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

[Lodesman]

Originally Posted by searenity38

I use water line depth. My transducer doesn't read depths less than a couple of feet very well so if I read from bottom of keel and I'm getting into skinny water I could be 1, or 2, or 3 feet off. Plus I like to compare to chart soundings. Just my 2c

The offset doesn't change where the transducer is mounted - while the transducer has a minimum, the sounder likely doesn't. My transducer is about 2 ft below the waterline, or about 4 ft above the bottom of the keel. I have it offset to read from the keel, and not only will it read down to zero, it will give negative readings, which is very useful when driving through the soft mud/silt that is sometimes necessary in Florida.

[petermjdavies]

Keel. When it says 0 we are about to hit.

[IanTrail]

Depth of water below the surface. That's what your chart + tidal calculations give you so I'd rather use a single datum rather than two different ones. 'Depth' to everyone on our boat means depth of water and we all know she draws nearly 2 metres.

[Coginchaug]

I set mine to actual water depth. I sail mostly in Maine where shallow water is rarely a problem and use my sounder primarily to calculate how much rode I need to put out. But it's no big deal either way -- I can add or subtract 5 or 6 even without a calculator.

[aqfishing]

Originally Posted by moseriw

Water level of course - to compare with maps!

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

[GarryD]

Depth under keel every time
Transducer is on almost bottom of hull - so need to allow for the difference between its position and the waterline, also distance to bottom of hull. Could get the measurements and calculation wrong.

No need to have a label next to display, or keep reminding the crew.

Zero means you're aground.
If you don't know whether it is keel or w/line based, keel based acts as a fail safe. If you assume wrongly that it is w/line you will still have water under you.

[aqfishing]

Originally Posted by lo2jones

Water depth is easier for navigating and anchoring. One only adds the height of the bow roller to calculate the scope before dropping the hook. With water depth, there are only two numbers required for navigating, with keel depth there are three.

Thats BS. A little extra chain never hurt anyone! Besides whether you add 6 or 10 feet its one calculation!

[aqfishing]

Originally Posted by RnJBSailing

We are in the Great Lakes so no tides to worry about and only in/out of rivers have any significant current. So given our sailing grounds we set ours from keel (which is 4'6"), we set it to 5' so we a little safety. We don't have to subtract the keel from waterline depth constantly to know if we are ok! Lake St Claire is shallow, except very high water years the last couple years, so we don't want to take time for our old brains or guest "Captains" to do any math!

AMEN!!!

[PADISail]

I am about as far as you can get from an experienced sailor, but if I had to deal with this, my logic would say offset for draft at keel - if not just a little deeper...
Just to institute some 'wiggle room', and decrease repair expenses.

The only way I would offset for 0 (zero), would be if the electronics had a VERY loud ALARM that could be set to account for keel + wiggle...

Without Alarm:
Our brains can do calculation very fast, when dealing with charting, and I'd rather be on the safe side when it comes to sinking or sailing...

For what it's worth...

[Sailing Ohm]

Water under the keel. There’s never any confusion that way. 0 means you’re running aground or pretty close to it.

I, the captain, know there’s actually an additional 6” but that’s not told to others onboard...unless 0 comes up

[Sailing Ohm]

I don’t understand the tide comments at all...
How is it easier to do mental math than let the computer do it?
I live in a tidal and shoal area and I am open to other ways if it can be explained how it might be easier

[s/v Jedi]

Originally Posted by searenity38

I use water line depth. My transducer doesn't read depths less than a couple of feet very well so if I read from bottom of keel and I'm getting into skinny water I could be 1, or 2, or 3 feet off. Plus I like to compare to chart soundings. Just my 2c

That’s a new one, we did not got to enjoy the sound of this before

So, when you set the offset for a reading of water under the keel, your transducer doesn’t work so well because it can’t read those shallow depths

Unless you remove the transducer and drill a new hole deeper under the waterline... the transducer doesn’t really move up or down

[RaymondR]

Originally Posted by moseriw

Water level of course - to compare with maps!

Maps of the wet bits are called "charts".

[Quebramar]

This thread got me thinking...

Depth under the keel seems to give this instant reading and assurance that «*all is fine here and now*» which is definitely critical.

My experience in tidal/streamy places with lots of sandbanks is that I need to know where I’m going and at what tidal state, as it might not end up happily... So I can definitely relate to the calculations of twelfths and reference to charts for a rapid evaluation of what is «*here and now*» and what it could be in say 1hr away from now.

Depth from waterline in my case helps me there, my draft in my mind being 2m (actually 1.9 but any step towards prudence is welcome), it is easy to react when I see the true depth of water, it had become a reflex: anyway near 2m is big danger!

Being able to understand what is the relation between zero-depth of chart and actual depth allows also to confirm my assumption on the actual state of the tide, which will help evaluating also the time left before inversion of the stream.

Actually things get more complex with swell and waves, as troughs take min. 0.5m from the available water in moderate conditions... and if there are shorter waves/swell, the weight of the boat could increase this effect.

That’s where the alarm comes into play, a convenient complement that tells me it is getting unsafe «*here and now*». Mine is set at 5m, more of a wake up call than an imminent crisis on the bow.

That’s just my way, handling a complex sailing area with hard sand bottoms that kill several boats every year [emoji29]

[s/v Jedi]

Here’s a bit more food for thought: if you want depth under keel, how do you envision that to work for boat with lifting keel, center boards etc? Will you change the setting every time you adjust the keel or centerboard?

About commercial shipping: they use water depth as well as water below keel.