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

[psneeld]

If you are really worried about grounding, choose a safe navigational depth for each stretch of your trip so that no way you will hit bottom as most of us are navigating with charts and not the depth sounder.

If you are in water where less than two feet matters, you better be going extremely slow or you will ground anyway or still pop that rock ledge/random rock that isn't charted well. Which probably we all do when poking around looking to anchor or enter known shallow areas like silted in marinas.

14 years of professionally ungrounding boats, I have heard a lot of stories from embarrassed skippers and they probably were mixed on what method of offset they used.

So while I have my method, and the other is OK with me...I just think it doesn't matter as good nav skills and situational awareness are far more critical. They are way more important than worrying about what should be a near instantaneous decision to slow/stop when the depth sounder gets near a number you should already have in your head long before it's really needed.

 

[mvweebles]

OP here. Thread has been really helpful and I appreciate all the comments.

The more I think about it, the more I think absolute depth works for me. The last year I've been in almost exclusively unfamiliar waters and while depth under keel is important in very shallow waters, just seems for my boating that absolute depth is more frequent (scope calculation being a good example, but there are others). Besides, when I ask for water depth, I want water depth. If I wanted depth beneath the keel, I'd ask for that.

When I get back go Weebles this fall, will definitely reset the sounder to absolute.

BTW - there were a couple comments about having two Sounders - one for absolute, other for beneath the keel. I understand the argument for different types of Sounders (eg 50hz and 200hz), but struggle with two different offsets. Sounds confusing as hell to me but obviously do whatever works for you and your boat. But it does remind me of a saying from my dearly departed father "a man with a clock always knows what time it is. A man with two clocks is never sure."

Peter

 

[psneeld]

Another tip for me was setting the depth alarm and knowing how to cancel start stop it ....QUICK! It was great for those stretches of ICW or totally unfamiliar, poorly marked waters where lower than my comfy navigational depth was crossed, but was a nuisance once in an area poking around shallows or in a marina/anchorage where I stayed awhile at less than the alarm depth.

I just got used to setting up that page, ready to silence the alarm. I would say that for depth alarms on complicated to use MFDs, a cheapo ($50-$200), depth only display with an in hull transducer set for best bottom return (probably 50kHz or lower). As long as it has an alarm and it's easy to shut down... that's great. you could always as just an alert or leave it uncovered if the math issue needs quick reconfirming/reminding.

 

[slowgoesit]

Ours is set to depth from the water surface. We know we draw 5 1/2'. Depth sounder set to water depth allows us to compare water depth to depth shown on charts. Works for us

 

[rslifkin]

Another tip for me was setting the depth alarm and knowing how to cancel start stop it ....QUICK! It was great for those stretches of ICW or totally unfamiliar, poorly marked waters where lower than my comfy navigational depth was crossed, but was a nuisance once in an area poking around shallows or in a marina/anchorage where I stayed awhile at less than the alarm depth.

I just got used to setting up that page, ready to silence the alarm. I would say that for depth alarms on complicated to use MFDs, a cheapo ($50-$200), depth only display with an in hull transducer set for best bottom return (probably 50kHz or lower). As long as it has an alarm and it's easy to shut down... that's great. you could always as just an alert or leave it uncovered if the math issue needs quick reconfirming/reminding.

The Lowrance MFD currently at my helm handles the alarm nicely. When it triggers it pops up a message on the screen and beeps. Once you tap the button to acknowledge, it silences and just keeps a little blinking reminder in the corner of the screen until the alarm condition clears (which happens when depth reads a foot above the alarm threshold).

 

[psneeld]

So did mine but some wakes/waves/irregular bottom depth would keep resetting it, even jockeying around in a slip tying up would, so turning off the alarm to better focus was nice. Not all machines may work the same but mine for me it was best to have the cancel function up on my MFD and ready to go as I usually didn't need it for anything else at the moment.

 

[rslifkin]

So did mine but some wakes/waves/irregular bottom depth would keep resetting it, even jockeying around in a slip tying up would, so turning off the alarm to better focus was nice. Not all machines may work the same but mine for me it was best to have the cancel function up on my MFD and ready to go as I usually didn't need it for anything else at the moment.

I can see that happening if the depth is really close to the alarm threshold. Mine has enough hysteresis that I haven't run into it though, most places where I'm in a continuous alarm condition are more like a foot below alarm depth rather than an inch, so it doesn't re-gain enough depth to clear the alarm.

