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

[sv_pelagia]

Water depth

All the above reasons, plus
- that's what you use when determining how much anchor chain/line to put out (plus bow height plus tide rise)

[Training Wheels]

Water depth

[s/v Jedi]

When you set it to water depth, there is no math involved to figure out if you’re aground or not. It is only a problem when you can’t memorize the drought of your own boat

When you leave it at 0 offset (sensor depth) then you get math involved. Charted depth, tide table, sensor reading, sensor offset. Four parameters of which the offset can be eliminated so that is an easy choice.

Water depth is the mainstream choice and easiest for those who use tide tables and check state of tide by comparing actual current readings. If you never work with tides, only check for water under the keel, then that is the best offset to use.

It dazzles me the examples people give here.... we normally don’t even have total depth than what they see under their keel. They take a 2’ sensor depth as a safety buffer. We spend months in waters that during low tides only leaves inches under the keel... or nothing during spring tides.

[Annie in WA]

we use water depth. calibration is by a 100’ tape measure with weight on end so no doubt about reading. Years ago my depth sounder broke and I improvised with a line and weight for depth. When we got into port I bought a 100 ft fabric tape on a plastic reel, when we are in skinny water we use the tape for exact numbers.

[Mike OReilly]

In reality, I do use an offset. My transducer is 2' below the waterline, so to get back to zero (water level) I have to add 2 feet.

[Dsanduril]

I see to calculations as about the same regardless of method of offset. How much water I need to anchor in is easier with keel depth, how much scope is easier with water level.

When anchoring I look at the tide tables and use them to get the difference from current tide to lowest tide (and highest tide) during the period I expect to be in the anchorage. That has nothing to do with water depth, just the amount of change I can expect from when I drop the anchor.

If I expect the lowest tide to be 'x' feet lower than 'now', then with depth set to zero when I run aground I can anchor in 'x' feet of water. No math needed, just an adjustment for swing area and whatever cushion I feel is reasonable.

OTOH, it does complicate calculation of scope for the rode, because I need to add depth reading + draft + freeboard + 'y' (tidal increase) to get to a scope value.

Six of one, half a dozen of the other.

[barnakiel]

Ours is set to the waterline. Otherwise I find it more difficult to quickly refer with the map.


b.

[Bycrick]

Water depth. One less piece of math when looking at charts. Easy to know that less than 7 ft I expect to hit something.

But so far, the most useful comment for me was "Post what the reading means next to the display." Same goes if you use UTC time for navigation. Or whether things are plotted/displayed using True or Magnetic. Especially if you have crew.

Personally, I set everything to display UTC, waterline depth, and all bearings developed electronically in True. All the magnetic compasses, even the electronic ones, read out magnetic. The GPS compass reads true, even though it runs the AP. Current variation posted at the Nav station and helm, so nobody forgets. Depth units usually in meters, but changed to agree with with any paper backup charts being used at the time.

YMMV.

[Wotname]

This thread highlights the different conditions in the different ares we sail in.

A few posts mention the ability (and simplicity) of comparing charted depths to the sounder. I am sure this is true in many cruising areas where the tidal range is small and regular and the charts are accurate.

However it is very untrue in other areas where the chartered depth bears almost no relationship to the actual depth without a lot of maths and even then it might not be reliable.

Once the tidal range becomes large and irregular and tidal stations are few and far apart, all bets are off. Other effects are river flows, river levels, wind strength and direction, atmospheric pressure etc. Combined, these can cause variations of many feet and can be unpredictable.

If you are using the sounder to prevent hitting the bottom, then setting the offset to display the depth under the keel, there is zero maths required. If you must know the actual water depth, simply add the boat draft to the reading - possibly the world's easiest arithmetic example.

[cal40john]

Save yourself the heartache of deciding, buy a fathometer like mine, there is no calibration setting.

[chrisr]

for us, always has been depth under the keel, and would never dream of anything else

if in shoaling waters i don't want to be doing mental arithmetic trying to work out how much water we have underneath us

cheers,

[wingssail]

Originally Posted by Mike OReilly

In reality, I do use an offset. My transducer is 2' below the waterline, so to get back to zero (water level) I have to add 2 feet.

Well, I'm really screwed up I guess. My sounder reads the depth from it's transducer (my main one, I have three), which is at the hull depth, -1.5 feet from the water level . It has no adjustment for keel, etc.

So when the actual depth of the water is 8 feet, my depth sounder reads 6.5 ft.

We draw 8 ft. So at 6.5 we touch.

Pretty screwed up, but it works for us.

[GILow]

I really don’t like doing subtraction under pressure.

Depth under the keel for me.

[fxykty]

We have variable draft depending on boards up or down so we use waterline as our universal 0. If boards down we need 2.5m water and 1.2m if boards up. We round down to 3m and 2m for clearance and there’s not much math needed (just the local state of tide). It’s all about what you’re used to, with neither being right or wrong.

Regarding scope, we only use water depth as our bridle attachment point is either just (relatively shallow depths) or well below the water level when the rode is extended. So we attach our bridle at the required scope, then the bridle allows for the height above the waterline.

[AndyEss]

Water depth for me.
Easily correlated with charts, easy to communicate to other boats, easy to set anchor scope.
My old Autohelm ST50 fathometer includes a alarm depth in addition to the sensor depth offset. My alarm is set at 9 ft, my boat draws 6 ft. The alarm is visual (blinking display, plus “shallow” so it doesn’t distract me with noisy beeping.
I still manage to go aground occasionally.
My new MFD includes CHIRP sonar. I bought and installed the transducer for it. As fascinating as seeing the bottom is, I find the detail to be a bit distracting. It’s great for picking up a rocky bottom to adjust anchoring.