You're beam reaching out, and back? If so, then you can do it with just the jib. If you have to point any higher than a beam reach, then you'll need some mainsail too, even if reefed, so you'll have a balanced rig. As a general principle, you can't sail upwind without a balanced rig, meaning roughly equai amounts of mainsail and jib. But if you're reaching, or broad reaching, then jib alone will work okay. Just make sure you can fetch whatever buoy you're shooting for with jib alone.