@vagabond87:
Now we are in mid point so to say.
Its just my opinion- I do think that Luffy, Zoro and Sanji developed haki even before time skip. Why? One thing is comment of Blackbeard in chapter 544 about Luffys haki growing. He has to have some way of comparing, they didnt fought before so I guess presence/aura(maybe CoO haki users can do it on will) might be enough in some cases to measure haki of another person. He meet Luffy at Jaya and then at Impel Down he said Luffys haki has grown. He had to have it even on Jaya going by the logic.
Next thing is Zoro being able to cut steel after learning to hear breath of steel- for me it screems that its haki related/ control over haki related thing. To cut steel with steel there must be some imbuing it with something to be able to slice and dice it. Like cutting rock without effort and then not cutting leafs sounds like different usage of CoA. Sounds for me. Imo :p Thats how I see that.
Last one is Sanji with his Diable Jambe- it would be most logical explanation if Diable Jabe not burning his leg and burning Jabra to the bone would be explained with haki. Out of those two it was Jabra who had presumably stronger body/flesh. Sanji burned through his tekkai not broken it with force. So its something fishy haki explainable there as well for me.
As for ingintion subject
! http://i12.mangareader.net/one-piece/520/one-piece-65109.jpg its picture from chapter 520 where match is used by CoA haki user to ignite herself.
So it was suggested that haki users can cover themselves on fire, question is how then Sanji fires up now? My guess would be- only by movement, by speed and friction somehow or that burning is as spirit related as it was with Zoros asura. Like manifestation of users nature in some way.
I do believe that Diable Jambe was move with usage of haki and friction to fire its up. Now most of attacks that we seen from Sanji can be explained by stronger manifestation of his spirit so to say, stronger haki and bigger speed thus also friction- in fight. Hell Memories being ignited with memories from Okama island by Sanji somehow points to that. As Zoro gotten his Asura, Sanji has his fire- now even those hell memories for example. But all in fights this connects with haki usage as person surely needs protection from fire that burns flesh of enemies.
I dont think that steam can turn into fire as it would burn out Luffys body. Or fire covering Sanji would burn him.
With Red Hawk my bet would be at that CoA hardening so more durable and harder arm + presumably far bigger speed of attack = extreme friction while attack is made ending up in igniting attack.
Other character was also shown to set on fire on the spot so to say.
From OP wiki considering this attack from Daruma who ignited himself same like Sanji few chapters later with hell memories.
! http://img2.wikia.nocookie.net/__cb20120924104107/onepiece/images/6/6c/FireballCutter565.png
- Hi-Daruma Cutter (火ダルマカッター _Hi-Daruma Kattā?, literally meaning "Ablaze Cutter")__: Daruma engulfs himself in fire by unknown means and then jumps towards his opponents, trying to bite them.[6] This is calledFireball Cutter in the FUNimation sub**_**
Haki is present is all Humans, that does not mean they can use it or will ever be able to use. At ID, Teach did make that comment because Luffy's will have become stronger and he had unconsciously fire Haoshoku Haki twice. At that point, non of them(SHs) could harness the power of Haki.
Phenomena's in OP can be explained by two things, Haki and/or DF. Haki is very clear, Haoshoku, power to overcome another person's will, Kenbunshoku, power to sense others, Busoshoku invincible armor and if the armor is strong enough, it can be use to attack. There is no point in Raleigh's explanation of Haki that states it can be use to generate elemental attacks. We have never seen anyone do it and probably never will. Luffy cannot be the odd one. If Haki is not responsible for the fire that came out during red hawk, then Gomu Gomu is the sole explanation.
The only person off to this rule is Sanji, which is why many critics have questioned Sanji attacks. If you read the MeraX2 thread, you will see those question been asked. If Sanji can ignite both legs at will, then what is the point of eating DF?
BTW, I don't like using anime as evidence. That show is filled with many stuff that didn't happen in the manga.
–- Update From New Post Merge ---
@zxcv11791:
Zoro ignites his sword in blue flame too, dunno if thats really haki based though.
Zoro didn't do any of such. The only time Zoro used a fire based attack was little garden after Usopp melt the wax with fire and Zoro attacked Mr5 with the fire burning on his blade.