I have read that the unit sometimes takes several seconds to "re-find" bottom if the sonar is left on when raising the TM ...
This can easily be rectified by pressing the power button and select "Sonar Off" to stop the transmitters while the xducer is out of the water ...
On a side note ... It shouldn't hurt anything to have the xducer out of the water and powered on ...the output power is not high enough to create heat that could damage the xducer (like in high power CHIRP xducers) ...
Rickie