This certainly is confusing to me. I think many want to try and help, but they need even more information.
I looked at everything and see you are testing on a strong lathe. So you must have not yet made or tested the blades yet.
What you are making sure looks to be at least TWICE as powerful of an alternator than the typical 10' mill alternators here. You have twice the magnets AND twice the poles. So it needs to be much bigger than just a 10' mill. I am guessing 14 footer.
Your 24 pole/18 coil layout is correct except it is tight with the magnets and coils and a little cancelling is going on from the bottom corners of the magnets. But I doubt that is the problem.
I am not as experienced as many here, but it looks like your coils are not thick enough. You mentioned only 21 turns of #13 per coil. The voltage (12v) you mentioned you are getting at 120rpm doesn't seem anywhere near high enough to me for that thin of a coil and that many poles. I would think you would be getting twice the volts at that rpm or more.
I suspect for a 12v system and that big of an alternator you are talking about BIG AMPS which means you also need a LOT bigger gauge (or equivalent) coils-- Like '3 in hand' #13 perhaps??. But the voltage you are getting from only one strand of #13 at 120rpm seems way to low.
Can you tell us what the thickness of your stator is? And more importantly what the Mag to Mag total airgap is? (from one magnet rotor to the other)
Are you absolutely sure all the magnet's polarities alternate, N,S,N,S,N,S,ect.?
Have you measured the AC voltage of each phase at a given rpm, to make sure they are the same. If one phase is lower, then a coil may be backwards causing bad cancelling and vibration, and lower voltage.
I'm afraid if you make only 10' blades they will stall very badly on that bigger 24 pole altenator.
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-Keep giving as much info as you can think of and they can help you get to the bottom of your problem. -Sorry I'm not more experienced with BIG coils.