I wanted to build a magnetic retrieval tool to recover ferrous objects underwater. I tried various "fishing magnets" but most of those don't work very well. They are simply not powerful enough. Or too heavy to repeatedly cast into the water. So I came up with a magnetic drag bar. Pictured is a stainless steel knife retrieved from Mobile Bay on July 4, 2013 using the tool.
Step 1: Materials Needed
550 paracord. 50 ft length and rated at 100 lbs. Available at local big box hardware store for about $4
Magnets. Two kinds are used in my project. I salvaged the kidney bean shaped magnets from disassembled hard drives work was sending to the shredder. "Free" other than the time to pull them. I'm guessing 5 lb pull. To augment, I ordered 12 additional ones from KJ Magnetics (part number: DX01-N52). 1 inch diameter, rated at 8.5 lbs pull. Figure $35 - $40 shipped if you can find a coupon code or not.
Metal bar stock (steel or aluminum). For $8, I got a 1/8 inch thick, 2 inch wide by 36 inch long steel bar from local big box hardware store. I choose steel. However, there's some good arguments for using aluminum underwater.
Newspaper. For painting on. Pick up a store advertisement flyer.
Sandpaper - laying around from another project. New say $4
Spraypaint - laying around from another project. New say $7.
WaterWeld. By the makers of JB Weld, it's a two part adhesive designed for use in and under water. that one can use to glue metals together so it'll hold in a submerged environment. Available at local big box hardware store. Ask a store associate as it's not consistent as to which dept (plumbing, or near the tape) even at the same name brand store in the same town. $6
Material cost: $64
Step 2: Tools Needed
Drill - I used a cordless hand drill
Leather gloves - protection when cutting and drilling metal
Hearing protection - when cutting and drilling metal
Measuring tape or Ruler
Nitrile gloves (when using the water weld stuff). These I picked up at Walmart. Harbor Freight stocks some good ones as well.
Safety glasses (for flying metal bits from drilling. You don't want a metal shard into your eyeball.)
Drill press - If you have one or access to one, then that'll make it go that much quicker.
V-DrillGuide by Big Gator. This makes it so easy to guide the drill for a proper hole to run the rope through.
Step 3: Cut Bar to Size
Cut metal bar stock to a length that makes sense to you. One factor will be a length you can handle. Another factor will depend on how many magnets you have. I choose a length of 18 inches.
Step 4: Drill Rope Hole
I used one C-clamp to hold the flat bar in place. I used the other to clamp the drill guide in place. Then drill hole in center to tie our rope to.
Step 5: Sand and Mark Magnet Locations
After cutting and drilling, lightly sand the metal bar surface for adhesive to stick. I used a Sharpie to mark where I wanted the magnets from KJ to go. For the 18" bar, I spaced them as follows: 2,5,8,11,14 and 17 inches.
Step 6: Attach the Magnents Using the Adhesive
Open the Waterweld package and read the instructions. Then put on the nitrile gloves. (Waterweld is safe for the most part assuming you don't eat it or something along those lines. As for the gloves, I just don't care to have any kind of adhesive on my skin.)
To use waterweld, think Play-doh. Cut a peice off with a plastic knife. You'll notice a white outside with a gray center. Squish together making a small ball Then smush the magnet onto the steel. Watch fingers as the magnet will be attracted to the steel.
Step 7: Attach the Magnents Using the Adhesive (continued)
This is where I re-used the hard disk drive (HDD) magnets I recovered. I attached these magnets in the vacant spaces. I put magnets on both sides. If I recall right, I gave the waterweld 24 hours to cure between turning the bar over.
Step 8: Paint
Paint a second coat.
Design Note: I used spraypaint as it's what I had laying around. This to help keep the steel from rusting. I did read about using "Plastidip". In retrospect, I wished I had.
Step 9: Rope
Once dry, tie it it to the paracord. As for what kind of knot, I would recommend a taught-line hitch to prevent your magnetic drag bar from coming off the line. Tie the other end to a wood handle/stick or some method of reeling the cord in.
Step 10: Results of Testing
The bar was able to latch on to and drag a claw hammer with ease. The second picture shows a metal fishing hook found with it near a boat launch.
This is my first instructable. After visiting the site for a long time, I finally got to contribute back. As I find new/different items, I may post here. If you have questions, please send me a message or discuss below.