Where the shaft is what is pushed through a hole in a boat to turn a propeller, the shaft seal is what is placed around it to keep the water out. For ages, this has been accomplished with a stuffing box into which a flax packing is wrapped and then compressed around the shaft to create a seal. The nut is tightened until the flood of incoming water is reduced to a drip—about two or three per minute—indicating that the seal material is being properly lubricated.
The problems with stuffing boxes are twofold. The least of the two is the need to pump out the water that leaks in. That’s annoying enough. But along with the lubricating water comes silt and sand, which never quite makes it into the bilge because it is trapped by the flax seal. And, because we don’t see it, we’re not bothered by it until, perhaps, years later when we discover our shaft has been heavily scored. All along, we’ve been re-tightening the nut to keep the drip at the correct rate when we were also increasing the pressure on the trapped abrasives that enhance the scoring.
The salvation for stuffing boxes is that this process can go on for many years with no real problems. All we have to do is periodically pick out the old flax, which also removes the collected abrasives, and replace it with new flax. When we tighten the nut, the flexible flax properly seals the shaft…even with the scoring.
If he operates in dirty waters, the powerboat owner will probably find himself doing this often. On a sailboat with a deep V-shaped bilge, however, repairs and adjustments may not need to be as frequent. Nevertheless, the owner will still have to learn how to stand on his head, grow a second elbow, and if he holds his mouth just right, he might get the job done in an hour or two.
A Better Idea
We evaluated shaft seals from six manufacturers, all of which take the sealing responsibility away from the stuffing box and place it within the shaft seal unit. None use flax packing or require periodic adjustment and all should make for a dry bilge over a long service period.
Some seals employ a positive lubrication system in which cooling water is piped from the raw water pump into the seal housing and made to flow out the stern tube rather than in the conventional method used on stuffing boxes. Some units use the prop shaft for the sealing surface while others take the seal completely off the shaft and place it elsewhere. And one unit doesn’t use water as a lubricant at all. Rather, it imitates the seals found in the lower housings of outboard motors and outdrives.
On a planing powerboat, a positive lubrication system is virtually mandatory because the venturi effect of the hull actually sucks the lubricating water away from the seal. On a non-planing hull, using a forced lubrication system is more of an option although the seal may still benefit from the cooling effect of forced water lubrication. The choice will depend on how hard the boat is used.
On a sailboat, a well-designed shaft seal can mean the owner will not have to worry about shaft leaks other than to take a periodic peek to see if anything is wet.
Rating Criteria
Our first concern is what happens when these seals fail and what would make them fail. Are they easily repaired? Can they be repaired in the water like a stuffing box or does the vessel have to be hauled?
Our next concern is with ease of installation, maintenance, clear instructions and potential pitfalls. If we are to have years of service, can we handle it ourselves?
Lastly, we must consider cost. We’re willing to pay for boat security, but will a one-time investment last the life of our boat or will repair and maintenance be a frequent and costly burden?
Rating Levels
All of the products examined work and should keep one’s bilges dry. To distinguish between the types, we created three ratings—Good, Better and Best
Good. Categorically, we think that any water-lubricated seal that uses a simple O-ring around the shaft can never be better than Good, and here’s why. Whether your shaft seal has forced water lubrication or not, there is always positive pressure on the wet side of the seal. The positive pressure on the non-forced system still exists because the seal is below the waterline. And the water still contains the same silt and sand that aggravates stuffing boxes. But because there is no drip flow under the O-ring, the abrasives will not collect at the same rate. However, the seal will eventually begin to leak because of wear, the abrasives will collect, and shaft scoring will occur. The longer you let it go, obviously, the worse it will get.
Now, to repair it, you have a dilemma. A new O-ring simply will not properly seal a scored shaft. This means that the O-ring will have to be moved to a cleaner place on the shaft or the shaft will have to be replaced (expensive). In most installations, we think repairs can be accomplished by shortening (or lengthening) the connecting hose plus replacing the O-ring, but the number of times this can be done is obviously limited.
