You can get by with the 115hp to troll but it's a lot more work. I had twin 115 mercurys on my 22' Larson. I ran depending on the situation one , or two sea anchors, or none at all. When fishing in the ocean I could slow a motor down to 500 rpms and fish all day with no problem. That worked wel because there were few boats close to my loction and there was a lot of room to move around. When I fished the tidewater buoy 10 up to Rice Island it became a different ball game. As Roger mentioned wind, tide, and river current all come into play. The seep and direction of the other boats also has an effect on the fishing situation. I was never able to match the trolling speed of the other boats and it became a problem for the other boats and myself. If you are in a pack of boats going 1.5 mph and your slowest speed is 2 mph it creates a conflict. To deal with it I would have to put the boat in nuetral, always shifting in and out of gear. That action creats wear and tear on the motor and takes the fun out of fishing. To get around that I would try to stay outside of any group of boats. Doing that can take you right out of the bite. When you do hook a fish when you have sea anchors in the water, it's one more thing that you need to get out of the water and back into the boat or risk fouling a line in them. I'm a slow learner I fished the boat that way for 24 years. My new boat has twin 150 Yamahas but more importantly it has a 9.9 high thrust kicker.