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Refitting a Depth Sounder Transducer

One Solution to a Problem

by Jim Hebert,

The Problem

After ten years of very heavy use, the Datamarine depth sounder aboard Voyager III, a 30-ft S-2 sloop, began to be intermittent. This spring (1996), it finally ceased to operate properly, displaying a reading of MSD or Missing Signal Detected. Checking with the factory representatives in Pocasset, Mass., they suggested that the transducer might be faulty.

Isolating the Trouble

Before replacing the transducer, I decided to make a simple check of the control head to eliminate the possibility that it might be the problem. A neighboring boat in the marina was equipped with the exact same model of Datamarine D/S, so I removed the control head from our boat, and connected it to the transducer aboard the sister ship. There the unit worked fine. The control head was good; our problem was either in the transducer or the cable connecting the transducer to the control head.

Making resistance measurements on the known good unit aboard the other boat showed that a good transducer exhibits a very high d.c. resistance, measuring in excess of 20 Megohms. (It takes a rather high-quality ohmmeter to even be able to make accurate resistance measurements that high.) The high impedance of the transducer prevented making any useful measurements of the cable's continuity, as an open cable would read practically the same as a good one. Checking our transducer and cable indicated a very high impedance, nearly infinite.

To eliminate the cable entirely, the cable was cut a few inches from the transducer, and the cable temporarily shorted at that point. The continuity was checked again, and the cable resistance was now easily measured. It showed the cable was intact with no opens or shorts. All indicators pointed to the transducer as the faulty element in the system.

With everything else shown to be in good order, our only choice was to replace the transducer. Fortunately, a local marine electronics store had the exact OEM part in stock for $110.

Replace All or Part?

At this point, some thought was given to purchasing an entirely new system. The cost would be about twice as much as just replacing the transducer, but then the entire system would be new. Retaining the old control head would leave us with a new transducer and a ten-year-old control head. Esthetics were also considered. The old unit was circular and matched the adjacent Knot-Log in size, shape, and color. Eventually, the additional difficulties of replacing the control head, which was mounted in the boat's cabin bulkhead just alongside the companionway, proved to be the deciding factor. We would just replace the transducer and leave the existing control head in place.

The new transducer was purchased and brought aboard for a quick check. Plugging the new transducer into the existing control head and lowering the transducer into the water brought the Depth Sounder back to life with accurate and steady measurements of the depth of the water in the boat well. All that needed to be done now was to replace the old transducer with the new one. I had been quite careful to be certain that the transducer was the culprit, because replacing it was not going to be easy.

Getting the Old Transducer Out

Unfortunately, the vintage of Datamarine transducer we had (c.1985) was of a design that did not permit the transducer to be extracted from the thru-hull fitting. The entire thru-hull fitting would have to be removed from the boat, and the new replacement unit installed in the existing hole. This change-out would require hauling the boat, since the work could not be done while the thru-hull was under water.

[Photo Voyager III in the haul-out slings] Arrangements were made with the boat yard, and we hauled the boat using a marine travel-lift, allowing us to keep the spar and rigging intact. The yard agreed to allow Voyager III to hang in the sling all day, if necessary, for which they charged us a reasonable fee. Getting the boat backed into the haul-out slip and getting the slings properly positioned took a little work, but that is another story.

The original installers of the transducer seemed to have used a high-grade marine bedding compound to seal the thru-hull to the fiberglass laminate of the boat's hull. Initial attempts to even back off the wing-nut retaining the thru-hull were discouraging. Some bedding compound was under the threads of the plastic wing nut, which, despite the "HAND TIGHTEN" legend embossed into it, refused to budge, even under the urging of a large hammer. For a short time it was a standoff: One plastic wing nut against two 225-lb-plus adult men, both of whom really wanted to remove that wing-nut. Eventually, the wing-nut gave in, and it was removed, leaving the thru-hull retained in the hull by only its bedding compound.

[Photo Transducer on hull bottom] A few whacks with a hammer showed how well it was still connected to the boat, as no evidence of movement was detected. Striking the strong plastic thru-hull/transducer with a rubber mallet produced an amazing amount of movement of the mast tip, as the whole boat shuddered under the impact. That plastic was very well connected to the boat's hull! Since the old transducer was non-functional already, it was decided to adopt a more invasive technique for its removal.

A 3/4-inch spade bit drilled a nice hole through the center of the transducer, until it reached the ceramic piezo-electric element inside. At this layer, all drilling stopped! Even masonry drills showed little effect. Fortunately, the ceramic element did not occupy full-width in the the transducer assembly, and a series of smaller diameter holes were drilled around the inside circumference of the plastic thru-hull/transducer assembly. A nicely sharpened chisel proved no match for the cycolac plastic of the thru-hull, and, after 30 minutes of assault, the mutilated remains of the transducer popped out the bottom of the thru-hull and onto the boat yard asphalt. A few minutes more and the thru-hull was completely gone, leaving a nice hole in the hull into which the new unit could be mounted.

