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