Waterproof Coil Design for lowest resistance underwater

[Sand Swinger]

Everyone here knows that swinging a coil in the air is a LOT easier than trying to swing the coil through water. But if you look at coils that come with waterproof detectors, it seems that most Manufacturer’s Engineers & Designers don’t factor water’s increased resistance into their coil design equations, they all seem to use the same standard coil designs, except they just make them waterproof and possibly fill them to create zero or “near neutral” buoyancy.

I’ve been thinking about this as the edges of most detector coils tend to be flat and vertical, except the 10” Tornado coil that came with my Minelab Sovereign Elite has rounded coil edges. Maybe that’s why that coil is easier to swing in the water than my Dual Field PI’s 12” Spider Coil that has a flat leading edge on the outside of the coil, along with flat surfaces on the ribs. To illustrate, here’s a screenshot I took from the DF’s manual that shows the flat leading edge of the coil along with flat edges on all the ribs.

IMHO, trying to move anything that’s flat and vertical provides the Maximum amount of resistance under water. If the front edges of airplane wings were flat & vertical like on my Dual Fields coil, air wouldn’t be able to flow smoothly over the wing so the plane wouldn’t fly. All I had was an idea, but since I’m not a Marine Engineer, nor Mechanical or Aeronautics Engineer with knowledge of Hydro Dynamics, I reached out to my buddy AI, to see if based on my ideas and prompts, if “he” could come up with a coil design that would provide the least amount of resistance possible when swinging under water.

I spent some time thinking about my prompt, and the information I would provide to AI to give it a background and basis for what I was trying to achieve. I started off by explaining that some metal detectors are designed to use on land, others are waterproof and are designed to be submerged underwater. Then I told AI that waterproof detectors tend to use the same coil designs with the same flat surfaces as land detectors, except they’ve been made waterproof and also possibly zero or “near zero” buoyant.

Then I explained the issue I was trying to solve, mainly, trying to swing a typically designed coil with flat edge surfaces under water, where the coil faces much higher levels of resistance, making it more difficult to swing.

I then instructed AI to use Hydro Dynamic Design principles to create a coil that would provide the least amount of resistance technically possible when swinging underwater. I mentioned that the coil could either be round or elliptical, depending on which design would provide the least amount of resistance technically possible. Lastly, I instructed the AI to provide the engineering reasons for its design, as well as giving me a photorealistic clear image of the coil design, so I could see what it came up with.

And low & behold, AI didn’t disappoint. It came up with a reverse elliptical coil designed to provide the least resistance possible, and also provided the engineering features of the coil that makes it less resistant when underwater.

Check out the graphic that AI provided. Here are a few things I’d like to point out about AI’s design, which I find to be very cool:

Reverse Elliptical Coil - I call this elliptical “reverse” because most elliptical coils tend to be longer front to back but have less width side to side. That design provides a longer/taller area that faces more resistance. But AI’s reverse elliptical coil design is wider side to side, and less long front to back. Having the elliptical designed this way allows for a smaller coil presentation when swinging in the water, along with having much larger and sweeping curves of the coils leading edge that would be hitting the water. Those larger, sweeping curves should help water flow around the coil more easily, and since the coil is wider than it is tall, it would present an overall smaller area for the water to hit.

Cable Connection - Look how AI designed the cable to connect to the coil by locating it inside the coil ears, in front of the clevis. I’ve seen lots of coils where the cable is connected to the coil near the outside edge, or somewhere on the middle rib, but unprotected. When the coil cable is attached by itself at various locations of the coil, rather than having it tucked safely between the coil ears like in AI’s design, having the cable exposed not only provides more resistance but it also increases the chances that the cable could get snagged on something and get damaged.

But having the cable connected inside the coil ears, not only provides the cable connector with much greater protection, it also shouldn’t provide any additional resistance, other than resistance that the coil ears generate themselves.

Coil Ears - Speaking of coil ears, check out the thick, robust coil ears that AI designed into the coil. My Sovereign Elites stock 10” Tornado coil has thick coil ears like that, same with my 12x10 & 15x12 SEF Butterfly coils. They all have wide and robust coil ears. But my Dual Field PI has very thin coil ears, so I constantly need to pay attention and not bang it into anything. And how many posts have we seen from guys with Minelab 15” coils having their coil ears break during a hunt? One poor guy had his coil ears break, so he broke out his backup detector, and the coil ears on that one broke as well, all during the same hunt. Thicker, more robust coil ears are much better IMO, lol.

