Try without an anchor. I have used a pontoon for 10 years and never use one in lakes except for very windy days and don't use one in rivers. To answer your direct question I use a 10 lb pyramid for the few times (once or twice a year out of 30+ times/year on the water.
I have an 8lb and 10lb for my 9' pontoon and before that used an 8lb on my 8' pontoon. I've heeded the advice here about anchoring in rivers and only anchor in a river while on the bank. In lakes I've found the 8 and 10 more than enough to hold me in all but the stiffest of winds.
You might also consider fabricating or buying a mesh "anchor bag" and fill it with rocks at the river. Nearly weightless until you fill it, just a thought. I've never used one, but one came stock with my newer pontoon. If that intrigues you shoot me a PM, it could be yours for cheap. It came with a line that I'm not using also.
:rofl:Caveman make tool! I love it. :thumb: How much did I waste on my fancy poured and molded anchor collection? bawling: Hell, some rivers I've visited even have rocks with holes already in them, pass a rope through and presto!
Beats me, all I know is that it is cheap and it works. If you want to get technical, put the can on a scale and poor cement to desired weight. I've also seen guys use cinder blocks. You can learn alot by watching "tarheels".
I tried out a "sea anchor" today on the raft. I used one of those cloth safeway bags with the rectangular bottom, it opened out perfectly with rope through both handles and slowed my drift markedly. I experimented a little with it, paddling against the 10 mph wind and anchor I made no headway. When I wanted to move, I tipped the water out and could navigate the raft easily. Seems like the perfect kind of bag to also throw some rocks in if you were on a river.
Use a 3.5# folding grapnel anchor in lakes. Plus a foot of chain. Holds an 8' pontoon just fine; if it doesn't hold, it is time to hit the scotch and remember when the waves didn't come up over the back of your waders!
With a boat go for 2 8-10 pyramids or concrete in a can. Great for mids.
In a river, I wouldn't anchor a pontoon except up on the bank.