Yeah, this can be a tough one. Here is one method. To get this, I drew a curve in the top view like your dashed line, and then went to the front view and rotated and scaled it into place. Then I used Edit/Trim with "Add trim points" to split the curve into 2 pieces. That gives you something like this:

Now you can use those two pieces as 2 rails for a sweep. Select the end edge curve of the propeller, run Construct/Sweep, and then pick the 2 pieces for rails. That will get you this:

It's not perfect because it is not entirely smooth with the propeller. You can probably get it a bit closer if you tweak the shape of that curve a bit. Eventually there will be some additional tools that will help make surfaces like this nice and smooth with each other, but those are not ready yet, although sometimes you can cut objects back and use Blend, but it is a bit unreliable now.

- Michael