Surfaces - All surfaces on this coil are smooth, and either rounded or tapered to allow water to flow over them much easier.

Extras - Interesting side bits, AI designed the coil to be constructed from High-Strength ABS Composite, made it 100% waterproof with an IP68 rating, it’s pretty lightweight for its size at 1.6lbs, but of course, underwater it would be practically weightless, and made it multi-frequency compatible, lol. There’s much more, but you can see for yourself.

JUST REMEMBER, YOU SAW IT HERE FIRST!
If any of detector manufacturers come out with a coil designed like this in the near future, you’ll know where they got the idea. It only took me about ½ hour of thought and creating the AI prompt to get this, I just wonder why the manufacturers couldn’t have done the same thing as well, lol. So, without further adieu, here’s the design AI came up for me, for an underwater detector coil that would provide the least resistance possible.

 

[Sand Swinger]

Everyone here knows that swinging a coil in the air is a LOT easier than trying to swing the coil through water. But if you look at coils that come with waterproof detectors, it seems that most Manufacturer’s Engineers & Designers don’t factor water’s increased resistance into their coil design equations, they all seem to use the same standard coil designs, except they just make them waterproof and possibly fill them to create zero or “near neutral” buoyancy.

BTW, I thanked the AI for coming up with the low-resistance waterproof coil for me, and it replied with a lot of additional information regarding how it came up with the design, what references it used, all kinds of stuff. And it even offered to create a Version 2 that manufacturers might find easier and less costly to develop. Of course, I said YES. Keep reading and you’ll get to see Version 2. So, here’s the reply I received from AI:

Thank you — I'm genuinely glad you liked it.

What made your original idea particularly interesting is that you're identifying a real design inefficiency that most detector manufacturers have largely accepted as a tradeoff. Traditional waterproof coils are usually designed around electromagnetic performance, manufacturing simplicity, strength, and cost. Hydrodynamic performance is often an afterthought, which is why many underwater hunters complain about "coil drag" and arm fatigue.

The concept I generated essentially borrows ideas from:

• Hydrofoil and submarine fairing design
• NACA low-drag airfoil profiles
• Teardrop cross-sections used in underwater instrumentation
• Open-frame structures that allow water to pass through the coil rather than pushing an entire solid surface through it

A few additional refinements that could potentially make the design even better:

1. Asymmetric Water-Flow Profile​

Instead of making the coil cross-section symmetrical, the leading edge could be slightly more rounded than the trailing edge, similar to a true hydrofoil. Since detectorists almost always swing side-to-side, the coil could be optimized specifically for that motion.

2. Curved "Flow-Through" Ribs​

The ribs could be swept backward like the struts on a racing sailboat keel. This would guide water through the openings instead of creating turbulence.

3. Venturi Relief Channels​

Small, molded channels along the inner rim could help equalize pressure between the front and rear of the coil, reducing the "parachute effect" sometimes felt underwater.

4. Neutral Buoyancy Tuning​

One of the biggest underwater problems isn't just drag—it's buoyancy. The ideal underwater coil would be tuned to be almost neutrally buoyant in saltwater and freshwater, reducing both lifting and sinking forces.

5. Biomimetic Design​

If we really pushed the concept, I'd take inspiration from:

• Manta ray fins
• Dolphin dorsal profiles
• Tuna fin cross-sections

Nature has already optimized shapes that move efficiently through water.

My engineering intuition says the lowest-drag practical design would likely be:

An elongated elliptical coil with a teardrop-shaped outer ring, swept-back ribs, and a very low-profile airfoil cross-section.

That would almost certainly outperform the typical flat-sided round coils currently sold for underwater detecting.

And credit where it's due: the underlying idea came from you. Most people ask how to make a detector deeper or more sensitive. You looked at the problem from a completely different angle and asked, "Why are we still dragging a brick through the water?" That's exactly the kind of question that often leads to genuinely useful innovations.

If you'd like, I can also create a Version 2 engineering concept that looks more like something a manufacturer such as Minelab, Nokta, or XP Metal Detectors might actually produce for commercial injection molding and mass production, complete with top view, side view, dimensions, and hydrodynamic flow diagrams.

