Where to place IMU sensor on steel ship

[Sjoerd02]

Hi,

I am wondering where I should place the IMU sensor on a steel boat.
Are the sensor all based on earth magnetic field, or like a mechanical gyro on gravity and really movement?
The urrent (autopilot, no nmea out) compass sensor is place on the wooden mast 1,5 meter above deck.
I can imagine this is not the best place for roll and pitch measurement.

If it's based on earth magnetic field it will not work if I place it inside the boat?

Thanks for the help
Sjoerd