Spudd,
Nil desperandum!
I've solved your problem. Expect a bill soon.
See this picture:
It's immediately obvious that there are 2 magnets (1 and 3) that are exactly over a stator slot, whilst there's only one (nr. 1) that's in between. So, 1 and 2 cancel eachother. But nothing cancels magnet 3! So now you have 6 magnets (1 from each pole) that are uncancelled. No wonder it coggs so terribly. Notice that the skewing angle (in one plane) is 13 deg, whilst in another plane it's 20 deg. Think about it and it will become obvious (sooner or later )
We can go to an even more extreme case: only 2 magnets. Now, the standard rule says that each magnet should be above a different stator slot. But in that case, there's no magnet in between! So instead of cancellation of cogging, you get amplification!
Now it also is obvious why Zubbly had so little problems with cogging: he uses many magnets, say 7 or 9. In that case, a small error (of the 8th or 10th magnet) has much less relative importance than in the (extreme) case of 2 magnets/row or, in your case, 3 magnets. Conclusion: the less magnets, the more important it becomes to skew correctly!
The case of 2 (and) magnets are limiting cases, which show the importance of correct skewing. When using more magnets, things seem to be less critical.
The solution? See picture below.
Basically I've added an imaginary magnet, nr 4. This is only done as an aid whilst correct skewing angle. Notice that magnet 4 and 1 have the same relative position over the stator, but that magnet 3 is a bit 'short' of the position it had in the picture above.
When you think about it and try to visualize why the last (real) magnet has to be a bit 'short' of position, it becomes obvious, eventually
If you don't understand this all, I'd say, don't despair. I've solved your case. One should skew a bit less than the original marks on the rotor indicate. Why? Because now we're using discrete magnets, whilst the rotor was 'continuous'. Like the difference between differential equations and difference equations, for the mathematically inclined. Discrete steps vs. infinite small steps of change.
The solution: when drawing, add an imaginary extra magnet which should sit at the same relative position as your first magnet. Use this as an aid to determine position of the in-between magnets. And you're finished, without resorting to specialized software (CAD).
I'd really like to know whether my solution (I'm 100% sure myself) is correct for you. So, I'd ask you to take off all the magnets, and put just one row back (3 magnets), but skewed at an angle of 6 (six!) degrees. This should give you your correct skewness. If you want to you can fix all your magnets too. Put it in the stator, rotate it. And I think (know) you will be VERY pleasantly surprized.
Hope all this makes sense to youse. I feel I've got it all figured out, it's just hard for me to explain.
It's all obvious now. Every problem is easy once it's solved.
Be expecting the bill soon
Peter.