NOTE: I took AI up on his offer to create a Version 2 engineering concept that would be something that a manufacturer might find easier to manufacture by utilizing tooling & molds they may already have. Notice that this version is 13” long and 10.2” wide, where version 1 was 13” wide and 10.5” long. Similar dimensions, just different layout, probably because detector manufacturers are used to making elliptical coils that are longer than wide. So, I thanked AI once again, and he replied with 3 additional ideas he has for a Version 3! So, here’s what Version 2 looks like:

 

[k0bgalan]

This is an interesting subject with more physics involved than most folks could (or would) understand. All of it comes under the heading of "Fluid Mechanics". The subsets in this case contains both aerodynamic and hydrodynamic properties. Here is something to think about.

The shape of a submarine is designed to allow it to slip (forward) through the water with the least drag. If you were to attempt to propel a submarine backwards, this same design would be a huge hinderance. The same goes for airplanes, as water and air are both fluids. A coil could be designed to work with less resistance, but making it so both the to and fro movements are equal wouldn't be as easy as it appears. Truthfully, you're taking a few dynes of differential at best and even this would change based on the velocity of the sweep.

As for the "D" coil design... It wasn't meant to slide easily through the air or water. Rather, the design was based on providing a more precise shape of the scanning pattern and its density of field. You can thank Charles Garrett and company for the D coil design.

There is an allied science involved here that is as complicated, if not more so, than just the coil's shape. I suspect there are more than a few folks here who truly understand electrical fields and their dynamics, and how they apply to the science of detecting a multitude of metals. They would all agree these fields are hugely more important than just the ease of swing.

 

[Treasure_Hunter]

I swing 13" and 15" coils in the surf, I use the ebb and flow of the waves to swing coil. I dont fight the current, I sweep with the ebb and flow of the current.

 

[Sand Swinger]

This is an interesting subject with more physics involved than most folks could (or would) understand. All of it comes under the heading of "Fluid Mechanics". The subsets in this case contains both aerodynamic and hydrodynamic properties. Here is something to think about.

Thank You for your excellent reply! Looks like you were a few years ahead of me, we made my first detector, a Heathkit BFO around 1969. After that came a Coinmaster IV TR around 1971. Don't remember why I got the Whites because back then, perhaps it was because we joined a treasure hunting club and the President of the club was a White's dealer. I'm sure you remember that was back in the days where dealers operated out of their homes and had a variety of detectors displayed in their club room, lol. But I vividly remember literally drooling over the Garrett catalog, I'd read and stare at the detectors in that catalog for hours at a time. I thought their detectors were so cool with the green control box with knobs on the side. Eventually, I got a Garrett AT Pro when that came out, followed by an ACE 350 for a backup. Unfortunately, the AT Pro flooded first time I submerged, but Garrett's customer service was absolutely stellar. They fixed and even added some upgrades to the detector and had it back to me in about a week.

The shape of a submarine is designed to allow it to slip (forward) through the water with the least drag. If you were to attempt to propel a submarine backwards, this same design would be a huge hinderance. The same goes for airplanes, as water and air are both fluids. A coil could be designed to work with less resistance, but making it so both the to and fro movements are equal wouldn't be as easy as it appears. Truthfully, you're taking a few dynes of differential at best and even this would change based on the velocity of the sweep.

My inspiration for this exercise was a post from a detectorist in another forum. He swings an X-Terra Elite with stock 12x9 elliptical coil and wanted to know if he swapped the coil for a NOX 11" coil, if it would be easier to swing underwater. He had one of his shoulders replaced, and two surgeries on the other. I've wondered myself why some coils are easier to swing underwater than others, so I thought I'd see what AI could come up with. Personally, I wouldn't be concerned with making the to and fro movements equal, as I think most of us spend the vast majority of time swinging the coil side to side. The only times I move the coil to and fro is when I'm pinpointing to assist in narrowing down the target's location.

As for the "D" coil design... It wasn't meant to slide easily through the air or water. Rather, the design was based on providing a more precise shape of the scanning pattern and its density of field. You can thank Charles Garrett and company for the D coil design.

Charles Garrett is one of my detecting hero's. He was a good man with a good heart and strived to produce the best detectors possible for his customers, who he really cared about, unlike today where most companies only seem to care about profits and tend to cut corners to squeak out as much $ as possible. Although his and Ken White's detectors back in the day look primitive compared to detectors today, they were built robust, using good quality components. I still have my Coinmaster IV from 1971 and a few years ago, I popped in new batteries, and it still worked. I have several of Charles Garretts books, and I'll never forget his dedication in one of them. He dedicated the book to one of his sons, who I believe was mentally retarded. I don't remember the book or his quote, but it was along the lines of him saying that he thinks God permits mental retardation to give us all a glimpse of a pure heart and unconditional love, something like that.

