Now that Christmas isn´t that far away I am sure many of you skippers have already made yourself a gift, am I right? A present for the boat, some new gear here, a little gadget there. And it´s fine: We work so hard that there´s more and more less nice things to be happy about. Even some premature Christmas unboxing bei it the smaller or the bigger items with the big day still two weeks ahead is absolutely okay. I did the same.
Last week I got a little cardboard box containing yet another part for my new boat. It´s the transducer unit. A little machine that will provide me with accurate informations on the depth and the boat speed (and also water temperature, for whatever reason I would need that). You already know this little black plug, sure. It´s not the sexiest of gadgets to write about, so why bothering? It´s a standard, dreary and boring part, right? Well, not so much! Not at all.
Under-appreciated piece of equipment
I´d say the transducer is among the most underrated parts of a sailboat. Yes, it might not be the most glamorous and one might think that it´s part of the common sense equipment not worth talking about, but when this little machine with a Bing doesn´t bing any more, well, then you suddenly find yourself in big time trouble! Boats can run aground or, not as bad but deciding when racing for example, speeds won´t be calculated anymore.
As I unbox the parts delivered by Airmar, I begin to grasp the importance of this piece. To be honest, this was my second attampt in acquiring these gauges for my Omega 42. Originally I thought it might be better not to have the DST 810 but to buy two separate pieces, each one especially dedicated to either speed only measurement or depth sounding. As my electronic equipment is Garmin-only, I went for the Garmin GDT 43 transducer and Garmin GST 43 logging device. My shipyard appreciated the fact that, if bought separately, there might be some more accuracy of each of the sensors but they asked me to change them into the one single combo-instrument: It´s only one hole in the hull therefore, not two. Okay, check!
How does a depth sounder work?
The „Machine with a Bing“, as immortal Monty Python would say, is quite a bit of technology. If you look at the underside of the DST 810 you will notice that one half of it is flat and nice, the other half is essentially a small plastic wheel that can rotate. Later more on that. The flat half indeed is the depth sounder.
The transceiver-unit is sending (ergo transmitting) and receiving ultrasound waves. These are basically „bings“ or „pings“, as Captain Ramius of the RED OCTOBER would say: „Only one ping, Jury!“ And this is also true for the Airmar DST 810 – the freqency of such pings transmitted is one ping per second. So, each second the transmitter will send out an ultra-compact sound wave that travels down into the water until it hits the ground. It will bounce off and be received by the receiving unit. A quick calculation of the given speed of a soundwave through salt water, which is roughly 1.500 meters per second, and the running time of that impulse (simple rule of three) equals the depth.
One per second, that means the sensor – in order not to confuse the echoes – needs clear reflections received back in well under one second and immediately fired into the NMEA 2000-network of your boat. This, and the fact that we cannot barrage the ocean´s floor with submarine-hunting-grade extra-strong Sonars, is the reason why the DST 810 only works sufficiently up until a depth of 330 feet, which are around 100 meters. Well, more than enough to check for anchorage and safe passage according to your boat´s draft.
Differences between the old DST 800 and the new 810
Most of you maybe sigh dismissively: „What´s the deal here?“, you may ask: „That thing looks exactly the same as my DST 800!“ Yes, it does, but the 810 has some interesting features and comes with a deciding progress: Boat speed measurement frequency! The old 800´s paddle wheel and gauge had also a „one ping per second“ frequency, meaning that speed through water was always a bit „behind“ and often felt off, kind of retarded.
That´s now corrected. With a measurement frequency of 5 per second starting at 0.3 knots (up to a max of 45 knots) the boatspeed-gauge will be much, much smoother and faster. Also, more accurate which indeed is important for racing and time-to-distance calculations for example. As it has the same dimensions, any of you can easily replae their old DST 800 with the latest 810, the thru hull is the same. Another improvement is Cast, the app. Yes, your depth sounder now comes with a proprietary app for your smartphone.
„Another plaything? Let me alone with that digital crap!“, some of you will sigh even more now. Well, I admit, I did so as well because I also kind of reject the „appification“ of sailing but in this case, it´s really cool: The calibration process of the transducer, the offset programming in particular, is much better and intuitive this way than using the old way via classic chart plotter. In my case, I think I won´t even have a chart plotter as part of my boat´s electronic equipment, so that´s pretty convenient. Secondly, a small detail and I hope you´ll never have to draw that Joker, but in case your network breaks down and all displays are black, the Airmar will still send its data to your phone´s Cast-app.
What´s in the box?
Of course, the transducer needs a socket to be mounted to. For the Airmar DST 810 there are basically three versions: Plastic, stainless steel and bronze. Depending on the material of the hull, either one of these is the product you´d go for. And there´s of course a price tag attached, but the differences are minimal. The box is around 500 to 600 Euros including VAT.
It is then only the so called thru-hull that comes in one of these three materials. The thru-hull consists of an outer and inner part that is put through a hole which needs to be drilled through the hull. This whole must be placed at a certain location, mostly directly in front of the keel fin of the yacht. It is absolutely advisable to choose an area where preferably no vortices are produced by the boat´s hydrodynamic flow. This, by the way, was another reason why my shipyard asked me to change from two separate gauges to this multi-sensor gauge.
Next to the thru-hull the package also includes a placeholder-plug. This is essentially an empty transducer-casing, easily distinguishable by the orange marking. There´s also a completely clean underside and no paddle wheel. This plug is used to close off the thru-hull when doing some cleaning, maintenance or checking on the transducer while the boat is still afloat. I´d also recommend to take out the transducer when the boat is haule out/in of the water or in winter storage on dry land: In this, the travel lift´s tethers cannot damage the instrument when accidentally put exactly underneath the gauge when hauling out. Also, in wintertime, the sensors are much safer inside the yacht when on dry land.
The old fear of flooding
A last word of reassurance for all of you boating starters who fear of water leaking into the boat. I know not a few skippers who, in literal fear of sinking, do not dare to take out the transducers when their boat is in the water. They rather let the blocked paddle wheel be the reason for no information on their boat speed and don´t pitch on the little snail or slug. Well, first of all: When taking out the transducer some water will indeed enter the boat´s bilge. But that amount is less than half of a soda can. Mostly, it´s a very little fontain and that´s it. Why? Because of this:
There is a little plastic flap inside of the thru-hull that closes and seals off most of the incoming water. The flap is closed in under one second by the pressure of the incoming water. It´s not completely watertight but more than enough time to insert the plug. 2 seconds, a little slurp of water that is soaked away with a nice bilge sponge. Basta. So – don´t fear your boat will sink! Nice news, I know. So, next time I see the guys in the shipyard I will deliver this little Christmas-gift to myself. For now, it has its perfect place of honor under our little green Christmas tree …
What´s the little or bigger yachting-gift you bought for yourself? Looking forward to your comments. Merry Yuletide, dear readers and cheers from Luebeck.
You might as well check out these related articles:
Hole in the bow? About a common misconception with bow thrusters (LINK)
Anatomy of a grounding (LINK)
Detecting a leak when sailing (LINK)