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post #11 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

I've done that several times and gone back to removing the offset.
If you use your depth sounder for navigation, comparing it to the charts, an offset can be really confusing.
It's certainly easy enough to re-calibrate, so give it a try, but keep in mind your depth finder won't gybe with the charts once you've added an offset.
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post #12 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

There are two reasons that you might want to read the true depth of water:
1. To compare with charted depths, adjusted for tide. This is useful for confirming your position and for following depth contours;
2. Anchor scope calculations (you also need to add the freeboard) - although it doesn't need to be that precise.

You may be able to calculate the true depth, e.g. if you set your offset to read the depth below the (lowest) keel. You will not be able to if you don't know the depth of the transducer.

Roger
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post #13 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

Quote:
Originally Posted by capta View Post
It's certainly easy enough to re-calibrate, so give it a try, but keep in mind your depth finder won't gybe with the charts once you've added an offset.

There's only ONE time that a DS will agree with a chart anyway. When the tide is at MLW or MLLW depending on the chart datum.

One is ALWAYS required to do some math at ALL OTHER TIMES.

And don't forget, the bottom usually slopes, right? So all those nice fuzzy lines on the charts with numbers on them are merely RELATIVE and you're usually between the lines anyway.

I understand navigating with depth contours quite well, and have used it often. Yes, one MUST know the height of the tide and make those3 corrections. But please remember, those depths were charted years, decades and sometimes even eons ago.

That's why all navigation courses carefully note that navigation/piloting should use as many different methods for determining position as possible.

I have two areas of skinny water I encounter regularly. For both, I have determined over the years what the tide height MUST BE for me to transit those two spots. Think of it as a DS reading done backwards!

So, as long as YOU decide what YOU like and what works for YOU, have at it.

Stu Jackson, Catalina 34, 1986, M25 engine, Rocna 10 (22#), Maple Bay, BC, Canada

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post #14 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

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Originally Posted by Stu Jackson View Post
There's only ONE time that a DS will agree with a chart anyway. When the tide is at MLW or MLLW depending on the chart datum.
Ah, you cold water, big tide sailors just don't get it. I believe it was +0.1 foot today on datum, or thereabouts (who checks?).
As they used to say, "Come on down..." and win the prize (your hair will not fall out from worry). lol
Now Darwin or The NE Queensland coast; there's a tide of a way different range.

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post #15 of 16 Old 09-16-2015
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Personally I set my offset to read what's under the keel and set the depth alarm for 2 feet so I'll slow my approach as I often explore 'Gunk Holes' in the Chesapeake.
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post #16 of 16 Old 09-16-2015
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Re: Depth Sounder Offset

To avoid running aground, key is knowing the measured depth beneath your keel. That you know it is the important point, not how you know it.

You can't know it without knowing the offset. It can be zero, or some value, but if you know how your sounder is set up, you can figure it out.

Whatever best assists you in doing that is the right setting.

Having "grown up" in the Navy with fathometers that are set to indicate soundings (depth beneath the keel), and with using "yellow" (caution) and "red" (danger) SOUNDINGS that are established based on MINIMUM EXPECTED SOUNDINGS (not water depth) beneath the keel, my sounder is set to indicate depth beneath the keel.

That's what I am comfortable with. YMMV.

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