My depth sounder failed a few days before hauling out of the water at the boatyard in St. Kitts. I was going to be in deep water most of the way so I wasn’t worried about it.

Unfortunately it is something that can only be fixed out of the water. This depth sounder transducer was permanently fixed through the hull and would require a complete replacement of the thru-hull.

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

I settled on the Airmar DST800 Triducer, a retractable unit that contains 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.

A “retractable” transducer requires a bronze thru-hull housing into which the plastic transducer secures, forming a watertight seal. The transducer can be removed while the boat is still in the water for cleaning or replacement. A spring-loaded trapdoor keeps water from rushing in while you have time to insert a watertight blanking plug. Retractable transducers are the way to go for this ease of maintenance & replacement.

Installation should be easy since the hole was already drilled and the appropriate size. Removing the old unit – that’d be the hard part.

Removing the Old Depth Sounder

There’s a large plastic nut holding the transducer firm against the hull and stuck in place with sealant. It wouldn’t budge. I hammered away at it with a chisel, eventually breaking it into a few large pieces that freed from the sealant.

The transducer still wouldn’t budge after getting the nut off. I couldn’t even identify the perimeter of the plastic flange from the exterior through the many layers of paint. I carefully chipped away at the paint until I could identify the flange and then I hammered away at that with a chisel.

My objective was to chisel between the plastic flange and the hull, breaking the entire flange off so that I could just pop the transducer out with a hammer. But I eventually realized that 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 the edge was. It required both more force and more precision.

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 again it wasn’t moving.

Removing the unit from the inside worked. The malleable sealant finally gave way after working the transducer back and forth for a few minutes with some vice grips.

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 way

Installing the New Airmar Depth Sounder Transducer

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. I wiped away excess sealant with mineral spirits once the housing was secure.

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 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 an NMEA 2000 port. The day before splashing back into 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 an NMEA 2000 network for one device. That’s right, you have to make a “network” and can’t just directly plug it in.

The 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 of sailing, I actually have a proper GPS that will display wind, depth, water temperature, and everything else I need or want. Morale boost!