Ran Aground Using Navionics.

[Benthic2]

Have you tried to verify the accuracy of your GPS ? You could do that with your cell phone, or dead reckoning.

Here's a free online depth chart of the lake as another source of info:
i-Boating : Free Marine Navigation Charts & Fishing Maps

You might want to stop in to the a few of the boat rental centers and ask them if any particular brand of chart is better or if there are any known inaccuracies. You can't beat local knowledge.

 

[Marco Flamingo]

Have you tried to verify the accuracy of your GPS ?

That's the first suspect. My Garmin shows accuracy when using the "fishing" (sonar) screen, but not the navigation screen. I can set an alarm when navigating so that it notifies me if it is off by X amount (usually set at 100' so that it doesn't go off too frequently). Having played with that a lot recently and watched the changing accuracy, I'm now more suspicious of the GPS reading (and even radar) than I am of the chart. I assume that the electronic chart is an electronic copy of an official chart. The official chart may have been around for 100 years and has been checked and updated periodically by the CG, NOAA, US Navy, etc. over the years. Starting with lead lines and stadiometers and now using sonar, laser, etc.

GPS looks to be accurate by showing you here you are on a chart. But if you watch the accuracy read out, it can bounce around in a way that it a little unnerving. And if the accuracy says it is within "37 feet," is that a 37 foot radius or diameter? And if the accuracy is alleged within 37 feet, how reliable is the accuracy of the accuracy? Is it based on some kind of a best guess algorithm computed from the number of satellites and their azimuths?

Reminds me of a hand-drawn roadside map I saw on a remote dirt road on Vancouver Island that featured a prominent "You Are Here." No I wasn't. I could tell just by the way the roads intersected and the surrounding hills (i.e., looking around). Just 'cause it says "You Are Here" doesn't make you are there.

GPS seems so authoritative that we are willing to accept its calculations based on orbiting satellites over dropping a piece of lead on a string. Reliance on outer space over eye sight. Sometimes we need to step back a bit.

 

[OldDan1943]

The suggested lead line does not have to be that long. Maybe 5-10ft longer than your keel depth.

People rely too much on technology.
I watched a video of a Tesla.... woman servicing her boyfriend while driving. He would look over her shoulder just to confirm he wasn't about to hit something.

I dont think there will ever be a RR effect on the ICW. Too many PWC and small undetected small fishing boats combined with those who have no concept of safe navigation.

 

[SteveK]

Marco, are you sure the alarm you are setting for 100' is not the off track alarm. As in the track should be and you are off track.

I cannot see how a GPS that is inaccurate know it is inaccurate without correcting itself. Like a dog chasing its tail.

 

[backinblue]

My Garmin GPS charts show very inaccurate depths in some parts of a river I boat in, but it could just be inaccurate chart data as someone else mentioned. In most cases it is accurate compared to sonar. Recommend setting up an alarm on your sonar to warn you well in advance of a grounding. Not enough that it becomes a constant annoyance, but deep enough to alert you of water depth you would not normally intentionally boat in.

 

[soin2la]

Along with all the usual charted discrepancies talked about, there also seems to be more and more boaters lacking skill and logic.

The radio chatter up here, between Hardwicke and Bramham Islands, is very disconcerting. When a pleasure boat travels through Cormorant Channel, into Sointula, thinks it is Alert Bay and wants to argue it, we know we have a problem. Hard to imagine how they managed to get this far.
https://mapcarta.com/24201540

…this week, in Desolation sound…

How busy is it down there now? I haven’t been through there in a month and it was busy then.

The record freshet on the Homathko River during the peak of the heat wave (40F) made the waters of Bute look like a scorched desert and I imagine there has been more debris down through Homfray, into Desolation Sound.

It’s busy up here, and around the corner, right on up to Hakai. The cable across Golby Passage at 4 feet above HHW, seems to be working well though.

I wonder whether crowd sourced data might solve at least partially….

Egads! The last thing I would rely on is the Twitterverse guiding me.

A fine example of the human factor, is the recent threads about groundings on clearly marked shoals/rocks in Prevost Harbor WA.

 

[Marco Flamingo]

Steve,

It isn't the off track setting for navigating to a waypoint. I have only tried that a few times and find it aggravating. If I'm in hand, I don't care if I'm off a computer generated line by 100'. That's why I'm in hand. It's like having a coloring book that sets off an annoying alarm if you go outside the lines. No thanks.

