Dripless vs Flax Stuffing Boxes
a- whats the difference, and
b- which is preferable, and why.
all my previous boats have had outboards, so i'm clueless. the new boat is a choey lee bermuda 30, and she's been on land for 10 years, so i'm expecting to have to replace most everything. i just heard about flax stuffing boxes the other day from a gentleman i met in a local yard and have all sorts of questions i don't even know about yet!! help!
you can find lots of diy packing gland articles with a simple google search.
By the way, it isn't "dripless"flax; it is drip less - they all require water for cooling, some more, some less. There is such an animal as a real dripless packing gland made by PYY which is also easily findable with a search.
There is a third....
Non-flax packed Stuffing Boxes.
To answer your question:
Avoid flax. Flax is a very fibrous chopped material that is imbedded in a waxy like substance. It resembles fibrous play-dough and is packed into a traditional SB
The other material is a tightly braided rope, that may be made from graphite fiber and may or may not have PTFE. It is designed to be as close to the performance as a dripless SB. It is packed into a traditional SB and adjusted tight to eliminate the drips.
A Dripless SB is a whole different design. There is not packing nut or flange and the principle is the a watertight seal is made by a graphite disk ring against a polished SS flange. It has to be bleed of air to make it work.
There is no good thing really about flax, so avoid it.
If you have a traditional SB, use one of the newer graphite packing material with PTFE and adjust it to get just a drip or two a minute during operation (when the shaft is turning). The downside is the the packing nut has to be adjusted periodically and eventually changed.
You can change to a dripless, but bear in mind, if a dripless breaks, you have a major problem on your hand. However, the chances of this happening are extremely small. With a traditional SB, even if the nut comes all the way off, you're getting about 2 gal/min, which a good bilge pump can keep up with for a while, while you fix.
In my boat, we have the traditional SB with the newer packing.
We have a PYI dripless. No issues for the past 5 years. It's made of a stainless collar that rotates against a carbon ring for the seal. A "rubber" (it's probably viton something) bellows keeps the collar and ring in contact. As you'd have to pull the shaft to replace the bellows (that's what would fail, if anything), checking it is required maintenance. But it is drip free.
I am dealing with a HARD bellows on a friends boat right now which is allowing it to leak under power.
While the bellows gave a great life it will require hauling the boat and replacing the shaft coupling as its NOT coming off due to the water leak from the dripless seal :)
We've had both... installed a PYI dripless on a boat with a V-drive - appreciated the feature given the almost impossible access to the SB below the oil pan. It worked fine, but anything that disturbs the sealing pressure from the bellows will allow leakage. If you plan to do in-the-water gearbox changes, or engine removal be sure to take precautions to avoid unloading the seal - it will leak very well indeed!
We replaced the traditional stuffing box on both our Sabres with a PSS shaft seal. It's what Sabre installs on their new boats. While the issues that others in this thread raise are valid, they are of low likelihood. We have never had an issue with our PSS, I check the bellows regularly, and made sure that the area around it was clear (should be anyway since there is a spinning shaft). Besides being dripless, another benefit of shaft seals is that they reduce the effects of minor engine misalignment since there are only two points holding the shaft vs. three with a stuffing box. The effect is increased cutlass bearing longevity. I don't plan to go back to stuffing boxes; if shaft seals are good enough for the Navy, that's good enough for me! IMHO, stuffing boxes are more worrisome because you always have to keep after that drip, and I HATE leaks of any sort!
I describe the benefits and process to install the shaft seal in the following two links:
BTW, I agree with others that flax in a traditional stuffing box should be avoided. Use the synthetic stuff. Flax is the traditional material because they didn't have anything better in the old days.
The traditional STUFFING BOX
Cons: needs periodic adjustment, needs smooth shaft surface to work well (no burrs, no galling or anyother shaft surface irregularities). Shaft should be removed and 'dressed' by a machinist when replacing packing.
Pros: can be packed (temporarily) with virtually any 'lubricated rope' that fits when in a dire emergency ... and still affect a 'seal' with the propshaft turning.
Inexpensive to operate. Technology is over 200 years old; therefore, quite reliable. Easy DIY changeout of packing.
Better pros: can be packed with braided PTFE (Gore-tex) for 'virtually dripless' operation.
Cheap, even when using GFO packing.
Virtually Dripless Packing by Gore GFO
SHAFT SEAL .....
Cons: CANNOT be fixed/adjusted while underway. If it fails while underway, you usually must stop the leak by 'draconian methods' ... and that all stop the shaft seal from working thus you cant use the engine. Those that have 'auxiliary' water cooling need to be 'occasionally burped' - installation of the high-speed seal requires careful 'precision'.
Pros: dry bilge.
I have a PSS shaft seal on both of my Newports. the 28' had one installed twenty yrs ago and it still is drip free. you do need to check that the clamps are tightened but other than that happy drip free sailing.
Rich - A clarification.
Shaft seals are self adjusting by their nature so they will not need to be adjusted once installed. Once the collar goes on and is locked, it's not going anywhere. We've never had to perform any adjustment in 20 years.
PSS no longer sells seals that need burping. They all come with a 3' hose that is led above the waterline. I was initially disconcerted to have an open hose in my engine compartment, but quickly realized that if I simply led it straight up and attached it to the underside of the cockpit floor, there would be no problem. Of course, I could take the hose off requiring burping, which is ok too. Do it once when the boat goes in the water and that's it.
One advantage to a shaft seal - you can't lose your shaft! I saw it happen once and the boat nearly sunk at the dock. The owner had engine work done and the mechanic forgot to tighten the coupling bolts. Slid right out the stuffing box when the engine was put into reverse. The collar on a seal would have prevented that. Pretty far fetched, but true!
|All times are GMT -4. The time now is 08:07 AM.|
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
(c) Marine.com LLC 2000-2012