Transducer Offset Question-

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Jun 11, 2018
Vessel Name
Indigo Star
Vessel Make
2006 Mainship 400
Our Mainship is on the hard for paint and while there I had made a point to get a somewhat more accurate measurement of our Airmar BV744 depth/Speed transducer's for setting the offset.

I think I would like to have the depth display to reflect the "depth from keel" to the actual seabed bottom.

I measured 3 feet from the transducer to the bottom of my rudder, (the rudder is actually lower than the keel a little)

So I should set the offset to minus 3 correct?

I say this because my thinking is by "electronically" lowering the transducer, I am reducing the distance from the transducer to the sea bottom correct?

The Raymarine manuals just say to "adjust as required" so am asking all the experts.

Thanks for your comments-


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You're on the right track. Confirming which direction to adjust will be easy once you're in the water. Enter the setting and see if the depth changes in the expected direction.
On my Simrad’s various depthfinders, +ve numbers increase the depth readout. Negative does the opposite. Initially, I set my offsets for depth under the keel as you propose. After using it that way for awhile, I reset the offsets to match the actual water depth. I found my old brain didn’t have to work as hard - matching depth sounder to charted MLLW depths. YMMV.
I like to see the total water depth. Then it matches the charted depth. But do it however you like and you can always reset it again if you decide you don’t like it that way.
I'm also in the camp of preferring the display to show total depth from the surface. Easier to compare to charts, and easier to exchange data with other boats about what to expect in areas where depths change over time.
I have no need to know the depth from the surface. Where I boat, the depth on the chart shows a single number, but in reality it is constantly changing based on the tide, barometric pressure, recent rainfall, prevailing winds, etc. Plus, it was recorded as that depth in 1932 (or whenever the chart data was gathered). Also, the number printed on the chart or appearing on the screen is not likely accurate within 20 feet (as with my GPS), so that number is advisory at best.

What I do want to know is how much water is under my keel. If I want to know the total depth for some reason, I add 3.5 feet. I suppose that gives me an approximation of what the number on the chart should be at that moment in time. Interesting in an irrelevant way.
It is easier to add draft when needed then to subtract it constantly while in shallow water.
I can see that. I don't actually do the math to subtract my draft though, I just have a number in my head that's "don't let it get below this". Basically I don't think of it as "I have 2 feet under the boat", I just think of it as "I'm in 5.5 feet, that's not enough water".

Depth alarm is set to 8 feet for me (4.5 feet under the boat) as a "pay attention!" warning. Depending on how well I know what's on the bottom, the "bail out" point is typically around 6 feet, sometimes as low as 5.
I hate to start with credentials but: I was a qualified ship driver on both Navy surface ships and submarines.

When I was serving, the US Navy’s fathometers were set for depth under the keel for good reason. When operating in littoral waters it is undesirable to depend on the watch stander to have to remember to subtract draft from the fathometer reading to report to the Officer of the Deck, especially under stressful situations.
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I measured 3 feet from the transducer to the bottom of my rudder, (the rudder is actually lower than the keel a little)

So I should set the offset to minus 3 correct?

We have two transducers, so we mix and match. One set to depth under keel, one set to water depth.

But the mechanics of setting each were completely different for the Furuno and Garmin units. Had to review both manuals, fiddle a little to sort out what each set of directions meant.

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