Bottom Imaging for Sailboats

[Ivanvet]

Originally Posted by Dockhead

Great; thanks!


Pardon my ignorance -- but what does "CHIRP sonar only" mean? So this is just fishfinder like picture of the water column under the boat only?


So side scan and struture scan or whatever requires a different shape transducer?


Can someone please explain what the different types are, and their capabilities, or steer me to some materials which explain this???


Assuming I can't have side scan or structure scan or whatever but just the water column under the boat -- does anyone feel like this is still worthwhile for this purpose?

This is a little summary of the types of fish finder transducers and how they can be used to ‘read the bottom’.
The ‘traditional sonar’ uses a typical transducer that transmits a single frequency in a shape of a cone. The lower the frequency the deeper you can scan. This ‘cone’ has an angle. Depending on this angle and how deep you are you will scan more/less area (if you are in shallow waters, the area scanned is smaller). In theory, if what you want is to have an idea of what the bottom looks like (soft Vs. hard), with traditional sonar you may have enough. The problem is that since we typically anchor in shallow waters (small cone), the area that you are reading is small, which means that you have to ‘scout’ around (that’s not too bad anyway). In this picture of traditional sonar, hard bottom has usually a ‘double bottom’ artifact, vs a relatively soft bottom where it is thicker and has only a ‘single layer’.




You can also see if the area is too weedy.



The next step up, which is what most transducers have nowadays is Chirp. This is the same as traditional sonar, but it uses multiple frequencies at once, giving you a better resolution.



Then you have down scan (Lowrance) or clearvu (Garmin) or down imaging (humminbird). All the same, with different names to confuse us (like with code zero sails). It also gives you an imaging of what’s happening under your boat, with very high resolution.




And then, you have side scan (Lowrance) or sidevu (Garmin) or side imaging (humminbird). This technology is great for fishing because it looks to the sides (duh) covering a lot of terrain. It needs a little bit of practice and fiddling around to get nice images but it pays off (for fishing).




StructureScan (Lowrance, B&G and Simrad of the Navico family) gives you an imaging of the bottom of the boat in 3D. I don't have experience with this one so I don't know how reliable it is.

I like using these transducers just because I do like fishing, but I don't think it's worth the trouble for what you want to do. In my case, I don't use an extra hole in the boat, I just bring the transducer from the dinghy to the boat and I mount it on the transom with a 'transom pole' (no holes). If you want something to fill the throug hull, then you are left with Chip (traditional 2D sonar), which, with a little bit of practice you can learn to read the bottom.

I hope this helps.

Ivan
S/V Balú

[a64pilot]

Originally Posted by NorthernMac

How much power do they use? Can it also NEMA our simple depth readings to other instruments?

That was my point about heat. At full screen brightness, the whole thing consumes 4.3W that’s not enough power to generate much heat.
My Dragonfly 4 was $105. It’s a stand-alone really good fish finder, and will show you exactly what’s on the bottom, throw a bicycle in and it will show a bicycle. A crab trap will be a box, it won’t show individual wires, I doubt it will break out tiny wires.
But at $100, it’s not a high end anything, it’s not going to interface with anything. I believe the newer ones are Bluetooth, because Bluetooth sells, doesn’t have much utility but people love to connect their gadgets to their cell phones.
If you spend real money for one, then I assume you will have much higher power and probably heat and maybe be put on the network etc.

[a64pilot]

I would expect one to work inside of a hull, but I would also expect it to lose most of or at least a great amount of the high frequency, and the high frequency is where the detail comes from.
I mean with 4W total the little dragonfly is supposed to work to 600 ft, but I Would expect that it starts to lose it’s fine detail imagining at pretty shallow depths, because 4W just isn’t much power.

But what really effects these things to an enormous amount is wave action. There were bridge spans from an old girder bridge sunk at Panama City as dive spots, on the very rare calm days it was amazing, you could see the actual bridge in great detail.
But on the average day with 2 to 3 ft waves, you could see definitely there was something there, but it was jagged broken straight lines at strange angles, because the boat was bouncing up and down.

Maybe expensive ones have accelerometers and can dampen boat movement out with software? But if not you will never see the photorealistic screen shots like in the ads, unless it’s mill pond smooth water.

[Dockhead]

Originally Posted by Ivanvet

This is a little summary of the types of fish finder transducers and how they can be used to ‘read the bottom’.
The ‘traditional sonar’ uses a typical transducer that transmits a single frequency in a shape of a cone. The lower the frequency the deeper you can scan. This ‘cone’ has an angle. Depending on this angle and how deep you are you will scan more/less area (if you are in shallow waters, the area scanned is smaller). In theory, if what you want is to have an idea of what the bottom looks like (soft Vs. hard), with traditional sonar you may have enough. The problem is that since we typically anchor in shallow waters (small cone), the area that you are reading is small, which means that you have to ‘scout’ around (that’s not too bad anyway). In this picture of traditional sonar, hard bottom has usually a ‘double bottom’ artifact, vs a relatively soft bottom where it is thicker and has only a ‘single layer’.




You can also see if the area is too weedy.

The next step up, which is what most transducers have nowadays is Chirp. This is the same as traditional sonar, but it uses multiple frequencies at once, giving you a better resolution.

Then you have down scan (Lowrance) or clearvu (Garmin) or down imaging (humminbird). All the same, with different names to confuse us (like with code zero sails). It also gives you an imaging of what’s happening under your boat, with very high resolution.


And then, you have side scan (Lowrance) or sidevu (Garmin) or side imaging (humminbird). This technology is great for fishing because it looks to the sides (duh) covering a lot of terrain. It needs a little bit of practice and fiddling around to get nice images but it pays off (for fishing).


