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Old 19-06-2012, 12:17   #1
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how do you compensate your depth gauge readings

considering that you sounder is sitting way below the water it is not telling you the depth from surface water to bottom, just reading from hull to bottom.

I have a nice color eagle EAGLE FISH EASY 320C which works well. I attached it inside the hull inside a contoured PVC pipe filled with toilet bowl wax. Depth read fine but temp reads low, I think.

My boat draws 3.5 feet and the keel is a foot below the sounder, so I have to think about the depth clearance too much. I would like it to read so if it reads 3.5 feet, then the keel would be touching. The unit has a lot of settings is this adjustable? Does anyone else have any thoughts?

ok found it on page 80 keel offset
http://www.eaglenav.com/upload/Eagle...781_092204.pdf
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Old 19-06-2012, 12:53   #2
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Re: how do you compensate your depth gauge readings

The keel offset can be negative or positive.
So you can set the depth sounder to read the depth of water, or the depth below the keel. There are sometimes heated discussions about which is best, I don't think it matters much as long as you know how it set up on any boat you sail.
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Old 19-06-2012, 18:07   #3
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Quote:
Originally Posted by sdowney717
considering that you sounder is sitting way below the water it is not telling you the depth from surface water to bottom, just reading from hull to bottom.

I have a nice color eagle EAGLE FISH EASY 320C which works well. I attached it inside the hull inside a contoured PVC pipe filled with toilet bowl wax. Depth read fine but temp reads low, I think.

My boat draws 3.5 feet and the keel is a foot below the sounder, so I have to think about the depth clearance too much. I would like it to read so if it reads 3.5 feet, then the keel would be touching. The unit has a lot of settings is this adjustable? Does anyone else have any thoughts?

ok found it on page 80 keel offset
http://www.eaglenav.com/upload/Eagle...781_092204.pdf
My keel is 1.3 meters. My depth guage reads 1.3 meters when the keel touches the bottom. My shallow alarm is set at 3 meters.

This way if I am sitting at the dock with 1.4 meters, load a bunch o crap on the boat that displaces it 0.1 meter the boat touches the bottom and the gauge reads 1.3. Makes sense to me.
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Old 19-06-2012, 18:18   #4
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Re: how do you compensate your depth gauge readings

Long discussion here:

True Depth or Depth Under the Hull ?
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Old 19-06-2012, 18:18   #5
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Re: how do you compensate your depth gauge readings

I don't adjust for depth--I adjust for clearance. In other words, my boat shows 0.0 when it touches bottom, but it will do that at a depth of 7FFW.

Note that I calibrated by boat in fresh water rather than salt. That's because it sits lower there than in salt. Since I take the boat up to the Delta every year, and since that's where it's most likely to touch mud.....
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Old 20-06-2012, 01:38   #6
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Re: how do you compensate your depth gauge readings

Quote:
Originally Posted by Ex-Calif View Post
My keel is 1.3 meters. My depth guage reads 1.3 meters when the keel touches the bottom. My shallow alarm is set at 3 meters.

This way if I am sitting at the dock with 1.4 meters, load a bunch o crap on the boat that displaces it 0.1 meter the boat touches the bottom and the gauge reads 1.3. Makes sense to me.
It does not any difference if the transducer is referenced to the waterline or keel depth.
Changing the displacement will not effect the reading when you run aground.
The extra weight sinks the transducer as much as it sinks the keel, the reference point is the transducer.
In the above example if retrenched to the keel. The depth reading would be 0.1, when the supplies were loaded it would drop to 0 and the keel would be touching.

In practice these sort of depth changes are less than the error in the reading and the adjustability of calibration.


Quote:
Originally Posted by Bash View Post

Note that I calibrated by boat in fresh water rather than salt. That's because it sits lower there than in salt. Since I take the boat up to the Delta every year, and since that's where it's most likely to touch mud.....
As above fresh and salt water calibration will not make any difference as the boat sinks further in fresh water, but so does the transducer. ( the very slight speed difference of sound in fresh or salt is also not a factor at keel depth)
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Old 20-06-2012, 01:50   #7
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Re: how do you compensate your depth gauge readings

Quote:
Originally Posted by Bash View Post
I don't adjust for depth--I adjust for clearance. In other words, my boat shows 0.0 when it touches bottom, but it will do that at a depth of 7FFW.

Note that I calibrated by boat in fresh water rather than salt. That's because it sits lower there than in salt. Since I take the boat up to the Delta every year, and since that's where it's most likely to touch mud.....
+1

I find this easier.

Using your depth sounder, for various purposes you might be wanting to know (a) how much clearance you have under your keel; and/or (b) what is the absolute depth of the water. So to get the information you want, you're either going to have to add your keel depth -- to know depth of the water -- or you're going to have to subtract your keel depth -- to know the clearance.

