Where exactly do you calibrate your echo-sounder to?

[Babylon]

Following on from a previous thread, I'm interested to know whether the majority of people calibrate their echo-sounder to the waterline or to under the keel? And, if under the keel, do you further complicate matters by adding an extra safety margin of say 0.2m to the calibration.

We're currently calibrated to under the keel plus a small margin, as this is how the thing was set when we bought her, and I've got used to reading how much water there is between seabed and fibreglass. I'd like to recalibrate to the waterline so that I've one less calculation to do when working out tidal heights, but I'm worried I'd forget that we've changed and run the ship aground!

Babs

 

[Twister_Ken]

Mine is under the keel, with no safety allowance. At 0.0 we stop.

 

[Boomshanka]

Water under the keel (with no additional 'adjustments') is my preference. Then set the depth alarm for 0.5m!!

 

[nct1]

Mine is under the keel, concentrates the mind no end

 

[FlyingSpud]

Water under the keel, but I add 0.2, which I then try to forget is there.

It always strikes me that, say, 20cm = less than 8 inches in old money, and that could just be a small bump on the bottom.

I can’t do the maths, but if the bottom is shelving at 20 degrees,, say, just how far extra do you go until 0.2cm has been lost?

 

[fireball]

I'm another one for "under the keel" ... no adjustments though - what would be the point of adding on a "safety margin" to find you need to go into that safety margin, then you have no idea if you're gonna hit (raymarine depth doesn't go negative) ...
I know the "purists" advise water depth for ease of navigation, but tbh with 2 chartplotters on deck I don't often use depth for (accurate) navigation - and it doesn't take a second to add the depth and draft ... whereas a quick glance wil tell me if I have got water under the keel (or not!)

 

[michael_w]

Waterline. Easier for thikkos like me to do the sums.

 

[CSFenwick]

Waterline - so when you do a tidal height calculation your sounder matches the answer (you hope!). If it's from your keel then you're forever taking off 1.x metres to see if it's right.

 

[Georgio]

Negative Keel offset, 0.0 and we are aground - tried and tested.

 

[FullCircle]

As a lift keel boat with an infinite variation on our draught, I use total depth of water.
Keel up it still varies. 1.1 lightly loaded, 1.3 everything full and 5 of us on it.
I have a large label plastered across the top of the depth sounder saying TOTAL DEPTH

 

[Cantata]

Depth under the keel. No alarms set, it would be going off all the time where we sail....

 

[BelleSerene]

If you're simply racing and don't need to do any navigation while you're at it, by all means set your sounder to read depth under the keel. Then at 0.0 you stop. Nice and easy.

If you need to do anything more complex, you need to know the depth of water. With your depth sounder calibrated to the bottom of the keel, you have an amount eg 1.8m to add or subtract from all your calculations. With your depth sounder calibrated to the waterline, you make tidal calculations, following a depth contour etc much easier.

Ditto if you're cruising. Then it seems to me there's no excuse for anyone who can read a chart to set the sounder to keel depth: when the sounder gets close to your keel depth (ours is 1.8m) you know you're running dry.

Regarding the safety offset of eg 20cm, I've always taken the view that I want to know the truth and then I'll make the judgment of how far to push it. Anyway, if you know you have 20cm to spare you're just going to push your luck 20cm further, aren't you? A more effective safety system is a depth alarm so you know (if you didn't already!) when you're entering dodgy territory and then you can call the shots by looking at the depth gauge.

 

[snowleopard]

Probably worth putting this one up as a poll.

FWIW mine is set to depth of water.

 

[sailorman]

00.0 = Zero = no maths to do

 

[TamarMike]

My NASA won't calibrate to waterline unfortunately. If it could I would, but as I don't trust it when depth is critical I use the lead line anyway.

 

[johnalison]

I must be the only person in the country who leaves it at depth-under-transducer. There are several reasons for this. First is laziness; I can't be bothered to change it. Secondly, sailing with only 0.5m showing on the dial gives me the willies, though I'm happy to sail in 2m of water. Thirdly, I've been sailing for yonks with sounders that can't be adjusted and am just getting too old to adjust to changes.

 

[Deleted member 36384]

It doesn't matter, just know if there is an offset and what it is. I wouldn't bother with a safety margin, should you set it to depth below keel, as it is redundant functionality.

Whatever you do make sure you know; previous comment on having a note fixed to the sounder stating reference datum is a good idea and I will adopt that - thanks for the idea.

Once you do set the offset, check it with a lead line when you are anchored or moored over a firm bottom. If nothing else it will provide confidence that the sounder is reading correctly.

The old adage "measure twice; cut once" springs to mind.

 

[bluebosunsmate]

Waterline!
Using keel then a safety margine is asking for trouble. Going over a rock or a bar in a chop!
You know what your keel depth is so add that to your safety margine then that translates directly to your charted depth!

 

[bobgoode]

Depth under keel...and I know when she reads 00.00 I have just touched!!

 

[Refueler]

I must be the only person in the country who leaves it at depth-under-transducer. There are several reasons for this. First is laziness; I can't be bothered to change it. Secondly, sailing with only 0.5m showing on the dial gives me the willies, though I'm happy to sail in 2m of water. Thirdly, I've been sailing for yonks with sounders that can't be adjusted and am just getting too old to adjust to changes.

I'm in Baltic so I cannot join your exclusive club ! But mine reads under transducer.

I did try to get it to read total depth, but then I started getting errors, then I tried under keel (which I have to say - I don't agree with) - but that wasn't possible ... too great an offset.

So I set it back to zero-offset which means that at 0.7m reading I touch ... my normal draft is 0.95m ... so I know to add 0.25m to any reading.

OK - lets actually look at true picture though ... tidal height is at medium nominal barometric pressure, no rain run-off or excessive water spill, without wind surge levels. In fact tidal level as calculated is rarely correct to such an accuracy for simple reason weather coniditions have significant effect on it.
So really does it make that much difference ? 25 ... 30cms offset ? Just wind or barometric alone can drive a level +/- 40cms or more at times.

Don't believe me ? Not my problem !! But I can say that when I see a 10m, 5m line on chart and I cross it ... my Echo-sounder is near enough to give me a rough idea of location !