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

[Tanqueray]

Originally Posted by Dsanduril

I actually suspect "depth below transducer" is the mainstream choice as most boats on which I have been a visitor have no offset configured. Installed from the box and left the way it came from the factory.

Only a few of us here on the forum with too much time on our hands discuss or think about it at this level.

I am pretty sure mine is not set (or set at 0).

The water in my area is so deep (Lake Michigan), and no skinny water entry for me, I hardly ever even think about it.

One day I anchored off shore for the night..wind shifted overnight and when I woke up, I could see the sand very clearly...I checked the depth gauge...about 4.2 feet...I draw 4.2. But I think the sensor is a foot or 2 below the water line...so that explained why no “thump” in the waves

Greg

[CyKlop]

Depth under keel.

It’s 2 am, you’re tired. Depth reads 2 meters. You forget which way it reads (or your temporary crew does). If you are set to waterline, and crew thinks it is clearance from keel, you may be in for a sudden stop. If you are set to, depth under keel and crew thinks it’s set to waterline, they will think they are about to bump and take action.

Which scenario would you prefer?

[SaltyMetals]

I dont care what others do but my boat is always set to depth under the keel. Personal choice.
I make sure any newcomers aboard know this.

Andrew

[wingssail]

Originally Posted by CyKlop

Depth under keel.

It’s 2 am, you’re tired. Depth reads 2 meters. You forget which way it reads (or your temporary crew does). If you are set to waterline, and crew thinks it is clearance from keel, you may be in for a sudden stop. If you are set to, depth under keel and crew thinks it’s set to waterline, they will think they are about to bump and take action.

Which scenario would you prefer?

Well Cyclop, it's not Either/Or. There are lots of scenarios. Most skippers have worked out how to deal with depths and depth readings on their boats.

In my case the tranducers (I have three running most of the time) are mounted on the bottom of the hull, which is 1.5 ft below the waterline. My draft is 8'. So when the depth sounders read 6.5' I'm touching.

Firstly, everyone on my boat knows that at 6.5' we touch. I make sure they know that. That's all they need to know. No tiredness changes that number. No calculations required. There is no room for confusion.

Secondly, we are not going to be navigating anywhere near that depth without skipper on deck and full alertness of the whole crew. Tired or not, shallow water sends adrenalin through our brains and we are not taking things as casually as you imply.

[blazing928]

I was depth under drives plus a couple of hundred mm, but now water depth with again 200mm fudge factor.

The reasoning is that once the sounder reads 0, under the drives you then can’t tell if you’re going any shallower or not.

Set the other way I know I have 1m draft, so if it reads 1m than 0.9m, it’s getting shallower.

[Simi 60]

Originally Posted by aqfishing

Glad you trust your maps so well and have the time to calculate the current tide vs the mean low soundings on the chart!

I would have thought these days with charting systems you'd have minimum depth set on charts.
Keep off the green, yellow and be very cautious with white bits.
Blue is good to go.

Saying that I have plenty of spots we have been that show as land on a chart but we have had 20ft or more of water under us when going through.

[Mr B]

Originally Posted by Simi 60

I would have thought these days with charting systems you'd have minimum depth set on charts.
Keep off the green, yellow and be very cautious with white bits.
Blue is good to go.

Saying that I have plenty of spots we have been that show as land on a chart but we have had 20ft or more of water under us when going through.

That happens here all the time, Every where is green, Land above water,
But I pick my way thru it,
Good GPS helps,
I dont have paper charts any more,

[dfelsent]

Mine is set to depth from the waterline.
The scary bits are when in shallows with tall eelgrass the reading are less than what I draw.
By a foot.
And yes I’ve calibrated by parking the boat on the sand with no grass.

[Scott Berg]

Interestingly enough I have two and one is labeled water under keel and the other water depth; flexible and useful

[CyKlop]

I agree that there are many ways to do it and the "right" way is what works for you.

In my life, I tend to do a lot of failure scenario planning and I try to make it so that the failure mode is always leaving a safety margin, i.e. "If I screw this up, which way do I want it to go?"

there are good reasons for the other way, though I disagree with them. The best one so far to me is the person who says that once it hits zero, he can't tell if it's still getting shallower. Of course, if it hits zero, you aren't going to be continuing on, so it kind of doesn't matter

As for the people saying it's harder to correlate to charted depths... how often is the water the charted depth? You're always having to offset charted depth by current tidal height, so doing adding and subtracting anyway.

[parkstone bay]

As an amateur, it's probably your choice, whatever floats your boat, as they say. Professional navigators will always use "depth from waterline"; many amateurs, (especially racers in tidal waters) like "depth under keel" . Neither is "wrong", but depth from waterline is - I suggest - the "righter" method. But, whatever protocol you use, it is CRUCIAL that you tell any other person who may take on a navigator's role. A note by the display ("Depth = under keel", or "Depth = from waterline", or similar), is really useful.

[Simi 60]

Originally Posted by parkstone bay

As an amateur, it's probably your choice, whatever floats your boat, as they say. Professional navigators will always use "depth from waterline"; many amateurs, (especially racers in tidal waters) like "depth under keel" . .

Evidence to support that claim?

[s/v Jedi]

Originally Posted by Simi 60

Evidence to support that claim?

No need for evidence because this is how it is taught in naval academy as well as all navigation courses

[Sailmon]

Responses make it pretty clear there is no one correct approach. It’s kind of like the debate between “north up” and “course up” in setting up chart plotting devices (MFDs, tablets...). The most important thing is to know which approach you prefer... and to make sure you have clearly communicated with others navigating or taking the helm. In our boat - cruising the skinny waters of S. Florida, Keys, Bahamas, we have elected for “actual water depth” BUT with a 1’ offset as a margin of error buffer... If the sounder reads 6’ we are actually floating in 7’ of water. It’s proven helpful in overcoming brief bouts of inattention (admiring an amazing sunset, being set by current on the beam, minor chart inaccuracies).

[Training Wheels]

This thread reminds me of the old Archie Bunker/Meathead skit of how to put on your shoes and socks! [emoji23]