Like Bash, I prefer the instant reading of clearance. All the more because I sail in an area with a large tidal range, so anyway I wouldn't be getting an instant reading of the absolute water depth -- you still have to dig up the rise of tide at that moment in your particular spot. But in any case -- the most urgent data is always going to be how much margin you have before hitting the ground.

I have two sources of depth data -- I also have an Echopilot forward looking sonar unit. To the confusion of many new helmsmen on my boat, I keep the Echopilot set to show not clearance, but depth of the water. So I have either datum at a glance.
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Old 20-06-2012, 05:35   #8
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Quote:
Originally Posted by noelex 77

It does not any difference if the transducer is referenced to the waterline or keel depth.
Changing the displacement will not effect the reading when you run aground.
The extra weight sinks the transducer as much as it sinks the keel, the reference point is the transducer.
In the above example if retrenched to the keel. The depth reading would be 0.1, when the supplies were loaded it would drop to 0 and the keel would be touching.

In practice these sort of depth changes are less than the error in the reading and the adjustability of calibration.

As above fresh and salt water calibration will not make any difference as the boat sinks further in fresh water, but so does the transducer. ( the very slight speed difference of sound in fresh or salt is also not a factor at keel depth)
Maybe we are saying the same thing but it actually does matter. The distance from transducer to keel bottom is fixed. The distance from waterline to transducer is variable based on load.

Maybe I wasn't clear. My guage is offset by the fixed distance from transducer to keel bottom.

The transducer always measures depth under the transducer. As you load the boat the distance to keel bottom stays the same, the distance from transducer to bottom reduces and the distance from transducer to waterline increases.

I do agree that for most boats this is in the margin of calibration error.

In my method, as opposed to the zero method to deterrmine "water depth" I have to mentally add about 2 feet, approximate distance from transducer to waterline. But others say, clearance is more important than a completely accurate depth.
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Old 20-06-2012, 05:54   #9
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Re: how do you compensate your depth gauge readings

Either way works be it offset to measure the depth beneath the keel, the depth of water or for that matter the depth from the transducer. I find it easier to set the offset to measure the true depth of the water. That way I can easily reference the chart and I hope that I can remember the draft of the boat.
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Old 20-06-2012, 06:57   #10
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Re: how do you compensate your depth gauge readings

Quote:
Originally Posted by lancelot9898 View Post
Either way works be it offset to measure the depth beneath the keel, the depth of water or for that matter the depth from the transducer. I find it easier to set the offset to measure the true depth of the water. That way I can easily reference the chart and I hope that I can remember the draft of the boat.
I used to setup mine for clearance. But now I set it up for actual water depth. The reason is when charting, or relaying depths to other boats, they don't know your keel legnth.

It leads to a lot of uneccessary confusion when three boats running side by side, one says they are reading 2 ft, and the other 2 are reading 7, and all three have keels between 4 and 7 ft.
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Old 20-06-2012, 07:05   #11
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Re: how do you compensate your depth gauge readings

Make your life easier, throw away that expensive depth sounder and buy a cheap one that can't be calibrated.
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Old 20-06-2012, 07:19   #12
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Re: how do you compensate your depth gauge readings

I always set my depth to actual. I have a hand held depth sounder that I use to double check that all is well.

Hawkeye Handheld H22PX
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Old 20-06-2012, 07:39   #13
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Re: how do you compensate your depth gauge readings

You learn some thing new every day,

My depth sounder was 2 feet out every where I went according to the GPS and my charts,

It didnt bother me as it was accurate, to both. It was just 2 feet deeper every where than was displayed,

Its just been calibrated to water level, Not the hull bottom, I have a 2 foot draught,
This is not a good thing, I was in 3 feet of water in the dark, heading for a man made breakwater, Uncharted as well, In Fiji,
which meant I had one foot of water under my hulls,

I will make sure my new one is calibrated to the bottom of the hulls when it is installed,

Thanks,
Brian,
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Old 20-06-2012, 08:22   #14
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Re: how do you compensate your depth gauge readings

In my head....
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Old 20-06-2012, 10:14   #15
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Re: how do you compensate your depth gauge readings

Quote:
Originally Posted by sdowney717 View Post
considering that you sounder is sitting way below the water it is not telling you the depth from surface water to bottom, just reading from hull to bottom.

.......? Does anyone else have any thoughts?.......
There are only four possibilities.
1. Offset for for hull clearance
2. Offset for true water depth
3. No offset, just measure form transducer to bottom
4. Offset for some other arbitrary point.

No. 4 is pretty useless for most of us most of the time.
No. 3 requires mental arithmetic every time to get the really useful information.
No.'s 2 & 1 are both useful but usually one is more useful than the other relative to a particular boater or situation.

So choose whether 1 or 2 is more useful to you and then do the mental arithmetic when you need the other.
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