I can't say anything about the Fu141, but 9-12 knots and 90kg with the US147 and 8.5 will be on the edge, especially in patchy winds.
I'm 87kg and use a 10.9 in 8-12 knots on the US147.
I also use a 8.8 on the US147, but that's more for +/- 11-15 knot conditions, again in patchy winds because when the wind gets more steady, I'm on my IS122 + 8.8.
Hope this helps.
IS87 / IS101 / IS122 / US147