On my Garmin, if I go into Settings and check on the GPS, it shows me the satellite contacts and relative signal strengths. Up in the left hand corner is a tiny accuracy /prediction/guesstimate in feet. After watching it for a few days, I bought an external GPS antenna. It's at the Post Office right now. I'm curious to see if it has any effect.

The changing GPS accuracy sometimes shows up when at anchor. I've had an odd swing show up on the track screen when I haven't really moved more than a few feet. The GPS shows that I bounced over 40' feet and back in a matter of seconds. I think I would have noticed that. It sometimes does the same when I'm tied up in the marina. I know I would notice that.

 

[High Wire]

The gps satellites are moving all of the time. Each gps unit is a little different on how it computes its position. If tied to a dock, zoom in as far as possible. The track line will leave a little bird nest after a while. More stable = small to none. Less stable = big nest.
My Garmin 5212 has none. My Humminbird 798 leaves about 30 ft radius. A couple times a year something goes amuck and shows me traveling 500+ knots!

 

[SteveK]

Marco, got it. I have geofence on a separate monitoring device which I set when leaving boat. Tied to the dock it plots the boat moving around, sometimes into the next slip. GPS is great but not constant for many reasons.

 

[moparharn]

Not sure if your Navionics chip lets you scroll between the three charts they typically use. My Navionics have always offered their chart, the Government charts, and the crowd sourced charts (least reliable). In the Great Lakes I typically use the Government charts and observe the datum. Vexilar sells a wireless transducer that I use on my boats so that I can pair my phone and iPad to the sounder. It is a cheap investment that has functioned perfectly for me so far. The icon for the charts is on the left hand side of the screen (iPhone) and looks like a stack of paper. The choices are Nautical Chart, Sonar Chart, and US Gov Chart. When I have doubt I go to the US Charts. For $150 the Vexilar sounder is worth every penny IMO. Bill

 

[rwidman]

In addition to what the others have posted. When running in shallow waters it is good to know what your sounder's offset is. If there is no offset then the depth indicated is depth below the transducer. To know depth of water or depth below the keel you will need to know the depth of the transducer relative to the keel or waterline. What I mean by keel it is the lowest part or the boat. That could be keel, rudders or props. The offset can be used to indicate depth of water, depth below the keel or anything you want. ...........

Simpler yet is to calibrate the depth sounder so it displays the actual water depth.

On most chart plotters this is an adjustment called "keel offset".

Dock or anchor your boat in shallow water. Use a boat pole (and a tape measure) to measure the water depth. Set the keel offset so the depth on your scree matches what you just measured.

 

[Gdavid]

Churned up plenty of mud but was able to get out of it without getting wet.

It is a good idea to change the impeller in your outboard after churning up mud and silt, any grit in the bottom can wear your impeller out pretty quickly.

 

[Delfin]

First...I'm new at this, and I fully expect my bad experience with the Navionics chart plotter to be my fault.

I have a 2021 Barletta LA-25 tritoon and use it on Lake Norman in NC. The boat came with SIMRAD and I bought a Navionics chip and installed it in the SIMRAD. I believe it was the inland lakes southeast version, something like that.

First outing with the new chip and within an hour I ran aground. The chart said I was in 12 ft or water but the sonar said 1.5 ft. Churned up plenty of mud but was able to get out of it without getting wet. Since then I have seen depths on the chart all over the place... saying 4' but depth was actually 18', saying 18' but was 4', etc etc. Most of the time it is close to the sonar reading but most doesn't count, because most of the time I don't want to run aground.

Why is the chart off by so much? Am I possibly not using the correct chip? Any suggestions?

Chart registry can be an issue. Up north a few years ago the chart showed us on the side of a hill on an island. Notified CHS and they acknowledged and corrected the issue. One safe guard is to define the route with all waypoints, then scroll out or in to stitch a new chart with a different scale and verify you aren't being led astray.

 

[backinblue]

Simpler yet is to calibrate the depth sounder so it displays the actual water depth.

On most chart plotters this is an adjustment called "keel offset".

Dock or anchor your boat in shallow water. Use a boat pole (and a tape measure) to measure the water depth. Set the keel offset so the depth on your scree matches what you just measured.

I agree and have calibrated mine to show true depth of water using a similar method. Some prefer to calibrate to "depth below keel" which doesn't make sense to me, but I guess it does for some who maybe are deficient in math.

 

[rslifkin]

I agree and have calibrated mine to show true depth of water using a similar method. Some prefer to calibrate to "depth below keel" which doesn't make sense to me, but I guess it does for some who maybe are deficient in math.