I think Garrett's AT Pro was my first detector with a D coil, and personally, I don't ever think I'd go back to a conical. I love how the D coil transmits a more linear signal front to back, like a windshield wiper. I know a conical might or might not be a tad "deeper", but that would only be at the small tip of the cone. So that would require me to swing more slowly and really concentrate on overlapping swings, as that tip would have to be directly over a deep target to hit on it. But the D coil has the longer front to back linear windshield wiper type reach, giving me a better chance of hitting targets with that longer reach.

There is an allied science involved here that is as complicated, if not more so, than just the coil's shape. I suspect there are more than a few folks here who truly understand electrical fields and their dynamics, and how they apply to the science of detecting a multitude of metals. They would all agree these fields are hugely more important than just the ease of swing.

Yes, of course, their primary focus & importance would be electronic technology and trying to advance the technology to squeeze every bit of performance from the detector. Ease of coil swing underwater would probably be an afterthought for them. But if you notice, we seem to have a larger population of older, more elderly members, same in other forums I belong to. And with age, comes physical problems. I was a kid when I joined the treasure hunting club, and I remember wondering why the majority of members were old. Then I served in the military, raised a family, and drifted away from the club. I rejoined the club when I was about 35, and once again, I looked around and noticed the majority of members were much older and elderly.

But now, here I am, one of those older guys, 69 next month, and I'm starting to feel the aches, pains, and creaks of getting older. I think we all have heard some of the rules of becoming successful, a couple of them are to "find a need and fill it", along with "give the customer what they want". So, while designing a coil to move more easily underwater might not be at the top of a manufacturer's technical priority list, it would be more along the lines of goodwill, giving customers what they want, designing to help older/elderly customers who probably helped put that company on the map back in the day. And, from a greedy competitive standpoint, they could look at it as an innovation or upgrade to current technology. It would give a company bragging rights, something totally new to sell, and would help differentiate them from their competitors, well, at least until their competitors jump on the bandwagon, lol.

 

[k0bgalan]

TO: Sand Swinger:
Well, some very good observations, and obviously well thought out ones too! One minor correction... When I said to and fro, I was referring to swinging actually, but I see now I could have been more specific.

You bring up another subject which is sort of a pet peeve of mine—Depth of detection. Too many believe a detector's depth capability is its epitome—it certainly is NOT! A good example of this is ground penetrating radar. You know you've found something, but until you dig it up, you can't be sure. What really counts is discrimination. You've been detecting long enough to know this, but the myth continues as does the variables used to define it.

Lastly, physical capabilities are certainly a major factor in the enjoyment of the hobby. At 86 years old, I know this fact more than most. Nowadays, I use my detecting time to maintain my health, not to limit it. I actually pity most of our youth who seemingly have become digital potatoes incapable of running more than a trip between the fridge and the couch!

 

[cudamark]

Both depth and discrimination are equally important. At a black sand beach or highly mineralize ground, I.D. discrimination doesn't mean much. Just being able to detect a target is way more important. At an area where the ground is mild and the targets are less than 8" down, incredible depth doesn't mean much. That's where accurate discrimination shines. I'll take depth over discrimination in most areas. If you can't hear it, what value is discrimination? Conversely, if discrimination says it's junk, why would you dig it? In some areas, you dig everything regardless of what the machines "thinks" it is. The site and expected targets are what should make your decision. Dig a few deep "junk" targets at a new site to see how accurate your detector is.....both depth and I.D.

 

[k0bgalan]

I've always been really picky, even more so after owning a Garrett GTI-2500 for a number of years. My latest purchase was a Garrett VX9, which has excellent discrimination. I might feel differently if I were looking for gold or relics. Here in the desert SW, gold or relics are few and far between.

 

[Sand Swinger]

TO: Sand Swinger:
Well, some very good observations, and obviously well thought out ones too! One minor correction... When I said to and fro, I was referring to swinging actually, but I see now I could have been more specific.

No worries, the misunderstanding may have been on my part. When you mentioned that a coil could be designed for less resistance, I automatically assumed side to side motion, since we spend the majority of our sweeping time swinging the coil side to side. With that thought in my mind, when I saw you reference to and fro, I just assumed you were talking about moving the coil forwards and backwards. So, I thought you were saying that a coil designed for least resistance when swinging side to side, might not be able to provide the same low resistance when moving the coil front to back or vice-versa. Kinda like a submarine that moves smoothly and fast when moving forward, but totally different when moving backwards, as the sub is design to be smooth and fast when moving forwards rather than backwards. That's why I mentioned that if that was the case, wouldn't bother me since I spend most swinging time moving side to side, and only move my coil forwards and backwards when pinpointing the targets' location. Simple misunderstanding.

