Depth sounder offset. True or depth beneath keel????

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Mar 21, 2019
United States
Vessel Name
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1970 Willard 36 Trawler
I realize this has come up before, but not for a while.

I have my depth sounder offset set to feet beneath the keel. Seems intuitive for my wife.

I recently entered a channel as lead where the other two boats had significantly deeper draft. So I was giving them depths but had to constantly add 4-feet to give an accurate actual depth. Seemed to be unnecessary calculus at an important time.

Thoughts? Argument for running actual depth vs offset for keel depth? I'm thinking of going back to actual depth and telling my wife to do the simple math and get over it.

What are y'all doing and why?

I want to look at the sounder and know what is under the boat, not how deep the water happens to be.
Peter, how often are you the lead boat giving depth soundings, how often are you by yourself and need to know if the bottom and boat will meet.

You probably had the time to reset for total depth before leading everyone in. Or you could have reset for the deepest draft boat and be calling out what is left below their boat. No one would have to do the math
I don't think there's a right or wrong answer. But since we're almost always in unfamiliar waters, uncertainty about water depth seems to come up

Example - we draw around 4.5 ft. As we enter an anchorage showing 12-feet, we need to reconcile for the chart that shows 16-17 feet (if it's accurately portrayed).

Second example. When we anchor. I'm on the foredeck, Cheryll is at the helm. When I ask her for depth so I know what rode to lay out, who does the correction?

Just wondering what others are doing, emphasis on those who explore unfamiliar waters.

Good question Peter and I agree that there may not be one good answer. DOMINO has two transducers, and one is set to depth and one to depth under the keel. I do not find myself with bias toward one or the other but it is nice to have the confirmation of both. I have the one transducer on LIBRA set to depth under the keel.
Like you I spend some time not only in unfamiliar waters but also in waters that apparently the charts are unfamiliar with! When charts are suspect, I find the depth under the keel most helpful. If I have trustworthy charts, I look at depth total as confirmation that my route is truly following the contour I intended on the chart.
See both arguments, but personally i always stick to actual depth. Just seems most relevant to know, compare to chart, tell someone else, know if you can dive off the stern, etc. But of course what you’re typically most worried about is whether you’re about to run out of depth. I’ve just always found it easiest / most logical to subtract my draft mentally when I need to.

But then I’m also one of those people who prefers to keep the plotter set on North Up, while reading the radar - separate screen - course up makes perfect sense, too.
I prefer having mine set to actual depth.

We draw 3.5 feet. Depth alarm goes off at 8 feet of water as a "pay attention" reminder. 6 feet is pretty much the no go point in most cases, although depending on knowledge of the bottom I may push it to 5. I never need to know about having 6 inches under the keel because I won't cut things that close, so I never do the math to convert to depth under the boat, just set a no go limit. With exposed props I'm not inclined to go feeling my way around places.

For getting in and out of questionable areas, surface depth is more useful. It's easier to report depths to someone else who is planning to come through an area. And if someone has told me what to expect for depth it's easier to judge if things are lining up as expected or if I might be in the wrong place. It's also more useful for anchoring to save the math.
I set mine to surface. Then when I keep my eye on the depth sounder, I make sure that I have at least 5 ft under the boat. My draft is 4 ft. I can also set my depth sounder to sound the alarm at X depth. Generally I set it to 8 ft, but when navigating shallow waters, I set it to 6 ft.
My preference is for depth under the keel (or in my specific case, the fins). This is how I have it configured.
If I were sharing and comparing depth information with a lot of others, I might feel differently. And if I were to spend a lot of time cruising in waters with significant tides, I might also feel differently - but my main cruising areas (Med & Baltic Sea) have insignificant tides, in general.
I have 3 transducers in Slow Hand. 2 are set to depth under the keel and one is depth of the water. Depth of the water is for communicating with other boats. I think in depth under the keel, clearance.

One of my best ideas was to mount one of my transducers as far forward as possible, maybe 8' from the leading edge. It allows me to slowly creep into skinny water, and stop if too shallow. The transducer has a depth of 2' whereas the skeg in the stern draws 4' 6".

