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Old 05-10-2017, 10:36 PM   #1
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keel offset used for depth sounder?

Recently bought a 2000 model year Grand Banks 42. I'm used to a depth sounder where the transducer offset is factored in (how far transducer is located below the waterline) so that the reading on the depth gauge is pretty much what the actual water depth is. I've found this very helpful in using charts - matching gauge water depth to charted depth as another check on my location. I know what my boats draw and it's easy mentally to know if I'm about to touch bottom.

This GB 42 is set up so depth gauge reads exactly what's under the transducer (no offset factored in). Spooked me first time I turned it on and saw 1.5 feet in the slip until I realized what was up.

So my question is - who out there adds in the keel offset (shows depth from surface) and who doesn't (shows depth from transducer location)?
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Old 05-10-2017, 10:43 PM   #2
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Third option, set the offset so the depth sounder shows depth from the bottom of the keel. This is what I do currently.

There are plenty of arguments for different ways of doing it and for a long time I had sounder adjusted to show actual water depth. I finally decided to just have it read water under the keel since that is what I really am interested in. In my area, with large daily tide variations and very infrequent depth updates, there is no way that the 4.5 foot difference is going to help with navigation.
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Old 05-10-2017, 11:00 PM   #3
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Mine are set for water under the keel.
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Old 05-10-2017, 11:26 PM   #4
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Mine is depth under keel.
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Old 05-11-2017, 06:14 AM   #5
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I agree that is what I have too, water under the keel
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Old 05-11-2017, 06:16 AM   #6
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I use actual water depth.

The alarm tells me when the bottom is too close....but of course any way it is set up, you still are doing a little math in your head. Whether deciding on navigational depth from the surface, your keel, or your sounder.....so just pick what you like.
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Old 05-11-2017, 06:35 AM   #7
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+1 water under the keel. Cruising often in very shallow water here and I want to know exactly what is left below my keel.

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Old 05-11-2017, 06:40 AM   #8
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My old, really old but good, Furuno sounder will only give depth under the transducer.
Its 1m below surface and 0.4m above the keel. But thats fine, on the Nav display on the PC I can adjust and display in large letters depth from surface.

We have, in common with a lot of tropical waters, lots of shallow water. Charts have 5m and 2m contours, so using depth of water makes sense for me.
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Old 05-11-2017, 06:54 AM   #9
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Under the keel for me too. I know I'm really shallow when the depth finder goes from 1' to 18' or more.
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Old 05-11-2017, 07:55 AM   #10
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Quote:
Originally Posted by FBoykin View Post
Recently bought a 2000 model year Grand Banks 42. I'm used to a depth sounder where the transducer offset is factored in (how far transducer is located below the waterline) so that the reading on the depth gauge is pretty much what the actual water depth is. I've found this very helpful in using charts - matching gauge water depth to charted depth as another check on my location. I know what my boats draw and it's easy mentally to know if I'm about to touch bottom.

This GB 42 is set up so depth gauge reads exactly what's under the transducer (no offset factored in). Spooked me first time I turned it on and saw 1.5 feet in the slip until I realized what was up.

So my question is - who out there adds in the keel offset (shows depth from surface) and who doesn't (shows depth from transducer location)?
Seems like setting it up to what you are accustomed to would make the most sense. Why force yourself into doing things differently?

To answer the question, I prefer setting the offset such that actual water depth is displayed for the same reason you mentioned but understand why others do it differently.
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Old 05-11-2017, 08:19 AM   #11
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There is also the option of using arithmetic when necessary with an offset of zero.
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Old 05-11-2017, 08:32 AM   #12
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I think it's best if you set it up the way you are used to it. Less chance of error at just the wrong time.
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Old 05-11-2017, 08:45 AM   #13
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Set to show the depth from the lowest point under the boat.

Which may be the keel, rudders or props depending on the boat.
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Old 05-11-2017, 09:20 AM   #14
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I always prefer depth under my keel.
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Old 05-11-2017, 09:38 AM   #15
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I want to know the depth below the keel.
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Old 05-11-2017, 09:40 AM   #16
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Quote:
Originally Posted by kev_rm View Post
There is also the option of using arithmetic when necessary with an offset of zero.
The problem with that is unless you know exactly how deep the transducer is mounted below the boat you don't truly know what the depth is.

And if you're going to go to the trouble of measuring that, you might as well measure your exact draft and either set the correct offset or set it to show true depth and skip the need to do the math in your head.
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Old 05-11-2017, 09:48 AM   #17
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Ultimately...everyone, no matter what method you use.... makes a conscious decision when considering depth....


We are all calculating a tiny bit or concentrating on what to do when whatever number shows up on the depthsounder is less than you want it to be.

is the usual look at that point.....
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Old 05-11-2017, 10:06 AM   #18
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Quote:
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I use actual water depth.

.
Ditto...
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Old 05-11-2017, 10:12 AM   #19
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I'm a bit surprise to see how many people report depth below keel. I'm not sure why I'm surprised other than perhaps to find myself in such a minority.

I report depth of water.
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Old 05-11-2017, 10:19 AM   #20
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Depth of water.
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