Use the cleat that Yarcraft posted. Mounted right on the bow of the boat, makes it very easy to secure the rope, no knots to deal with.
I do think you are cutting yourself short on not using the cleats mounted on the sides of your boat. If you are working structure, in a river, with current, it's my feeling you WANT to change the connection point where the rope is tied off, to cover the structure you are fishing. Moving the rope from the right side of the boat, to the left side of the boat, to the center bow position, will move your boat a small amount allowing you to fish verticle in a slightly different location. Current fishing is all about current seams, if you are 5 feet away from a current seam, you get no fish, move 5 feet, and you could have the hottest day ever experienced.
In each rope position, fish the length of your boat, if you catch a fish, put down exactly in that spot again. Then when you are done fishing the length of the boat, with the rope tied in 3 different positions, let out an additional 15-20 foot of rope, and do it again. Start on the up current side of the structure, then fish the top, then fish the down current side of the structure.
I see people fishing river currents all the time from one anchor point, never moving in their boat, when they catch nothing, they pull anchor and leave. They might have been within a few feet of finding the current seam that holds fish, and never gave themselves the opportunity to catch fish. My thoughts anyway...