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

[Dsanduril]

Originally Posted by Bycrick

Now, we are getting into the area where the numbers approach being meaningless. ....
Measuring inches of depth accurately with a recreational boat depth sounder? Get serious.

While Fischer and Kasparov are duking it out to what will ultimately be a draw because they've both shown that no matter whether you select keel or a particular waterline you will still need to calculate/estimate actual depth, Spassky comes in from the cold and wins the match with an elegant move that no one expected.

[Lodesman]

Originally Posted by s/v Jedi

120” is what some here claim, because the offset was set to waterline level before loading up! So that is a problem because in reality it is not 60” under the keel but 56”

I don't think anybody made this claim

[Peterbreit]

I set mine to read water depth under the keel. Plus a few inches to account for 230 gal of water, 50 gal of fuel and my lady's shoes ;-))
Also I am sailing with different crew every weekend and was getting of tired to answer obvious questions "How much does your boat draws again"?
So when we go over the boat check-out I tell my crew the reading is water-below-the-keel. No mathematics needed. KISS

[Siberian Sea]

Sounding on charts are often not accurate or detailed enough to be trusted in terms of just a few feet overall. One thing I hate to do more then anything is run aground. I want to know exactly as possible how much water is under my keel and this is where I set my depth calibration to read. I know my sailboat draws 6 feet I can always add that to know total water but the first thing I want to is how close to the bottom my boat is. Why do you think the manufacturer recommends you place the transducer as close to the lowest point on your hull as possible? It’s not to coincide with charts it to let you know when your getting close.

[zstine]

Originally Posted by Siberian Sea

Why do you think the manufacturer recommends you place the transducer as close to the lowest point on your hull as possible? It’s not to coincide with charts it to let you know when your getting close.

with all due respect, the manufactures don't care if you calibrate waterline depth or depth below keel. They recommend placing it as low as possible to avoid interference from bubbles, which tend to stay higher in the water.

[Siberian Sea]

Originally Posted by zstine

with all due respect, the manufactures don't care if you calibrate waterline depth or depth below keel. They recommend placing it as low as possible to avoid interference from bubbles, which tend to stay higher in the water.

Sorry but your mistaken, I used to work for Furuno and we recommend placement to give accurate readings below the lowest point on your hull so you how much water exists between that point and the bottom

[s/v Jedi]

Originally Posted by Siberian Sea

Sounding on charts are often not accurate or detailed enough to be trusted in terms of just a few feet overall. One thing I hate to do more then anything is run aground. I want to know exactly as possible how much water is under my keel and this is where I set my depth calibration to read. I know my sailboat draws 6 feet I can always add that to know total water but the first thing I want to is how close to the bottom my boat is. Why do you think the manufacturer recommends you place the transducer as close to the lowest point on your hull as possible? It’s not to coincide with charts it to let you know when your getting close.

So when you see 1’ you know you have 1’ of water below your keel. You can also add 6’ draft which would mean total water is 7’... right?

Now if you would set the offset so that total water depth is shown, you would see 7’ so you know you have 1’ of water below your keel. You don’t have to add anything to get total water depth because it already shows that.

Now, what does inaccuracy of the sounding on the chart have to do with either option? In both scenarios the real depth is measured, not a number from a chart

[CarinaPDX]

This discussion is devolving into silliness. There is nothing about depth, either with the sounder or chart, that is accurate to an inch or two. Nor are most seabeds smooth enough to provide a depth to inches. And there is never any guarantee that there isn't something on the bottom that was recently deposited, such as a lost anchor, broken outboard, sunken boat, or any manner of human detritus. Anyone counting on clearing the bottom by inches is on a fool's errand.

Greg

[s/v Jedi]

Originally Posted by CarinaPDX

This discussion is devolving into silliness. There is nothing about depth, either with the sounder or chart, that is accurate to an inch or two. Nor are most seabeds smooth enough to provide a depth to inches. And there is never any guarantee that there isn't something on the bottom that was recently deposited, such as a lost anchor, broken outboard, sunken boat, or any manner of human detritus. Anyone counting on clearing the bottom by inches is on a fool's errand.

