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Old 06-05-2011, 07:27   #1
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True Depth or Depth Under the Hull ?

I am sure this post exposes my "noob-ness" but I couldn't find anything on here or google regarding how Depth Finders are generally setup. So I defer to those with much more experience.

Do you generally set your Depth Finder to true depth from the surface or depth under the hull?

I realize there is an offset on most (maybe all depth finder's) and part of me likes the idea of being able to look at the depth finder and know what is under the hull.

As a charterer in the Caribbean it makes sense to know how the depth finder is setup before setting out ... or are they always set the same way?

Your thoughts are appreciated ... and I will take any flaming kindly.

Thanks in advance.
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Old 06-05-2011, 07:30   #2
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Re: True Depth or Depth under the Hull?

Myself, I prefer to set up the depth to true depth. After checking the depth sounder with a lead.
and Knowing the draw of my boat, there is no problem to have the depth under the keel.

I prefer this because the charts give the true depth and that the depth finder is also a navigation aid.

But this is personal. I know very experienced captains doing just the reverse.
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Old 06-05-2011, 07:37   #3
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Re: True Depth or Depth under the Hull?

Same decision as Alecadi for the same reasons. Knowing the actual depth is a useful navigation tool so keep the sounder reading actual depth.
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Old 06-05-2011, 08:05   #4
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Re: True Depth or Depth under the Hull?

Makes perfect sense. Thanks for the thoughts. I will make sure to ask during checkout how the depth finder is setup.
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Old 06-05-2011, 08:06   #5
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Re: True Depth or Depth under the Hull?

There is no rule here. You can set it however you want. My current sounder has no adjustment. My previous sounder had an adjustment which I set so that it read the keel depth above the bottom, no mental subtraction was necessary. For me how far my keel is off the bottom is the most important number.

Where I work there is roughly a higher high and lower low (semi-diurnal tides) 7 foot tidal range therefore the chart depth rarely matches the actual depth.
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Old 06-05-2011, 08:15   #6
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Re: True Depth or Depth under the Hull?

Yup, no standard "rule", but it sure is a good thing to know how the boat you're on is set up. It is more than just navigating, too. You need to know in order to set proper scope when anchoring, too. A 5 or 6 foot difference equates to around a 30 to 40 foot difference with a 6:1 scope. If you're in a relatively shallow anchorage with a big tidal range, you could easily find yourself with too little out.

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Old 06-05-2011, 08:20   #7
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Re: True Depth or Depth under the Hull?

I leave mine set to clearance rather than depth, and I calibrate it in fresh water rather than salt. That way, errors are always in my favor.
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Old 06-05-2011, 08:25   #8
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Re: True Depth or Depth under the Hull?

Quote:
Originally Posted by LifeJacket View Post
I am sure this post exposes my "noob-ness" but I couldn't find anything on here or google regarding how Depth Finders are generally setup. So I defer to those with much more experience.
It's totally up to you and the ease it is for you. True depth (under keel- good for shoals) and actual depth (from the air- good for anchoring) both need to be calibrated while the default is from the sender. If you forget and you have a 5' keel, this could be a problem if you're not situationally aware. Many people like to set it to 2' below the keel, but then again maybe you like to do the math.
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Old 06-05-2011, 08:46   #9
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Re: True Depth or Depth under the Hull?

I prefer mine set for true depth, with one caveat. We have just over 3' draft, (with the board up, 7' with it down), and our transducer is on the bottom of the hull, just above the 1' minikeel which houses the CB slot. On ours, as well as most depth sounders, If the transducer is closer than 2' from the bottom, it gives highly inaccurate readings... Like reading 15' when it is really 4' deep! In clear or partly clear water, I can see what is happening, but in really brown water, and with my preference for shallow anchorages, it can get me into trouble.

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Old 06-05-2011, 09:01   #10
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Re: True Depth or Depth under the Hull?

i just use mine as a reference, I touch bottom when it reads 3' which is actually 4.3' of water. I dont venture into areas where i read less than 5' unless its a protected area and the tide is rising.
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Old 06-05-2011, 09:59   #11
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Re: True Depth or Depth Under the Hull ?

I have mine set to depth under keel. While doing the math to figure clearance when set to true depth, it is one added distraction I would rather not have to deal with when going into shallows.
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Old 06-05-2011, 10:03   #12
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Re: True Depth or Depth under the Hull?

Quote:
Originally Posted by David M View Post
... For me how far my keel is off the bottom is the most important number ...
Paraphrasing & mangling Ron White*:
It’s not how far the keel is off the bottom - it's THAT the keel is off the bottom.

* "... It's not THAT the wind is blowing - it's WHAT the wind is blowing ..."
http://en.wikiquote.org/wiki/Ron_White
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Old 06-05-2011, 10:07   #13
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Re: True Depth or Depth Under the Hull ?

Quote:
Originally Posted by Astrid View Post
I have mine set to depth under keel. While doing the math to figure clearance when set to true depth, it is one added distraction I would rather not have to deal with when going into shallows.
You don't need to be a rocket scientist for "doing the math"
if your draft is 5 and if you're depth read 7 you're still good

start worrying when it gets at 6...

That's where I put the alarm depth on mine...

And always look at the chart to see on which depth you are going anyway...
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Old 06-05-2011, 10:25   #14
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Re: True Depth or Depth Under the Hull ?

I had meant to say, while doing the math to figure clearance is easy, it is one added distraction I would rather not have to deal with when going into shallows.
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Old 06-05-2011, 10:44   #15
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Re: True Depth or Depth Under the Hull ?

I set mine for 1 foot below the keel, that way if it says 0 I haven't bumped yet, then it's by brail. But I only draw 3'8"
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