View Full Version : grey Seal's stern tube
Wood shaft log hole sander & epoxy painter; made for use with an electric drill:
After the first coat of epoxy the PVC stern tube would no longer slide thru, so I had to sand the inside a bit. The hole was given a second coat of epoxy and the tube was painted with same as it was slid & rotated into place.
Stern tube in:
That's a pc of heavy walled steel tube inside the PVC it's only there to make sure the PVC stays 100% streight, and will be removed tomorrow. ID of the PVC is 1", OD, 1 3/8"; I am planning on a 3/4" shaft. I have a custom made bearing housing, that I may install tomorrow. This stern tube is "pretty permanent" I guess. :rolleyes:
Cutless housing installed:
Both acorn nut will eventualy be bronze, but these were all I had.
They sit on 3/8X5 bronze hangerbolts; do these need to be in bedding compound?
Final alignment/shimming hasn't been done yet; I'll do that when I have a pc of 5' X 3/4" solid stock,
to mimic the prop shaft.
03-14-2009, 06:02 PM
You do amazing work, Gert. Thanks for all your photos and text throughout this wonderful project. I have always admired Grey Seal.
I shall file that idea away for future reference Gert. Looks good.
03-16-2009, 10:54 AM
Good stuff . I'm watching with interest too .
I've wondered about using plastic instead of bronze for a shaft tube, and will be very interested to Know how it works out. I've always done tubes by threading the outside of the tube, and boring and tapping the back of the stern bearing to thread onto the tube, then using a "lobsterboat" style stuffing box, the type mounted on a short piece of rubber hose that slides over the inboard end of the bronze tube, and is hose clamped to it.
I wonder how you're attaching the PVC tube to the stern bearing, and what kind of stuffing box you'll use. I've always avoided stern tubes that stick out a long way from the shaft log, or stern post, because they seem vulnerable to damage. Also, if the engine is going to be on flex mounts, the closer the stuffing box is to the stern bearing the less it will move around. I always worried about the engine dancing around on its mounts and wiggling the tube enough to fatigue it. Which was the reason for the hose-mount stuffing box.
For what it's worth, I've often heard that boats change their shape so much between sitting on land and floating that there's not much point in being overly particular about shaft alignment until the boat is launched.
It sticks out this much because the inboard end isn't finished yet. Think of it as a "liner"; the hole is 40" long. I am considering a flanged bronze shaft log for the inboard end because the rigidity of this thing also started to concern me after the install. I have 20 20 hind-sight :).
The problem I've seen with fixed stuffing boxes occurs when the engine is on soft mounts. If the stern bearing is a cutless bearing, it's has some ability to move. Unless the shaft is really short, 1/2" of motion at the engine isn't enough to change the shaft angle at the stern bearing enough to cause unusual wear. But if you rigidly fix the midpoint of the shaft, you can have trouble. Say the stuffing box is mounted at the middle of the shaft, and the engine moves 1/2" on its mounts. That means that the shaft has to move the same amount at the prop end. If it can't, it will either exert a strong load on the stuffing box, or the shaft will have to flex.
That's why I like the flex-mounted stuffing box.
I started thinking about this years ago, when replacing hard mounted Atomic four engines with one and two-cylinder diesels on very soft mounts. The most common stern gear back then was a hard mounted stuffing box inside, and either a stuffing box for a stern bearing, or a cutless bearing. The trouble would crop up as either broken or loose mounting bolts on the stuffing box. The problem disappeared when a soft-mounted stuffing box was installed.
But there might easily be other explanations that make at least as much sense. After all, I grew up on a cattle ranch.
For give my ignorance here; but is there supposed to be a cutlass bearing at BOTH ends of the 40" long stern tube? I had assumed NOT. From the cutlass bearing to the shaft coupling at the motor will be about 55"
By soft mount stuffing box do you meen the dripless (PSS) ones with the "bellows"?
03-18-2009, 08:09 PM
What is the designed duty cycle of this setup?
Maybe you have it in a sailboat where the intent is to run it for 10 mins untill you get the sail up?
If so, it will most likely be ok.
Or are you intending to run this with substantial power for hours at a time?
03-18-2009, 10:00 PM
Had you considered using G-10 pipe for this application?
It's available up to about 40" lengths.
The most common arrangement is a cutless bearing in a bronze casting, just like you have it, as an outside stern bearing.
