[Aitor]

It has taken half a day but at last we are on the same page.

Maybe the problem is that one of the rods is underloaded.

[Marc Fauvet]

half a day is a record ! 

as for the 'underloaded', i'd say that #2 is loosing potential line weight from the fly leg on the ground whereas, #1 has all its line on the ground. no legs.
cheers,
marc

[Lasse Karlsson]

It has taken half a day but at last we are on the same page.

Maybe the problem is that one of the rods is underloaded.

Hi Aitor

Underloaded? thought the big spring theory was exploded   

I'd go with the notion that in the OH cast, the majority of the line is accelerated in one direction, in the Roll, only a part of it is, and some of the energy will be wasted making the rest of the line move in the direction we intend. Since I'm crap in physics, I'm probably wrong, but I really like your explanations, so I'm looking forward to it   

[Aitor]

Hi Lasse,

No, you aren't wrong (or, at least, I see this issue in the same way). 

These things are difficult even for instructors, so what I use in my classes is a simile based in throwing a couple of balls. It seems to get the point across at first sight. Will try to shoot a meaningful video some day.

[Lasse Karlsson]

Hi Aitor

You could try it with two rods, one where the line is straight behind you and the other where the line is setup for a rollcast. That one works pretty well, it's no problem casting two rods at once  They don't even have to be identical 

Cheers
Lasse

[Will Shaw]

I'd go with the notion that in the OH cast, the majority of the line is accelerated in one direction, in the Roll, only a part of it is, and some of the energy will be wasted making the rest of the line move in the direction we intend.

I'm actually quite worried now because I was actually thinking the same as Lasse. Come to think of it I don't know who should be more worried, me or Lasse.

W.