They are especially effective for trolling. When fish hear the sound of the roaring engine of a boat, most will swim to a safer location. Great news for the fish, but hardly the ideal situation for the fishermen who is trying to present his bait to the fish.
Letting out more line may seem to be the solution. However, you are still limited to covering only the area directly behind the boat. Also, depending on your rigging, letting out more line often means that your bait will sink deeper. This could lead to it getting caught in weeds or on the bottom, or may just not be ideal for the species of fish you were targeting.
With a planer board, you have complete control over the depth of your bait at any distance you want to place it, up to 150 feet or more away from the side of the boat. To control the depth, simply increase or decrease the length of the line between the planer board and the bait or lure. To increase or decrease the distance of the planer board from the boat, you can reel in the line or let it out more.
The shape of the side planer board is such that trolling it through the water, or letting it out into a current, will cause it to move to the side. Much like the shape of an airplanes wing allows the airplane to ascend and descend. Depending on whether it is a port or starboard planer board, either it will veer to port (the right if you are facing the back of the boat), or to starboard (the left when facing the back of the boat).
This method of fishing is highly effective but still fairly unknown and underutilized. If you use the planer board properly, you can improve the number of fish you catch, and have tons of fun reaching places you never thought possible without a single snag!