Boat towed PI Metal Detector coil - What are the issues and effectiveness?

[Shipwrecked]

What is your experience with a boat towed PI metal detector coil?
How effective is this detection system for locating targets underwater when being towed?
How effective would it be in locating large non-ferrous targets such as aircraft wreckage buried in a sandy bottom up to 10 to 12 feet?
Would like to hear from those who have actually had experience in towing a PI metal detector and understand the issues.
NOTE: Looking for non-ferrous material, so a magnetometer won't work in this situation.

 

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[old man]

Shipwrecked, Depends on what manufacturers unit that you use. Some are exceptional and others? Well, I wouldn't waste the money. Also, If you know of a plane that is made of non-ferrous metal. Let me know who makes it

 

[Dell Winders]

It's easy enough to calculate. For example, if a 7 foot PI coil will detect an automobile, or the side of an Aluminum building at 10 feet thru the air,you might get 9 foot, or less detection distance thru the sand, or mud. Which means that if your coil is towed 4 feet off the bottom it is only going to detect very large targets such as an aircraft that are buried 5 feet, or less beneath the sand or mud.

I have found that adding a Tim Williams data logger, to a PI, and other Metal detectors gives it up to 30% more depth detection. Arc-Geo Loggers Dell

 

[old man]

What is your experience with a boat towed PI metal detector coil?
How effective is this detection system for locating targets underwater when being towed?
How effective would it be in locating large non-ferrous targets such as aircraft wreckage buried in a sandy bottom up to 10 to 12 feet?
Would like to hear from those who have actually had experience in towing a PI metal detector and understand the issues.
NOTE: Looking for non-ferrous material, so a magnetometer won't work in this situation.

Shipwrecked. How deep is the water where you are looking Is it less then 100 feet??

Depending on how large your target is, there are units that will detect over 40 feet deep. Personally, I wouldn't waste the money on a JW Fisher, unless you are looking for an airplane.

 

[Shipwrecked]

Old Man: Thanks for your reply. The aircraft is a C47. I have been told that it did not have much ferrous content, maybe that is not so. Still to be determined. Anyone on the forum know more the metal content of C47's. Water depth is around 30 ft in fresh water. If it can be found with a mag that would be great. Any detector and coil size suggestions would be appreciated.

Dell Winders: I like the Arc-Geo Logger. Thanks for the info.

 

[comfortably numb]

To my knowledge C47's were built from aluminum, (non-ferrous). The engines and some other parts would have ferrous metal (steel). If the engines are near it a mag should pick them up.

 

[old man]

To my knowledge C47's were built from aluminum, (non-ferrous). The engines and some other parts would have ferrous metal (steel). If the engines are near it a mag should pick them up.

comfortably numb, I know planes are made out of aircraft aluminum. I was only joking, because I really didn't think he was looking for an airplane. Actually, If that is what he is looking for: I would use a side scan sonar first. Or a Mag.
But if he is looking for other types of non-ferrous metal. I would recommend the Pulse Star II. Which is made in Germany, but it's sold by Kellyco near Orlando. I think you've used mine with John.

When they first came out, they claimed to find a cannon from 40 feet away. I don't know about the cannon. ( They detect ferrous and non-ferrous metal and let you know what type of metal you are getting a reading on. ) but, I do know that it will detect a large amount of silver from over 50 feet away.
Merry Christmas to everyone and Happy Hunting.

 

[Shipwrecked]

Old Man: I have thoroughly searched the area with a high resolution side scan sonar several times and there is no obvious sonar target. Only a few pieces of the aircraft wing ever came into shore. The aircraft was observed landing on the lake and the crew successfully got out just before the C47 sank out of sight. (NOTE: I have +35 years of side scan experience and have located over 200 shipwrecks including light aircraft and cars.)

The Pulse Star II looks good but appears to be a detector for land use. I need to tow the detector coil close to the lake bottom.

Back to my original question: What are the issues and effectiveness of boat towing a PI Metal Detector Coil for locating large non-ferrous metal wreckage?

 

[aquanut]

Shipwrecked,
We have built an underwater sled for the Pulse Star II to ride along the bottom equipped with a one meter coil. It performs as we had hoped.
Aquanut

 

[Jolly Mon]

I am very much an amateur, but I have a JW Fisher Pulse 8x and have used it fairly extensively with both the 18 inch circular drift coil and the 8x 48 towed coil. I believe keeping the coil close to the bottom would be your greatest challenge. I am not so sure the Fisher 8x48 coil might not detect a huge object like an airplane at something approaching 10 feet. But if you tow it across the bottom, the bottom has to be CLEAN or it will hang-up repeatedly and drive you crazy. The Fisher Pulse 12 is designed to detect large objects at up to 16 feet, I believe. It and the Pulse 10 are deployed using a tow fish. A huge factor will be bottom topography/ cleanliness...in order to get 12 feet of actual detection depth with the Pulse 12 with a tow fish, you have to be able to consistantly keep the fish 4 feet from the bottom. That may or may not be easy to do...in a lake or river bottom full of stumps and logs and limbs it may be impossible...but if the bottom is sandy, fairly clean and gently sloping, you might get great results, even with a cheaper unit like the Pulse 8x...lack of discrimination should cause no problems in this case... you can disregard all targets smaller than an airplane...LOL...

 

[old man]

Old Man: I have thoroughly searched the area with a high resolution side scan sonar several times and there is no obvious sonar target. Only a few pieces of the aircraft wing ever came into shore. The aircraft was observed landing on the lake and the crew successfully got out just before the C47 sank out of sight. (NOTE: I have +35 years of side scan experience and have located over 200 shipwrecks including light aircraft and cars.)

