This is what I have found also with the ST 90. It really likes a 13x6 prop. I have tried various props on the ST 90, but always went back to the 13x6 as it always seemed to generate the best performance with a 13x6. If the SK 90 will generate as good as performance using a 14x6 I will be happy, but havn't run my SK 90 yet to test some various prop configurations. The SK 90 is mounted on my new Super Sportster 90/120 that I built from kit, and once I finish getting the radio equipment installed I will fire the SK 90 up. It should be interesting to compare to my other Super Sportster 90/120 that had a Super Tigre 90 on the nose of it swinging the 13x6 apc.