I suspect that you've used a disk primitive object to make your rudder.  disk primitives are one sided only.

I would suggest that you make your (first) rudder from a two section open ended cylinder.  Form the overall shape in cross section mode with one of the "end" cross-sections.

copy that cross section to template, then move to the next two cross sections and Cross Section->conform to template.

Then scale one of the *end* cross sections *slightly*.   Copy this new shape cross section and then conform the other *end* cross section, leaving the middle section "normal".

What this does is give you a rudder with some "thickness" to it (suggestion: work with an overly wide rudder, don't scale to the correct "thickness" until the last step).

Finally, since you're working with an "open"  cylinder, in cross section mode, navigate to first one, then the other, *end* cross section and Cross-Section->Make POlygon.  if you don't "see" the end poloygon, go into polygon mode, and navigate to the "invisible" end polygon and flip its normal.

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I'm glad you could make sense of the posting.

The rudder suggestion is only that.

You could increase the number of segments and instead of making "end" polygons, you can scale the end cross sections to a point, etc.