So, what's best? Lip seals require no rotating ring, and in fact, have no moving parts at all, but they depend on a pristine propeller shaft surface to maintain a seal. Polishing of the shaft may be required before installation, and any corrosion, nicks or pits can compromise the seal, allowing leakage and premature wear to the seal itself. Face seals require a rotating ring and adjustment of the bellows preload, but because of O-rings within the ring, they can be mounted to less-than-perfect shafts and still maintain a good seal. A disadvantage shared by both is the need to uncouple and withdraw the shaft for seal replacement, sometimes necessitating hauling the boat. This expense can be delayed, at least once, by mounting a spare seal in a carrier on the shaft itself at the time of the original installation.