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Old 07-18-2020, 12:05 AM   #1
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setting transponder offset

I get what they are wanting to do but do i want to know how much water is under the keel or do i want to know how deep the water is from the surface and then mentally decide how much room i have under the boat?
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Old 07-18-2020, 12:09 AM   #2
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Which ever you like.
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Old 07-18-2020, 04:55 AM   #3
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I have my offset configured such that it shows the depth beneath the lowest point on the bottom of my boat. I don't want to be doing mental gymnastics every time I'm in shallow water. Too much risk for error! This has worked out well for me for years and I've never had any thoughts of changing it to depth below the surface.
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Old 07-18-2020, 04:55 AM   #4
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Which ever you like.

Exactly.


Water below keel lets you focus more easily on how close you are to running aground, but requires mental math to compute water depth & correlate to chart depths.


Water depth lets you more easily correlate to chart depths, but requires mental math to figure how close you are to running aground.


Take your pick which you prefer.
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Old 07-18-2020, 04:59 AM   #5
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... & correlate to chart depths.

Fully agreed! But by the time I'm in water that shallow, going 0.10 knots, I don't really care about what the chart says for depth below the surface at the exact point where I am. (Except for tide planning, if relevant for the cruising area involved).
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Old 07-18-2020, 06:24 AM   #6
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I use water depth. For me the sounder is as much a navigation/charting tool as a don’t run aground tool. I do the math for state of tide much more often than needing to know exactly how many inches til the scraping sound on the bottom.

Draft is one of the boat’s grail numbers, dimensions and capacities we can all recite even if in a coma. Cognitively speaking, my draft figure essentially becomes the “zero.” When I look at the sounder, it’s perceived as relative to my draft, not zero and no conscious math at all. But, that’s me.
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Old 07-18-2020, 07:51 AM   #7
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I like redundancy for a critical system like that, so I had two seperate systems, one set for each as they were side by side. Having one set to overall water depth helped navigate from the chart, and the under the keel one of course helpful in skinny water.

I have the one on the Whaler set to total depth as well. There were occasions where we used it to survey potential anchorage depths that hadn't been surveyed in decades or not charted. The mysterious end to the chart at St. Mary's, Georgia was a classic example. Turns out the water stays quite deep well upriver. So we could anchor the big boat well away from the pack.

So the "whichever you like" is the best answer.
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Old 07-18-2020, 07:59 AM   #8
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I have one in the bow for traveling in shallow water that displays keel clearance. It also has an alarm set 2' deeper. Have one mid ship for water depth, so when somebody in a boat behind me asks, I can give water depth without doing math. Then there is the third one with a graphic display of the bottom.

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Old 07-18-2020, 08:03 AM   #9
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I have one in the bow for traveling in shallow water that displays keel clearance. It also has an alarm set 2' deeper. Have one mid ship for water depth, so when somebody in a boat behind me asks, I can give water depth without doing math. Then there is the third one with a graphic display of the bottom.

Ted

Wow, Ted! I guess you certainly have this topic covered!
If I ever get a forward-looking sonar, I would change my existing depth sounder to water depth instead of depth below keel.
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Old 07-18-2020, 08:12 AM   #10
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I keep mine set for total depth. I know what my "stop and carefully reverse" depth is, so it's easier to do less math for anchoring, etc.
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Old 07-18-2020, 08:20 AM   #11
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I expect one's choice is influenced by how skinny the water is where they cruise.
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Old 07-18-2020, 08:32 AM   #12
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Instructing boaters and pilots taught me ....people tend to think and approach operations in a widely varied manner. Some can do math quickly in their heads, others not so much.

So bottom line is, do what works for you.

I use a depth sounder to tell me how deep the water is....that number is important to me for lots of reasons.I use the alarm to tell me when there is less between my keel than I want....
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Old 07-18-2020, 09:12 AM   #13
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We set both depth instrument offsets (fwd) and midship (deepest) for both by physically measured distance between the surface of the xdcrs to the bottom of keel. Fwd and mid readings match (level keel). What we read is actual keel water clearance. No math required no chance of errors. Simple.
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Old 07-18-2020, 10:03 AM   #14
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Quote:
Originally Posted by twistedtree View Post
Exactly.


