You can test Whirlwind damage yourself fairly easily if you're not inclined to believe me. Take a low damage weapon for low damage spread on the weapon itself, equip a piece of gear that has +minimum damage higher than your weapon's max damage, and it'll raise your minimum weapon damage higher than your max weapon damage. This will eliminate the random damage variance outright Then swing at a zombiie in quest one before your reach New Tristram and you'll see damage.
Hasta is 18-26 weapon damage. +9 minimum from ring brings it up to 27. 880 str is +880% wd, or * 9.8. Frenzy and Whirlwind are both 110% wd, or * 1.1.
27 * 9.8 * 1.1 = 291.06, checks for Frenzy.
291.06 / 3 = 97.02, checks for Whirlwind.
If it was 55% weapon damage per hit, it'd have come out as 145.53 damage per hit, which is not even close.
For the record, I've also noticed this 2 hits per whirlwind (you can see in on the zombie above actually). So I don't know why it's divided by three, as two would make more sense to me. But whatever.
Interesting. So that means there are 3 hits, but only two are displayed by the game engine? Also how would that work for all sorts of procs, does whirlwind land 3 or 2 hits, that can trigger ctc?
I did not test the numbers, that is the source of confusion. Also info about 55% per hit was available on several forums, which turns to be wrong (people probably assumed same as I did).
I was playing around with it a little more, and I think I worked it out.
It seems it's most likely simply a case of those zombies being too small to eat a third hit. Diablo seems to eat three hits most of the time just fine.
WW alternates hands when dualwielding, so if you equip different attack speed weapons, and watch the charater details screen, you can see the speeds alternating as you're Whirlwinding, and you can usually see it doing three hits.
Whirlwind is listed at 16 Fury Cost. You can actually cast it with less. If you equip Unforgiving and start at zero Fury, once it ticks up to five, for a split second, WW remains greyed out, but somewhere between 5 and 6 Fury, the WW icon becomes colored allowing you to cast it. Makes sense it would be 5.33 Fury cost per hit of WW. We know from gear Spirit and Hatred at the very least count decimal places, so Fury doing the same shouldn't be too surprising.
What I gather from all of this is that when you cast WW, it attempts to spend 16 Fury to perform a three hit WW, and if your Fury doesn't permit, it'll allow you to do partial WWs.