The Pulse Star II looks good but appears to be a detector for land use. I need to tow the detector coil close to the lake bottom.
What are the issues and effectiveness of boat towing a PI Metal Detector Coil for locating large non-ferrous metal wreckage?

Shipwrecked, If you couldn't find it with a side scan, then I would advise using a mag. I'm sure there is still plenty of ferrous metal still inside the plane, plus you have two engines and landing gear out there somewhere. Like aquanut said, the pulse star can be used with a 100 foot tow cable with a sled. But, I personally don't think that is the best thing to look for a C-47. The Mag would be my first choice. I wish you luck and Merry Christmas,

 

[Shipwrecked]

Old Man: Thanks. I will have to investigate what the ferrous content is in a C47. The mag for me would be the easiest. Happy Holidays to you.

Aquanut: Could you provide some details on your experience with towing your 1m coil underwater? What type of cable are you using? What is the coil inductance of your 1m coil? What speed are you towing? How much weight is the sled? What is the deepest target into the bottom that you have located? Fresh or salt water? What is the detection limit of the 1m coil in air. etc etc.. Your experience and info would be appreciated.

 

[Salvor6]

Hello Shipwrecked. I built the sled to tow the Pulse Star coil for Aquanut. It is made from 2" PVC. I had to pour cement in the tubes to make it sink. It weighs about 70 pounds. The 100' cable is a Pulse Star accessory available from Kellyco. It is coaxial cable. I don't know the inductance of the coil. We towed it at idle speed, about 2mph in both fresh and salt water. It will detect about 20' down. A C-47 has tons of ferrous metal on board. You should be able to detect it with a mag. Thats your best bet.

 

[Salvor6]

Shipwrecked where are you located? Maybe we can help out.

 

[Trembull]

HowStuffWorks "Douglas C-47"

 

[aquanut]

Jolly, I think Salvor6 answered most of your questions. As far as an air shot, I had to get within 4 feet of my aluminum pontoon. It is much more effective underwater.

 

[Shipwrecked]

Jolly Mon: Thank you for your info on using towed PI Metal Detector. A few years ago I was given a Pulse 8 coil, towfish, and cable but no control unit. I have been thinking of acquiring the control unit or just building one. Sounds like a large coil is needed for the deeper depth that I might need. My shipwreck playground is in the Great Lakes.

Salvor 6: Thanks for sending the pic of your sled design. Very Nice
aquanut: Thanks for the detection info

You can check out the stories of our shipwreck discoveries and those of others on our website Shipwreckworld

HAPPY HOLIDAYS and good luck to all in the 2013 search season. -- Thank you for your inputs to my questions. Jim

 

[stevemc]

I have a large coil PI a Pulse 2000, with a 1 meter coil, I have it fiberglassed into a rounded rectangle 5' wide and 3 1/2' long. Made in the US but not anymore. I lengthened the cable with a type of shielded coaxial cable like they use for cable TV . On land in air it will pick up an automobile at about 25'. I have several PIs with 10" and 9 1/2" coils and they will all pick up the auto at about 5 1/2' to 6' in air max. The large coil will pick up an aluminum beer can at 6' in air and in salt water about 8' unfortunatly. They will pick up hi power lines 100s of feet away, and wont work under bridges or anywhere with rebar and metal reinforcing, as way to much metal. They work good away from powerlines and buildings etc. PIs work better in water and ground than in air. Especially salt water. I found that I had to have the coil at 10' or above the bottom to not get the 1000s of beer cans that are out there. They will drive you crazy. I have used it on beaches and dug up some nice aluminum beach chairs and crab traps-huge holes dug with shovels, and I have found cannon sized objects on a well known 1715 fleet beach-to pin point you just hold it up to pinpoint-pretty high, but they were below the waterline, way below. I feel if you can keep it at 10' above the bottom you can do good. Here is a picture of my big coil, without the sled. In freshwater you shouldnt have a problem, but in saltwater when it is off the bottom even a foot it "waaa waaaa" constantly, but you can hear hits through the catter-which is just trying to ground balance in the saltwater, which it cant do until the coil is down on the bottom, like any PI or any UW TR or VLF detector does, but is more noticable with a large coil detector.

 

[stevemc]

A magnetometer will work to find this plane, which is the same as a DC3. They had twin 8 , 9, or 10 cylinder radial engines which have iron crankshafts and connecting rods. Most other parts are aluminum, all bolts were steel, and valve train was steel. About the whole plane is aluminum. Still enough iron/steel to let a sensitive mag find it at a fair distance. If you grid the area, you should pick up something. I am surprised that side scan has not found anything. Maybe it was salvaged by the military by hooking it. Or by modern divers for the aluminum. Most of the airplanes I have seen in the ocean are still in good shape.

 

[Shipwrecked]

Hi stevemc: Thanks for the great input on the operation of your PI Metal detector underwater. What is the inductance of your large coil and/or how many turns of #? awg wire does it have. Also, what voltage is being applied to the coil? Could you post a pic of your sled?

In the area where the aircraft went down there is a lot of movement of sand on the bottom. The bottom is very flat so anything protruding would be detected by the side scan. We are fairly confident that the plane has not been salvaged in the past.

Regarding the C47: Does anyone know if the metal contained in the engines is of ferrous content and have an estimate of the weight?

I believe that the Pulse 2000 is no longer manufactured. Do you know anything about the Powerscan 2Pro unit?