On older boats, it isn't unusual for them to have used a heavy-dury stuffing box on the outside as a stern bearing, to locate the shaft and take the sideways thrust of the propeller, and then a second stuffing box on the inside of the shaft log, stern tube, whatever you want to call it, that can be adjusted to keep the water out.
Many boats that had a shaft log like yours appears to in the picture didn't have any liner (or shaft tube) inside the wooden shaft log, relying on the watertightness of the wooden log. If the log checks, you've got a leak...
Often the stern tube would be made out of a piece of lead pipe (well coated with paste white lead) that was small enough to fit easily into the bored hole. Then a well-greased tapered hardwood piece would be driven through the lead pipe, stretching it to fit tight against the wood. This works because lead is very malleable, and because lead pipe in those sizes used to be common in household drain plumbing. I'm not advocating this as a method, just describing it as a historical curiousity.
I think I remember that the rubber cutless bearing was developed by B. F. Goodrich rubber around WWII. Before that, stuffing boxes and other methods were used for stern bearings. Cutless bearings were quickly widely adopted because they work so well.
A conventional stuffing box has a bore, that has to fit the shaft with slight clearance. At the end of the bore is the recess that the rounds of flax packing are pushed into, and the packing nut that tightens against the flax and makes squashes it against the shaft. Both parts are critical. The bore looks like a loose plain bearing (brass bushing), and that's how it's used when a stuffing box is used as a stern bearing. But its main purpose is to hold the shaft steady so that the packing can make a seal.
The most common type of stuffing box is a bronze casting with a mounting flange that can bolt to the inboard end of the shaft log. There are two situations where this doesn't work very well:
1) When the boat is so flexible that it twists enough to "un-align" the shaft. I worked as sternman on a wooden 34' lobsterboat years ago, which had a cutless stern bearing and flange mounted inside stuffing box, Detroit 4-53 engine on solid mounts. In the fall, when taking up traps for the winter, we'd load her quite heavily back aft. Loaded like that, running in a quartering sea, you could hear her shaft go "wuhwuhwuh" at the end of each roll, as the hull twisted and the shaft came out of alignment, then go silent as she rolled through vertical, and then go "wuhwuhwuh" at the other end of the roll.
2) When the engine is mounted on very soft flexible mounts, as many small diesels are. In this case, it isn't the hull that twists-it's the engine that moves.
The common way to address these problems is with a "self-aligned" stuffing box, as pictured at:
The part on top is a welded metal shaft log that can be bolted to the inside of hull planking for twin-engine installations. The rubber hose fits over the spigot on the end of the shaft log, and the stuffing box slides into the other end of the rubber hose. It has a bore to align the shaft to the packing, and a clamping nut to squish the flax.
With this design, the shaft can move sideways, either because the engine is jumping around on its flex mounts, or because the hull twists, and the rubber hose allows the stuffing box to move with the shaft.
There are two common ways to mount a self-aligning stuffing box to the shaft log:
a) You can but a cast brass flange that bolts to the inside of the shaft log, just like your stern bearing bolts to the outside. It has a spigot that the rubber hose pushes over, and is clamped down to.
b) The liner or stern tube is the correct size for the rubber hose to slide over, and sticks out of the shaft log by a couple of inches, giving a place to use hose clamps to tighten the hose to the stern tube. I've only seen that done with bronze shaft tubes. I'm willing to believe that plastic is strong enough, but it's a very critical part of the boat. If the stuffing box fails, you have a big leak until you jam a rag or something in around the shaft, and drive it into the shaft log. Then you can't use the engine.
You can get dripless neoprene lip seal type "Las-drop" stuffing boxes in this style, as seen here:
I personally don't like this type of seal for wooden boats. They seem like an expensive solution for a non-existent problem. But they seem to sell well, so maybe I'm wrong. If I had a boat that had a perfectly dry bilge, I'd put a little pail under the stuffing box, and dump it over the side every so often.
There's a limit to this, because at some point the shaft will move so far that it will bear against the inside of the shaft log/stern tube.
03-29-2009, 06:08 PM
hi gert-you're way ahead of me -i'm still setting up frames-your boat looks great-i expect to see a launch within a year-good luck
03-30-2009, 01:54 AM
Hey Gert. I haven't been around much lately but it's great to see your progress! She's looking very nice, indeed. Keep posting those photos!
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