Hi Lang - one way to get a curve that actually hugs the surface is to sweep a line along your helix so that you have a kind of fin type protrusion and then use Construct > Curve > Isect to get a curve where the fin piece intersects the outer surface.

See here for a few examples where Danny used this technique:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4084.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=3530.2

Anyway, that's one possibility for getting some curves that spiral around but actually hug your cap's outer surface. Then you use those curves for further constructions like doing a sweep along them.

You also might form the curve on the surface just by drawing a freeform curve and placing a lot of points snapped on to the surface too.

To get the tapering effect it may be easiest to construct 2 such curves for either side of the piece and then those can be used as the rails for a 2-rail sweep. Sweep using 2 rails where the rails come together can be an easy way to construct that kind of surface.

- Michael