How to Install a Transducer

THIS is a how-to article based on my project to refit the first Picaroon with a new depth sounder.

The product I am fitting here is a Garmin 160c fishfinder. This is a small, relatively cheap, portable unit that is aimed principally at the open-boat fishing market.  It took me some digging to confirm that one can, in fact, install it permanently in the cockpit bulkhead of a cruising sailboat, in what is called a ‘flush mount’.

First of all, this assumes you already have a hole in the hull that is the right size for the transducer you are fitting.

When the Picaroon’s ancient depth sounder display finally gave up the ghost, I discovered that I would need not only to replace the head unit, but also the fully functional transducer as well, because Datamarine, who made Picaroon’s sounder, went out of business.

If you are replacing a defunct sounder, and the transducer is still operational, ask the manufacturer which head units it will work with. Parts from different manufacturers, or different eras, don’t necessarily talk to each other properly.

What You Will Need

Firstly, you are going to want to buy a new depth sounder and transducer–if you haven’t done that yet, completing this project will be tricky. This job is easier with two people.

  • Hammer
  • Pipe wrench
  • Scraper
  • Medium grit sand paper
  • Marine silicone caulk
  • Clean rags
  • Paper towels
  • Beer

Removing the Old Transducer

Sounds like the easy bit, doesn’t it? Well, prepare to be disabused. A typical installation will consist of the transducer in a threaded housing, a washer, and a hull nut. The housing is inserted from the outside, the wash is slipped on from the inside, then the hull nut is screwed down over the washer–just like a nut and bolt.

I got the hull nut off easily, although you might find others are not so. As you can see from the earlier picture, the old hull nut had big hefty wings on–I would guess that more often than not, you’ll be faced with something less substantial to yank on, which will totally suck for you. Any through-hull is likely to be anointed with liberal quantities of caulk, and this tends to gum up the threads and generally glue things in place.

Once your hull-nut is off, the thing might just knock out quickly with a few taps from a hammer. Alternatively, it’ll take two of you an hour and a half of cursing to get the thing to even budge, like it did on the Picaroon. I found a pipe wrench to be the best tool for torqueing the thing free.

Remember, once you have the hull nut off, the only thing that is holding the transducer in place is caulk, so you are trying to break that bond

Don’t be too annoyed if it’s hard to get out, though–that’s why it didn’t fall out and sink your boat last summer.

Preparing the Surface

Use a sharp scraper to clean off any remaining caulk or other detritus around the hole, both inside and out. Make sure there is no grease, paint, goo, crap, shite, or anything that will get in the way of the massive caulk overdose you are about to administer.

Once the surface around the hole is clean, sand it lightly to give the caulk something to grip onto, then wipe clean with a rag.

When this is all done, you should be left with a smooth, matte surface that’s free of anything that will stop the caulk from bonding. You don’t need to grind it down to bare glass, though.

Fitting the New Transducer

First, feed the cable through the hole from the outside. Now, thread the washer (and spacers, if you are using them), then the hull nut, along the cable, so they are ready to fit onto the transducer housing.

Get your caulk–I used 3M’s Silicone Marine Sealant–and spread it on the transducer housing. You want to make sure it’s on the lip, where it sits against the outside of the hull, and well up the threaded body–there should be caulk showing above the hull nut when you’ve tightened it down. Don’t feel you have to skimp on the caulk here. Any excess can always be wiped off later. Make sure you fill the threads of the housing thoroughly.

Now, press the transducer housing up into the hole. Smoosh it around to bed it into the caulk–it should stay in place, but this is one of those places it’s useful to have another person on hand.

Inside the hull, slip the washer onto the housing, then any spacers, then screw the hull nut all the way on. For plastic units, screw it on hand tight (until you can’t get it to budge further without tools)–don’t use wrenches or you risk breaking the plastic. I was installing a plastic transducer into a solid glass hull. Different rules apply for metal transducers, and metal or wood hulls, so make sure you read the instructions properly.

Wipe away any excess caulk from both sides of the hull, and you are done. Give the caulk about 24 hours to dry before you launch, paint, or anything else common sense tells you will hurt it.

Et voilà! You have a new transducer snug and secure.