Stuffing box or dripless shaft seals?

[Draughty]

My vote goes to the Volvo-penta seal; cheap, effective and long lasting.

[mvweebles]

For archival purposes, a summary on dripless - not intended to change any minds of the die-hard traditionalists, just provide some facts and comparisons for anyone considering a Dripless:

There are two types of dripless systems - a face seal type (ex: PSS, Lasdrop), and a lip seal type (ex: Volvo, Tides). The Face Seal type has two pieces, the Rotor Assembly which attaches to the shaft and spins with the shaft, and a Stern Tube assembly that attaches to the stern tube and is stationary. Each piece has a machined surface that forms a seal. This type requires compression, which is provided either by a bellows with spring on the Stern Tube Assembly (PSS) or an internal spring assembly on the Rotor Assembly (Lasdrop).

The Lip Seal type is a single piece Stern Tube assembly. Sealing is provided by an O-Ring or V-Ring against the shaft (common type of seal in industrial applications). There are no moving parts, but they are more susceptible to silt and debris to eventually score the shaft. Some Lip Seal products rely on multiple seals within the assembly -

Lip Seals have the benefit that on some (Tides), a spare O-Ring can be installed and pushed into place if needed. The downside of a Lip Seal style is they are more susceptible to silt and debris wearing the surface of the shaft. For this reason, many industry commentators (such as Practical Sailor) recommend the Face Seal style over Lip Seal, though less of an issue at slow-speeds of displacement vessels.

In my opinion, of the Face Seal variety, the Lasdrop appears to edge PSS for two reasons: First, Lasdrop uses a two piece annular clamp on the shaft to hold the rotor assemble in place while PSS uses set-screws to clamp the rotor. These set screws are the weak point of the system and should be renewed with each adjustment, so carry spares (and carry them in a place you'll find them when needed). Second because Lasdrop uses an internal spring on the rotor assembly, the bellows is a non-proprietary length of straight hose to connect to the stern tube. Lasdrop uses Trident silicon hose which is incredibly strong and solvent resistant and is a good choice for a traditional stuffing box.

Planing speed vessels create suction on the cutless bearing which would cause the shaft seal to run dry. To remedy, all shaft seals for hi-speed vessels require some form of lubrication (some use a small oil reservoir, most use water from the exhaust). This is a non-issue for displacement speeds, and the hose barb on the seal not needed, though it can act as a vent so no 'burping' is required (run the vent to above waterline along the boat's centerline).

[MalteseFalcon]

You can purchase directly from PYI cost should be around $300 US which is nowhere near $1000 Canadian it also includes free shipping, not sure if free shipping to Canada or just to a USA address

[Rhumbline]

My C&C 38 came with a dripless which I was told by the seller would last forever. I've since learned that is not true and the expected lifetime is about 10 years, and when they fail they can fail catastrophically. IMO the old style packing is much simpler, cheaper and safer. I'm thinking of changing it out on my next haulout - before it becomes an expensive problem.

[Kammo]

If the seal leaks at higher RPM you may need to fit a ball valve or restriction to the water feed on the seal. It is common for the water pressure/flow to be too high causing the seal to leak. They only need enough water to lubricate the seal and and log bearing (if fitted).

[Capt. Mike Maurice]

The safest stuffing box uses two, commonly available bolts, one on each side that can be adjusted with a common wrench or maybe pliers. The packing can be replaced while in the water and almost anything be used in an emergency for the packing. Whereas the dripless systems if they have serious problems may require a haulout and the parts may not be readily available.

Nordhavn uses the two bolt system I described. I like dripless systems and have had them on many boats. I have had major trouble with them a few times. I would not have one on a boat to make blue water passages. These are essentially the issues.

[Jim Cate]

FWIW:

There are other face seal models available. The Crane unit on our boat is much simpler than the vaunted PSS, has no set screws involved, is smaller, has a very rugged bellows and in addition incorporates an emergency clamp which can seal off leakage if the bellows is compromised. This latter device obviates the worries about catastrophic failure at sea, but one must note t hat when used the shaft is immobilized and motoring is not possible.

I replaced ours at 13 years service (longer than mfg's recommendation). It was still serviceable but would drip at times. I'm happy with t he service and with the associated risk level.

Jim

[contrail]

You CAN replace a Volvo seal in the water, although you will take in some. If one starts leaking, you can also put a hose clamp on the inboard end to buy some time. Don't overtighten. I have had a regular stuffing box, went from that to dripless, which was a big improvement, to Volvo, which was an even bigger one.

[r2sail]

I had a similar problem it was caused by a broken motor mount that allowed the engine to move around enough at hirgher RPMs to allow water to come in, if it doesn’t leak at lower RPMs that might be your problem

[CassidyNZ]

About ten years ago I had occasion to be creative about a leak on the rudder stock. It had a packing similarto the prop shaft gland assemblies but needed replacing as the threads were damaged.

