Q&A Transducer installation

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Q: Should I mount my depth-finder transducer inside my hull?

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Q: Should I mount my depth-finder transducer inside my hull?

A: Maybe. We’ve done it for years on both our large and small boats. But this method of installation doesn’t work well or at all on some boats and for some purposes.

Depth finder manufacturers often recommend against doing this. The hull can interfere with the transmission of the signal and the depth finder’s sensitivity. The question is: How much will it degrade performance and are you willing to put up with that degree of degradation for the benefits of an inside installation? The benefits include not having to haul your boat, not putting another hole in the hull, and ease of replacement.

If you mount the transducer inside, mount it over a section of the fiberglass hull with no foam or any other coring. (Don’t try it on a wooden or metal hull.) The fiberglass should have no bubbles, voids, inconsistencies or other defects. The thinner the hull at the point of installation the better, as to its effect on the transducer.

Manufacturers often recommend that you epoxy the transducer to the chosen spot, if you mount inside. The problem with epoxy is that when you must replace the transducer, it may be in the one spot in the hull that works best where you already have a relatively permanently mounted useless transducer. You can break it loose and chisel and sand the epoxy (if you can reach that spot), but that’s a lot of work. Or if you determine, at sea trial, that you’ve epoxied it in a bad spot, you’ll probably damage the transducer while breaking it free.

I’ve used various substitute bonding methods that work to various degrees. To find a good spot on my Mako I had to reach way back into the “ ’tween hull” area. This is a spot that I can barely see, much less access to work easily, and it never stays dry for long.

To see what I could get by with, I tried BoatLife Life-Calk to bond the transducer, because the product sets up in moisture. I should note that this was not recommended by either manufacturer. To my amazement, the transducer read through it, although not when running fast. Also, I had to rebed the transducer frequently, as readings degraded over a few months. (Life-Calk, a good product, isn’t marketed as a good medium for depth finder transmissions.) Star brite says its Quick Cure Marine Polyurethane Adhesive/Sealant (No. 83301 or 83321) would work well for this purpose.

On Chez Nous, our 53-foot Gulfstar motorsailer, we use a water box. I’ve always found these to work well. If I used one in the Mako, I’m certain I’d be able to read the depths at high speeds. However, it would be very difficult to install a water box in the spot I’ve chosen for the transducer on the Mako.

There have been quite a few articles written on “building” water boxes. The idea is that you construct a watertight box around the spot on your hull where you want the transducer to sit. You fill it with water and put the transducer, aiming down, through a hole in the top of the box. The transducer then has only water and the hull through which it has to read. If that hull spot is a good spot, as discussed above, the interference will probably be negligible, and you’ll probably be happy. If it isn’t a good spot (and you won’t really know until you take the boat out for a test run), then you’ve got to build a water box in a different, hopefully good, spot.

We solve this problem by using plastic yogurt or cottage cheese containers. When we feel like spending really big bucks, we might buy a suitably sized Rubbermaid container. We cut out the bottom and form the resulting lip to meet the contour of the hull where we want to mount it. This is the only hard part. We then seal the lip to the hull with a silicone sealant, fill it with water, and cut a hole through the top for the transducer cable and body.

The water might evaporate over time or leak slowly if you didn’t get a good seal, but it’s easy to refill. On one of our boats we couldn’t get a watertight seal because we had trouble cutting the bottom to fit the contour, so we left the bottom of a container intact and glued it to the hull with silicone sealant. This worked, although I assume it wouldn’t work well in a fast boat at high speeds.