StructureScan (Lowrance, B&G and Simrad of the Navico family) gives you an imaging of the bottom of the boat in 3D. I don't have experience with this one so I don't know how reliable it is.

I like using these transducers just because I do like fishing, but I don't think it's worth the trouble for what you want to do. In my case, I don't use an extra hole in the boat, I just bring the transducer from the dinghy to the boat and I mount it on the transom with a 'transom pole' (no holes). If you want something to fill the throug hull, then you are left with Chip (traditional 2D sonar), which, with a little bit of practice you can learn to read the bottom.

I hope this helps.

Ivan
S/V Balú

Thanks for taking the trouble to write this!



Sounds like Side Scan or Structure Scan would be really useful for surveying difficult anchorages. But can't be done via a through hull, correct?


So that leaves Down Scan? I think that this would still be very useful. You wouldn't see as much terrain at once, but you don't necessarily even need to see the whole anchorage anyway -- just check and confirm that the bottom in that spot where you plan to drop the anchor is clear.



Another useful thing would be if underwater cables are visible. I once brought up an unmarked and uncharted cable on my anchor -- a terrifying moment. In Finnish waters.

[Manateeman]

Dockhead. I see that in 2014 there was an earlier discussion on this subject in which you also mentioned lead lines. You were correct then that a really good picture of the bottom requires power. Very true. High quality sonars are getting smaller and less costly. For a really nice picture of the bottom, please look at what’s available for commercial survey or commercial fishing.
I’ve sent inquiries to friends knowledgeable in imaging sonars. They suggest you might find an older Norbit unit as survey companies frequently upgrade their equipment. I had a good laugh when they sent me a photo of a huge Norbit transducer attached to the bottom of a kayak.
I wouldn’t put it past the Navy to Velcro one to a manatee.
Regards.
Mark a manatee without earbuds.

[johnn33]

This summary may be of interest
https://bluerobotics.com/learn/under.../#introduction


Perhaps not for all of us...

John

[Sea Huddle]

Sounds like a huge expense and a guessing game each time you enter an anchorage. Easier to keep a spare anchor and use a trip line. Btw, I've never lost an anchor and I imagine most haven't. Play the odds in Vegas!

[pianopraze]

Originally Posted by Dockhead

But i wish I could just LOOK at the bottom. I know some fishing boats have all kinds of fantastic toys like side scan sonar. My B&G Zeus MFD's have some kind of sonar box built into them. Is there some device I could use to image the bottom, which would fit into a standard 51mm through hull?

Yes,

you can literally look at the bottom, and for less than $1000:

https://www.digitalcameraworld.com/b...erwater-drones

[QaiLo]

There are drones out there with cameras that will give you a visual on the bottom, or what ever. We used them to inspect piling and the underside of docks. Not terribly expensive.

[Wolfy]

I use a 500ft depth rated low light level, colour video camera attached to the chain links on a bracket approx 8ft above the anchor its position gives a clear view of the anchor itself and what's immediately below it.
The viewing monitor is at the helm which gives me the warm fuzzies as I watch the anchor lower slowly on to a favourable section of seabed then fall over and "bed in" as far as I want it to be when I start backing down.
The camera stays on location with the "ring" led lights on until it's time to recover.
I use some cheap tywraps to secure the small video cable to the chain every 10ft...they're easy to break off during recovery including the camera and bracket.
The whole systems costs less that a descent depth finder...taking orders now😉

[StoneCrab]

I am always skeptical of historical references.

Like the HVAC guy who responds to your questioning an OBVIOUSLY bad idea with "son, I've been doing it this way for 30 years". That doesn't make it right or good.

While yes, mariners of old did use sounding leads; often to great effect, there are a great many shipwrecks from days of yore. How many of these were due to ineffective anchors or bad guesses about the bottom?

I rarely have any worries underway. It is when I get underway, thinking that I am firmly anchored that keeps me up at night. Short of diving on the anchor, picking your spot is just an educated guess.

The HVAC guy and the anchor set has fooled me quite a few times. I've got plenty of stories to tell in both cases.

[Jammer]

Originally Posted by NorthernMac

What about one of these?

Tie a line to it, plop it in the water and WiFi to your navioncs on your phone or iPad


https://www.amazon.com/Vexilar-SP100.../dp/B00CJJBM66


I have their other sonarphone product, mine suction cups to the back of a boat and it’ll paint sonar right onto navionics, great for bird dogging bigger boats, and of course as a fish finder too

This seems like it would work well for your needs

I have the suction-cup transom mount version of the same thing and use it on my canoe. It isn't side scan -- it provides the same data you would get from a typical thru-hull mounted transducer.


The pod version is perhaps useful for measuring depth when you don't have a boat or in areas where you can't get a boat, such as when fishing from shore, or when trying to map an area close to a dam.

[goguygo]

I've wanted to connect one of these to a crab pot for entertainment purposes, they are cheap and it may work...
https://www.ebay.com/itm/Eyoyo-30m-U...4338da2888525f

[Jammer]

Originally Posted by Dockhead

Sounds like Side Scan or Structure Scan would be really useful for surveying difficult anchorages. But can't be done via a through hull, correct?

There are side scan thru-hull transducers. For sailboats with substantial deadrise they recommend installing a pair of them:


https://www.simrad-yachting.com/simr...thruhull-dual/


They are expensive, and I am unsure whether the (also expensive) structurescan compute module is necessary.

[Togreenfeet]

This might be out there but have you considered an underwater drone? ROV? They are becoming more and more affordable with 150m tethers. I'm partial to the QYSEA FIFISH V6......... there are lots more.