Limited Clearance Collars are made to fit onto the driveshafts where space is limited, such as on sail drives. They are precision die cast to fit tightly around the entire propeller shaft. Water between the collar and driveshaft can lead to the anode degrading from the inside, causing the collar to loosen and spin, losing all metal contact and becoming useless. Other manufacturers may use small BB’s to make up for a poor fit, lifting the collar off the drive shaft and creating the same effect.