I guess it all goes back to the saying, "If what you are doing is not working, you have to do something else". One thing I think most of are guilty of is next-spot-ites. We leave a proven spot, that is laden with bait fish, to hopefully find an active fish on the next spot. Some days when the fish are active, and you can do no wrong, this might work. But it probably makes more sense to assume the fish are there in the most likely place, and to figure out what they want. Or at least be there when the feeding window opens.

Often, when I'm trolling, I look at the distance from my inshore lure, to the shore, and wonder just how many fish are residing in that zone that are not seeing my lure. Darren always had a tactic of going back and casting a shoreline that should hold fish and did not produce on a trolling run, with the premise that the fish are shallower than he was able to get. I don't agree with the planner board concept. I see no way to measure or control the depth under the lure, and don't know how you can run a lure successfully, unless it's a very shallow runner. Comparing this to casting, you are still only covering one breakline, vs. straining the depths from very shallow. I am often reminded, and frequently bore you with the details, of fishing the second week of November with Brett C. and Gord. After 2 1/2 days of trolling unsuccessfully, we ultimately figured the fish were located very shallow. Probably in a foot or two of water. Another observation I've made is that when casting for fish this time of year, they are usually very "presentation specific". Meaning that if you don't show than what they want, you might as well go home. In the case with Brett and Gord, It was very slowly retrieving a Jake. Painlessly slow. If you twitched it, or reeled it any where near normally, no dice.
Other times (different trips) it was a very slow twitch and pause with a neutrally buoyant lure, and the fish took it on the dead pause.

Other times the fish go in the opposite direction. Deeper. Especially when the water temperatures dip below 40. I would say as a general rule, most of us use 10 feet as a starting point for trolling. Doesn't mean you are always trolling at 10 feet, but are attempting to. Sometimes you end up in 4, and sometimes you cannot find anything as shallow as 10 for a stretch. Half a boat width can be the difference between 4 feet or 14 feet. Sometimes ignorance is bliss. Can't tell you how many fish I have seen caught when someone accidently slips up shallow. Even nail a fish when freeing a snag. When I look back at it, sometimes the first time troll a spot, crashing too shallow, was my most successful time. After I learned the structure, and started getting finessey with the shallow part, I have never been as successful. Adopting the old "Spoon Plugging" concept of straining the depths, it would make sense to take that proven spot, piled up with bait, and work the structure increasing the depths with each pass. The direction and angle of your pass can make all the difference. Can't say how many times I have just come off a pass, and nailed a fish 50 yards into heading back over the same structure in the opposite direction.

Besides depth, other things to consider when evaluating the results of rod position. Lure speed on the turns. First one to the fish. Position relative to the structure. The outside rod is not always deeper. And, of course, whatever lure you are running in a given position. Lots of variables to consider when trying to establish a pattern. Agree with Rockpt that the 3-4 mph is a good starting point. Typically going faster when the fish are more aggressive, and slower when the water temperatures drop and/or the fish are less aggressive. I am constantly changing my speeds. Often a function of turns. A large benefit of running the boat from a tiller handle. When trolling walls. I run so close that I'm afraid the tip would hit if the lure lost tension, and the rod straightened out. I don't find trolling walls all that successful. Seem the perpendicular presentation of casting the wall is more successful that the parallel presentation trolling presents. Maybe some of that is time in the fish's face.

Successful trolling is much tougher than casting. You are in the fish's face a whole lot less than with casting, using the boat to position the lures. Off a few feet, and game over. I am constantly creating a mental image of whet's under me, and how the lures ore contacting or relating to it. Wonder how the Humminbird 3D units would benefit this concept?