Which transducer?

[WildBill]

The 20 degree's is the angle of the cone pointing toward's the sea bottom. The 20 degree transducer is a narrow angle beam or narrow cone. You would use a wider beam in shallower waters. The narrower the beam or cone the more concentrated the signal so therefore the deeper it will see. The question should be what are you using it for? I had a dual switch on my lowrance with a wide angle transducer and a narrow angle 20 degree thru hull transducer. I mainly used the stock wide angle dual frequancy transducer that came with the unit it fished great up to about 300'.