Shoot-Thru transducer placement for depth sounder?

[dalepo]

I've been trying to install a new Hawkeye DepthTrax 2B depth sounder in my 22' Spindrift sailboat, a standard fiberglass production boat built in 1984. I've had no luck finding an interior location where the transducer successfully gets a reading through the hull.

I assume this means either that 1) the fiberglass hull is cored throughout and so won't transmit sonar, or 2) I've just chosen the wrong spots, and need to find some that are less likely to be thickened / reinforced.

Any reliable way to tell whether a hull is solid fiberglass or cored, or to find a part of a hull that is solid glass? I would much prefer not to drill a hole in the bottom of my boat to make this thing work!

[sailjumanji]

To test, make sure there is no air between the transducer and the boat bottom. It won't shoot thru air. And as you said, it won't shoot thru foam. To test, you can either fill the bottom with an inch or so of water, or you can put the transducer in a gallon bag half filled with water and place it on the bottom inside.

If your issue is foam, you can cut away a circle of the inner/topside skin, scrape out the foam, and then backfill with epoxy. That will make it a solid area to shoot thru. I have done this on two Corsair tri's that we owned, and it worked great. I then squeezed out a cone-shaped silicone pile, and pressed the transducer into it, and let dry. You want to work it down and not have any bubbles, which is why the cone shape. If at some point in the future you have to remove - we never did - it is much easier to remove the silicone than using epoxy or something more permanent.

[dalepo]

"If your issue is foam, you can cut away a circle of the inner/topside skin, scrape out the foam, and then backfill with epoxy."

This sounds like a promising idea and one I haven't seen before. What's the best tool / technique to use for cutting away the circle of inner skin?

I don't have a good sense of the thickness or structure of my hull, and I could imagine this kind of operation sinking a boat if not carefully performed!

[sailjumanji]

I used a standard hole saw but removed the bit. Will just have to hold it steady. Once you get the cut started, the skin cuts easily. The foam can then be scraped away with just a sharp screwdriver, a putty knife, Drexel, etc.

[sailjumanji]

Ive also used the standard hole saw drill WITH the bit. That is steadier, but you just want to make sure the bit is short enough to not go thru outer skin. I did that once accidently, but it was a trailer boat. So just put some tape over the hole before we poured the epoxy inside.