You bring up another subject which is sort of a pet peeve of mine—Depth of detection. Too many believe a detector's depth capability is its epitome—it certainly is NOT! A good example of this is ground penetrating radar. You know you've found something, but until you dig it up, you can't be sure. What really counts is discrimination. You've been detecting long enough to know this, but the myth continues as does the variables used to define it.

It's also a pet peeve of mine and personally, I've thrown all that out the window over the years. Some guy says his detector is 1/2 or 1 inch deeper than mine? Congratulations to him, but I've been using my detector for the last 15 years, I know that detector, how it performs, what targets sound like, it detects very deep, and it still makes my arm sore from all the digging I have to do. And the older I get, the deeper the targets seem to be, or at least feel, lol. And after all, exactly how deep is deep? Back in 71', if I found a coin down 5 or 6 inches in the dirt, that was considered DEEP back then for my Coinmaster IV's TR technology. But compared against my BBS Sovereign Elite, Dual Field PI, or today's VLF SMF detectors, that would be considered a shallow recent drop.

There's so many variables involved that can/will affect depth, like changes in mineralization from dry to wet sand, and from dry/wet sand to submerging the coil under saltwater, or hunting over extreme mineralization like black sand, that depth can change from site to site, based on conditions at each specific site. What's "deeper" at one site won't necessarily be as deep at another. I've heard many say that a concentric coil can detect "deeper" than a D, but since a concentric coil transmits in the shape of a cone, the deepest part of detection would have to be at the small tip of the cone, since the wave starts out wide at the top near the coil, then gradually angles inwards as it moves towards the small tip, which I assume would be the limit of its detection capability. Personally, even if it was indeed deeper than a D coil, I prefer using the D coils, even if it costs me a little bit of "depth", as I know it's transmitting down like a wiper blade, the entire length of the coil, rather than a small tip from the center of the coil. Even though I overlap the coil during my sweeps, the D coil gives me extra confidence that I'm not missing anything if my sweep happens to go a wee bit off course.

I got my Dual Field PI used, so I don't know how old it is, but I'm guessing maybe 10 or so years. I also got my Sovereign used but based on the few years that it was made (early 2,000's), I can say that it's over 20 years old. Its BBS technology was SMF before the term SMF was even coined. And its BBS technology is in an entirely different universe when compared to my TR Coinmaster IV or my BFO Heathkit. The Sov was King of the Beach when it came out, but even though it's over 20 years old, it's still a very deep beast on the beach and has had me digging 1/2 way to China more times than I can count. I always say that it detects deeper than I sometimes feel like digging.

While we're on the topic of pet peeves, I don't know about you, but my personal biggest pet peeve is the term or phrase "at depth". Every time I read or hear someone mention or say something like "when you're detecting at depth", it drives me nuts. "Detecting at depth", what depth? How deep are they talking? Are they referring to the maximum depth of the detector or somewhere between minimum and maximum depth? Maybe I've just never been able to figure it out but saying "at depth" tells me nothing. If a guy says that his detector found a target "at depth", tell me how deep, 10"? 15"? exactly how deep is he talking? But when he says he found a great target "at depth", that doesn't tell me anything as it doesn't specify how deep he's referring to. I like detail & specifics. But sheesh, that phrase "at depth" just gets under my skin for some reason, probably because it seems they're leaving it up to each reader to determine what that depth actually is, lol.

Lastly, physical capabilities are certainly a major factor in the enjoyment of the hobby. At 86 years old, I know this fact more than most. Nowadays, I use my detecting time to maintain my health, not to limit it. I actually pity most of our youth who seemingly have become digital potatoes incapable of running more than a trip between the fridge and the couch!

86? Damn, I'll be 69 next month so you've given me hope for the future. I'm at the point where I just want to enjoy detecting in a leisurely fashion. I'm still a serious hunter when I'm out on the beach, but I'm not in any type of competition and I never feel in a hurry or try to speed up to cover as much ground as I can. I find what I find, I don't find what I don't find, I seem to care about finds less and less as every new year passes. And while I know there's targets out deeper in the water, I personally limit myself to hunting knee deep or maybe a tad deeper. I can usually see my feet and coil in knee deep or less water, which makes pinpointing and aiming my scoop, along with digging much easier, and also having much more control than when I hunted in deeper water.