I machined up a bronze cap to fit onto the boat side and a conventional double lipped grease seal on the cap side, obviously with a seal appropriate for the shaft size. The collar was long enough to take two seals, a grease nipple between and waterproof grease. That whole assembly still works perfectly with not a drop of water getting in.

So now I’m thinking of a similar device on the propshaft where the gland assembly is getting pretty ragged. It will be a Teflon tube sized to take 4 double lipped grease seals, the first three facing the sea to keep out the water. The fourth (at the top) faces the other way to contain the grease. A collar between each to maintain correct spacing, grooved to carry grease to the shaft. A grease nipple between each set of seals and waterproof grease injected. The tube will connect to the hull using the same heavy rubber pipe that the gland packing now attaches to so that the seal assembly has some flex capability.

Given that the clearances will be reasonably snug and filled with grease, my thinking is that even if a seal fails, very little water will get in. If no seal fails (which I’m pretty confident of) no water will get in.

My reasoning is that if seals like these can run almost indefinitely on my vehicles, why not on a boat?

Interested to hear thoughts. I have modelled the assembly on Solidworks, will try to post some pictures.

[CassidyNZ]

So, I did a drawing of an earlier version that has just 3 seals but it displays the principle. Also part alignment in the drawing not so accurate - it’s an assembly of Solidworks parts I quickly did in Autodesk Fusion 360. My latest version has 4 seals but I haven’t found the drawings.

I could make that up in a morning and it would not cost fifty bucks

[RUSTYNAIL]

I recently did a complete renewal of my PSS, due to it being 8 years old. I found that the engine mounts had settled a bit, and the shaft wasn't centered in the fiberglass shaft tube. The carbon had worn down at an angle. The cutless bearing was also worn at an angle. I'm glad I redid the whole system. The most fun was removing the shaft coupling. I like a dry bilge, but for going around the world, I think I'd stick with a stuffing box and extra stuffing material.

[Reefmagnet]

Originally Posted by CassidyNZ

So, I did a drawing of an earlier version that has just 3 seals but it displays the principle. Also part alignment in the drawing not so accurate - it’s an assembly of Solidworks parts I quickly did in Autodesk Fusion 360. My latest version has 4 seals but I haven’t found the drawings.

I could make that up in a morning and it would not cost fifty bucks

My initial thoughts are that an unsupported prop shaft shaking around would flog out the conventional seals very quickly in some set ups. A rudder stock generally has more support and larger OD and certainly less lateral movement than a prop shaft. I looks like it would be easy enough you to prototype and test, though so might be worth a shot.

[CassidyNZ]

Originally Posted by Reefmagnet

My initial thoughts are that an unsupported prop shaft shaking around would flog out the conventional seals very quickly in some set ups. A rudder stock generally has more support and larger OD and certainly less lateral movement than a prop shaft. I looks like it would be easy enough you to prototype and test, though so might be worth a shot.

Yes a rudder stock certainly is very stable especially given it never actually completes a full rotation. But I’ve put a dial gauge on my prop shaft and under normal operation there is very little run-out.

I’m not sure I understand the “unsupported” condition - if the engine mounts and the cutlass bearing are in good shape the shaft is actually fully supported. Plus, of course there is the same element of movement provided by the rubber pipe connecting the seal tube to the shaft tube (boat) as a gland assembly so if the shaft does occillate under stress (gear shifts, etc.) the seal assembly can follow.

Yes, it is quite easy to build. Putting it on? Not so easy. The shaft coupling hasn’t been off for at least the 12years I’ve owned it

[malbert73]

Originally Posted by CassidyNZ

Yes a rudder stock certainly is very stable especially given it never actually completes a full rotation. But I’ve put a dial gauge on my prop shaft and under normal operation there is very little run-out.



I’m not sure I understand the “unsupported” condition - if the engine mounts and the cutlass bearing are in good shape the shaft is actually fully supported. Plus, of course there is the same element of movement provided by the rubber pipe connecting the seal tube to the shaft tube (boat) as a gland assembly so if the shaft does occillate under stress (gear shifts, etc.) the seal assembly can follow.



Yes, it is quite easy to build. Putting it on? Not so easy. The shaft coupling hasn’t been off for at least the 12years I’ve owned it

I think any tiny shaft wobble would put a lot of stress on the rubber seals. Tides marine solves that with an integral solid bearing that takes the movement of the shaft. And all it takes is a single lip seal to keep out the seawater. So yours is pretty overengineered from seal perspective- you probably don’t need that many seals though guess it can’t hurt.
So I’d add a solid bearing (maybe delrin/etc ) inside the pipe to take the shaft movement