Water below keel lets you focus more easily on how close you are to running aground, but requires mental math to compute water depth & correlate to chart depths.


Water depth lets you more easily correlate to chart depths, but requires mental math to figure how close you are to running aground.


Take your pick which you prefer.
I wonder if anybody will ever come up with the extremely simple firmware to display BOTH should the operator wish? I nominate Humminbird for the job.
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Old 07-18-2020, 11:07 AM   #15
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Just going to add my "ditto" to all the other comments. It is entirely a matter of personal preference. Use whichever makes the most sense to you.


Just be sure, if you ever let someone else pilot your boat, that they know which choice you have made!
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Old 07-18-2020, 11:19 AM   #16
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Which ever you like.
When I first saw Comdave's comment, I thought it was uncharacteristically "cavalier" of him. I now see it's the most accurate and succinct response in this entire thread!
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Old 07-18-2020, 01:06 PM   #17
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When I first saw Comdave's comment, I thought it was uncharacteristically "cavalier" of him. I now see it's the most accurate and succinct response in this entire thread!
I certainly didn’t mean to be cavalier about it, but rather matter of fact. My boat draws 3’8” with the transducer about 2” below the surface. I can mentally add 2’ for water depth or subtract 2’ for water below keel and that gives me 6” margin for keel clearance. We were doing PATON patrols a couple of years ago and spent several days running in water that showed 3’ or so on the depth finder so that meant that we were in water about 1’ below the keel. Worked out fine.
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Old 07-18-2020, 01:28 PM   #18
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I certainly didn’t mean to be cavalier about it, but rather matter of fact. My boat draws 3’8” with the transducer about 2” below the surface. I can mentally add 2’ for water depth or subtract 2’ for water below keel and that gives me 6” margin for keel clearance. We were doing PATON patrols a couple of years ago and spent several days running in water that showed 3’ or so on the depth finder so that meant that we were in water about 1’ below the keel. Worked out fine.
No justification needed! That was a compliment!!

It truly is a personal preference. I spend more than half my time poking around shallow waters, tidal harbors, rivers and canals, so being able to easily see the few inches under my keel* is quite important to me. On the other hand, I have found it a useful compliment to AIS and radar to be able to follow depth contours on the chart sometimes when caught at sea in the fog. In these rare situations, I have to do a little mental math.

* Asterisk is because I really don’t have a keel. i have two stabilizing fins that thankfully extend below the rudder and prop. These have proven to be a great benefit when exploring shallow waters.
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Old 07-18-2020, 04:25 PM   #19
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I went with actual water depth. Since my draft is 4.0 ft it’s pretty easy. >4.0= good. < 4.0= bad. My alarm is set according to where I am. Georgia 8.0 ft. SC 7.0 ft, Florida 6.0 ft. I like early warnings.
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Old 07-18-2020, 07:44 PM   #20
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As everyone said, it's a matter of choice. I'll tell you my preference and how I arrived at it.

At first the boat had the depth set to the amount of water under the keel. I thought this was a great idea, and so did my boating partner. Cool, we'll know just what's under the keel! Off we went.

What got annoying was the constant:

"What's the depth?" "Eight feet." "Is that under the keel or on the chart you're looking at?"

"I see we're in 20' now." "Is that 20' on the chart or 20' on the depth sounder?"

Even worse in one of those semi-panic situations where you find yourself in shallower water than you were anticipating:

"Six feet, let's get out of here!" "Six feet?! Is that under the keel or...."

Ad nauseum.

We changed the depth sounders to read depth from the surface and suddenly all was wonderful. We both knew we drew 5' so subtracting that mentally was instinctive, and now our depth sounder matched the charts and we always knew what we were talking about. Ahhhhh.

Frosty
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