Crab trap line

[crabby nick]

What is the preferred main line to use for a topless trap and how long should it be?

[U812OHNO]

1/4" nylon will work.  Decide what depth you'll be crabbing in and then add a couple three feet.  Just make sure your float is strong enough to keep your trap from sinking should you accidentally drop the trap off in 15' of water when you only have 10' of line down to the trap.  I've got a collection of traps that I've found out there that were floating just under the surface of the water.  I'm not much of a trap guy anymore, but I would do 20' of line from the float to the trap, and then depending on the time of year, determined what depth I would crab in.  If I was crabbing in 8' of water, I would wrap the rest of the line around the float and keep it there with a rubber band.  This way, I had one line that would work all season long/all different depths.  Hopefully someone else will chime in on this and offer up a 2020 version of what I used to do.   
 

[mbrage97]

20 feet of 3/8 inch poly.  It floats and the wider diameter is easier to pull.  I wear firm grip work gloves.

[Crab Shack]

I took the advice of Rick (Kingcrabber) and got some macramé, it's strong and soft on the hands.  Only negative is that it floats, so if you're steering challenged......be careful!  

[KALknowsRAE]

I use the #7 Franklin Crabbing Rope. It isn't too bad on the hands and it sinks pretty fast. You have to melt the ends when you cut it or else it will fray badly. They sell it at Miller's Island Crab Pot Supply. They sell floats too. And just like U812 says, just add a few feet more than the depth you will be crabbing in.