not work and if you have a GPS plotter you can take the heading information from there instead. Most plotters can output COG Course Over Ground information to the autopilot. The TMQ compass can use this information to get a stable heading on a steel ship.  You should know that  COG (course over ground) information is part of the RMC sentence. COG is not a sentence itself. So look in the manual of the plotter if it can output the RMC sentence to an external instrument. We have used this method on several steel boats when nothing else helped. It works really good on most steel ships.

The reason this works together with the TMQ ELECOM is the special features inbuilt in the compass. It’s the accelerometers and gyros that outputs the heading information to the autopilot. The magnetometer is only used to make it long term stable. Otherwise the compass would slowly drift away. Since magnetometers measures the earth magnetc field they are not suitable on steel ships which themself can have a magnetic field. For this reason you can use a GPS to make the compass long term stable and the ELECOM compass has such an inlet for GPS data. By connecting your GPS plotter to the compass you get the perfect compass for steel ships. When the boat speed is over two knots the compass will take the information from the GPS and under that speed it will take the information from the magnetometer. The compass will take the speed information from the same RMC sentence as from where it gets the COG.

Please take a look at the picture to the left that tries to explain this. Normally the IMU part (read about that here) takes the information from the magnetometer. The magnetometer is however very sensitive to magnetic disturbances. That is the reason why the compass when it receives COG information on it’s inlet can use that information instead. However under 2 knots a GPS can not give a good heading information so the compass switches back to use the information from the magnetometer instead.

Note that for the AP47 there is a special short cable that has to be inserted at the compass inlet in the display to get the GPS interface. This has to be ordered seperatly.