Greg

Yet there are thousands of sailboats stuck in the mud every low tide. Not everything is the same in every place in the world.

[Reefmagnet]

Originally Posted by Siberian Sea

Sorry but your mistaken, I used to work for Furuno and we recommend placement to give accurate readings below the lowest point on your hull so you how much water exists between that point and the bottom

They obviously never sold transom mounted transducers.

[Simi 60]

We have a commercial quality Koden fish finder and it runs surface down in meters but the volume of the alarm is pitiful, I guess they dont want to scare the fish

We have an ancient Datamarine with a heart stopping alarm that is in feet and has keel offset
Fantastic, basic depth and works a treat and when it says 1ft we are close to rubbing.
One for sale on ebay, tempted to get it for the spares locker

https://www.ebay.ca/itm/Datamarine-D...0AAOSwHZJfZ3rd

[puffcard]

Yep, set to true water depth. We're shoal 4' 2" draft. But what I found is boats with 4 foot drafts drive around like they're 3 ft and boats with 6 foot drive around like they're 8ft. We are east coast and bahamas, most of the time anchoring in 10 ft or less. So most times anchoring, a quick look at depth and times 5 we good to go, + the bridle. Most don't know how far down their transducer is, or what's really below the keel, talking inches is funny. With total water depth, drop a fishing lure over the side let it hit the bottom. Lower tip of pole to water level, pull up and measure line. Set sounder to match. If setting depth to under keel it's called "Keel Depth" or "Depth to Keel", not water depth. Submarines usually used Depth to Keel.

[Martkimwat]

Originally Posted by Siberian Sea

Sounding on charts are often not accurate or detailed enough to be trusted in terms of just a few feet overall. One thing I hate to do more then anything is run aground. I want to know exactly as possible how much water is under my keel and this is where I set my depth calibration to read. I know my sailboat draws 6 feet I can always add that to know total water but the first thing I want to is how close to the bottom my boat is. Why do you think the manufacturer recommends you place the transducer as close to the lowest point on your hull as possible? It’s not to coincide with charts it to let you know when your getting close.

The reason they advocate it being as low as possible is to have a clear view of the bottom without the risk of obstructions and the possibility of interference from surface air bubbles, There is in fact an equally valid reason for placing it up as high as possible, since a lot of sounders will not provide a reading when the bottom is less than 4 feet from the sounder due to the peculiarities and limitations of the sonar system which basically gets 'swamped' when the returns are that close. The sounder, if external is also less vulnerable. my personal view is what is good for the professionals is good for me. I don't believe you will find any professional mariners using depth under the keel. The argument that using waterline will be invalidated by the vessels variable loadline is spurious since the offset you have put in the device will remain the same and therefore the depth the keel below that offset will not change, the waterline might but the offset to the keel distance will not. Do those that advocate using depth below the keel, make an allowance for their variable waterline depth due to loading when calculating how much water depth they will have in a particular place or when contour sailing? of course they don't. if you sailing that that level of 'accuracy' you are foolhardy to the nth degree, there are too many variables outside your control. regarding sailing into shallow waters and feeling your way, if you have accurately set your offset to the boats designed waterline and measured your keel depth from that same design waterline, nothing changes. If your boats designed keel depth is 1.8M (6 feet), when your sounder says six feet or 1.8M, you will touch, doesn't matter whether you are heavily laden or not. Your actual waterline might be 6 inches above the design waterline, therefore your draft is 6'6" (2M) and thus you would touch in that same depth of water, but your sounder will still say 6 foot/1.8M since you have set the offset to the design waterline which has not changed.
I happen to be a great fan of Tom Cunliffe who has years more experience both as a professional sailor, and Yachtmaster instructor/examiner and he categorically espouses the use of offset to waterline not keel, for depth.