Putting In the New Transducer,
and Finishing Up

After cleaning the debris from the hole and surrounding area, the new thru-hull was installed and bedded with 3M-5200 sealant. The previous installers had done a great job, fashioning a wooden backing plate that kept the transducer nicely aligned with vertical, and they had chosen a good spot in the hull, where the bilge was almost flat. Their hole size matched the new unit perfectly, so the re-fitting was very simple, now that the old unit was completely removed. The new thru-hull slipped in place, with the 5200 bedding making a strong and water-tight seal. The new plastic wing-nut was carefully hand-tightened, and the new transducer element with O-ring seals installed, The hull was once again water-tight. Bottom paint was applied to the exterior portion of the thru-hull but not to the face of the transducer element itself.

Another hour or so and the cable was re-rove through its twisting path under the cabin sole, up through the engine box, behind the quarter-berth, across the cabin bulkhead via a concealing trim piece, and into the instrument box cover where the control head was located.

The new design of the transducer will permit the unit to be replaced in the future without having to completely remove the thru-hull. The electrical portion of the transducer can be removed from the fitting, and a replacement can be installed without having to haul the boat. The cable, however, is a one-piece design, so it will have to be re-run, a time-consuming but not too difficult task.

In the course of fiddling around with the old transducer and control head, the SHALLOW GAIN control was adjusted (more or less randomly) in an attempt to cure the original problem. With the new transducer in place, the gain needed re-adjustment. A few weeks passed before anyone noticed that the unit was reading incorrectly in really shallow water, announcing a depth of 7.0 feet when the boat was actually in more like 3.5 feet of water. Adjusting the SHALLOW GAIN control cured that problem, and things were finally back to normal with the Depth Sounder.

3M-5200: The Right Choice?

Using 3M-5200 sealant may or may not have been the best idea, depending on whom you ask. (See John Hughes' website for more information.) Several people I consulted before undertaking this replacement warned me that if the old transducer had been bedded with 3M-5200 it would be very difficult to remove. I tried to contact the dealer from whom the boat had been originally purchased, since their yard had installed the Depth-Sounder transducer, to ask them what type of bedding compound or sealant they generally used. As it turned out, I couldn't get hold of them before we hauled the boat. When the old thru-hull was such a chore to remove, I assumed it had been bedded with 3M-5200.

I asked a couple of knowledgeable people around the yard what to use for this application. The most common answer was "5200". So, as I described, the new transducer was indeed installed with that sealant, or perhaps I should say, that adhesive.

A short time after Voyager III was back in the water and everything was thoroughly dried and working well, I did get a call back from the yard who had installed the original thru-hull. Yes, they remembered the boat, and the salesman was pretty sure that the bedding used was not 3M-5200! He said, "if we had put that in with '5200, you'd never get it out!"

This started a trend, and it seemed that the next few people I told about the project all commented that they would have used something less adhesive for bedding in the thru-hull. Too late for Voyager III's thru-hull; it's very well cemented to the hull with 5200. I just hope I never have to remove it!

What Killed the Old Transducer?

Repairing the Depth-Sounder turned out to be quite a bit of work and expense, two things I'd like to avoid in the future. What killed the old transducer? The best guess is simply old age. A transducer is like the head of a drum. Whenever the Depth-Sounder is on, electrical pulses strike the transducer's piezo-electric element and are converted to vibrations, or in this case, ultrasonic vibrations, just like a drum head makes sound when struck. The transducer is struck hundreds of time a minute by these electrical impulses. Eventually, it just wears out from the beating it is taking. Voyager III, being a club boat, is used much more often than the typical sailboat of her size, and she is in use virtually every day during the Michigan boating season, which can be stretched from early May to late October if you are really hung-ho about sailing. I began to think about the hundreds of hours the Depth-Sounder is left running while the boat lies at anchor overnight. Instead of running the D/S all that time, I now turn it off when it really isn't needed, in the hope that such conservation might stretch the life of this transducer a few more years. Another possible cause of damage: operating the D/S when the boat is not in the water. The presence of the water loads down the transducer, so having it pulsed when it is not submerged may produce excessive amplitude vibrations from the transducer, with the potential to damage it.


After reading this article, Al Gunther of Bainbridge Island, WA wrote me to comment about his experience with a similar job on his Catalina 27 Corleto:

Hi Jim,

I just read your story on replacing a transducer. I replaced mine on the last haulout. The yard had a pipe wrench that had a four foot long handle, which did the trick for me in about 20 seconds. My through hull nut was located behind the fuel tank in a corner. I was able to just barely get to it with a slim end wrench, and turned out the through hull from below.

I opted for 3M 101 for my new transducer, which I located about 4 feet forward of the keel. I plugged the old hole with epoxy and fillers. I like having the transducer up forward, and it angles slightly forward also, so I can "feel" my way into an anchorage.

Al Gunther

Copyright © 1997, 1999 by James W. Hebert. All rights reserved!

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