What are the sounder settings? You can get a false reading with gain too high or range set wrong. Try with auto range and auto gain. Otherwise if the actual water depth below the surface is 17 feet, the depth below transducer should be 13.1 feet, which should display as 11 feet below keel if you have selected depth below keel as you preferred depth in settings. So go back and try depth below surface and see what it reads as a check.