[Mr B]

My depth is set to one foot under the hull, The right side one,
Being in the middle of the boat,
It gives an accurate sounding as the middle doesnt fluctuate up and down like the Bow Or transom may do,

I have a two foot draught,
I went out this morning and had 4 feet under my Hull,
I came back this evening and the depth was 00 Feet,
It was an extremely low tide,
I scraped my hulls in a few places in the Marina channel,
No, the channels are not flat and level,
The main channel I had 2 feet under me,
But I kept to the left side almost grazing the red markers,
The Green markers side I would have run aground for Sure,
When I berthed in the marina, I had Zero water on the depth gauge,
I normally have 3 feet on the gauge at low tide in the Marina,
Measured with a plumb Bob on a string in the middle of my Cat,

[Lodesman]

Originally Posted by Martkimwat

There is in fact an equally valid reason for placing it up as high as possible, since a lot of sounders will not provide a reading when the bottom is less than 4 feet from the sounder due to the peculiarities and limitations of the sonar system which basically gets 'swamped' when the returns are that close.

Do you just make this stuff up? There's a minimum readable depth (my Airmar says 1.5 ft) but that has to do with the allowed time between ping and return, similar to radar.

The sounder, if external is also less vulnerable. my personal view is what is good for the professionals is good for me. I don't believe you will find any professional mariners using depth under the keel.

When you say "professional mariners" are you talking about sailing instructors and charter/delivery skippers, or are you talking about ships' captains as well. If you're including the latter, you're wrong.

The argument that using waterline will be invalidated by the vessels variable loadline is spurious since the offset you have put in the device will remain the same and therefore the depth the keel below that offset will not change, the waterline might but the offset to the keel distance will not. Do those that advocate using depth below the keel, make an allowance for their variable waterline depth due to loading when calculating how much water depth they will have in a particular place or when contour sailing? of course they don't.

With ships, the transducers are mounted at the bottom of the molded form, very near the keel, within inches of the bottom, but metres from the waterline. But which waterline? Fresh? Summer? Winter North Atlantic? Lightship? Fixing it to a virtual waterline invalidates the reasons you advocate using the waterline as the reference point. Fore and aft draft will be recorded after loading/unloading before sailing. These are prominently displayed on the bridge, and can be easily added to the depth on the sounder, for planning purposes, or tying depth into a fix. Much easier than taking the sounder figure, subtracting the chosen W/L figure, then adding the actual draft.

if you sailing that that level of 'accuracy' you are foolhardy to the nth degree, there are too many variables outside your control. regarding sailing into shallow waters and feeling your way,

You're invalidating your whole argument for setting it to the waterline. Certainly with recreational vessels, where W/L is not going to change by much choosing keel or W/L as reference points is really a matter of personal choice. But the W/L crowd are all saying it allows an accurate comparison between the depth read off the sounder to the chart. Now you say that is foolhardy. If you're going to be doing math to get there, what difference does it make if you have to add keel depth?

if you have accurately set your offset to the boats designed waterline and measured your keel depth from that same design waterline, nothing changes. If your boats designed keel depth is 1.8M (6 feet), when your sounder says six feet or 1.8M, you will touch, doesn't matter whether you are heavily laden or not. Your actual waterline might be 6 inches above the design waterline, therefore your draft is 6'6" (2M) and thus you would touch in that same depth of water, but your sounder will still say 6 foot/1.8M since you have set the offset to the design waterline which has not changed.

I don't think anyone has disputed that point. But what benefit is there in having the sounder saying 6ft when you're aground, if the water depth is not actually 6 ft?

I happen to be a great fan of Tom Cunliffe who has years more experience both as a professional sailor, and Yachtmaster instructor/examiner and he categorically espouses the use of offset to waterline not keel, for depth.

Does Mr Cunliffe give a logical reason for this, or are you just following dogmatically? I'm curious for those yammering on about contour sailing, do you correct your depth readings for heel? I seriously doubt it, as I also doubt anyone is fixing by soundings alone, if they are unable to add their own draft to the other calculable offsets.