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I think I cause some confusion by calling the 6 "the Evinrude" not realizing the 9.9 is also an Evinrude.

The PN I provided is for a three blade prop for the 6 where the blades look very much like the ones on the 9.9 prop you have. I think your problem is more the prop than the motor when using the 6. If you can get the prop and motor into the powerband 6hp should be plenty. With the correct prop you should be able to get up to hull speed with ~4hp. With 50% more power you should be able to counteract a normal wind. Also those large flat blades should reverse much more effectivly than the cupped higher speed blades you currently have on the 6.