David, I work in the seal/packing industry and have done some research on the different packings vs mechanical seals for boat shafts. I am preparing a boat for long distance cruise and have decided to go with packing if only for its simplicity and ease of maintenance. Although I have complete confidence in mechanical seals. I have also looked at TFE lip seals since I provide so many to industrial and motorsport applications. As far as TFE impregnated packing there are lots out there. Some even claim not to need any lubrication (ie no drip in the bilge). I will not go that way myself. I will probably use a natural ramie fiber impregnated with tfe. Product name Ramilon 4586. Its advantage over cotton and flax is that its 8 times stronger and does not change its volume in water. Its advantage over some of the synthetic fibers is that it will not wear the shaft as bad.
There is always a compromise and your type of sailing is probably the deciding factor. I did a drawing a few years ago that was for a stuffing box set up for a mechanical seal with a packing backup. The packing would be in place but not tightened unless the mechanical seal failed. I like the mechnical seal because of the NO shaft wear factor. I like the packing because of its simplicity and ability to handle some shaft runout. The two together would be a good combination but I never pursued it any further. No real commercial value, something I will look at again in retirement. Hope this answered some of your questions. Feel free to call me at 800 449 4729....Don Gray...Signal Technology Systems, NC
I''m a mechanical engineer and being very familiar with pump mechanical seals and packings as related to industry I thought that switching to a premium brand pure teflon packing would be a smart move. NOT!!! I replaced the old worn out flax/wax packing in my Mirage 29 with its Volvo 2 cyl. diesel and 1" dia. prop. shaft in the Spring of ''99 and had nothing but leakage problems throughout that season. I conferred with pump experts who agreed that shaft rpm/fpm was well within the acceptability of the teflon packing. The shaft and gland/nut were in great condition. In theory it should have worked but it dripped at an alarming rate and got progressively worse with each passage under power. Repeated gradual tightening only succeeded in extruding the teflon out the gap between the nut and shaft. I finally broke down and changed back to cheap flax and wax packing while still in the (fresh)water. Instant success! A little initial tightening and then one follow-up and scarcely a further drop in the bilge even at 2500 rpm and hours of running. It seems that the tried and tested flax swells enough to provide the right compression without overheating and with no further tightening. In this case modern technology is not the answer!