Better. Any system that moves the seal off the shaft and places it elsewhere completely avoids shaft scoring and is therefore better than O-rings on the shaft.
Best. These systems move the seal off the shaft plus provide substantial shaft-bearing surfaces for automatic alignment. A sealing surface off the shaft is great, but if it is not properly aligned, it is destined for excessive wear and premature failure.
Click here to view the Dripless Shaft Seals Value Guide.
Before You Begin
All of the shaft seals evaluated here employ a hose or other mechanism intended to connect to the stern tube. Before you charge ahead to retrofit an existing stuffing box, be sure to examine your particular set-up so that proper connection to the stern tube can be established. Not all stuffing box configurations are alike and your through-hull hardware may have to be changed.
Norscot
Made by the Ibsen Co., Norscot standard and non-standard shaft sizes are from 3/4″ to 3″, with prices from $182 to $494.
These units do not use water as a seal lubricant but rather they imitate the oil-lubricated shaft seals found in outboard motors and outdrives. Two replaceable O-rings are used on the ends of an oil-filled cast bronze housing that is connected to the stern tube with a rigid hose. Dextron ATF oil is fed into the unit by a reservoir mounted away from and above the waterline. The simple elegance of this system—although it does use the shaft as the seal surface—is that there is no positive water pressure on the rear seal. Instead, the positive pressure is within the unit and generated by the elevated oil reservoir. This means that when the seals do wear because of time and usage, no abrasives will collect to score the shaft surface and the O-rings can be simply replaced.
In addition, the wearing can be easily detected by a steady drop in the oil reservoir level.
The negatives are these: The portion of the shaft where this unit mounts must be pristine and free of burrs and cuts—even those that have been polished out. Very often, shaft installers manhandle the shaft with water pump pliers or Vice-Grips, which leave small digs even in stainless. Polishing these out still may not be sufficient to allow these seals to seat correctly.
Secondly, the shaft must be aligned to the center of the stern tube because this unit has no provisions for self-alignment. This alignment has to be accomplished after the unit is slid onto the shaft, before it is installed, and after the shaft is re-connected to the transmission.
A spare front seal could be stored on the shaft and installed when needed while the boat is in the water, but if the rear seal has to be replaced, it will be necessary to haul the boat and the installation process repeated.
Bottom Line: Given a clean shaft and proper alignment, we see this unit yielding an excellent life under all usages and shaft RPM. We rate it between Better and Best.
TrellCom Marine
Made by Svelda Industries, TrellCom Marine shaft seals come in sizes between 3/4″ and 3″, with list prices from $278 to $704.
These are brass units with force-fed water lubrication that requires between 1–2 gallons per minute of water flow per inch of shaft diameter. They employ a lip seal made of acrylonitrile (Buna-N) synthetic rubber against the shaft instead of an O-ring. The seal is held in place by a retainer ring for ease of maintenance. (A lip seal is wedge-shaped and curls against the shaft surface a bit like a windshield wiper). The unit also contains a short segment of a cutless bearing-like guide also made of Buna-N, which helps keep the unit aligned.
We expect this unit will perform well at any RPM, but because it uses the shaft surface for its sealing surface, we fear shaft scoring will be a problem as the lip seal wears in silty waters. It will be easy enough to change the hose length to make the new seal ride on a clean surface, but the vessel would have to be hauled, even if a spare seal is stored on the shaft.
Bottom Line: In accordance with our rating scheme, this unit falls in the Good category.
Strong®
Tides Marine makes Strong® shaft seals in standard sizes of 3/4″ to over 6″, with list prices from $130 to $750 for the 3″.
Strong seals are made of rugged polycarbonate, with forced water flow seal housings that employ a nylon alignment guide plus a hefty O-ring seal against the shaft. A spare seal is provided, which is housed in a package designed to clamp onto and spin with the shaft 1″ forward of the working unit.
Because the seal change cannot be done quickly, the vessel will have to be hauled to complete it properly. The instructions emphasize the importance of the shaft surface being free of any burrs or nicks that might damage the O-ring surfaces of both the working unit and the spare.
