Surely it doesn’t matter which way up the phone is orientated provided it’s held in a fixed position and Calibrated in that location so you have your reference point.
As I understand it from the documentation on the iPhone (as an example), the Core Motion data from the 3-axis accelerometers and gyroscopes show any changes in orientation through the API. Otherwise all those driving games that require you to drive the car by tilting the phone would only work if it was upright.
I mount my phone to the mast in a Lifeproof Case and I’d suggest that for most dinghies it wouldn’t be possible to mount the phone horizontally in the centre of the boat – unless you want to risk standing on it!
