Back-up Depth Sounders?

[Kings Gambit]

I've heard before about how you could supposedly determine your position from depth soundings. What am I missing - looking at a chart I don't see how that works.

Here's a position from a recent trip I took. I was inside the red circle, reading 14' of depth. It was an incoming tide supposedly 1' above MLLW, so I should be looking for places charted as 13'. So I could've been anywhere along the green line, which is not a very precise position, and not correctly where I was -
View attachment 185363

I don’t think that’s exactly what was said; here, at least. I think Don’s point was that if your depth sounder fails, you can know the depth under you from your chart, if you have your chart position. If you are coming toward shore and your sounder goes out, but you have a chart track, position, you’d know how close to approach, etc., if it’s all charted correctly. With fog, it’s the same except that there is no evaluating the situation further looking toward shore for changes in water hues, or other indicators. Without the sounder in fog, you‘d have ONLY the plotted position on which to rely, although possibly with fog signals. Everyone here voicing an opinion agree that relying on a sole source of “input” while piloting is not best seamanship. Lacking sounding you would have no confirmation of your depth. Breakout the lead line (or digital) before coming closer. We’re talking here about a fm or more of variance, not a foot or two.

 

[Davidasailor26]

I don’t think that’s exactly what was said; here, at least. I think Don’s point was that if your depth sounder fails, you can know the depth under you from your chart, if you have your chart position. If you are coming toward shore and your sounder goes out, but you have a chart track, position, you’d know how close to approach, etc., if it’s all charted correctly. With fog, it’s the same except that there is no evaluating the situation further looking at the shore for changes in water hues, or other indicators. Without the sounder in fog, you‘d have ONLY the plotted position on which to rely, possibly with fog signals. Everyone here voicing an opinion agree relying on a sole source of “input” while piloting is not best seamanship. Lacking sounding you would have no second confirmation of your depth. Breakout the lead line before coming closer. We’re talking here about a fm or mor of variance, not a foot or two.

All good points. The assertion that you could approximate your position based on depth wasn’t explicitly here; I’ve just seen it elsewhere and Don’s post just got me thinking about it. And I’m sure the usability of the depth data does depend greatly on the region where you are. Where I sail everything is pretty much within a fathom of everything else, except the channels. Where I grew up sailing there wasn’t such a thing as 2 fathoms of depth. Places where there are sharper contours will have different considerations both for the usability of soundings and the need for them.

 

[WayneH]

The old depth finder on my boat would flake out at the dammest times. Moving from the old riverbed out to the dredged channel, it would sometimes report 5 feet of water when the chart would indicate 35 feet. This becomes a great annoyance when you're coming into a bay that has shifting shoals and suddenly the DF would report 1.5 feet and you go aground at 1.7 feet under the transducer.

A friend on mine taught me about a manual back up DF. Fishing pole, a non-weighted cork and a weight to make the cork float correctly. On the Gulf Coast, I have the weight set 6 feet below the cork. That gives me a foot of depth at low tide. If I cast the weight out in front of the boat, I get instant indication of Go/NOGO depending on whether the cork floats upright or on its side.

Why do I use this method? Because charts are not well made on the Louisiana coast. We spent the night on the back side of Raccoon Point twice. N29° 03.946' W090° 56.617' Creeped in, using the fishing pole method. DF showing 1.3' due to the freshwater streaming off the land. We can see the island but the chart doesn't reflect reality very well and indeed shows 1 foot of water. Eased in towards the island and dropped anchor about 100 feet off from the island. The 10foot tiewrap on the anchor chain when underwater before the anchor hit bottom. Nowhere inshore of the island does the chart show 10 feet but it's deep behind the island.

I've replaced the instruments on the boat since that last trip. Too freaking nerve wracking when you can't trust your instruments. We like to gunkhole and find locations where we can be undisturbed by powerboats and water roaches.

And the time is coming when I will have to reset my depth gauge for larger tidal swings.

 

[Kings Gambit]

The crude lead line I made many years ago was for use in Tampa Bay and Tierra Ceia Bay, and proximities. Chiefly when entering nearshore Mangrove waters on the Grady White, which had no depth sounder, to confirm chart depths indicating “passes”, etc.

 

[Charlie Jones s/v Tehani]

I use two different methods. Of course i have an electronic sounder, mounted and funtioning Then I have a lead line- a 2 pound weight on a 1/4 nylon 40 foot line, of which I usually do maybe 12 feet (I draw 3'6"). I use it off the bow as I'm creeping in
Then I have an old zebco reel on a short cheapy rod, A cork fastened at 5 feet and a weight heavy enough to pull the cork under. Feelinng my way into an anchorage, I use the dinghy, cast ahead and if the cork goes under- enough water. If it floats -too shallow. Used to have a wife who would drive the boat. now I anchor and test ahead..

