Transducer Seducer

By Tom Neale, 7/7/2011


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For years I’ve installed my depth finder transducers inside my fiberglass hulls. It’s easy to replace the transducer if it goes bad, it’s less likely to go bad in the first place, it’s not going to be damaged by hitting an object such as a stick or log while underway, and you don’t have to put that extra hole in the bottom of your boat. There are also drawbacks. Most transducers are designed to read only through clear water. However, at best, you should expect less than optimal performance if it’s reading through the hull. If you put it in the wrong place inside the hull there may be so much interference with the transducer that your depth finder will be unusable. Interference can be caused by air bubbles in the laminate, uneven laminate, coring and more.

Water Box from Yogurt Box.
But I do it anyway. I’ve found that if you do it well, you’ll at least get good readings at slow speed if you’re not trying to read too deep. My motorsailer is “slow” in this context and I’ve generally found that if the water is too deep for the signal to hit the bottom, I don’t have much to worry about. And its hull is solid with no voids in the laminate.

Most people who do this use the traditional method of building a water box and securing the transducer inside so that its signal finds no interference save what may be in the laminate itself. This is the best way to go if you get it right. But that’s a big IF for me. You can never know if the spot you’ve built it on is a good spot until you build the box and try it out underway. If you’ve chosen a bad spot, as, for example, an area where there’s a cavity or other anomaly in the laminate, you’ve got to start again from scratch. So I’ve experimented with easier methods of fixing the transducer inside the hull. I’ve even used Life Calk by Boat Life to glue transducers to the hull and found, to my amazement, that it works—not very well, but well enough for my purposes. And using a product like this is easy because you just have to clean and degrease the surface, coat the transducer with the product, and press it down on target, pressing out all bubbles. You don’t need a lot of access space and this can be very important. And if you need to move it-- no problem.

However, interior mounting in my MAKO has been less satisfactory because at fast speeds the transducer won’t read well, just as I would have expected. Also I’ve found I’ve had to redo it every few years because the adhesion has become impaired with age and bouncing around, impairing the conductivity (which I hadn’t expected to be there anyway with this sort of bonding). So this spring, I decided to bite the bullet, take the chance, and build a water box. I chose my spot as carefully as I could, not being able to use the old spot which I knew worked well, because I couldn’t reach that far in between the hulls to do the work. I look for spots where there’s no sign of uneven layup, no glass strands visible under the resin and no extra layup such as you’d find to secure a stringer.

My water boxes aren’t exactly high tech. But they’re easy, they’re cheap and they work. In the “Tips” section below I’ll explain how I do it. Once done, if it turns out that I’ve picked a bad spot in the hull, it’s not a big deal to redo it somewhere else with another container.

Tom’s Tips on Making a Cheap and Easy Transducer Water Box

1. I use a plastic yogurt or cottage cheese container. And I don’t buy them from West Marine. Since my wife eats the stuff (and makes me eat it), they’re free. I save the top.

2. I cut out the center area of the bottom, leaving around a half inch or so of the inturned rim. This gives the container support and gives a broad surface for sealing. The hole cut out of the bottom must be large enough for the transducer, so it’s reading only through water and hull—not through the container bottom.

3. I smoothly sand and clean the hull thoroughly with acetone or a similar solvent at THE chosen spot. BEWARE of harmful fumes.

Click Here for More Tips

This time the job went very well. I turned on the depth finder, expecting to get a very good signal, barring a bad choice of location. Instead, I got two different readings, repeatedly, that were fairly consistent. One was the correct depth; the other was the depth with several feet subtracted. BUMMER! I removed the transducer from the container and hung it over the side to rule out a transducer malfunction. The depth finder read accurately. I decided to run the boat for further testing. It read fairly accurately and did so at much greater speeds than it had while secured with Life Caulk. So I’d at least gotten a good spot. This didn’t make sense.

The creek bottom at the dock was very soft mud. I’ve known for years that this can mess with a depth finder’s mind. But usually it’ll show “missed” or many variable readings—not what I was seeing. And the bottom hadn’t messed with that depth finder before, nor did it when I hung the transducer over the side. So I did the best thing to do on a pretty afternoon. I decided to worry about it later. I wanted to run my Mako and I knew where I was going and how deep the water was.

Last weekend I spent an hour underwater working under my motor sailor. When I’d finished the work, I decided to hang out at the very bottom of the creek, just because I like to. You never know what you might find down there. This time I found a very cold layer of water at the bottom, very sharply differentiated from the much warmer water above it. The line between cold and very warm was several feet off the bottom. I usually find this when I dive, but seldom so sharply defined. It was early summer, we’d already had a lot of heat, the winter had been very cold, the water had recently been very cold, and there is very little current or other disturbance in that creek. And that’s what was giving that depth finder the fits. It seduced the transducer into thinking there was bottom where there wasn’t.

Goes to show you, there’s always a curve ball that Mother Ocean (even Momma Creek) can throw you. Be ready to catch.

And what worries me is that life jackets, personal locator beacons, Aps that do everything from telling you whether it’s raining to whether it rained a few minutes ago, online licensing courses and so much more, much of which is bureaucracy byproduct originated in government cubicles, aren’t going to make that much difference unless…

All of these things and more can make a huge difference and I’m not knocking anything that helps. But unless we lose the attitude that we should keep shoveling hordes onto the water who think that “out on the water” is a place where you don’t have to know what you’re doing except turn a key and that there are no rules (after all, there are no lines in the middle of the road out here) and where special skills aren’t required because, “Hey man, water is soft ya know,” it’s not going to get any better.

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