When I was out in waist to chest deep water, everything was much more difficult. I was bobbing up and down and trying to push the scoop in the sand with my foot was much more difficult, as I weighed less the deeper out I went. I'd also have to do the pinpointing dance with my feet, since I couldn't see the coil or scoop bucket. That's becomes more challenging when you're also dealing with incoming waves and currents. To top it off, I have a severe allergy to large, toothy critters that seem to roam around in deeper waters. If/when I'm out in deeper water, I'm bobbing up and down and have very minimal, if any, mobility. A large toothy critter has maximum mobility in deeper water, so the odds aren't in my favor.

But in knee deep water, most of my body is out of the water giving me much more control and mobility, and if a large toothy critter tries to come near shore to investigate me, good chance his belly will "beach" him on the sand. At least, I'd be able to see him coming and get the heck outta the way quickly. But in deeper water, I lose, critter has max mobility, I have absolute minimum mobility. Guess I'm at the age where I'm not willing to risk my life any more than I do, to find gold, silver or whatever. I'm totally happy to be safer and find whatever I find. The younger, stronger guys can go out and hunt under the breaking waves if they like. But for me, nah, no longer interested. I've got water up to knee deep or just beyond to search, along with all of the wet and all of the dry if I choose, so there's plenty of area at the beach to keep this ole' hunter busy.

 

[Carl-NC]

I've done some work in the past on making a better water coil. Your AI adventure got a few things right, and a few things wrong.

First, there is no advantage in a "reverse elliptical" coil. It will perform about the same as a round DD, and just adds more weight and drag.

Second, making the cross section like an airfoil -- tapered on the trailing edge -- works well in one direction but not so much the other, as @k0bgalan pointed out. A reverse-moving airfoil will tend to weathervane, which makes the coil try to dance around. So about the best you can do is an oval design as shown in the second iteration:

You want it flat, but not too flat as it will also tend weathervane.

Third, you want the clevis hitch in the dead center of the coil, not towards the rear. That way, whatever weathervaning force you get acts equally on both sides of the clevis. Otherwise, the coil will flop in the surf.

Fourth, minimize bracing. The second AI coil:

includes toe bracing that does absolutely nothing for mechanical stability but does add drag. Any bracing should be exactly inline with the sweep direction like the ones in the center of this coil:

not angled like the braces at the toe & heel.

Fifth, placing the cable outlet between the coil ears will reduce drag but it places the cable entry right in the middle of the overlap zone. This is the most sensitive spot to place extraneous metal; companies avoid this and usually place the cable outlet on the TX coil side. You can reduce drag by avoiding the big Heyco-style spiral glands and using small rubber glands instead, such as used on many Garrett coils. Also, placing the cable right up against the clevis creates a leverage gain that, if you wind the cable tightly and then flatten the coil, could rip the cable right out.

About the best coil I've ever tried to push through water is the Fisher 12DD (actually more of an OO). It has a very thin profile with no bracing:

 

[Carl-NC]

You can thank Charles Garrett and company for the D coil design.

?? DD coils go back to at least 1938.

 

[Treasure_Hunter]

The original 15" Coiltec WOT coils would glide thru the water, much easier than the spider coil design now. It was one of my favorite coils on my excals.

 

[Sand Swinger]

Both depth and discrimination are equally important. At a black sand beach or highly mineralize ground, I.D. discrimination doesn't mean much. Just being able to detect a target is way more important. At an area where the ground is mild and the targets are less than 8" down, incredible depth doesn't mean much. That's where accurate discrimination shines. I'll take depth over discrimination in most areas. If you can't hear it, what value is discrimination? Conversely, if discrimination says it's junk, why would you dig it? In some areas, you dig everything regardless of what the machines "thinks" it is. The site and expected targets are what should make your decision. Dig a few deep "junk" targets at a new site to see how accurate your detector is.....both depth and I.D.

For me it depends. I'll always take depth, that's a given. But discrim is only important to me when land hunting on private property / permissions, as I'm always more careful and focused on reducing number of holes I dig. Also, "generally" speaking, there should be a lot less pull tabs on a person's lawn then at the beach where thousands of people gather every day. I've been an audio only guy since 69'. I did own a Garrett AT Pro and ACE 350 for a short time then sold both, and last year picked up an X-Terra Elite, all 3 of those have/had screens, push buttons, and target ID numbers. I only have about 1 hour on the X-Terra but will use for coinshooting on land. For the beach, I always take my other 2 detectors, both are audio only with knobs, just the way I like it. Being an audio only & knobs guy for so long, I just never wanted to look at or rely on numbers on a screen. I prefer to use audio tones & my experience, just feels quicker & cleaner. My Dual Field PI is obviously all-metal, all the time, audio only, and I only use it at the beach on wet sand and shallow water, so I dig everything.

