I had the same problem as you Jman, only I have a 14.5' runabout with a VRO 70hp, and the same set up as Dave with the Minnkota, only my foot pedal set up is modified and in the dash
My problem was when I fished slower river water I couldn't get below 3 to 3.5 mph after leaning down the Idle circuit, and turning the Idle down, I have 4 different props for it, (13,15,19,and a 21)so I went to the 13" pitch it only came down to 2.8 mph...Being poor I took 2 - 5 gallon buckets and drilled 1" holes in them(Just on the bottom to start) with a hole saw they slowed it to .7 mph up stream so. I just kept adding holes to the sides till I got my .9 to1.4 mph...Bring your cordless drill its not as bad as it sounds
Just remember to pull them out of the water before you take off......something Else, you'll want good rope and clips and don't trust the bucket handles take the handles off and drill holes to mount 3 points