3/25/2012 – How deep is it?

Note: The following is one of those technical repair-type posts. If you’re not into that stuff, I suggest you just skip this and go directly to the next posting: “Chicken Enchiladas ala Siempre Sabado”. Of course, you could read this posting anyway just to humor me.

A couple of times toward the end of last summer’s cruise, the depth sounder on our Garmin GPSMap498 had given us erratic readings. The display would start to flash, indicating that it wasn’t getting a good sonar return. Then it would quit blinking and give us a reading. A few times, this reading was very suspect. When we sailed across from La Paz to Mazatlán in December, we also experienced these iffy readings. Doesn’t matter so much when you’re out in the middle of the Sea of Cortez but when you reach land and want to anchor somewhere, it’s kind of nice to have an accurate fix on how deep the water is. However, we managed to make it from the Stone Island anchorage to our present position in the La Isla marina without any problems in spite of the on-again, off-again depth readings. Fixing the depth sounder was added to my to-do list.

December, January, February and the first couple weeks of March went by, in which time we had no need for a depth sounder, being securely tied to the dock. But, with our imminent departure nearing, the depth sounder repair started to work its way back up toward the top of the list.

I was pretty sure that the problem wasn’t with the GPS itself. Actually, I was just hoping that was the case since the only practical recourse would be to replace the unit. So, instead, I concentrated on the transducer. The transducer emits a sound wave (2 actually) that bounces off the bottom. It then receives the bounced signal and calculates the depth based on how long it took the signal to return. It’s sonar. You’re all familiar with sonar from the old WWII movies like “Run Silent, Run Deep” or the more contemporary “Das Boot”. Works just like that except you can’t hear the ping.

Not much I’d be able to do with the transducer as you can’t really take it apart and fix something inside. Not that I’d have any idea what to fix or how to fix it even if I could open the thing up. So, I was going to concentrate on wiring issues. The unit is located under the sink in the head. I thought I remembered there being some wires spliced together down there. That was undoubtedly where the problem was.

I dug out all the stuff that’s stored in that locker and did indeed find some wires butt-spliced together. However, they weren’t for the transducer. They were for an old speed indicator that had long since ceased to work. I clipped the wires and stuck the stubs out of the way. The wire to the transducer was solidly potted into the head of the unit. No real room for problems there. So, I followed it to the other end. This had to be where I’d find a problem. But NOOOOOOOOOOoooooooo… That end plugged into the lead from the GPS with a very solid fitting. The contacts inside the fitting were even clean.

I could only think of one other thing that I could do to the transducer short of simply replacing it. This unit is set up to send it’s beam through the hull so no holes have to be drilled. To do this, a piece is essentially glued to the inside of the hull to receive the transducer itself. This piece acts as a reservoir to hold a liquid that the beam is sent and received through so that it doesn’t have to travel through air which would apparently distort it or something. What if the reservoir had lost some or all of its fluid? That seems like it could cause these erratic readings. By the way, the readings had, since our arrival in Mazatlán, gone from erratic to non-existent. Just a flashing “0.0 feet”. This had to be it. Right?

So, I reviewed the installation instructions. The fluid to be used was either propylene glycol (RV water system anti-freeze) or mineral oil (like baby oil). We didn’t have any propylene glycol on board but we did have a bottle of baby oil. And it only called for 71 mls. So, I unscrewed the top of the transducer, revealing the inside of the reservoir. The pinkish look of the fluid told me that the installer had used the anti-freeze option. There was no way to tell by looking at it whether or not it was the right amount so I just soaked it all out using paper towels. When the reservoir was clean and dry, I measured out 71 mls. of baby oil. Not having any metric measuring cups, I had to do a conversion to tablespoons. I ultimately came up with 71 mls. equals 4 tablespoons plus 2.4 teaspoons. I measured in the appropriate amount of oil, cleaned up the inevitable mess, replaced all the stuff in the locker, put my tools away and went up to the cockpit to confidently fire up the Garmin. Wait through the warm-up period while it acquires satellites and then….

“0.0 ft” (flashing)

Well CRAPPAGE! That’s not what was supposed to happen at all.

So, I figured that I must have screwed up my measurement and I proceed to repeat the whole process. Finish up, clean up and fire up.

You know exactly what comes next, don’t you?

“0.0 ft” (flashing)

Okay, that’s it for me. It’s got to be a faulty transducer. What are the chances that the same transducer is still for sale? And, even if it is, what are the chances of getting one in Mazatlán in a reasonable amount of time and at a reasonable cost?

I crack open a well-deserved cerveza and then fire up the MacBook. I googled “Airmar P79 transducer”. I get a ton of returns. I check a few out and find that this transducer is made for lots of different depth sounders and it’s readily available at a bunch of US outlets. Prices range from about $110 to over $200, depending on where you buy it. Okay, that’s a lot but it’s way cheaper than a new GPS/Depth Sounder. Now, how do I go about getting one?