Once I find water deeper than draft AND tide range, I anchor. That tide range is very important. In Texas waters it's usually around a foot. In the Carolina's it can easily be 11 feet Makes a HUGE difference

OH see- Wayne H #23 post above for the cork thing- I suspect I'm the friend he mentions

 

[Kings Gambit]

I use two different methods. Of course i have an electronic sounder, mounted and funtioning Then I have a lead line- a 2 pound weight on a 1/4 nylon 40 foot line, of which I usually do maybe 12 feet (I draw 3'6"). I use it off the bow as I'm creeping in
Then I have an old zebco reel on a short cheapy rod, A cork fastened at 5 feet and a weight heavy enough to pull the cork under. Feelinng my way into an anchorage, I use the dinghy, cast ahead and if the cork goes under- enough water. If it floats -too shallow. Used to have a wife who would drive the boat. now I anchor and test ahead..

Once I find water deeper than draft AND tide range, I anchor. That tide range is very important. In Texas waters it's usually around a foot. In the Carolina's it can easily be 11 feet Makes a HUGE difference

OH see- Wayne H #23 post above for the cork thing- I suspect I'm the friend he mentions

Clever

 

[SoSound]

I haven't ever felt the need for a back-up depth finder. If I lose my depth finder, chances are I've lost all my electronics and I've probably got bigger problems.

It happened to us two weeks ago. But then, I have a history of "...we've never seen (an instrument) failure like that before...". Started our cruise from Tacoma and all was well, everything working just fine. We took a bit of a beating for the first couple of hours with higher winds than expected, and opposing currents, but at the end of the day, still no problems. Start the second day, no depth data, nothing. We're headed for a marina at Port Townsend, so no problem, we'll continue on and troubleshoot when we get there. Two days later, West Marine in Anacortes had a replacement depth transducer in stock, and that fixed the problem. But, I definitely did not lose all of my electronics when I lost my depth data.

A very timely thread! Thanks, @Kings Gambit!

 

[Mr. Dave]

Charts are pretty precise depth indicators.

I don't know, the Cayuga Inlet channel in Ithaca is pretty silted up, lots of boat keep striking bottom, and getting stuck, in water that ain't even close to the stated depth on the charts.

As we only draw 2.5 feet, I don't fret, but it's a real problem for others.

 

[Kings Gambit]

My sounder has two alarm settings: deep at 80 ft @ shallow at 12 ft. It probably should be 20 ft, and it was once. Frequently approaching rapidly shoaling water could justify possession and use of a relatively long lead line, such as 12 fm (72 ft). We see this much depth close to the islands here. The anchoring area just seaward of the moorings at White Cove (Catalina Island) is about 60 ft, and shoals to 20 ft in just a few hundred feet closer to shore. One that long would be handy approaching the anchoring area with failed transducer. It might be far too long for use on the SE U.S. coast, such as around most of FL and other Gulf states. A “safe” distance from the Ventura coast where seas approach coincides with the 5 fm contour.

 

[SoSound]

My sounder has two alarm settings: deep at 80 ft @ shallow at 12 ft. It probably should be 20 ft, and it was once.

I had part of this conversation with @jssailem at Port Townsend (It was great, to finally meet you, John!), we used to have our shallow alarm set at 15 ft., or less. We had it go off in Canada last year, and by the time I could react I was seeing silt stirred up in the water behind me... What I'd like is a "wake up call", somewhere between "Depth data missing", at more than 700 ft., and "Shallow".

 

[Kings Gambit]

I had part of this conversation with @jssailem at Port Townsend (It was great, to finally meet you, John!), we used to have our shallow alarm set at 15 ft., or less. We had it go off in Canada last year, and by the time I could react I was seeing silt stirred up in the water behind me... What I'd like is a "wake up call", somewhere between "Depth data missing", at more than 700 ft., and "Shallow".

I think there should be two programmable settings, with an alarm notice at each one. Set the deeper one for 80 ft.

 

[capta]

I've heard before about how you could supposedly determine your position from depth soundings. What am I missing - looking at a chart I don't see how that works.

Here's a position from a recent trip I took. I was inside the red circle, reading 14' of depth. It was an incoming tide supposedly 1' above MLLW, so I should be looking for places charted as 13'. So I could've been anywhere along the green line, which is not a very precise position, and not correctly where I was -
View attachment 185363

The problem arises when us pleasure boaters use charts made before GPS. The powers that be are updating charts used by commercial vessels, but not much is being done to update charts for us. We just aren't a big enough market for the expense. So, your GPS is giving you your actual position withing a few feet, but when they made the charts, they were hand drawn from soundings taken on a compass bearing from a fairly accurate, but not GPS accurate position.
I've seen sat pics of an island beside the hand drawn old chart of the same island and you wouldn't believe they were the same island.

