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?

Which ever you like.

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.

Comodave said:

Which ever you like.

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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.

twistedtree said:

... & correlate to chart depths.

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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).

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.

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.

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

O C Diver said:

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

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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.

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.

I expect one's choice is influenced by how skinny the water is where they cruise.

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....

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.

twistedtree said:

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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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.

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!

Comodave said:

Which ever you like.

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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!

ScottC said:

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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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.

Comodave said:

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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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.

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.

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

I draw the same 3'8" as comodave. So I make it easy and mentally assume I'll hit when the depth reading gets under 4 feet. Being that my props are below the keel, 5 feet is the "nope, time to bail out" point. Depth alarm is at 6.5 feet as a "hey dummy, start watching your depth"

I know I can be onboxiously pedantic at times, but it is a transducer, not a transponder.

..and we set ours to water depth to easily correlate with chart data. We know any number less than 5 is bad.

Personally I like to know true depth of water. It's not a huge calculation to just subtract my draft to know how close I am to the bottom. My draft is not going to change much, so it's easy to look at water depth and know how close I am to the bottom. Knowing true depth is better for comparing to chart depths and also when calculating anchor rode scope.

Conversely, I find it just as easy to add the depth of the sounder to the reading IF I want to know total depth. Running around in bottoms only a foot or so from my skeg leaves little time for the business of math.

rgano said:

Conversely, I find it just as easy to add the depth of the sounder to the reading IF I want to know total depth. Running around in bottoms only a foot or so from my skeg leaves little time for the business of math.

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Pretty much the same thing I guess. Thing is, if I know I have 3' draft, I read the depth and don't worry till it gets below 5' or so. Not like I need a calculator for that.

danderer said:

I know I can be onboxiously pedantic at times, but it is a transducer, not a transponder.

..and we set ours to water depth to easily correlate with chart data. We know any number less than 5 is bad.

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I do the same. I couldn't resist posting this. LOL

Pedantic is an insulting word used to describe someone who annoys others by correcting small errors, caring too much about minor details, or emphasizing their own expertise especially in some narrow or boring subject matter.

From my teaching perspective....it's not even math to some...you remember your draft and when the sounder gets close to the number in your head (draft) a mental alarm goes off.


Now the question is what "difference" in actual water depth and draft (like your sounder alarm trigger depth...also know as navigational depth I think) do you prefer?


So the math is done before hand...whether you remember one number or the other.

I want to know the depth of water beneath the keel.
Chart noted depth will not keep me from running aground.
If the chart says 10ft and I draw 4ft, I know I have 6ft beneath my keel.

OldDan1943 said:

I want to know the depth of water beneath the keel.
Chart noted depth will not keep me from running aground.
If the chart says 10ft and I draw 4ft, I know I have 6ft beneath my keel.

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I guess we all think differently and I respect that. But if you know you draw 4ft and your transducer is reading true depth, wouldn't you know that if your depth is approaching your draft that you are in trouble regardless of what a chart says. Don't forget Lindsey's law: “when your draft exceeds the water’s depth you are most assuredly aground.”

With my depth set to read actual water depth, I don't bother doing the math to determine depth under keel. I just keep 3 numbers in my head: If it gets down to 6, slow way down and be careful. If it gets down to 5, then idle speed or less and it's probably time to bail out unless there's a good reason to continue. If it gets down to 4, expect the props to hit bottom any second, as there are only a few inches to spare.