How Low Can You Go?

Depth gage showing 0

Well, we’re not aground yet.

Forgive me while I geek out a little here.

You see, at heart I’m kind of a gadget guy. I love technology and all the wonderful things it provides.

Take for instance the SONAR (SOund Navigation And Ranging) technology that almost every boat uses to determine how deep the water is beneath it.

Used by many marine mammals and developed for use with vessels in the early 20th century, SONAR sends sound waves through the water and times how long it takes to get echos back from other objects in the water. The longer the time, the more distant the object.

The downside of technology is that because it generally makes our lives easier, it is easy to become dependent upon it.

So when the 36 year old depth gage on Shannon decided it was time to retire, it really put a cramp in our sailing. I mean it really makes me nervous not knowing just how deep the water is under our five foot deep keel. It would be a real bummer to come to a sudden stop after hitting a rock just below the surface.

I will admit that boats have been sailing waters for centuries using nothing more than a lead weight tied to a string to see how deep the water was, but I’m not that much of a traditionalist. I like my gadgets!

And so begins our search for a new depth gage.

I had no idea that there are so many decisions to make when selecting a new depth gage.

A Little Technical Background

(Warning: technical details begin here. If you aren’t interested in them scan ahead to get back to the story..)

For starters, there are two main directions to choose from. The older, traditional sailboat depth gage that does little more than display a number and offer a low or high depth alarm. This is basically what I’m replacing, although mine was so old that it had little tubes that displayed the numbers, not an LCD or even an LED screen.

The other category of depth-instrument is the “fishfinder”. These are more like little computer screens that show the depth of water in feet (or fathoms or meters) in addition to showing all of the echos bouncing back from the SONAR signals. These show a much more detailed view of what is under the boat. The funny thing is that even though these are more complex and give better information, they are generally less expensive than the traditional “depth only” sailboat systems.

Then there is the choice of transducer, which is the part of the system that creates the sound waves and then listens for them to come back. These come in a handful of configurations.

There are single-frequency transducers which are geared either towards better detail at shallower depths (200mhz) or less detail in deeper water (50mhz).

Many fishfinders now come with dual-frequency transducers that can send and receive on both frequencies so the user can decide which is more important, depth or detail.

Recently there have been a new class of transducer introduced that is a phased-array which transmits and receives on a wide range of frequencies and gives a very clear image of the bottom, including any vegetation, rocks, and of course fish. However, these are very expensive and used only by the most serious of anglers.

Once a frequency has been chosen, then you need to decide what kind of transducer to use, transom mount, through-hull or in-hull.

Most power-boats hang the transducer off the back of the boat (the transom) in the water, where it will be out of the turbulence from the propeller.

The other mounting options are a through-hull where a hole is drilled in the bottom of the boat and the transducer is secured by a large nut and glued into place. This gives the cleanest signal and provides good detail to the instrument.

Finally an in-hull transducer is glued to the inside of the boat and it shoots the sound waves through the hull. This installation requires the most power and provides the least detail to the instrument. What it doesn’t do though, is require any holes to be drilled below the waterline of the hull.

So, now that you have a basic understanding of the world of marine depth instruments, let me share what has been happening with Shannon.

Back to our story…


View of evening sky

Looking East from Shannon in Everett

A couple of weeks ago the depth gage kept bouncing back and forth between “00” and what seemed like a reasonable depth. At first it was quite shocking to see the depth gage reading “00”! I mean what underwater mountain was I sailing over? The water in the Puget Sound where we were at was supposed to be hundreds of feet deep.

So we fiddled with the two red knobs on the front of the instrument (the labels had long ago worn off and no owners manual could be found on the boat or online) and we could occasionally get some kind of reading.

Well it didn’t take long to realize that this just wasn’t going to cut it when we started trying to find a good place to drop anchor.

I put in a call to Signet Marine to see what they could tell me about our depth gage.

Good News and Bad News

I knew I was in trouble when the customer service rep, Aaron asked me if it had two red knobs on the front, then mumbled about never having actually seen that model. Sure enough, the only documentation he could find was the wiring schematic of the instrument. No user manuals, no technical manuals, nothing.

He did offer me a free inspection of the instrument if I sent it in to the factory in Southern California. Well, since it wasn’t really doing me any good where it was, I figured it was worth a shot.

Let me take a moment to share with you a little nugget that a friend of mine shared with me. He introduced me to the phrase, “boat yoga”. You see, it seems that just about any time something on a boat can be put together in such a way that it is almost impossible to ever get to again, then that is exactly how it will be put in.

Back to our story. Aaron was nice enough to let me know that I could get the guts out of the instrument by simply removing the three small screws that are positioned around the outside of the back of the case. This would allow me to leave the case in the fiberglass wall so that I didn’t have to figure out how to keep the rain water on the outside of the boat.

Sounds easy enough, right?

Well, the instruments are housed in a wooden box that gives about 3/4 of an inch clearance top and bottom. No problem for the first two screws as they are at the 10 oclock and 2 o’clock positions. But, the third screw is at the 6 o’clock position. With less than an inch clearance between it and the box that it is in. Ugh.

But wait! I remember reading about a similar incident aboard our friend’s boat Del Viento. The answer to the problem was an offset screwdriver! Off we go to the local Lowe’s to get just such an item.

But no! The offset screwdriver is too long to fit between the box and the screw.

Okay, now what? Well, I finally realized that a hose clamp is thin enough to fit into the screw head and small enough to fit between the screw and the box. So, with a hose clamp straightened out I was finally able to loosen the screw enough to grab it with needle-nosed pliers and work it out of the case while crouched down low enough to actually see what I was doing. Boat Yoga!

Off goes the instrument to the factory. About a week later Aaron calls back. Good news and bad. Good news is that the instrument is working perfectly. Bad news is that it means the transducer is going bad.

So, we have a 36 year old instrument that relies on tubes that works, and a solid-state transducer that isn’t. Wonderful. More bad news: the transducer is a 180mhz model that hasn’t been made in decades and cannot be found.

Okay, well I can see that Shannon is now in the market for a new depth gage. So the hunt begins. It is then that I find out all of the possible options and decisions that must be made.

The cheap part of me says that I should just get the least expensive replacement that will tell me how deep the water is below the keel. That is what was in there and that is all that I really need.

The gadget-lover in me says that this is the perfect opportunity to upgrade! Get a fishfinder that has a GPS in it and that can display color charts also! After all, that would be a lot safer than relying on just paper charts alone. Not to mention that it would give another speed display (through the GPS) as a backup to the 36 year old speed indicator that sits next to the now empty case of the old depth gage.

So, the internal debate rages on for now. I strongly suspect that the gadget-geek is going to win this one It really would be nice to have electronic charts available in the cockpit and for less than the cost of a new depth gage from Signet Marine, I could have depth, GPS, moving maps and more.

Stay tuned for more on this story…

3 thoughts on “How Low Can You Go?

  1. Doug Mabry

    Come on Mike, just go with the rope and lead weight. Your sextant will enjoy the company. 🙂

    As an added bonus Kelly could practice yelling “Mark Twain”!!!!!!!

    1. Mike Routen Post author

      Yeah, but the ground comes up so fast around hear by the time you finish yelling “Mark Twain” you’ve run aground. 🙁

      1. Doug

        But just think of the photo opportunity with her balanced on the bow throwing the line to check the depth. 🙂

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