Raymarine autopilot + ST4000+ Owner's Handbook Manual page 71

56 ST4000+ Wheel & Tiller Autopilots: Owner's Handbook
D5401-1
Compass - recommended location for non-steel hulls
Finding the most suitable location
To identify the best compass location on your boat, start at the pitch
and roll center then move the compass up and/or aft until you find a
location with minimal magnetic disturbance.
Note: To check if the intended mounting location is free from
magnetic influence, tape a handheld compass to the chosen location,
then turn the boat through 360°. The readings on the handheld
compass and your boat's main steering compass (if accurately
aligned) should not differ by more than10° at any heading.
Steel-hulled boats
On steel-hulled boats you must mount the compass at least 1.2 m (4 ft)
above the main deck or wheelhouse. Otherwise the compass will not
work correctly because the hull will prevent it from sensing the
Earth's magnetic field. The following illustration shows
recommended mounting positions for steel-hulled boats.
Note: The higher above the waterline you mount the compass, the
more the boat's pitch and roll will affect compass performance.