[wes]

Mullins,

Nice post and good questions. I will give a crack at it as I have a bit of CNF experience myself and also have watched Martin dive CNF in person, though he has changed his stroke a bit on the video since I saw him in person last.

First, Rslomkow is correct with theoretical fact that wave drag does not affect us when swimming undewater, this is why the same nuclear sub that con do 50 kts on the surface can do close to 90 kts while submerged.

This no wave drag underwater affect, though, is the same for DNF and CNF so doesn't explain the different styles and speed of strokes with DNF usually being very slow and dance like while CNF looks more labored and like they are "sprinting" underwater. The main difference, and its a huge one, is that DNF swimmers are typically weighted to be perfectly neutrally bouyant, while CNF swimmers weight with just enough to enable them to get down to the sink phase of the swim. I use 13 lbs for my DNF training and used 4 lbs for my last 48 M, US CNF national record swim (I'm still pretty immodestly proud of that!!)

The reason for this, and Martin and Will would be similar, is that as you descend on CNF, the air in your lungs compresses and you lose bouyancy and lots of it. Martin has about 12l in his lungs so going down to 80m this 12l shrinks doe to pressure over 4x so ends up being 1.2l big. This loses bouyancy for Martin of about 20 lbs if my math is correct (feel free someone to do exact calcs). So to compensate for this Martin will want to be maybe 10 lbs positively bouyant at the surface so he is only 10 lbs negative at the bottom plate.

This in turn has a huge affect on the stroke. At the surface and off the bottom Martin is in effect first "pulling" down with him a 10 lbs float and then pulling up with him a 10 lbs weight. This means if Martin glides at the start he will quickly start to "glide" backward right up to the surface. This means very quick connected "choppy" strokes to get down and then glidier strokes as MArtin nears near neutral bouyancy and then pure gliding down and the reverse back up. This is also why many people do CNF with only a partially full lung on inhale which minimuzes the bouyancy shift as there is less air (see FRC diving for threads on this or read all posts by Sebastian Murat).

Cheers Wes Lapp