I think Arne is right – anyway, I can say I had a very similar situation with my little SJR sail - “…the sail…wants to set too far forward from where it should be…”.
The SJR should not need the conventional hauling parrels, as it is designed to drape almost by itself, without them, but I was forced to try adding a standing parrel to the yard, because the sail kept slouching forward. (With SJR there is no choice – the sail has to be set “where it should be” because of the slot). So, I added this parrel, (edit: I believe it is called a Throat Hauling Parrel, anyway, its purpose is to haul the yard back). And then I found, same as Annie, that it had become unacceptably difficult getting the last part of the sail hoisted, though the sail could haul up and set very comfortably with one reef in.
Since the sail had to be forced into position regardless, I never found out if it would have caused lee helm. I didn’t want to add a running parrel to this rig, but something had to be done. At first I did not realise what the problem was, but it had to be something to do with the geometry of the set-up, and I realised in the end it was because in the last couple of metres of raising the sail, the halyard angle became too great, resulting in a large component of the halyard force pushing the yard forward. The halyard attachment point of the yard was already as far forward as I could make it.
The halyard angle was too great, simply because the mast was too short.
This problem was not intuitive to me when I made the mast. I had made sure the halyard blocks would not be chock-a-block, and that there would be enough halyard drift to allow the yard to rotate – but I found out too late that, in my case anyway, this is still not enough.
(The clue for me was the realisation that with one reef in, there is heaps of halyard drift. That meant the angle of the halyard is less, and thus the unbalanced horizontal component of the halyard force is less.)
Luckily, I did not need to add a running parrel to haul the yard forward back, because (1) the SJR mast is vertical, which alleviates some of the halyard angle and (2) because of a peculiarity of my tabernacle arrangement I was able to raise the mast up higher, and gain the extra halyard drift that was needed. The problem was 99% solved, and if I get around to it, I will raise the mast another few inches more this summer.
My reason for chiming in is not directed at Annie (who is onto it, and knows more about these things than me anyway) but as a warning to anyone else who is building for the first time a low yard angle rig such as the Amiina Mll SJR, or quite possibly any low yard angle rig: though it may not seem necessary, the mast height must be at least as high as the peak of the yard, and higher still would be better. This is to ensure that the angle between the halyard and vertical (not the angle between the halyard and the mast) is kept within an acceptable limit. I think Arne can probably advise what would be the optimum halyard angle, but for the Amiina Mkll rig, that minimum mast height quoted above would be my rule of thumb.
(With SJR you don’t have the luxury of altering mast rake (actually, forward rake exacerbates the halyard angle problem) and you don’t have the luxury of being able to haul the sail forward or aft. So allow for plenty of halyard drift and achieve the minimum halyard angle. You can always shorten the mast later (but you probably won’t)).
PS A little bit more thinking about the geometry of it, leads me to believe that this requirement for halyard drift is greater with the low yard-angle rigs, and maybe less of an issue for the high peaked yard of, say, the Johanna rig. I think Arne was aready alluding to that. (If I am correct, this makes two reasons why the Johanna-type rig gives maximum sail area for minimum mast height.)
Forward rake also adds to this halyard angle problem. It is interesting to draw some diagrams and look at the relationships between yard angle, sail balance, mast rake, halyard drift and halyard angle.
The more these parameters can be harmonised, the better.
(PS thanks to Arne for pointing out my mistake in terminology, which I hope I have now corrected. I am not familiar with the terms YHP, THP, HKP etc because the split junk rig has a different system for controlling the drape of the sail - and my copy of "Hasler/McLeod is locked away and I couldn't look it up!)