You can't have too many transducers. Lost the transducer for the video bottom sounder last spring. It was very nice to know I had two others to read the depth of the water.

We have two sounders, so we set one each. We can see both easily enough, but I tend to focus on the one closest to me (depth below keel). Admiral has a good view of both, too...

I don't think there's a right or wrong answer. But since we're almost always in unfamiliar waters, uncertainty about water depth seems to come up

Example - we draw around 4.5 ft. As we enter an anchorage showing 12-feet, we need to reconcile for the chart that shows 16-17 feet (if it's accurately portrayed).

Second example. When we anchor. I'm on the foredeck, Cheryll is at the helm. When I ask her for depth so I know what rode to lay out, who does the correction?

Just wondering what others are doing, emphasis on those who explore unfamiliar waters.

The only right answer is "there is no right or wrong answer"...we both have said while being instructors, the most important thing is to recognize how a person thinks. Thinking one method over the other is a fool's errand as some people alert or recognize one danger faster than another.

Old timers often comment how the "newer" generations can't do math in their the answer "it doesn't matter" doesn't apply to them as they need the quickest alert that triggers their brain into recognizing grounding or danger is imminent.

Pick your poison.... I always want to know the depth of the water so I can relate it to the charted water depth plus/minus the tidal difference.... helps me locate my position if I trust the charted depths. But it doesn't matter as I have run so many different boats where I have no clue what the offset is, I just have to use experienced precaution to keep from doing something stupid.

That's not a good answer for most boaters, but it's one reason old timers tell newbies that there is no good substitute for decades of vast experience absorbing boating . 😊
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If it were me ( and its not) I would continue with the depth under your keel and let the followers do the math.
Your draft varies as your fuel and water is used up. In skinny water that maybe quite important. Setting at a “depth under keel “ is almost never accurate. Simple depth is accurate. To be safe if using that number you should only use your maximum depth when full. That means an additional guess at your true clearance depth. Also charts you’re running may not be accurate. Additional calculations are required to immediately see if they are off and how much. Finally I find most crew and friends I have were trained conventionally trained where true depth is most commonly used. I don’t want confusion or having them need to change their habits. Lastly when anchoring find it’s easier to use true depth.
I want my read out to be understandable immediately when looking at a chart. I only want to add in a compensation for tide. I believe in KISS.
I have only used true depth with no offset throughout my boating life.
Agree that like north up v course up it’s a matter of preference. But with that I generally run one screen north up and another course up. I don’t have that option with depth.
Would remind folks to think about where your engine cooling water intake is regardless of what you use. For me that’s as important as what you use. You don’t want to be sucking in sand,mud or grass if you can avoid it. For new folks on my boats find it easier to tell them we need 6’ although my intake is really 4’ down.
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Agree, everyone has to use what makes him comfortable.
When anchoring stern to (i.o.w. with a land line to the shore) I want to know the depth below the keel. But when I am going through a canal, river or have to deal with tides, I want to know the true water depth so I can cross check it with the charts.
Chart depth has been mentioned a few times. Correct me if I am wrong but chart depth is based on low mean tide, so it is not precise either since low tide can be lower than chart datum.

Setting at a “depth under keel “ is almost never accurate. Simple depth is accurate.
Why is one accurate and the other not? They are both reported from the same transducer. BTW, it does not matter how loaded or unloaded a boat may be since the difference from transducer location to the bottom of the keel remains the same.
Our brains work differently. Water depth from the surface is accurately measured. It’s the same regardless of what your draft is. Your exactly right you can easily work backwards to get that number know what offset you put in. But it’s another calculation when comparing to a chart reading or anchoring. I use depth from surface when figuring out how much rode to put out not depth from bottom of my keel. I use the surface depth reading when comparing it to chart. For both of those I can skip the step of figuring surface depth. Of course I had to enter into depth sounder reading the offset to the transducer. But I only needed to do that once. As long as my MFD is working I don’t commonly use depth to follow a depth contour if depth is over 8’ but I do anchor very frequently as well as compare my depth to charted depth whenever in skinny water. Rather do one calculation than two. Again a matter of preference and how you were trained. My wife hates north up. I like it. I don’t want to do calculations I don’t have to. At this point using depth under keel feels unnatural. Doesn’t make it wrong.
Funny.... some here are worried about inches when another's wake or surge from an inlet, squat, etc..etc makes that kind of accuracy moot.

