Shaft Seals

On the face of it the packless shaft seal (PSS), which is dripless, seems to be the obvious choice, but there are one or two caveats:

Firstly, there are two types of PSS, depending on the anticipated speed of the shaft. For anything over 12 knots hull speed, it is recommended that the seal have a nipple fitted for the injection of water to keep it cool. This is usually tapped from the engine cooling water. Low speed boats do not need it. The Slipper Launch is definitely low speed, and besides, the electric motor is not water cooled, so a nipple-less seal could be used. But, these seals need to be filled with water rather than air, so have to be manipulated (basically bellows pumped) every time the boat is put back into the water after trailering. The gland type seal, because it leaks, is self-priming in that regard. Alternatively, a high speed PSS seal could be used with the injection nipple connected to a through hull fitting above the water line, which would allow retrograde priming.

Secondly, when a gland type seal fails, it does so slowly, by allowing gradually more and more water to drip through. When a packless seal fails it could theoretically do so dramatically, and allow a large amount of water through suddenly, possibly overcoming the capacity of the bilge pump. The people with experience with these seals, however, tell me that they have never seen such an occurrence.

Moreover, there is a benefit in having a usually dry bilge and considerably less friction with the PSS. They require very careful fitting of the components over the shaft tube and onto the shaft itself, and a stable motor which will not tend to displace the sealing rings with movement, but as these elements can easily be achieved with the electric motor I think the PSS is the way to go.

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