[pair of jacks]

I currently have a transom mount tranducer for my ff and the readings are ok while at rest, but really bad at anything over about 12 knots- I've been thinking about a shoot thru hull Airmar for my 21- maybe I'm confused but I though Peter was asking about a shoot thru not a thru hull but most of the advice and pics were for a thru hull- does anybody currently use a shoot thru on a 21 ?