Jami Jokinen wrote:
Holy cow! The inner diameter is 131mm, and the lower part of my mast will be 130mm. I suppose this is unusable, because:
Back on topic, I was thinking about the collar suggestion, and refer back to the compression ring idea ... and how they could be used as deck collars as well.
Two face to face sandwiching the fibreglass as a collar, then one as a step.
These are actually slip fit "Welding Necks" used in gas, oil, industrial fluid industries.
(Interestingly, most come with finished surfaces and face-to-face gaskets to size, as they were designed to bolt large pipes together. Pipe sizes that pretty well match round mast sizes).
You say there is only 1mm clearance? I'm actually thinking that is a good thing, about what you would want (if it is 1mm total, it is .5mm on each side). What I dont know is how thick the protrusion is but it looks plenty thick enough to machine.
So - all approximates for the sake of discussion - work with the idea of machining a broad ringed but shallow band on the inside of it, say, 3mm deep (as in 6mm total) and 26mm high, it could be possible to make a 3.5mm thick shim, a bushing actually, out of a strip of metal by wrapping it around your mast until it follows the curve. You want to leave a compression gap between the ends.
Actual dimension are open to discussion, say 25mm high.
The compression ring/bushing will fit that into the ringed groove.
The sides of the protrusion look thick enough to drill and tap a useful thread into them at an equal distances, if not welded some stainless steel nuts on around the side, and install grub screws into them. I'd go for a short and fat as you can fit.
Now, install the bushing into the groove, drop the mast into it, tighten the bushing up as a compression ring equally all round, using some Loctite on the screws and, lastly, once it is all set, run a ring of silicon gunk around the gap at the top. You may find it seals perfectly as it is.
With a close tolerance above, you should get near perfect alignment on the keel at the step.
You could even use two such necks as a deck collar, one above (pointing up) and one below the cabin roof (pointing down) clamped together and sealed, using two such bushings/shims systems, say, 4" apart.
The advantage of that is you have twice the amount of surface area to spread the forces across, and they would align up perfectly, so you end up with the same effect as having a single longer pipe with a collar welded around it.
This is similar the set up they use on motorcycle front forks. At the end of the stanchions, inside the fork legs, there are bushings/shims managing huge and persistent forces (imagine 1,000lbs of motorcycle and riders, travelling at 150mph, dealing with 1,000s of bumps a minute). They last for 10,000s of miles of such punishment.
Now go back to the old fashioned way of banging in chocks of wood. How does it compare? How much actual contact area does it have to spread its load across? The only advantage wood might have is absorbing is shocks, the disadvantage being it deforms when it does - fixed by being tapped in further - (before eventually falls apart and need replaced).
To do the wood chocks really nicely, I'd say you'd probably need someone with a big wood lathe - so there's not that much difference in manufacture and this way you might end up without having to have a big gaiter on it.
Possible problem ... fitting any curvation on the cabin top. Would work best where the front hatch has to be used/replaced with a perfectly flat surface. Someone else would have to comment on the fibreglass's strength. You'd probably want a broad layer of ply to spread any loads.
Absolute overkill from an engineering point of view, but just offered as brainstorming though ... if they are cheap, they may be worth playing with.
Your mast would fit absolutely straight, top and bottom, so leave the final bolt and glue up until last. But the flanges are actually design to feed a pipe (mast) into them.
Is that clear? Critique away at will ...