You may not have ST800 instruments on your boat. If you have an earlier version then it may not have a screw cap as described by all the other posts.
On my Classic 30 both the transducer and the blank were secured by a stainless steel pin pushed transversely through holes in the tube and a hole in the body of the transducer/blank. When inserting the transducer/blank it was only necessary to line up the holes before replacing the pin. What really mattered was that the transducer was put back the right way round and there was an arrow on top to show you which way was forwards.
BTW, Paul, the difference between true wind and apparent wind is the boat speed over the ground NOT speed through the water.