well, you need something more than isometric exercises for your core Ristar, they don't necessarily have to be crunches tho.
But your right, you need rotational work in there
Weighted side twists
(To do these get a 5-15 pound plate/dumbell)
Lie with feet fully extended and arms behind head, holding weight,
Crunch up, bringing your arms over your head and reach out towards your legs,
Then, twist around 90 degrees in both directions, really reach out.
Do them slowly, and put them in sets of 8-12, there great because they hit ur rectus, obliques, and really help your core rotation strength)