 

[Shrew]

My preference is to set the offset to Actual Depth. This allows the sounder to match the charts. I know my draft (3' 6") , and my shallow depth threshold (5-6 ft) is based on how many feet I feel comfortable under keel. This also allows me to easily add the 5 feet of pulpit height when calculating scope for anchoring.

Transducer depth (default) is useless to me since my keel and running gear are much lower than the transducer. If I anchor based on transducer depth, i'll be short scoped.

To me true depth is the easiest and makes the most sense. Just another person's opinion.

 

[SteveK]

@Shrew
Suppose the waterline to bottom of keel is 4 feet. Suppose the transducer is located 1.5 feet from the waterline.
Did you enter an offset so the sounder displays 1.5 feet more than the transducer sees?
In my case I would enter 2.5 offset to display below keel depth.

@slowgoesit
The chart depth remains constant. If there is no tide then the display depth will stay constant over a fixed location. Other than a guide prior to entering shallows how else would chart depth matter.

 

[psneeld]

It is all simple math to calculate ANY or the parameters we may be looking for at any time.

Now I can see some worried about going aground or being out of the channel, heck some people use all different methods of putting red and green thingies on the dash to remind them how to follow channel markers (I wonder who reminds them to switch when they change channels? )... but really ...post the difference between the depth from keel and actual water depth and place it next to your MFD..... you still have to pick a "safe navigational depth" all the time.

Like I said...it doesn't matter as a number should be in your head when to chop the throttle...either distance to the keel or the true water depth...NOT ROCKET SCIENCE....so either offset is fine as long as you pay attention and are situationally aware of what to do when that number is NOT what you want.

People go aground for a lot of reasons, but I never once heard in hundreds of groundings it was confusion about which water depth offset was used (unless it was extremely different on that vessel and the driver had no clue how the depth sounder offset was set...as in a delivery captain)

 

[O C Diver]

I find this thread quite entertaining. First of all, depth on a chart is the normal low water. Any depth on a chart that is in a tidal area, is going to fluctuate based on tidal stage. So it's unlikely that your depth will match the chart depth.

Secondly, in a shallow area where you are monitoring the depth constantly, why would you want to keep doing the math? With keel depth, all you need to do is read your clearance. I've got 4', now 3.5', now 2.5', now 3', now 2', now 1.5', now 3', etc. With a 4.5' draft boat, why not eliminate the math and focus on other equally important things.

Regarding multiple depth sounders, why would you not check the same depth display each and every time? This is no different than engine gauges. You look at the same display each time for the same information. Having multiples is about redundancy, not different places to look for the same information. On Slow Hand, surface to bottom depth was located on a smaller different display away from the normal depth display. This was so that it purposely wouldn't be used accidentally.

Ted

 

[psneeld]

removed

 

[mvweebles]

Why would you want to keep doing the math? With keel depth, all you need to do is read your clearance. I've got 4', now 3.5', now 2.5', now 3', now 2', now 1.5', now 3', etc. With a 4.5' draft boat, why not eliminate the math and focus on other equally important things.

For me, there are more times where I want to know the depth of the water than distance below my keel. Scope for anchoring comes immediately to mind. And I've spent quite a bit of time in unfamiliar channels where I'd like to confirm whether the chart data is roughly accurate. But again, it's a personal decision. If you're always in familiar waters and only adjusting for tide, perhaps it's not a big deal.

"Regarding multiple depth sounders, why would you not check the same depth display each and every time? This is no different than engine gauges."

I'm not a single hander like you are. If it were just me checking the gauges, I agree - no problem knowing what the reading is. But I have a wife who is frequently at the helm - and always at the helm when we anchor. Also, I am a believer in a system of record - a single source of truth. With that as a baseline, two different readings creates confusion which could have consequences. I can imagine the conversation with two sounders:

ME: "What's the depth?"
WIFE: "Which display?"
ME: "The small one on the left."
WIFE: "That says 17-feet, the other one says 22-feet."
ME: "That's not possible. The one on the left is absolute depth and will always be more."
WIFE: "Oh....you meant that 'left'.....sorry. 22-feet."

Granted 17-feet is plenty regardless if it's beneath the keel or absolute depth, but I asked the question for a reason. For me, having two different readings is confusing. Would be like having one in feet, one in fathoms (simple math there too). But whatever works for you. All I can say is what works for me.

Clearly there is no right answer. All I can say is how I arrived at what works for us in our cruising areas. If the circumstances change, decision may change.

Peter