The stuffing box is difficult to reach and inconvenient to repack or retighten. This has lead manufacturers to create a mechanical seal that keeps the water out while allowing the prop shaft to rotate. Many people have installed these, and mostly they love them. I've got my reservations, as a failure on one of these devices may be sudden and catastropic. I prefer to use a high-tech packing material (along with standard flax) to give me a no-drip stuffing box that fails slowly and gracefully. I'll leave it to a fan of these mechanical seals to write a description of these units and their advantages. (Please send it to me and I'll add it to the site.) Also see Dripless Shaft Seals Have Advantages by Terry Johnson for a good description of these seals, and some of their advantages and disadvantages.
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