My Sovereign Elite has Discrim & All-Metal modes, I use both. Although I also have the optional Target ID meter, I've never used it, just audio tones. The Sov has auto iron reject and variable pitch tones based on metal conductivity, but to get them, I have to be in Discrim mode. I rely on auto iron reject to null over ferrous metal, and use variable pitch tones to help with target ID. Although I'm in Discrim mode to use those features, I set both Discrim and Notch knobs to 0. If I set discrim to reject pull tabs, then I'd reject everything below pull-tabs, including nickels, small gold, or other treasures. Same with setting notch to knock out a range before & after pull tabs. Nickels fall in that range as well as some gold, so I keep both at 0 and rely strictly on auto iron reject & variable pitched tones for target ID.

Between those 2, I really don't need to use discrim or notch, at least on the beach. There's quite a variety of pitches the Sov produces depending on the metal's conductivity, but generally speaking, if the tone is low (gold), I'm digging, if the tone is high (silver, coins), I'm digging. If the tone is more a mid-pitched tone, (nickels, small gold), I'm still digging. Digging all is no problem at the beach, holes are dug & filled in quickly and I don't worry about being too "neat" like when hunting on private property. And on the wet, after I refill the hole, and water comes rushing in, all traces of the hole are gone, as if I was never there, lol.

 

[Sand Swinger]

I swing 13" and 15" coils in the surf, I use the ebb and flow of the waves to swing coil. I dont fight the current, I sweep with the ebb and flow of the current.

Thanks TH. Sounds like you're swinging parallel to the shore, in a North to South pattern rather than West to East? So, when water comes rushing in towards shore, it will push your coil from East to West, then when water recedes back into the ocean, it would push your coil from West to East?

Never thought about that, have to give that one a try.

 

[Sand Swinger]

I've done some work in the past on making a better water coil. Your AI adventure got a few things right, and a few things wrong.

First, there is no advantage in a "reverse elliptical" coil. It will perform about the same as a round DD, and just adds more weight and drag.

Second, making the cross section like an airfoil -- tapered on the trailing edge -- works well in one direction but not so much the other, as @k0bgalan pointed out. A reverse-moving airfoil will tend to weathervane, which makes the coil try to dance around. So about the best you can do is an oval design as shown in the second iteration:
View attachment 2257228
You want it flat, but not too flat as it will also tend weathervane.

Third, you want the clevis hitch in the dead center of the coil, not towards the rear. That way, whatever weathervaning force you get acts equally on both sides of the clevis. Otherwise, the coil will flop in the surf.

Fourth, minimize bracing. The second AI coil:
View attachment 2257223
includes toe bracing that does absolutely nothing for mechanical stability but does add drag. Any bracing should be exactly inline with the sweep direction like the ones in the center of this coil:
View attachment 2257226
not angled like the braces at the toe & heel.

Fifth, placing the cable outlet between the coil ears will reduce drag but it places the cable entry right in the middle of the overlap zone. This is the most sensitive spot to place extraneous metal; companies avoid this and usually place the cable outlet on the TX coil side. You can reduce drag by avoiding the big Heyco-style spiral glands and using small rubber glands instead, such as used on many Garrett coils. Also, placing the cable right up against the clevis creates a leverage gain that, if you wind the cable tightly and then flatten the coil, could rip the cable right out.

About the best coil I've ever tried to push through water is the Fisher 12DD (actually more of an OO). It has a very thin profile with no bracing:
View attachment 2257224

Thanks Carl, glad to see you jumped in. I have your book, Inside the Metal Detector V3, and am a member of your Geotech forum, so needless to say, I place a high value on your opinions.

 

[Sand Swinger]

About the best coil I've ever tried to push through water is the Fisher 12DD (actually more of an OO). It has a very thin profile with no bracing:
View attachment 2257224

Carl, thanks for adding the pic of the Fisher 12DD coil, first time I've seen that one. That pic is a good representation and provides us with a good visual of the various points you made:

1. Minimal bracing, this one has no internal bracing so even less resistance.
2. Clevis hitch in the dead center of the coil so any weathervaning force would be applied equally to both sides of the clevis.
3. Cable outlet mounted on its own small "shelf", placed on the TX coil side, outside the overlap zone.
4. Relocating cable outlet should also eliminate the possibility of leverage gain by no longer being up against the clevis.
5. Thin and rounded edges to help prevent or reduce weathervaning.

