Hey Elliot,
They are there to balance the boat on her lines. Check this thread: http://www.pearsonariel.org/discussi...vice-and-input

I wouldn't have noticed visually, but apparently Lucky Dawg used to squat aft a bit. What I did notice was that under sail, the lazarette was constantly awash with about 5 gallons of water and at rest, water pooled at the aft toe rails rather than flowing to / going down the scuppers. I experimented with adding more and more until finally I had 240 lbs of tube sand (about 20 bucks at Home Depot) in the forepeak. That eliminated both of those issues. Since they are used for winter pick-up truck traction (I presume) they are triple wrapped and less likely to spring a leak than construction sandbags. It doesn't happen often, but on occasion, Lucky Dawg can time waves so she wallops the water with her bow pretty joltingly. The sandbags are heavy, but snuggled in there very gently - if you can say that of 240lbs of mass. I would worry about the cinder blocks only because they have sharp corners - points of contact with the hull being more concentrated vs. over a long smooth sandbag line.