To aid in protecting the seal surfaces during installation, Tides Marine provides a removable plastic hat that fits inside the unit as it is slid onto the shaft. They say nothing about lubricating the shaft during this process, but clearly some liquid soap will help. Nor is any mention made about pre-aligning the shaft to the center of the stern tube before final installation, but here again this is required because the connecting flex hose is still quite rigid.
Bottom Line: Because of its beef, we rate this unit at the top of the Good category.
PYI Packless
The PYI Packless Sealing System shaft seals come in standard sizes from 3/4″ to 3-3/4″ with list prices from $170 to $1,320.
These off-the-shaft sealing units employ a polished bearing surface on a carbon graphite flange that is connected to the stern tube with a flexible hose employed as a spring bellows. The flange is pressed against a stainless steel rotor bearing surface that itself is O-ring-sealed and secured to rotate with the shaft with set screws. We examined two units—one with forced water lubrication for planing hulls and the other without forced water lubrication.
These simple yet elegantly designed units should yield a long service life with little or no maintenance. If any seepage were to occur, it is easy to compress the bellows and wipe away contaminates that may have collected on the sealing surface, and then let the seal reseat itself with usage.
If there is a downside, it is this: We’re not enthused over the nylon nipple used to pipe in the forced water lubrication. For us, nylon hasn’t aged well and always seems to crack at the most inopportune time.
Neither are we delighted with clamping the stainless steel rotor bearing to the shaft with set screws. Set screws need a bite to hold securely and on stainless that’s difficult to do. Even if they do establish a bite the shaft surface will be burred in exactly the wrong place…because this fixture may have to be moved to get the proper bellows pressure. Here, one of the O-ring seals within the stainless steel rotor would be made to slide over the burrs no matter which way it is moved. (Sets of spare O-ring seals are provided).
We voiced this concern to PYI and they said there have been many who have expressed the same worry. However, they said, the cup screws employed (as opposed to pointed) do little to mar the shaft and there have been no reported failures.
Although it’s contrary to intuition, PYI makes a major issue of NOT lubricating the shaft with oils before the hardware is slid on and this is correct. Oils under the O-rings, even if not rotating against the shaft, will allow a water leak. Use only liquid soap because it is soluble in water and will quickly dissipate during service.
Bottom Line: We like these units and rate the forced water cooling model in the Best category. The other non-flow model we rate in the Better category, only because we wouldn’t recommend it for a planing powerboat. For a sailboat, it won’t make a difference.
Duramax
Duramax Shaft Seals come in standard sizes from 3/4″ to 6″ (metric sizes 20mm to 120mm) and cost from $200 to $817 (for 3″).
We give Duramax high grades for the best instruction manual. It carefully walks the user through a proper installation. In addition, the literature is very helpful in guiding the user to the selection of whatever he needs to alter his stuffing box to make it shaft seal friendly.
The Duramax forced water-lubricated units with brass fittings are the biggest of the bunch. They use a sturdy polycarbonate clamping mechanism with an internal O-ring to connect to the stern tube—as opposed to a simple hose. Their off-the-shaft nylon seal is spring-loaded by a short section of rather stiff bellows hose that presses against a stainless steel bearing surface affixed to the shaft with a hard rubber clamp that is also sealed by an O-ring. We like this method better than set screws.
The instructions are very detailed as to setting the bellows compression. There is no specific provision for shaft alignment, but although they recommend the shaft be centered to the stern tube as close as possible, they claim the unit can withstand misalignment and shaft whip as long as it doesn’t hit the inside of the guide structure.
Intuitively, we feel that the actual nylon sealing surface is a bit undersized. It’s only 3/16″ wide, riding against a 7/16″ stainless steel bearing surface; if wider it would distribute the load over a greater surface, thereby yielding an increased life.
Bottom Line: We rate Duramax in the Better category.