I do the same as well. And surface depth readings are preferable, as they're easier to match up to a chart or to what someone else told you to expect. And it's an easy number to give to another person if they're wondering about depth in a shoaled area.

 

[backinblue]

I do the same as well. And surface depth readings are preferable, as they're easier to match up to a chart or to what someone else told you to expect. And it's an easy number to give to another person if they're wondering about depth in a shoaled area.

Also helpful in calculating anchor scope. Not that you couldn't do a little extra math and get there, but why bother? I also agree with ease of comparing to chart depths,

 

[diver dave]

BEWARE the Navionics "feature" called Water Level Correction. It allows every depth number on the chart to be instantly changed by an offset of your making.

 

[SteveK]

I agree and have calibrated mine to show true depth of water using a similar method. Some prefer to calibrate to "depth below keel" which doesn't make sense to me, but I guess it does for some who maybe are deficient in math.

That is an unnecessary comment.

We are not all the same with preferences. I prefer to see at a glance how much is below my draft. Also it makes for shocked passengers when we travel in one foot or less.

 

[OldDan1943]

True depth vs water under the keel is a personal preference.
I prefer water under the keel because the keel will ground first.
If I am in skinny water, going slow, I want an instantaneous reading of what's there.
The ICW is famous for shoaling, especially around the 'cuts'. We may to feel around searching for a path. We dont want to end up on a cut's delta nor the bank of a cut nor a 'bump in the road.' Again, just a personal preference.

 

[Marco Flamingo]

That is an unnecessary comment.

No, it makes sense. Mathmeticians change the offset to add value X so that the sounder calculates the surface, then they subtract the draft of the vessel, factoring in the changing vessel load and sea conditions, to estimate a numeric range representing the amount of water presently under the keel.

Boaters just offset the draft.

 

[rslifkin]

No, it makes sense. Mathmeticians change the offset to add value X so that the sounder calculates the surface, then they subtract the draft of the vessel, factoring in the changing vessel load and sea conditions, to estimate a numeric range representing the amount of water presently under the keel.

Boaters just offset the draft.

Nah, I just know "alarm beeps at this number and I should be nervous, if I get to this number, expect to feel a thump"

 

[SteveK]

No, it makes sense. Mathmeticians change the offset to add value X so that the sounder calculates the surface, then they subtract the draft of the vessel, factoring in the changing vessel load and sea conditions, to estimate a numeric range representing the amount of water presently under the keel.

Boaters just offset the draft.

so it makes sense to call out others for being mathematically challenged??? I think you misread/misunderstood.

 

[backinblue]

My math comment was meant as a joke. I know it's a matter of preference, but it just makes more sense to me to show true depth of water. If I know I am in 5' of water with 3' of draft, I already know how much water is under my keel. When in doubt, remember Lindsey's Law: “when your draft exceeds the water's depth you are most assuredly aground.”

 

[OldDan1943]

Soo-Valley, my math gene died about 5 years ago.

 

[SteveK]

Soo-Valley, my math genie died about 5 years ago.

Your spelling too, sorry for your loss.

 

[Marco Flamingo]

My math comment was meant as a joke

So was mine.

My Garmin just recently started dropping the depth reading frequently. Slow and shallow (<40') used to always get a steady reading. Not anymore. Time for some troubleshooting as the offset, whether surface or keel, doesn't matter right now.

 

[backinblue]

So was mine.

My Garmin just recently started dropping the depth reading frequently. Slow and shallow (<40') used to always get a steady reading. Not anymore. Time for some troubleshooting as the offset, whether surface or keel, doesn't matter right now.

Is the face of the transducer clean?

 

[bligh]

So this body of water has likely not been charted with GPS. Originally they had to use transits and triangulation methods to measure both the longitude of the lake as well as its elevation. Being so far from the ocean, this was not an exact science and the lake is not exactly where the chart says it is. While the lake itself is likely charted well even though the soundings are old, you are likely not on the lake where the GPS shows you to be. If you study the chart and landmarks and shoreline of both your surrounding area and the chart itself, you will ikely find you are off an 1/8 of a mile or less. You will need to just use the chart for reference and knwo that your position is off a bit and to be more aware of shoals and other hazards.

 

[rgano]

The charts on western rivers only display the height of the bottom above mean sea level. Then you subtract that charted level from the pool level (also in feet above mean sea level) to obtain the actual depth of water. Pool level is broadcast in some areas and obtained at locks as well. Pool levels can rapidly decline with a water release from a dam. Was Lake Norman possibly at reduced water level?

 

[psneeld]

On a Lake Norman web site it discusses a seasonal 8 foot difference in lake levels.