Article by Capt. Steve Katz
Shaft Seals
If your boat has an inboard engine, you most likely have a stuffing box, dripless seal or something similar to provide a water tight seal between the rotating propeller shaft and the hull and around the rudder posts. If you have an outboard or inboard/outboard (I/O) we will discuss similar seals on those engines.
Traditional Stuffing Box
Not much has changed since the 17th century when the stuffing box was used to seal bearings on windmill pumps! Our typical marine stuffing box is comprised of a locknut and packing nut that holds multiple rings of braided “flax” rope impregnated with wax and lubricants. The stuffing box is attached by a hose to the shaft log. The flax inside the stuffing box, when compressed, forms a seal to keep the water out of your bilge. A small amount of water is necessary to keep this flax from overheating and failing.
Adjustment – the packing nut must be regularly adjusted to account for wear of the packing. If it is too loose then too much water will leak past the seal into the boat. If it is too tight then there will be not enough water to cool the packing and cause damage, resulting in excessive amounts of water entering the boat and possible shaft scoring. If your stuffing box is leaking more than 10 drops a minute, it needs to be tightened or serviced.
Packing Replacement - When the flax packing wears, it will need to be replaced. My first experience as a teenager doing this was not fun. I was sent into the bilge because I was the only one who could fit and was handed two big wrenches and a sharp pick while the others on-board gave me instructions from the comfort of the salon. This was a fast way to learn the procedure with water streaming in while I removed the multiple layers of flax with a pick. I was really worried that I might be the cause of the boat sinking in the slip if I didn’t work fast enough!
Flax packing is available in many sizes; normally 1/8” to 5/8”. The size must match your boats hardware. If you do not know the proper size, you can remove one layer of flax and measure. Once you have the correct size, cut multiple strips (usually 3 or 4) that are long enough to go around the shafts circumference, butting ends at a 45 degree angle (no overlapping). You can use the actual shaft as a guide to properly size the flax. Loosen the locknut to allow the packing nut to be removed. Using a pick, remove all of the old flax and clean the area as necessary. Install one ring of flax at a time; be sure not to let the seams from one layer line up with the next layer. Use a soft tool to seat each layer of flax. Once the stuffing box is full, hand start the packing nut until tight. Remove the packing nut and check the flax to be sure it is seated properly, and then re-tighten. When properly adjusted, there should not be any seawater dripping when the shaft is stationary and only a few drips (1-3 a minute) a minute when the shaft is turning. You should check and re-tighten if necessary after running the boat for a few hours and again after a few trips.
ABYC standard procedures cover shaft seals and shaft logs and state that “Graphite impregnated packing material shall not be used because of the possibility of galvanic incompatibility with the shaft material” While many boaters have chosen to use graphite packing, I would suggest more frequent inspection if you use graphite material.
Dripless Shaft Seals
If all of the above sounds too complicated, then modern shaft seals may be for you. Manufactured by a few different companies in slightly different configurations, they consist of a flexible seal (face seal or lip seal) lubricated by pressurized water, usually from the engine’s raw water system. While these seals themselves do not require regular maintenance, inspections are necessary to ensure the components and supporting systems are in working order. It is most important to check the amount of pressurized water supplied to the seal, about one gallon per minute at idle for a Tides Sure Seal. A loss of cooling water will quickly ruin a dripless seal. If you have twin engines, be sure you have a crossover hose between the seals to assure cooling water to both seals if only one engine is running. Other maintenance includes checking the condition of the shaft tube hose, water supply hose and associated hardware. While these seals usually last years, when replacement is necessary the boat should be out of the water and the shaft needs to be removed from the gear to replace the seal. Many boaters choose to install a spare seal on the shaft at the time of original installation, which allows seal replacement while the boat is in the water, without shaft removal.
Outboard and I/O Seals
While these propeller shafts are different in design, they too have shaft seal. In this case, the seal is keeping the seawater from the oil lubricated lower unit. A seasonal gear oil change can indicate the presence of water in the gear oil, a possible sign that the propeller shaft seal is leaking. Often I find fishing line or leader material wrapped up behind the propeller, which can cause damage of the shaft seal. If there is water in the gear oil or a leak is suspected, a lower unit gear case pressure test may be necessary to find the leak.
Captain Steve Katz, ABYC Master Technician, is owner of Steve’s Marine Service serving Annapolis and Ocean City, MD.