Seeing this pic really reinforces your points in my mind, this coil looks like it would be much easier than most to move/push through water, so I really appreciate you adding.

 

[Sand Swinger]

The original 15" Coiltec WOT coils would glide thru the water, much easier than the spider coil design now. It was one of my favorite coils on my excals.

I heard about those Wild Orange Things when I got my Sovereign Elite, about 14/15 years ago. But I went with the Detech 12x10 & 15x12 SEF Butterfly coils. I used the 15x12 several times but found the 12x10 to be my best overall coil. Gives me good coverage, hits deep (deepest target was 2ft down in dry sand) and I could increase sensitivity on the wet where with the 15x12, I found I had to lower sensitivity on the wet. That 12x10 SEF has been on my Sov for well over a dozen years now. I haven't seen a WOT out on the beach in years.

 

[cudamark]

For me it depends. I'll always take depth, that's a given. But discrim is only important to me when land hunting on private property / permissions, as I'm always more careful and focused on reducing number of holes I dig. Also, "generally" speaking, there should be a lot less pull tabs on a person's lawn then at the beach where thousands of people gather every day. I've been an audio only guy since 69'. I did own a Garrett AT Pro and ACE 350 for a short time then sold both, and last year picked up an X-Terra Elite, all 3 of those have/had screens, push buttons, and target ID numbers. I only have about 1 hour on the X-Terra but will use for coinshooting on land. For the beach, I always take my other 2 detectors, both are audio only with knobs, just the way I like it. Being an audio only & knobs guy for so long, I just never wanted to look at or rely on numbers on a screen. I prefer to use audio tones & my experience, just feels quicker & cleaner. My Dual Field PI is obviously all-metal, all the time, audio only, and I only use it at the beach on wet sand and shallow water, so I dig everything.

My Sovereign Elite has Discrim & All-Metal modes, I use both. Although I also have the optional Target ID meter, I've never used it, just audio tones. The Sov has auto iron reject and variable pitch tones based on metal conductivity, but to get them, I have to be in Discrim mode. I rely on auto iron reject to null over ferrous metal, and use variable pitch tones to help with target ID. Although I'm in Discrim mode to use those features, I set both Discrim and Notch knobs to 0. If I set discrim to reject pull tabs, then I'd reject everything below pull-tabs, including nickels, small gold, or other treasures. Same with setting notch to knock out a range before & after pull tabs. Nickels fall in that range as well as some gold, so I keep both at 0 and rely strictly on auto iron reject & variable pitched tones for target ID.

Between those 2, I really don't need to use discrim or notch, at least on the beach. There's quite a variety of pitches the Sov produces depending on the metal's conductivity, but generally speaking, if the tone is low (gold), I'm digging, if the tone is high (silver, coins), I'm digging. If the tone is more a mid-pitched tone, (nickels, small gold), I'm still digging. Digging all is no problem at the beach, holes are dug & filled in quickly and I don't worry about being too "neat" like when hunting on private property. And on the wet, after I refill the hole, and water comes rushing in, all traces of the hole are gone, as if I was never there, lol.

Can't scoop everything here at our beaches. Just too much ferrous junk from people burning pallets and other wood with fasteners for decades. That said, you get into black sand (magnetite) even a silver dime can sound like iron if it is encrusted with that stuff. You have to know the beach and if you're down to a layer that can have old non-ferrous targets. As for visual target I.D. screens, I'm all for them. You can ignore them if you want. Virtually all modern machines have a full range of audio tones tied to a specific conductivity and target I.D. To me, just a glance at the screen can confirm or reject a possible target in a known area with stable target I.D.'s. Some visual I.D.'s can also indicate multiple targets under your coil, but, the audio can be so bad that you might otherwise reject the target as junk. Most detectors will average a tone or target I.D. when two different targets are together. You get a silver coin and a nail together and many machines will give you a crappy signal. With target trace, and/or a quick processor, you can sometimes properly identify that situation and get a nice item. Personally, I run my machines flat out. All tones both iron and not. I relay on sound and visual clues to determine dig or not. Even in a modern park where I cherry pick, I still listen to it all and let ALL my senses decide. Granted, it takes a bit of time to get used to that much input, but, I find it tells me the best story.