Metal Detecting with Underwater Tow Sled

[Home Run]

Hello to all,
This is my first post on treasurenet so please be gentle...My question is about towing a PI detector coil on a sled behind a boat. The wreck I am searching for is in 100-150' deep water and am wanting to detect cannons & anchors etc within my budget (Magnetometers will do the job but priced way above what I can afford right now). So my question is can you run a search coil up to 150' away from the PI control unit and how would it affect its performance? Any advice would be greatly appreciated.

 

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[Tom_in_CA]

I suppose you could get a PI detector customized to have 100 to 150 of cable-wire line, leading down to the coil, ...... in a "perfect world". But the devil will be in the details. Because whether or not it's technically feasible, is just the beginning of your woes. Because the pulse will sound off (as it's designed to do), on all sorts of metal. And at the bottom of ANY typical water-way where human activity has long-gone-on, will be oodles of various metal items. So your detector is going to give you signals on various aluminum cans, nails, hubcaps, junk, etc... And how do you plan to keep it hovering off the bottom at the usual few-inches above bottom? I mean .... you'd have to basically drag it (lest how do you know you're even at the bottom?) And the minute you get a signal, and you're trying to handle the thing from 100 ft. above while your boat drifts around ? You can never double sweep the spot, etc..


I tried something similar to what you're asking, from a mere 3 ft. deep in water (with the coil submerged over the side of a rowboat as we searched for a weapon in an LEO assist). And it was an exercise in aggrevation. The minute you get a signal, you have to hold on to the boat with poles to keep it stationary, so you can attempt to "x marks the spot". And then turn around and try to dig the target with long-handled scoops was another exercise in aggrevation: it just created a "polling" push and pull with each attempted swipe. Likewise: the minute you get a signal, you merely have to go down and investivate (scuba?), which just begs the question then: why not just go down, to begin with, with standard pulse machines and simply scuba around on bottom?

So I think you're dreaming. Magnetometers, maybe. But a pulse coil at the end of a 100 ft. cable-wire? nah.

 

[Home Run]

Thanks for your advise Tom in CA, basically I'm not trying to find small objects. I want to tow a 1500x1500 coil to locate the large iron objects such as anchors, cannons, use my drop cameras & ROV to help determine the general location the ship settled (My understanding is that a large coil won't pick up the small stuff, correct me if I'm wrong). So we can't attempt to dive on it, unless we know where it sank. The logistics of conducting a search in that depth isn't so much the issue for me, it's whether I can run a long tether from the unit to the coil.

 

[Jason in Enid]

1500 WHAT? inches? feet? Millimeters? The largest coils I have seen are about 6 feet.

Tom Is correct, there will be LOTS of junk (yes, big junk) it will be sounding off on, but that's the smallest of your problems.

#1, how are you going to keep this close enough to the bottom? You have to realize that debris, rocks, reefs, stick up. Snag that coil and it's done. If you aren't close to the bottom with it, it won't work.
#2, how are you going to work at those depths? Are you a diver? Do you have any idea the available working times at those depths?

 

[Home Run]

1500mm (5 feet), like I said the logistics isn't the issue, I have prototype designs for towing close to the bottom and negotiating snags & rocks, however the search area is actually hard sandy featureless bottom, also due to the remoteness of the area the ship sank any sounding off will be of interest. Yes I dive and do know available working times at these depths, hence why am after any info on my original question.

 

[Tom_in_CA]

Home-run, thanx for some clarifications. ok, a few observations/questions:

a) Start with that 5 ft. coil, and see what the smallest object it can pick up is. I mean, for example, is that cannon and anchor sized and bigger-only ? Or is that sized coil capable of finding items down to toaster sized items ? Soda-can sized items? etc... Because once you enter into the realm of soda can and toaster sized items, I think you'll be surprised how much modern junk can fall into that category-of-size, that litters ocean bottoms. Well .... at least around harbors, piers, etc.... Maybe not in the open ocean middle-of-nowhere, granted.

b) but even assuming this 5 ft. coil won't sound off on anything smaller than cannon/anchor sized, I'm still skeptical of ability to hover the thing (even with your sled idea), to the correct height. And once you get a hit, can you imagine the aggrevation of needing to circle around to criss-cross the spot, to "x-marks" the spot ? (to make sure you didn't just get random flutter, etc...) ?

c) and the minute you get a hit you want to investigate, what good is under-water cameras (with I suppose 100 ft. cables for them too?) going to do? If the object has even a single inch of sand covering it, all you're going to get is a nice picture of the bottom (assuming your "sled" hasn't stirred up sediments which merely cloud any such picture, to begin with !).

d) So you go down to investigate the "beeps". Ok. Then that merely, again begs the question, why not just go down with scuba gear to begin with and metal detect? 100 to 150 ft. is child's play depth for any experienced scuba person (don't even need special gasses at those easy depths). And yes, I realize you can't swing a "5 ft. coil" on a hand-mounted standard underwater beach pulse machine. So here's how you can just as easily replicate that desire to "only find big objects". Simply have the coil be several feet higher than the bottom of the ocean floor as you scan. Because singular coins, cans, and small items will be too far from the coil to be heard, when held that high. But a cannon or anchor WILL be large enough, to give a signal despite the user holding it that high off the ground.

e) yes I realize that covering vast amounts of real estate acreage isn't possible with simple scuba and paddling around down at bottom depths with scuba. So for that reason, is why you're thinking of the boat method, to hone down where you want to investigate further, right? But no, I don't think your 100 ft. cable to standard detector is feasible. In a perfect world, perhaps. But realistically? No. You can certainly try it, but in the end, with that kind of money and modifications, why not just get a magnetometer?

 

[Rangerbb]

Sounds like you are trying to use the wrong tool to get the job done.

 

[cudamark]

With 150' of cable connecting the coil with the control box, I would imagine you would have to custom-make the detector to account for the difference in signal strength and resistance. Why not attach the detector to a camera submersible and detect that way?

 

[Jason in Enid]

OH yeah, if you are talking about detecting up to 150' deep, you will need about 250' of cable because that coil isn't going to be straight down while under way.