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  #1  
Old 07-18-2012
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Checking depth sounder calibration (Clipper Cove silting question too)

On Sunday entering Clipper Cove the depth finder was down to 1.9 feet at one point. The charted depth there is more like 12 ft (we kept close to the pier at the north east corner, as you are supposed to). The tide at the time should have been +3 ft.

Is it silting up that badly?

I want to check the calibration of my depth finder. It's a newfangled NMEA 2000 one with adjustable offset via the display. I've thought of using a sounding line with a weight, while in the slip. Has anyone else done this?
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

yep. You should do it that way at least once! Who knows if it was ever set for keel offset when installed, or setup correctly when installed.

I was just on a large megayacht type boat up in Alaska, I put a crab pot off the stern and it sank 5' and stopped. So I told the owner 'um there's only 5' of water under the stern"

He replied "really! The sounder said there was 12' below the keel"

(which begs the question of Why the F did he anchor in 12' of water with a 7' draft in Alaska with a 10' tide... but I digress)

ANYWAY, since we were hard aground and I had nothing better to do I measured and the deph sounder was dead accurate, 12' of water, but it was actual depth not 'under the keel' as the PO or broker had told the owner.

Of course; SEVENTY feet closer to shore at the stern it was a little shallower
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

I installed the transducer and networked it via NMEA 2000 to the chartplotter and a Garmin GMI 10 display.

I guesstimated the offset and programmed that, also adding a couple of feet for safety. I used 5 ft for the offset, and the transducer is 3 to 4 ft above the bottom of the keel. So it should be within a foot, two at the most. But still, I should check it I think.
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

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Originally Posted by MarkSF View Post
I installed the transducer and networked it via NMEA 2000 to the chartplotter and a Garmin GMI 10 display.

I guesstimated the offset and programmed that, also adding a couple of feet for safety. I used 5 ft for the offset, and the transducer is 3 to 4 ft above the bottom of the keel. So it should be within a foot, two at the most. But still, I should check it I think.
You will be done with the testing faster done than writing these messages here
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

But I'm at work so can write lots of speculative messages, can't do the actual test. Plus this thread took me over 1000 posts (joking)
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

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But I'm at work so can write lots of speculative messages, can't do the actual test. Plus this thread took me over 1000 posts (joking)
congrats!
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

Thank you, thank you, 1005 now.
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

I have never bothered with that offset nonsense. I just drove my boat up to some thick mud bottom and when it touched I was aground, read the depthsounder: 4.4 So, if it nears 4.4 I'm going aground. No math involved.

Clipper Cove is NOT silting up. If you didn't run aground, it doesn't matter what your sounder read.

My 25 years of experience indicates that for a yacht with a nominal 6 foot draft, one needs zero water (in the tide table) or higher to get in or out. Simple.
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

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Originally Posted by Stu Jackson View Post
I have never bothered with that offset nonsense. I just drove my boat up to some thick mud bottom and when it touched I was aground, read the depthsounder: 4.4 So, if it nears 4.4 I'm going aground. No math involved.

Clipper Cove is NOT silting up. If you didn't run aground, it doesn't matter what your sounder read.

My 25 years of experience indicates that for a yacht with a nominal 6 foot draft, one needs zero water (in the tide table) or higher to get in or out. Simple.
Do you happen to know anything about the wreckk in the south west corner? One of my charts has it, the other doesn't. The one that does says wk, 1ft depth.
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Re: Checking depth sounder calibration (Clipper Cove silting question too)

I just measured mine last weekend with a weight and some line. Was off about 2' which is approximately how far the transducer is below WL.

I like mine to read the exact depth. There are anchorages here in Maine where it is not unusual to have a foot below the keel at low. Sometimes we have to do a little math to see if we are anchoring in enough water when the tide goes out.

"(which begs the question of Why the F did he anchor in 12' of water with a 7' draft in Alaska with a 10' tide... but I digress)"
Xymotic, I do this all the time. But only when the tide is low. ;-)
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