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post #1 of 16 Old 09-14-2015 Thread Starter
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Depth Sounder Offset

I have a Southerly 110 - It's a variable draft boat - 2'4" to 7'2".

The depth sounder is about 6" above the bottom of the boat and, currently, it reads from its location.

I think I want to offset the depth sounder by 6" so it reads from the bottom of the boat. So, if I'm reading 3' I know I have 3' under the bottom of the boat regardless of the keel position.

Does this make sense? What problems might I get myself into if I read depth from the bottom of the boat?
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post #2 of 16 Old 09-15-2015
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I don't see any negative effects if the transducer is on the bottom of the hull or anywhere else for that matter. Speaking with common sense of course.
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post #3 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

If I understand what you're saying you want to show depth below the hull.. but not offset at all for the extra keel draft?

Will that give you and indication of 'clearance' when the keel is fully raised? If so then you'll get a valid reading down to just barely floating, and you'll have to keep in mind the extra 5 feet you need when the keel is down. If you offset for the deep keel setting you'll be reading '0.0' with the keel raised in less than 5 feet of clearance and not really know where you are depth-wise. Either way you need to be aware of the differences as to keel position. Since you're in a shallow draft area offsetting to the hull bottom/minimum draft probably makes sense.

I might add another 6" to give you a bit of a 'err on the side of caution' factor...

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
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post #4 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

Keel offset is typically adjusted so that the keel (at its lowest 'down' position) is at 'zero' reading on the depth sounder when the boat is 'just touching' the bottom. Another way to state keel offset: Mathematically it makes the depth sounder 'appear' to be mounted at the very bottom of the keel (when fully 'down').
In this way you wont have to be always mentally calculating a 'go/no-go' when it comes to depth readings as a 'zero" reading means that youll be touching/hitting the bottom when your keel is all the way down.

Additionally you can make the setting of depth where the depth alarm comes on / activates ... and at any depth you wish, in addition to the keel offset. I personally use an additional 3ft. of water below my keel for when the alarm activates .... and I sail a lot in water that may be 6" or less below my keel (Bahamas, etc.). In so doing I only have to 'mentally calculate' when that depth alarm is sounding.

Last edited by RichH; 09-15-2015 at 12:14 AM.
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post #5 of 16 Old 09-15-2015 Thread Starter
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Re: Depth Sounder Offset

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Originally Posted by Faster View Post
I might add another 6" to give you a bit of a 'err on the side of caution' factor...
I like this idea.

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Originally Posted by RichH View Post
Keel offset is typically adjusted so that the keel (at its lowest 'down' position) is at 'zero' reading on the depth sounder when the boat is 'just touching' the bottom.
Right. But that means I'd be doing "negative" math if the keel was all the way up. The other "issue" is there is no absolute keel depth. I have a "keel up" and "keel down" button and 6 LED lights. It's a guess as to the actual depth.

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In this way you wont have to be always mentally calculating a 'go/no-go' when it comes to depth readings as a 'zero" reading means that youll be touching/hitting the bottom when your keel is all the way down.
I understand what you are saying... And with a fixed keel, I'd definitely agree.

I'm hoping it's just a matter of preference and I can use depth with keel up and not get myself into any trouble.
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post #6 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

For what its worth, we like to set the offset so it reads actual water depth from the surface. I've done this on every boat I've owned. Takes away the confusion for us. Then the depth should be approximately equal to what your chart says (at least at mean low), so it's useful for navigation, and with a center boarder, you can do simple math. If I'm drawing 6 feet, and my sounder says 6 feet, I'm on the bottom.

YMMV.
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post #7 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

I'm with capecodda. I like to know the actual depth. Most helpful when anchoring and wanting to know how much anchor line to let out.
SHNOOL and ChristinaM like this.
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post #8 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

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Originally Posted by JimsCAL View Post
I'm with capecodda. I like to know the actual depth. Most helpful when anchoring and wanting to know how much anchor line to let out.
With variable draft, this is what I'd do. You will have to do a comparison against the know drafts for your boat anyway, and I'd find it easier to remember the actual draft than have to know them from the bottom of the hull.

You still need to add the freeboard for anchor scope calculations though.
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post #9 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

I also have a swing keel boat with draft variable from ~2' to ~6'. Now into my 20th year with this boat, I am comfortable reading actual depth and allowing for the amount of keel out to deal with shallow water. As the OP understands, variable draft allows us to go where we can get into trouble, since chart soundings in very shallow water are not very reliable--at least in my experience.

Apparently the Southerly does not allow for determing draft precisely with the keel in an intermediate position. In my case, the keel pennant is visible as it emerges from the coachroof enroute to the electric winch that hauls it. I've marked my pennant to identify actual keel draft in 1' increments, which is best done when your boat is in the travelift.

So, I am comfortable reading actual depth to provide an unambiguous reference to chart data. BTW, I tried setting my chartplotter to magnetic north up, but quickly went back to true north up, because that's how printed charts are set up.
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post #10 of 16 Old 09-15-2015
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Re: Depth Sounder Offset

It simply doesn't matter. All this recurring discussion about keel offsets always confuses me. Sure, it'd be "nice" to know that your depth sounder reads exactly what your keel depth is, but is quite meaningless.

When we got our boat, I found a patch of skinny water with a mud bottom, and slowly drove the boat into the shallows. When we hit the bottom I saw that the DS read 4.4.

That's all I need to know.

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