Lasdrop
Nautical Specialties’ Lasdrop shaft seals are made in standard sizes from 3/4″ to 4″ with list prices from $95 to $675 (Dry Seal models); $179 to $1,550 (Original models); and $245 to $1, 975 (Gen II models).
We reviewed these three forced-water lubrication models in two types; one uses the shaft for sealing with a hefty O-ring, spare seal provided (Dry Seal model); two employ an off-the-shaft sealing mechanism with a graphite compound against a stainless steel bearing surface (both Original and Gen II models). For shaft alignment, all three utilize a cutless bearing-like section built into the connecting hose.
The difference between the two off-the-shaft seal types is how the sealing surface is loaded. The Original model employs a sturdy flexible hose as a spring bellows that presses the graphite seal against the stainless bearing that in turn is clamped to the shaft with a hard rubber split ring and sealed with an O-ring. The Gen II model uses a specially made shaft fixture with a built-in stainless steel spring that makes the stainless surface press against the graphite bearing surface, which is affixed with a rigid hose to the stern tube.
At first, it was difficult to choose between these two very well-engineered techniques but then we found something to pick on. The Gen II’s unique stainless steel bearing with built-in spring is housed in a nylon case and clamps onto the shaft with a split section.
It is also sealed off with two O-rings that necessarily have to slide on the shaft because of the spring. Now, once set, there is no reason for the seal to slide onto the shaft except as the bearing surface wears. (Nautical Specialties tells us the Gen II spring will outlast the Original model’s bellows spring). Nevertheless, we still prefer the flex hose model for its simplicity. Besides, we really like the hard-rubber clamping mechanism with its assuring O-ring seal for holding the stainless steel bearing surface in place.
As for the basic Dry Seal on-shaft-sealing model, it is simple, well-made, self-aligning and connected with a rigid hose. It has a heftier than average O-ring seal but it is not our choice in dirty water, even if a spare seal is provided.
Bottom Line: The Lasdrop Original and Gen II rate in the Best category, while the Lasdrop Dry Seal we put high in the Good category.
Conclusion
Clearly, any shaft seal that provides a dry bilge is better than one that does not. A dry bilge also eliminates the headache of figuring out what to do with oily water that by law cannot be discharged over the side.
We found nothing that would limit the speed and RPM of any of the units reviewed, but for a high-speed planing hull, a forced-water lubrication system—because of its ability to cool—is a must.
The Lasdrop and PYI products top our list.
On a non-planing hull or a sailboat, any of the units should perform satisfactorily with a long service life but we still recommend those units with off-the-shaft sealing mechanisms as opposed to those that seal against the shaft—especially if the vessel is to be used in silty and sandy waters. None of the manufacturers we talked to have run extensive tests-to-failure in silty and sandy waters although most have bench tested them for many thousands of hours. All tell us that the flushing mechanism in a forced-water lubrication system is all that is needed to keep the seals free of abrasives. Maybe so, but when a seal does eventually fail, we’d prefer it not damage our shaft, no matter how slight.
Lastly, is the issue of cost. These products establish boat security, and because they are all different, we cannot highlight a Best Buy. Engineering is what counts here, not price.
Contacts- Duramax, Duramax Marine, 16025 Johnson St., PO Box 67, Middlefield, OH 44062; 440/633-1616, fax 440/627-5365. www.duramax-marine.com. Lasdrop, Nautical Specialties Inc., 14081 Timberview, Shelby Twp., MI 48315; 810/781-9843, fax 810/781-9845, e-mail [email protected]. Norscot, Ibsen Company, 15026 Dansmore Ave. N, Seattle, WA, 98133; 206/364-2284, fax 206/362-4970, e-mail [email protected]. PYI Inc., 12532 Beverly Park Rd., Lynnwood, WA. 98037; 425/355-3669, e-mail [email protected]. Strong, Tides Marine Inc., 3251A SW 13th Dr., Deerfield Beach, FL 33442; 800/470-0949, www.tidesmarine.com. TrellCom, Svelda Industries Inc., 3588 Main St., Keokuk, IA, 52632; 800/553-7036, 319/524-8430.
