EchoSounder Offset

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ash
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EchoSounder Offset

Postby ash » Wed Jul 16, 2008 10:00 pm

What depth do you show on your echo sounder, depth of water or depth under the keel?

I've just got round to replacing my Seafarer 501, and my Stowe Navigator 1 with a Nasamarine Clipper Duet but found that the offset only works one way. You can add offset so that depth shown is depth under the keel but you can't have 'negative' offset so that depth is water depth. (I suppose this is a disadvantage of buying relatively cheaply)

I don't suppose that I should worry too much, with the Seafarer I was just reading to the nearest metre and if you got a complete circle of light then you were just about to touch bottom.

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little boy blue
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Postby little boy blue » Wed Jul 16, 2008 11:22 pm

depth under the keel.

i`m not sure if you could get negative offset unless you used a transom mounted transducer just at the water surface. however my experience has been with cheap sounders as well so hopefully the better heeled forumites in our midst can enlighten us :)

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Postby Allegro » Thu Jul 17, 2008 10:22 am

Depth under the sounder, no offset applied. I know (from experience!) that we go aground at about 0.9m on the sounder. We draw 1.2m - for any other use of the sounder (how much anchor chain to put out, for example) I tend to assume I'm seeing water depth and ignore the 0.3m error as being insignificant.

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moodysailor
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Postby moodysailor » Thu Jul 17, 2008 10:36 am

I use depth under the keel - with a 1.7m fin keel on the East coast I need to work to small depths at times and don't want to be doing arithmetic.
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Postby ash » Thu Jul 17, 2008 11:51 am

moodysailor wrote:I use depth under the keel - with a 1.7m fin keel on the East coast I need to work to small depths at times and don't want to be doing arithmetic.


That sounds right, calculating height of tide or anchor chain scope should be a more relaxed affair.

I seem to remember reading of one disadvantage of using depth under keel. If you are slightly inaccurate with the setting, and find yourself still afloat with the reading at zero, then you don't know if the situation is getting worse as it won't show negative figures.

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Allegro
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Postby Allegro » Thu Jul 17, 2008 1:11 pm

ash wrote:
moodysailor wrote:I use depth under the keel - with a 1.7m fin keel on the East coast I need to work to small depths at times and don't want to be doing arithmetic.


That sounds right, calculating height of tide or anchor chain scope should be a more relaxed affair.


Well, yes, but when you're wanting to know whether or not you're about to go aground you don't do arithmetic anyway, do you, so offset is irrelevant provided you know the numbers involved. For example, if my depth sounder's reading 1.5m I know I should be moving really slowly, and if its reading 1.0 I should probably be in reverse! I don't need to do a sum every time because those key numbers are always the same.

So I think "Offset or not" is a red herring, really, the answer is "Whatever, provided you know!"

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Postby Olivepage » Thu Jul 17, 2008 11:01 pm

" If you are slightly inaccurate with the setting, and find yourself still afloat with the reading at zero, then you don't know if the situation is getting worse as it won't show negative figures."

Don't follow the logic of this

If the dial says 2m and you draw 2m then you should be aground but aren't you still don't know when you will touch - until you do.

In both cases you know you're very close to being in the sticky stuff and should do something different.

Of course - you don't have these problems with a lead line.

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Postby sahona » Fri Jul 18, 2008 11:35 am

Smugly... I can set it both ways, but use depth under the keel. This was set on our drying grid by running the instrument before we floated off, and offsetting to zero. HOWEVER, echosounders can be erratic depending on the reflective quality of the bottom, and there may be big stones in the mud... Unfortunately, I can't set the shallow alarm to anything lower than 3 feet. I think the guys that buy fishfinders ( 'cos they're almost as cheap) may have the right idea. We regularly get less than 0.8 feet registering in our bit of Troon marina.
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Postby aquaplane » Fri Jul 18, 2008 3:40 pm

My solid state depth sounder is fixed at 1.1 meters, with the keel down.

When the alarm goes off, I wind the keel up a bit and look over the side to see which way to go to get out of the situation.