 

[capta]

It happened to us two weeks ago. But then, I have a history of "...we've never seen (an instrument) failure like that before...". Started our cruise from Tacoma and all was well, everything working just fine. We took a bit of a beating for the first couple of hours with higher winds than expected, and opposing currents, but at the end of the day, still no problems. Start the second day, no depth data, nothing. We're headed for a marina at Port Townsend, so no problem, we'll continue on and troubleshoot when we get there. Two days later, West Marine in Anacortes had a replacement depth transducer in stock, and that fixed the problem. But, I definitely did not lose all of my electronics when I lost my depth data.
A very timely thread! Thanks, @Kings Gambit!

You obviously have not been keeping the black box full. Gotta work on that.

 

[SoSound]

I think there should be two programmable settings, with an alarm notice at each one. Set the deeper one for 80 ft.

Would a deep water alarm sound when the depth is decreasing to 80 ft.? I know that the shallow water alarm does not sound when the depth is rising, it sounds only when depth is falling.

 

[jaysweet]

Charts are pretty precise depth indicators.

I don't know, the Cayuga Inlet channel in Ithaca is pretty silted up, lots of boat keep striking bottom, and getting stuck, in water that ain't even close to the stated depth on the charts.

As we only draw 2.5 feet, I don't fret, but it's a real problem for others.

I agree. While I am often reassured to see my depth sounder agree perfectly with the chart, we just went aground in supposedly 7 feet of water (our boat has 3'10" draft) over Labor Day weekend. Around 1/2 hour after we were pulled off by a towboat, that same day, we were in a major well-travelled dredged channel which reports 2.2 meters of water in 2010 on the chart. We were behind a large ~40 foot sailboat squarely in the channel, when all of a sudden, they stopped dead. I talked to them as we snuck by slowly on the outside: they drew 5.5 feet and had gone aground. Unfortunately, there was no way I could help them in a busy channel with an 18 hp engine.

The Chesapeake is a muddy mess, and many of these depth soundings are very, very old.

 

[jssailem]

Alarms are nice, but not a substitute for situational awareness. With waters sometimes 700 feet deep in the Puget Sound, anything beyond 300 feet my DS gives up. If I come upon a non charted seamount, it is likely going to get me just like uncharted shoaling. Sometimes it is like strolling in a mine field.

On the other hand less than 80 feet and the DS is golden.

 

[jaysweet]

My sounder has two alarm settings: deep at 80 ft @ shallow at 12 ft. It probably should be 20 ft, and it was once. Frequently approaching rapidly shoaling water could justify possession and use of a relatively long lead line, such as 12 fm (72 ft). We see this much depth close to the islands here. The anchoring area just seaward of the moorings at White Cove (Catalina Island) is about 60 ft, and shoals to 20 ft in just a few hundred feet closer to shore. One that long would be handy approaching the anchoring area with failed transducer. It might be far too long for use on the SE U.S. coast, such as around most of FL and other Gulf states. A “safe” distance from the Ventura coast where seas approach coincides with the 5 fm contour.

This talk of setting depth alarms at 80 ft sounds pretty funny to the Chesapeake Bay sailor's ear. Here, the major shipping channels are dredged to maybe 55 ft, and most of the rest of the middle bay is 20-25 ft, totally leaving aside the creeks, rivers and inlets. I did charter once out of Anacortes, though: the west coast is a totally different beast.

 

[Michael Davis]

This talk of setting depth alarms at 80 ft sounds pretty funny to the Chesapeake Bay sailor's ear. Here, the major shipping channels are dredged to maybe 55 ft, and most of the rest of the middle bay is 20-25 ft, totally leaving aside the creeks, rivers and inlets. I did charter once out of Anacortes, though: the west coast is a totally different beast.

55 feet is just getting into good anchoring depth!

 

[jssailem]

So true Michael. Chesapeake sailors have a lot of water that is some areas you can walk in and not get your knees wet.

It is a fun place to explore. They have soft shell crabs and bluefin crab cakes. And warm summer evenings that seem to last till midnight and beyond sitting by a bay.

 

[jssailem]

Michael, most of the time the concept of a good fleece to throw on, in the AM so you can sip a cup of coffee in the cockpit would be foreign. They also only know Orca from the movies or in google images.

We have so much wonderful water to explore it is sometimes overwhelming.