Do you calibrate you depth sensor to water level or keel?

[David B]

Originally Posted by AuroraGH

Got a new plotter, and had to replace the depth transducer since it had become a random number generator.

All working but my plotter has a calibration setting for "offset" which the manual says I could set for water level (sensor is 2 ft down) or depth of keel (4 ft below sensor).

What did you all choose to use?

Keel - absolutely!

I want to know that when it reads 0.1 of a metre, that's what we have under us. If for interest sake I want to know the actual depth, it's not hard to add 2.1 metres to the reading, but really for me, the depth instrument is there primarily to let me know if we can get into a shallow area or not.

Anchoring is after we get there, and navigation is before we choose to go there.

[Bruce K]

I set mine to waist height when standing on deck amidships; this gives the same depth as a well handled lead line, which is what I am used to.

[Adelie]

Originally Posted by fxykty

Huh?

Why do you need tide and charted depth to figure out the depth of water under your keel only when the offset is to the waterline? Isn’t all you need is your draft?

I don't know about Gary's situation, but generally I want to know what the depths are going to be like for some time to come whether I'm anchored or underway.

My background is sailing in the Seattle area where the tidal range at springs is somewhere around 12' near Port Townsend and 20' near Olympia.

Here in Southern California the spring tidal range is 7'-7.5'.

In Granada in the Caribbean the spring range is like 1.25-1.5' and the Bahamas and Bermuda are like 4.5-5'.

I get that in a lot of places with with small tidal ranges forecasting future depths is not that critical, in others it can be very important when tidal height can be changing by 3' or more per hour.

Forecasting future depths where I am at right now is seems easier to me when starting the math from waterline rather than bottom of keel.

[Stu Jackson]

Altogether too many math challenged skippers?

[GILow]

Originally Posted by Stu Jackson

Altogether too many math challenged skippers?

Well, I can’t speak for the others, but I have a degree in Pure and Applied maths, and I still set my depth sounder to depth under the keel.

I think it’s just one of those things where you do what feels right for you.

[Tillsbury]

Originally Posted by CarinaPDX

It seems to me the answer reflects one's perspective on depth sounders. If you are using it to only avoid groundings then the depth under keel seems appropriate. For those of us who learned to use depth as another source of piloting input the depth from surface, to compare against charts, is the way to go.
Greg

This is another valid point. I learnt to sail in the UK east coast, where 3 metres is "deep water", and where our Decca would regularly place us 10 Miles inland if we bothered to look at it. When all you have worth using is a paper chart and a depth sounder, you need it to match the chart and tide as closely as you can. More than once I've navigated entirely by sounder in thick fog.

I do have the advantage of being a hundred years ahead or so in units, so am able to spot that a depth of 2 will find me aground, rather than have to do what people seem to think is tricky by subtracting 6 or 7...

[Lodesman]

Originally Posted by Martkimwat

I think you'll find any professional sailor including sailing instructors, will tell you it's better to set you offset to waterline. You should know your keel depth of by heart and so easy to know when you are going to touch the mud! when you are contour sailing you only need to correct for tidal height.

I think you'll find most professional sailors are used to keel-referenced depth, particularly in cargo ships, where the waterline and draft are extremely variable.

[StuM]

Originally Posted by Lodesman

I think you'll find most professional sailors are used to keel-referenced depth, particularly in cargo ships, where the waterline and draft are extremely variable.

I think that depends on your definition of "sailor" as opposed to "seaman".


I assumed Martkimwat was referring to professionals who sail (sailing instructors, delivery crew, charter skippers, etc and was not referring to commercial shipping crew.

[finefurn]

I've don't it both ways. The depth on the charts are the depth at low tide. But the bottoms can change and it's not always reflected of the charts. Therefore I set mine for depth under keel, because that's what's going to hit first

[Lodesman]

Originally Posted by finefurn

The depth on the charts are the depth at low tide.

Actually the depth on the charts is the depth at chart datum. Typically you have to add high and low tide figures to the charted depth to get actual water depth (or subtract a negative tide )

[flightlead404]

Originally Posted by Dsanduril

If anyone else will be operating the boat then a note needs to be posted .

This thread is like asking "whats the best primer" or "whats the best engine oil"

Seems like if different operators is an issue, depth under the keel is the safest as is the operator things its the other way around they just have "extra" water under the keel.

[Lodesman]

Originally Posted by StuM

I think that depends on your definition of "sailor" as opposed to "seaman".


I assumed Martkimwat was referring to professionals who sail (sailing instructors, delivery crew, charter skippers, etc and was not referring to commercial shipping crew.

Perhaps he was, but then again, I would think any person who regularly bounces around between different platforms with different keel depths might not want to depend on memory and math under stressful conditions with critical "safety" information. Plotting fixes or calculating rode requirements generally have less immediacy.

[Montanan]

As to calibration one can chose either waterline or bottom of keel. But be sure that you accurately measure from the actual position of the sensor relative to your chosen reference calibration offset, up to the water line or down to the bottom of the keel [assuming the sensor is not mounted to the bottom of the keel where there would not be any calibration to the bottom of the keel].

I like to confirm that the calibration is correct by seeing if the depth sensor yields a negative number when I plow the boat high and dry on the rocks. A negative number reassuringly confirms that you have indeed bumped bottom when one calibrates to the bottom of the keel and very usefully indicates by how much one has to refloat the boat. That be called navigating by braille.

[Mr B]

I dont see how the tide effects depth under the keel,
As long as I have a foot of water under my keel, Im afloat,
Irrespective of the tide levels,

In this Picture behind French Island,
I had a full tide of 6 feet, And I was scraping my hulls,
Even the channels didnt have 6 feet of water in them,
Thats 2 feet of water under me from the waterline,
It was touch and go to get out of that area,
I wont be going around French Island again,

I dont know how yachts get in there and out with four or even seven feet of water draught,

[Moontide]

I do waterline as I still remember how to subtract 4 to get depth to the bottom of the keels. Figure you either have to subract when underway or add when when anchoring. Guess it would be less math to use the keels, but maybe that is helping stave off alzheimers.