Need some trouble-shooting help.

I needed to replace my 15-year old depth sounder with something that would work reliably at shallow depths, most importantly, when the depth gets below 2m and I am about to run aground. Also, I don't like putting holes in the hull, so I wanted an in-hull sounder that would shoot through the hull.

In my previous boat (Austral 20), I fitted a Lowrance Hook 2 5x inside the hull, and it worked very nicely, especially in shallow water. Being a cheapskate, I thought I would try again with the Hook 2 4x, which has a smaller screen and a smaller bullet, and it is cheaper.

At this stage, you might rightly point out that the Hook2 bullets are designed to be mounted in the water off the back of the transom of a tinny. They are not designed to shoot through hulls, and their primary purpose is finding fish, not measuring depths, although they do the latter function as part of the former. But, my experience shows that you can mount a Hook 2 5x in-hull and use it to get the depth in shallow water. The 5x uses a more sophisticated sonar system, which may make the critical difference in shallow water.

My problem is that the Lowrance Hook 2 4x is not getting reliable readings at shallow depths. It blanks out at about 3m (?) depth, which is just when things get interesting. I had glued the bullet to the inside of the hull with some Fixtech, but I took it off (and slightly damaged the casing) and re-positioned it with some blue-tack to try to improve it. I've put it in the bilge, a little forward of the old through-hull fittings, in front of the keel, and I cleaned the area with a wire brush and a wipe-down before fitting. Now, it turns on and fires up, but I get no depth readings at all. I only tested it by sailing around inside Manly Harbour at low tide, so probably did not get deeper than 3m. I have yet to see what happens in deeper water. I might have to dangle the bullet off the boat to check it, but that would mean undoing the wiring, routing and soldering.

I'd like to know if this is a problem with the Hook 2 4x unit or my installation. If it is a problem with the unit, I might just have to bite the bullet* and get a better one, although I'd like to know it will work before buying it. Other problems could be the blue-tack or something in the hull that interferes with the signal.

Has anyone else had experience with mounting a Lowrance Hook2 4x as an in-hull depth sounder?(* apologies for the pun)