A year or so ago, I ordered a water pump rebuild kit from a place in Florida and had it sent to La Paz. It arrived in a reasonable amount of time but the duty was something like 25% and that, combined with the shipping just about doubled the price which was none too cheap to begin with. I’d heard horror stories about shipments getting hung up forever in places like Guadalajara and, although only about 10% of cruisers’ horror stories are true, I didn’t really want to take the chance if I didn’t have to. So, I took a hike over to Total Yacht Services which is near Marina Mazatlán. These guys do boat repairs and also sell some parts and such and have a very good reputation among the Mexico cruisers.

I told them what I needed and they said they shouldn’t have any problems getting it. I looked through their West Marine catalog, found the appropriate part and noted that it was listed at $109.99. And, although Airmar makes these transducers for lots of sounders and the wiring can change depending on which sounder is used, the one that West Marine carried was specifically for the Garmin and even had exactly the same part number and customer number as the one I would be replacing. This should be a simple case of plug-and-play. I placed the order and they said it should be here in about a week.

A couple days ago they called to let me know the transducer had arrived. I was pleased to find that, even with shipping and duty, the total rice was only $128. Of course, there was also the Mexican IVA tax that brought it up to $142 but still, not too bad a markup.

The next morning, I was ready to try it out. We had just stuffed the lockers that the cable ran through full of food and I didn’t really want to unpack all that stuff right now. So I just plugged the transducer in, ran the cable along the floor and plugged it into the GPS. Turned the unit on, waited for it to warm up and…

“0.0 ft.” (flashing)

WTF???? That can’t be right. What the hell is going on here?

I started reading through the instructions again and noticed that on the newer version, they had eliminated mineral oil as a transfer medium. The only option now was to use propylene glycol. I couldn’t really believe it could be that simple, but OK.

Now remember, non-toxic propylene glycol is used in RVs when the owners want to winterize their water systems for storage. And where are we? Oh yeah, Mazatlán where it might get down to the mid-40s in the dead of winter. Maybe. Not a lot of reason to winterize anything. Not to mention the fact that most of the RVs down here belong to gringo snowbirds who are living in them, not storing them. So, the chances of finding propylene glycol were, at best, slim. However, it can also be used to pickle watermakers for storage so, maybe…

A visit to a number of stores turned up lots of regular antifreeze (ethylene glycol) but none of the non-toxic stuff. A query on the morning VHF cruisers’ net also didn’t turn up anything except some faulty information about there being lots of it at Soriana. What to do?

I started thinking about why they use propylene glycol. Well, first off, since not all boats are kept in toasty Mexico, Airmar certainly wanted a medium used that wouldn’t freeze solid and break something. Okay, I can see why antifreeze is used, but why the non-toxic stuff instead of good ol’ Purolater? The only reason I could see, besides maybe user safety, is that ethylene glycol antifreeze isn’t usually as clear as RV antifreeze. Maybe the turbidity would screw up the signal. I don’t know. OK, how about just using something like cheap tequila? Wouldn’t freeze and it’s transparent. It might work unless the plastic of the transducer or the rubber in the o-ring is alcohol-soluble. Unlikely but maybe. I finally decided to try just plain old water. I soaked all the oil out of the reservoir, cleaned the residue with an all-purpose degreasing cleaner, and then measured in the appropriate amount of water. Then rather than putting the new unit in, I decided to try the old transducer. My reasoning was, if the water does the trick, I wouldn’t have to re-run the cable. So, I installed the old transducer, plugged the cable in to the GPS, turned it on, waited for it to warm up and then….

“0.0 ft.” (flashing)


But then…

“12.3 ft.” (no flashing)

I waited, holding my breath. Surely it was going to return to 0.0 soon. I waited. And waited some more. Then I went and found something else to do, returning a half hour later. It was still getting a good reading. I tossed a weighted line over the side and measured the distance to the bottom. It was the same as the sounder was showing. Holy, crap, it worked!

Since then, I’ve had the sounder on for hours at a time and it hasn’t gone back to flashing 0.0 yet. It’s been 2 days. I’m beginning to think that it’s fixed. Of course, this brings up the question of “why the original failure?” Maybe the fluid level really had dropped and bringing it back up to the right level was all that was needed, providing that the correct medium was used. And, why was mineral oil OK before but not now? I mean, it didn’t work at all for me and yet it must have worked sometime in the past or it wouldn’t have been an option on the original instructions. And, if it used to work, why doesn’t it work now? What happened to have Airmar completely remove it from the instructions? I don’t have the answers to any of these questions and, as long as the sounder continues to give me accurate readings, I guess I don’t really care all that much. However, I’m hanging on to the new transducer as a form of insurance. My experience so far is that, if you have a working spare (of anything) aboard, the original will not fail.

Wish us luck.


About sryoder

Steve & Lulu... retired. Had enough of the cold wet dreary fall/winter/spring in the Pacific Northwest. Bought a boat, fixed it up, sold our home and sailed to Mexico in November, 2010. Been here ever since except for occasional forays to the States (summer only, thank you) to visit the kids, parents and siblings. If you're looking for a sailing blog, this is the wrong place. This is a traveling, hunkering in, eating blog. Sailing is just how we get from place to place when we can't walk.
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