How do I set the keel offset on the Is0091?

Press the ON/OFF MODE keypad until 'KL' appears, then use the up or down arrow keypads to adjust the offset, up to a maximum of 19.9 feet. For example, if the bottom of the keel is 2 feet below the transducer and you want to read depth below the keel, set the display to read 2.0 FT. Once a keel offset is programmed, the depth below the keel will activate the shallow and deep alarms. (Page 3–4)