I have a garmin 240 that came with my used lund explorer boat. Is it normal for it to loose the bottom signal when motoring down a lake. What angle should the transducer be set at ?

It sounds like you could use a manual for that Garmin. Look near the bottom of this Garmin support pages.

http://www8.garmin.com/support/userManual.jsp?market=2&subcategory=All&product=All