As these are relatively large diameter seals they are normally split seals. Being split they generally have high leakage rates compared to non split faces. The leakage is collected and pumped away.
That my understanding, I don't know how they deal with a failure but if you really want to know I can find out for you as I do have a contact in the industry.
On seals used for this application it exist a deformation of the seal, into the housing, to do the job to avoid the ingress of water into the system. By the way, all these elements are ready to withstand friction. Nevertheless, friction, in this case, will not be very important since:
- Rotation on a shaft propeller is low
- Seal is continuously cooled by water
Once installed, as you say, you have got a contact-pressure against the shaft. This one, due the material properties and friction of the seal, gives to the final system (engine or whatever rotates the shaft) a minimal torque that can represent about a 5% loss of power (value to be calculated on each case). So shaft continues rotating without any trouble
To do this job you've got several kind of seals, including mechanical seals.
Most popular ones are packings and PTFE or plastic bushes, but, depending on criteria, you can go further. As you go down (and pressure increases) you need a seal more capable to withstand such pressure. In this case you have got special rotary seals (some are split to be easier the installation) to do the job
Even I have seen a propeller using just an O-Ring as the unique seal.
We have made applications on submarine equipments for critical environments, and having, inside, analytical instruments. In these cases, you can apply up to spring energized seals due that these are, mostly, made in PTFE, so power-loss (due low friction) decreases dramatically compared to a split seal.
On almost every case, most important point is to keep centered the shaft due that pressure is not such important as it should be a right guiding of the shaft.
