The pontoon is entirely successful. I used 8mm chain and 5kg Bruce anchors, lighter than your specification, but far less backbreaking to lift and check, and rock-solid even in the worst weather: gales with 80 mph gusts and three months locked in four inches of ice. The anchors were well bedded-in with the powerboat. We’ve found that heavy gear tends to skate, whereas the lighter anchors just dig deeper and deeper. If the pontoon can stand the strains of ninety acres of moving ice during the thaw, the load imposed by the dinghies (even a novice trying to fillet it with a Wayfarer) is minimal.