A boat's compass uses the earth's magnetic flux to drive the floating card - any local magnetic forces can distort or even dominate at the compass, ruining the compass accuracy. Loudspeakers, engine blocks, electric motors, electric cables and the like can all distort the reading and in the case of steel boats the hull also has an effect.
We faced similar challenges when positioning our Raymarine EV-1 Sensor Core. This unit contains a fluxgate electronic compass, a multi axis solid state gyro and a GPS receiver.
The Sensor Core is the real intelligence that drives our autopilot system - it was important that we found a satisfactory location for the unit - the same goes for any magnetic compass. Raymarine's installation guides said it should work fine where our previous electronic compass was mounted, but we wanted to check alternate sites. In the end we measured the lowest magnetic flux field right where the old compass was mounted - out on the stainless davits mounted on the stern. Unfortunately this meant running the STng network cables another 10 meters through the boat and out onto the davits, but that's another story...
Multi Measures App
Incidentally, Tesla is the name for a measured unit of magnetic flux density, named after Nikola Tesla, a Serbian - American physicist and engineer.
What makes a good Sensor Core or electronic compass location ? It should ideally close to the vessel center line, with access for the required network cabling and with a decent view of the sky so the internal GPS can operate. However the real driver here is lack of magnetic interference - that's the fundamental requirement.
Using the Multi Measures app we were able to wander around the boat, watching the display in real time and testing all the possible installation locations. When testing each location its a good idea to operate all the boats electrical systems, to make sure that no interference is generated when say the genset is started or when the water heater is switched on. The Multi Measures app also includes a tilt measuring function, truly useful for mounting things horizontally or vertically on boats that often have no useful reference lines.
Gemeco Marine Installers App
We also used the very capable Tesla meter included in the iNstall marine installers app by Gemeco. The Gemeco company distributes a wide range of marine electronics and NMEA network products in the USA, so this app is clearly aimed at marine electronics technicians.
iNstall includes a generous number of tools, references and calculators to make marine electronics installation a little easier - transducer details, cable sizing tools, tilt measurement and a "mix'n'match" guide for sorting out product compatibility.
The Tesla meter within the app allows you to zero the meter scale once a background level is established, then see the differences as electrical systems or machinery are activated.
Sadly for us, these handy Tesla meters proved that our inspired "alternate location" choices were not so good after all, and the Sensor Core ended up installed back out on the davits, where it works just fine.