Seems to me there a lot of hand wringing with some and the reality is that experience teaches some things in boating should never be measured in inches.

Often I was driving keel boats in areas where the bottom was sand or mud.... did I care how close I was to the bottom? Heck no.... but when operating where the bottom was hard, rocky, or questionable when it came to debris...then heck yeah.
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Depth under keel is always correct, as the transducer to keel relationship doesn't change. Transducer to waterline changes with load, so surface depth accuracy varies. I set mine based on a fairly light ship waterline, that way it will err on the side of reading low when heavily loaded rather than reading high when lightly loaded. The 2 inches or so of difference been light and full load isn't significant enough to be a big deal for us.
My old Mainship had a hollow keel and xducer was mounted inside on that keel. I k ew depth below thw keel.
My Albin had the xducer mounted a couple of feet above the bottom of the keel on the hull.
I never went aground in 30 boating years.
So, to me its a moot point
Agree just preference. For me it’s been not if but when but I like exploring and have been in places where charting has been sketchy.Slow is pro. Dancing through coral heads and shifting sands has been most troublesome. US east coast, even the ICW , hasn’t been bad as it’s just mud or sand so doesn’t hurt anything. Still you’re right grounding is a very rare event. Not even once a year. Not the end of the world. I done it once since owning this boat. On our first long transit (RI to Va) came into Deltaville at 2am tired having been running round the clock. Went on sand as we wandered around trying to figure out which slip was ours. With a full keel and a 540hp engine got off in a minute . Single engine trawlers are so much easier to deal with than sailboats when stuff like this happens. Hauled the next day and didn’t even lose any bottom paint. Slow is pro in skinny water.
My Furuno video sounder is old, simply shows depth below transducer. The 'ducer is 1m below the waterline. On the TZ screen I display water depth using a 1m offset. I look at both readings about the same amount of time. As to keel, its 0.3m below the 'ducer. I just pay close attention when in <2m of water.....
Mine is set at water under keel. I have a sticker on the sounder display reminding me of that.

- In the Salish Sea, most of the time we have lots of water. For those times when it is shallow, I want to know how much between the bottom and my keel.
- Water under the keel doesn't depend on the current load of the ship. The transducer goes up and down with the hull.
- I do have to remember to add that 4.5' of keel depth when determining how much scope to put out when anchoring.
I always put the "real" ( from surface)because : nothing to do for my old brain ( until I still remember my ...draft)
for example a "bathymetric ligne " of 20 m just looking 20m ( eventually corrected by the tide at this moment). not need to make plus to the keel or plus to the transducer.
Seems an even split among us with decent justification for either decision. Truly a matter of preference.
Like some of the others posters we have two transducers, one set for depth under keep, the other depth. That gives me the flexibility or option to pay attention to either one without doing math. Simple enough I know, but when you're running in very shallow water I'd rather minimize distractions and save my limited brain power for everything else. Unlike absent-mindedly cutting a 2x4 too short, grounding a boat can be a lot more expensive. Aside from that convenience of both displays though, I'd never want to depend on a single transducer or reading. I assume the previous owner was a fishing guy because he had a fishing display and the MFD rigged to each transducer. I have had one or the other go out. Not very often, but in shallow water I really don't want to be blind on depth. When we did our last New England/Hudson run I found the Navionics charts to be remarkably accurate on depth (charts closely matched the live depth readings in other words) but I wouldn't want to gamble.
I vote under the keel depth.
Under the keel depth for me too. Seems to me that when you are worrying about how much water you have, you are concentrating on other things as well - knowing instantly how much water you have to play with saves a bit of brain power for other things. The anchor chain calculation can be done at a slower pace with less distractions/other things to think about.
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