These Optimist buoyancy bags are made of polyurethane coated nylon and are about half the weight and significantly stronger than buoyancy bags made of PVC or PVC coated nylon. Some bags are available with a twist lock tube valve.This valve is attached to a piece of tubing. With this valve the bag can easily and quickly be inflated and deflated. 48L bags are slightly heavier than 43L bags but after a capsize there is a lot less water in the boat.