So, I was moving some reels between rods tonight after tying a couple of different combos based on my assortment of rods and reels.

 

I was pretty sure one of the reels was spooled with mono and the other with fluorocarbon.  But I wasn't sure which was which.  "I know" I said to myself, "mono floats, so let's clip some line and test it".

 

Sample one, floated on the water on surface tension.  Pushed it under to break the surface tension and it sank to the bottom.  Must be fluorocarbon I thought.

 

Sample two, floated on the water on surface tension.  Pushed it under to break the surface tension and it sank to the bottom. Huh.  I was pretty sure one of them was mono.  Maybe I have bad memory.

 

I needed a control sample. Clipped some line from a spool of 12 lb. mono, floated on the water on surface tension.  Pushed it under to break the surface tension and it sank to the bottom.  Huh?  I was always told and believed mono floated. Well, my spool of mono does not.

 

Experimental science has caused me to question the mono floats narrative.  Thoughts?

Braid floats,  mono sinks slowly,  FC sinks like a rock.

I think the thought is that mono floats on the surface during the retrieve when fishing.

specific gravity of water is 1.00

specific gravity (density) of nylon mono is 1.10

specific gravity of fluorocarbon is 1.78

 

specific gravity of seawater is 1.03, so mono is 30% more buoyant in salt. 

 

Sinking mono fish story from 19-y-o.  Fishing from a lighted tee-pier on Lake LBJ in the summer, I cast out 1/16 oz. Panther Martin on Daiwa Minicast, stripped line from the reel and leaned the rod on the rail.  Sat down and let everything sink.  Then retrieved along the bottom of the sandy cove and up to my light.  I caught a white bass every third cast.  They weren't happy with the hot water in the cove, but wanted the bait in the light - they hugged the bottom, darted up to feed, and ran back to the deeper colder water to breathe.  My retrieve exactly followed their trail.  The dock was lined with people, and I caught the only fish.  Filleted 17 before I went to bed and slept in the next day.  (of course my dad went fishing without me)

 

 

Monos ability to remain on the surface of the water is its best quality for one reason: it will only sink as your lure pulls it down. When the line stops sinking, it means that your lure is on the bottom. This is very helpful information. Braid will do the same. 
 

The sinking nature of fluoro is its blessing and its curse. On the plus side, your bait sinks faster. On the negative side, the entire line will sink meaning it can be hard to tell when your bait is done sinking. 

Monos dia and density essentially makes it buoyant.  It will sink given proper weight and lack of tension.  FC has a higher density and smaller dia than an equiv mono, which is why it sinks much faster.

PE lines generally float when dry, but the characteristics can vary greatly, particularly the lines that soak up water like a sponge, maybe 832 sponges give or take.

  

Years ago, I bobber fished for King Salmon in Alaska.  This was before spectra braids were available, and mono was basically the only line choice ( there was Dacron braid, but I never tried it). The mono would slowly sink, making it difficult to mend the line and set the hook.  A trick I learned was putting wax on the line by running it through a grove in a block of wax.  I also tried fly flotant, but the wax worked better.  A few years later super braids which float well came out, and waxing mono became a useless forgotten trick.  Mono has almost neutral buoyancy in fresh water, but does slowly sink.  Fluorocarbon is heavy and sinks fast, which is why it has some slack line sensitivity.

@king fisher old braided silk fly lines were mastic-varnished then waxed.  They didn't float, but sat on the surface tension.  You have to carry a chamois to keep them wringed-out.  

I used to pick the good ones from antique reels, refurbish and sell to cane-rod fishers.  

I fished them a few times just to say I did it.