The alarm doesn't go off often and I'm always going dead slow if it does.

Anchoring in tidal waters can be fun. I have to do a sounding by dropping the anchor slow when I think I'm close enough in. Then let enough chain/warp out accordingly.
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ash
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Postby ash » Mon Jul 21, 2008 12:22 am

Olivepage wrote:Don't follow the logic of this

If the dial says 2m and you draw 2m then you should be aground but aren't you still don't know when you will touch - until you do.


But you would know if it's still 2m or if it's dropping to 1.9, 1.8, etc.

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Reading "OUT"

Postby ash » Mon Jul 21, 2008 12:41 am

If I can be allowed a slight thread drift.

Tested the Duet on Saturday.

In deep water it spends a lot of time flashing OUT which I can understand if it can't get a return from a soft bottom.

Up to 50M, it shows depth for most of the time.

Between 50 and 100M it shows OUT for the majority of the time, but will show depth. The deepest that I noted was 90 something.

The bit that I find concerning is at over 100M it shows OUT most of the time, but when it does show a figure it's very low - varies but less than 2M.

I suppose that the novelty will wear off, and I'll only look at it when the chart says I need to. It's a bit of a pain as my end end of the Loch is very deep, and because it's a combined unit it's always going to be powered up. I'll need to persuade the first mate that she needs to ignore the reading unless it's steady.

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More Fred Drift

Postby Nick » Mon Jul 21, 2008 1:05 am

The bit that I find concerning is at over 100M it shows OUT most of the time, but when it does show a figure it's very low - varies but less than 2M.

I think this is pretty standard.

Drifting away, we are still having no luck getting our NASA target to read when the engine revs are above tickover, although it is perfect under sail. We have ferrite rings on the main power output cable from the alternator and on the power feed to the instrument.

Is it time to try a new unit?
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Postby Olivepage » Mon Jul 21, 2008 7:57 am

"But you would know if it's still 2m or if it's dropping to 1.9, 1.8, etc."

Oh thats absolutely true.

But the question is what would I do about it. Trusting an electrickery machine to measure 4inch (0.1M) accurately isn't it.

I would be looking at doing something different - like reversing away and waiting for a bit more tide.

Suppose it depends where you are to an extent, I guess the Essex and Suffolk boys are used to having 3inch of water under them but in my neck of the woods grovelling about in 2 ft of water isn't something you do very often so I tend to go for a wide margin.

And besides SWMBO gets really annoyed if I go aground, she just will not accept that its her fault.

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ash
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Re: More Fred Drift

Postby ash » Mon Jul 21, 2008 9:19 pm

Nick wrote:Drifting away, we are still having no luck getting our NASA target to read when the engine revs are above tickover, although it is perfect under sail. We have ferrite rings on the main power output cable from the alternator and on the power feed to the instrument.

Is it time to try a new unit?


It would be handy if you could borrow one as a test.

Does anything else show signs of interference? VHF, domestic radio.

Can you temporarily feed the Nasa Target from a battery which isn't connected to the alternator? To see if the interference is coming through the 12V supply or the transducer cable.

FWIW - I have the standard 40A alternator, no fancy charging regimes, O1B2 switch on the port side of the box above the engine, both batteries in the bilge, fuses and neutral block in the box above the engine. The transducer is in an oilbath under the Vee berth, starboard side, and the cable runs through the heads and under the starboard bunk. My Nasa is on the starboard bulkhead.

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Postby Olivepage » Mon Jul 21, 2008 11:50 pm

"Drifting away, we are still having no luck getting our NASA target to read when the engine revs are above tickover, although it is perfect under sail. We have ferrite rings on the main power output cable from the alternator and on the power feed to the instrument. "

Sounds unusual, do you have any clever charging or regulation fed from the alternator?

Have tou tried a ferrite bead or perhaps better a ferrite ring with a few turns around it on the lead to the transducer.

Trouble with buying another sounder is that if you don't know the cause it may well affect the new toy.
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