This is a bait diver, similar to commercial ones available at most tackle shops. It is trolled with your line attached to one of the tail holes, with the leader and terminal tackle off one of the belly holes. It floats when not pulled, but will dive under tension.

A small weight should be placed in the tube portion of the diver before gluing on the cap. The weight slides forward and back and helps the diver to point the right direction. Commercial versions use a ball bearing or steel marble. I have had success with those, but I also found that a 9mm FMJ bullet works very well and slides back and forth cleanly. (note, I say bullet, not cartridge- I mean the actual lead bit you use when reloading, not something with gunpowder and brass or a primer)

The default size will dive to around 20 feet, but you can easily scale this up to a larger diver.

There are 3 files- You'll primarily just use the body and cap, as these can be printed without supports and allows you to place the weight inside without the cap. I have also included a version where the cap and body are one unit. Sizes smaller than the default won't need a weight, so you may be able to just print that one if you want a small version.

Update: I've just uploaded V2. This adds a little more airspace back into the body, which helps with getting it to float & dive properly.