Talk about timing!  Not that I wanted my depth sounder transducer to fail on me, but when it did, it was only a few days before hauling out for storage.  I only had to sail a few miles along the coast of St. Kitts without it to get to the boatyard, and I knew I wouldn’t run into any problems with the depths as the coast drops off quickly.

I think I may have had something to do with the initial failure.  I had crammed all sorts of storage boxes in the bilge, resting on top of the transducer, and the cable coming out of the transducer was bent and squished pretty good.

Unfortunately it is something that can only be fixed out of the water.  This depth sounder transducer was permanently fixed through the hull and not one of those retractable kinds; I’d have to completely remove the entire thru-hull.

While sitting in Nevada patiently awaiting my return to the boat, I had to find a new transducer.  The criteria was simple.  It had to be compatible with my new Garmin GPSMAP 527 unit and also fit in the existing 2″ hole that contained my inoperable DST50 transducer.

I settled on the Airmar DST800 Triducer, a retractable unit that houses a depth sounder, speed log, and water temperature sensor.  I didn’t have any of these!  And they’d all work with my new GPS.

“Retractable” means that you install a hollow thru-hull housing, bronze in my case, and the actual plastic transducer unit slides into this housing, forming a watertight seal.  The housing contains a spring-loaded valve so that the transducer unit can be removed for cleaning and replacement if necessary, while the boat is still in the water.  The valve is not 100% water-tight, so the Airmar package also comes with a blanking plug to install once the transducer unit is removed.

Since the hole was already drilled out, installation should be easy.  It’s removing the old unit that’d be the hard part.

Removing the Old Depth Sounder

old transducer

After breaking the plastic nut on the old transducer.

I couldn’t even unscrew the plastic housing nut inside the hull, and that’d be the first step to getting this thing out.  I hammered that away with a chisel, breaking it up into a few pieces that eventually became free from the sealant.

After getting that off, the transducer still wouldn’t budge.  From the exterior, the fairing in the hull that the transducer was mounted in was completely smooth – you could barely make out where the transducer flange was mated to the hull.  I carefully chipped the paint off to identify the plastic flange, and then hammered away at that with a chisel.

The objective was to chisel between the plastic flange and the hull, breaking the entire flange off so that I could just hammer the actual depth sounder unit out of the hull.  But for some reason the outer edge of the flange was fiberglassed to the hull?  Maybe to make it slightly more hydrodynamic and give the boat an extra 1/100th of a knot?  That’s why I had a hard time identifying exactly where it was.  I had to start my chiseling with the inner area of the flange and then work my way out to the edges where the fiberglass started.

thru hull hole

Finally got that old depth sounder out of the hull.

After some careful pounding into the hull with a hammer and really sharp object, the entire flange broke off.  I tried hammering the transducer out of the hole from the outside, but it wasn’t moving.

Removing the unit from the inside worked.  Using some vise grips and working the transducer back and forth in every direction, the malleable sealant finally gave way after a few minutes and it popped out.

broken depth sounder

I obliterated the old depth sounder in the process.

Back outside, I ground away the fiberglass edges that used to be around the flange and sanded it smooth.  I also removed all of the old sealant with a razor and Dremel then cleaned it with mineral spirits.

So in summary, what worked for me to remove an old plastic transducer:

  • Remove the nut inside the hull – break it into pieces with a chisel
  • Break away the flange outside the hull, again with a chisel
  • Work that transducer in every direction, from inside the hull, to get the sealant to give   

Installing the New Airmar Depth Sounder Transducer

new thru hull

No such thing as “too much sealant” here.

I applied the new sealant, BoatLife Life Caulk, to the flange and threading of the housing.  A lot of it!  Installing the housing from the outside of the hull took a little twisting to get the housing to go in properly and squeeze out any excess sealant – you want to make sure it gets in all the threads and fills in all the gaps.

Hopping back into the boat, from the inside I installed the plastic washer and hand-tightened the metal hull nut.  Once the housing was secure like this, I wiped away excess sealant with mineral spirits.

I let all this cure overnight and waited until the next day to pop the transducer in and paint the hull.

Make sure you inspect the o-rings on the transducer before you put it in the thru-hull; these are your best line of defense against water intrusion.  The Airmar DST800 comes with a small tube of lubricant to make sure the seal is tight; Vaseline will work as well.


The new transducer, before paint.

The transducer slides in easily, and is secured with a plastic cap nut.  Make sure the arrow on the transducer is pointing forward before completely tightening the cap nut, then secure the cap nut with seizing wire so that the nut won’t come loose.

The NMEA 2000 Surprise

I guess I didn’t know as much about boat electronics as I thought I did.  I bought this specific Airmar depth sounder with the NMEA 2000 cable connector option, because the Garmin GPSMAP 527 has a NMEA 2000 port.  The day before splashing back in the water and heading south, I tried plugging the transducer cable into the GPS unit and boy was I surprised!  Both the cable and the GPS had male connectors.  I guess I should have looked at this sooner.  I needed a special kit and it was going to take almost two weeks to have one shipped from St. Martin.

Screw it, I’m going south and will deal with it there.

So once I got to Grenada I went into Budget Marine and ordered a Garmin NMEA 2000 Basic Network Starter Kit (wish I purchased it in the US after paying Grenada prices).  This kit has everything you need to make a NMEA 2000 network for one device.  That’s right, you have to make a “network” and can’t just directly plug it in.

nmea 2000 starter kitThe NMEA 2000 kit comes with a power cable (because it needs a separate power supply), some T-connectors to connect all the cables, end fittings for the T-connector ports not being used, a backbone cable to locate your network in an inconspicuous area, and one NMEA 2000 connector cable.

The installation of the NMEA 2000 network was actually very simple compared to the NMEA 0183 connection I used with my anemometer (wind transducer).  And this now gives me the option to add other NMEA 2000 devices to my GPS by simply adding another T-connector to the ones I already have.  Easy as that.

Just don’t fall into the same trap that I did.

And now, after 4,000 miles, I actually have a proper GPS that will display wind, depth, water temperature, and everything else I need or want.

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