Back-up Depth Sounders?

Don S/V ILLusion · Sep 26, 2020

Charts are pretty precise depth indicators.

shemandr · Sep 26, 2020

We used to carry a lead line. Someone gave it to me. We would take it out in the dingy to scout an anchorage. I never did use it off the boat.
Given KG's scenario, the next thing I'd do is crap my pants. Yeah, a back up sounder would be good but what if it gave a third depth? Which one are you going to believe? That aside, I would re-plot the position and go with that. All sounders can be adversely affected by thermoclines, soft muddy bottoms and probably other phenomenon.
I've actually been pretty much in that situation coming into Stonington CT. - and I anchored. This was before chart plotters but I used my GPS to plot my position. Problem was the GPS was reading in minutes and tenths of minutes and that chart read in minutes and seconds. There was no way I could do the conversion in my head with the level of stress I was experiencing. Once anchored it was easy to figure out. I made a course to Academy rock and turned West to feel my way into the anchorage.

Michael Davis · Sep 26, 2020

I have a friend who likes to anchor off the beaten path. He has a depth sounder, a GPS chart plotter and a computer/printer for his dinghy. Loves to go into uncharted place(we have a lot of those on the BC coast) and make his own charts, knowing he will be undisturbed at anchor.

Hunter216 · Sep 26, 2020

Little out of my “depth” here

as I don’t really need a sounder where I boat but the tech interests me.

The accuracy of using a GPS fix to locate your chart position and therefore depth might depend on how old your GPS is. L5 band GPS has improved things quite a bit in the last few years.

If your thinking redundancy have you considered adding a forward looking sonar? From what I understand they don’t look out very far <100meters but that might prove useful if you are approaching an area that is of concern.

Don S/V ILLusion · Sep 26, 2020

Kings Gambit said:
True. But we should consider the what if’s. What if it is very foggy and you’re on paper chart with no radar, close to shore. You have GPS, but you have to plot your position. The sounder does not agree by a fm or two with the depth your plotted position has you over. Now what?

Lots of ‘what ifs’ there. If all that was concurrently a problem, I’d do what we told boaters via radio when they called the CG for help or rescue because they were lost - that’s why God invented the anchor.

shemandr · Sep 26, 2020

Since I am sailing OPB's primarily and doing RC work, I would consider a portable unit.

Will Gilmore · Sep 26, 2020

Don S/V ILLusion said:
Charts are pretty precise depth indicators.

In fog, on the coast of Maine, I was taught to compare the depth under the boat with that on the chart to confirm where you are. At the time of my lesson, I was certain we had sailed to far and missed the channel we were looking for. After the lead line showed only eight feet under our keep, I found a likely anchorage on the chart that was three miles farther past where we were trying to go. As I peered through the fog, I confirmed it was an anchorage as I could just make out a couple of boats on bouys. We were right between them.

I consider a depth gauge, of some type, an essential piece of navigation equipment. Having a backup wouldn't be bad. I'd go for the backup that took up the least space and weight, cost the least and was never going to break when you needed it.

-Will (Dragonfly)

Hunter216 · Sep 26, 2020

Kings Gambit said:
That could be useful.

Now that I’ve got you thinking about spending boat bucks

here is a fairly recent article.

Sonar and sounders: 4 new models that are good for more than just fishing

Developments in sonar technology are now allowing us to see the shape and structure of the seabed, including vegetation, in increasingly clear detail

www.yachtingworld.com

capta · Sep 26, 2020

With GPS as affordable as it is today, and considering the reliability of modern day (and older ones, I still see the old flasher types now and then),
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.

rgranger · Sep 26, 2020

Okay... maybe I need some sleep but when I first read your post title I thought... "why would your present sounder not function when the boat is in reverse".

NYSail · Sep 26, 2020

My wife bought me a hand held battery unit that works great..... not expensive and pretty cool to have.

Davidasailor26 · Sep 26, 2020

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 -

Head Sail · Sep 26, 2020

NYSail said:
My wife bought me a hand held battery unit that works great..... not expensive and pretty cool to have.

That's what she told you it was? lol

shemandr · Sep 26, 2020

A fair point. Using depth to fix your position depends on the bottom contour changing. Not all areas are suitable. You would also need a line of position from some charted feature. Suppose you knew you were on the depth contour of 11/13 feet. That with a LOP would give you a reasonable idea of your position. It would also eliminate a lot of area that you could say "I'm not here." That's not as good as knowing where you are but it's better than nothing. Presumably you would also have a DR plot that would contribute to your confidence. Obviously we all would prefer the DBO accuracy of the GPS or Chart Plotter.

capta · Sep 26, 2020

Davidasailor26 said:
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.