What's your heeling angle? & Is it a deep full-keeler, or shoal draft? Are you pinching a bit by the jib tell-tales?
If you are heeling beyond 20 degrees, that's why you are sliding sideways -- and a leeboard won't help, cuz it will be crabbing just like the keel. At that point, reefing the main and switching to a smaller headsail will keep you pointing higher (and probably making better speed). Pinching doesn't work so good on these boats because the keel and (presumably keel-hung) rudder aren't lifting foils. May need to keep it footing and accept lower pointing angles.