Hey PaulE ;D,
When I had my 11 foot fly fisher, I used 2, 10 pound, round downrigger balls and the worked great. They sink in the mud well and you don't move once they are down! There are also the lead pyramid anchors and they sink well into the mud and if you anchor in a place that has less mud, the four corners of the pyramid anchor will grab better. As far as how heavey they should be, I would say 2 lead, 10-12 pounders should work. If you have several people in the boat you might want a little heavier. :thumbup: