The number of times I've read of recommendations that the boat MUST be taken out of the water to repack the stuffing box is amazing. Most likely from folks who haven't done it. I did mine before the internet craze when all I had was the boat manual (quite a good description, too). I bought the stuff, figured out how to take the old stuff out (drywall screw - the piglet pigtail things don't work on my box, they're too big) and spent an hour or two upside down on my head with water leaking it all the time. The boat remained afloat. Even if my bilge pump was broken, or if the batteries were dead, there simply wasn't enough water to worry about.
Catastrophic failures would, however, occur with a PSS shaft seal.
However, your boat, your choice.
Failures of stuffing boxes? Unless the thread go wonky, which I've never heard of happening, if your packing is gone, throw a shoelace in until you buy some more packing.
It can be fixed "under way" but you'd want the engine in neutral or off to stop the shaft from spinning. Can be done anywhere on the water, not necessarily at your dock.
Stu,
I've "done it", on perhaps hundreds of different vessels, power and sail.... You really need to work on more of a variety of vessels to make broad statements like that. There are plenty of boats that I simply won't re-pack in the water as it is just not safe to do so, and I am darn comfortable doing re-packs.
Take the Sabre 34 MKI for example, did three this spring and hated every second of every one of them.. On the boats with port side shafting, most of them, this is a 1 handed re-pack where the nut does not even have room to move forward the 1/4" thickness of the packing. It is physically impossible to get two hands in there to re-pack unless your a Cirque De Soleil "little person" who can contort themselves into a shoe box...
It can take a few hours to re-pack these boats and water coming in would just make it take longer and make for a real PITA of an install. I would turn down that job if an owner asked me to do so in-water.. This is further compounded on the S 34 MKI by an engine sump that is isolated from the rest of the bilge, and they have no pump in there, to prevent spilled oil from being evacuated. Bristol 35.5's, Cape Dory's, many older CCA boats. etc. etc... Many boats can be easily re-packed in water but many are like going to Vegas, are you feelin' lucky.....
Even the late 80's Ericson 34's have a stuffing nut that barely moves forward a 1/4" but there is enough access to do these in-water, if you know what you're doing....
Just because it can be done in-water on many boats does not mean it can on all and there are many older CCA type Ericson's out there where this may be uncomfortable for most owners to tackle. This should not be based on anyone's comfort level but the person re-packing the box. I would not advise anyone not comfortable in their skill level to try this on all but the most simple to access boxes.
The other problem is that some of these older stuffing box hoses are so rotted out that I would not touch them with wrenches, in the water, to break the nuts free for fear of tearing the hose. This has happened to me, but luckily only on the hard.. I really would not want that to happen in-water... Also many builders, Catalina included, used sub standard non-stuffing box hose which is very, very thin compared to the proper hose. Most Ericson's I've worked on have the substantially thicker hose made specifically for stuffing boxes.
That being said catastrophic failures are VERY rare in traditional or PSS seals and if you replace the hose every ten or so years (6-7 with PSS), with the right stuff, it will likely never happen. The one failure I do see related to stuffing boxes is shaft log failures. Some builders bonded bronze shaft logs to the fiberglass hull, Mariner did this, and the bond eventually fails. I have seen Cape Dory's with a similar failure. These failures are usually a trickle though before it becomes catastrophic.
I see a lot more seacock failures than stuffing boxes. Had a Marelon valve about six weeks ago week with a broken handle that was leaking around the handle stem. The most common packing box "failure" I see is improperly tightened locking nuts and the stuffing nut backs off. I have seen boats sink due to this because the pump keeps up while the owner is still on-board but it quickly kills the batts once they leave.
Replace the hose every so often, use T-Band or AWAB hose clamps, properly tighten the locking nut, and this is really a non-issue, unless your boat catches on fire and the hose melts but then you have other issues more pressing.......