A question about trolling spinnerbaits for muskies.

The answer is both. The advantage by using a flat line is in that you can keep the lure below the surface by adjusting rod holders (also hand held) to keep the rod tip subsurface. On problem you have with spinnerbaits and speed is them staying in the water. A planer board will work great to get the lure too the side, but depending on the lure, you may have to use in-line weight between board and lure to keep it down.