Is your x70 on the bow now?
Again, I recommend that you transom mount the 'Bird's transducer. It will lork MUCH better. If you MUST shoot thru, then I recommend any good 2 part epoxy. There is a lot of good reading on that subject on the net.
I got this from the Lowrance website:
Shoot-thru-hull transducers are epoxied directly to the inside of fiberglass boat hulls. The sound is transmitted and received through the hull of the boat - but at the cost of some loss of sonar performance. (You won't be able to "see" as deep with a shoot-thru-hull transducer as one that's mounted on the transom.) The hull has to be made of solid fiberglass. Don't attempt to shoot through aluminum, wood, or steel hulls. Sound can't pass through air, so if there's any wood, metal, or foam reinforcement, it must be removed from the inside of the hull before installing the transducer. Another disadvantage of the shoot-thru-hull transducer is it can't be adjusted for the best fish arches. Although there are disadvantages to a shoot-thru-hull transducer, the advantages are considerable. One, it can't be knocked off by a stump or rock since it's protected inside the hull. Two, since there is nothing protruding into the water flow, it generally works quite well at high speed if it is mounted where a clean laminar flow of water passes over the hull. Three, it can't be fouled by marine growth.
I recommend anyone to look at this link:
http://www.lei-extras.com/tips/sonartut/Default.htm
Mini