Are "compression tubes" really necessary for mast throughbolts?

So I've taken my deck-stepped mast down for the first time in ten years to repaint it, and to address some flexing in the deck underneath the mast step.

When I last painted the mast in 1995, I installed aluminum tubes around each of the through-bolts that pins the two sets of spreaders to the mast. The thinking was that these would prevent excessive compression from the nut from collapsing the mast section. These tubes are not particularly thick - I think I got them from a machine shop.

Having removed these through-bolts now after 19 years -- the mast did not fall down over that time, though it was taken down twice for the winter -- my impression is that the "compression tubes" were not necessary nor would they have been effective had they been called upon to do anything.
* They're corroded to the stainless bolts, preventing any inspection of the (anaerobic) environment inside.
* They're thin, certainly providing no significant strength to the fastener or providing real resistance to overtightening.
* They're not perfectly sized to the width of the mast - they're a few 64ths shorter, though perhaps that was my error.
* Finally, with nearly 20 years of internet content having been built up since 1995, there seems to be very spotty mention of these things being used with Ericsons or other boats. I did find some mention of people using Garolite tubes and some Rube Goldberg methods of getting them up the mast, but generally compression tubes seem like the exception rather than the rule.

So, do I need to replace them when I replace the through-bolts? The through-bolts, by the way, are as pristine as the day they were installed, save for the compression sleeves corroded around them!

No IF

The whole purpose is to prevent someone, when rigging the mast, from overtightening the bolts that go through the mast, and compressing the mast, which significantly which will significantly weaken it.

You could have bolts that only allowed the nuts to be threaded on the correct amount. This is done with a bolt that ins't quite threaded enough and a die.

You can use any suitable material, g-10 for instance to make them out of if you want compression tubes.

You could tighten them only the right amount and not use anything. Then again someone might come after you and tighten them enough to distort the mast wall.....

Guy
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I had the same problem on the mast of my E25. I ordered aluminum tubing from Onlinemetals.com, cut it to the precise length, and then installed two new stainless bolts (with Tef-Gel) for the upper and lower shrouds. I was able to make the lock nuts nice and snug and saw no flexing on the sides of the mast.

Roscoe

Thank you for the insights. Is a tight fit around the through bolt a necessity? Thinking I can find a g10 tube from McMaster to fit around the thickest bolt, and use the same size around the smaller bolts.

Still seems like jamming a tube up the mast that touches both sides of the inside of the mast will be impossible, whereas jamming a tube up the mast that is slightly narrower than the inside of the mast will not be useful in reducing mast compression.

tenders said:

Thank you for the insights. Is a tight fit around the through bolt a necessity? Thinking I can find a g10 tube from McMaster to fit around the thickest bolt, and use the same size around the smaller bolts.

Still seems like jamming a tube up the mast that touches both sides of the inside of the mast will be impossible, whereas jamming a tube up the mast that is slightly narrower than the inside of the mast will not be useful in reducing mast compression.

Click to expand...

I've seen one installed: many years ago on a mast for a previous boat of mine. As best as I can recall, what the guy did was to run a small diameter line through one of the holes in the outside of the mast, fished it up and out one end of the mast (I think the top), threaded it through the compression tube, and then fished the other end of the line out the hole on the opposite side of the mast. Then, by tightening the line by pulling on both ends, he was able to situate the compression tube in line with the holes and, with a little fiddling, install the bolt.

Exactly how tight it was inside the mast I cannot recall at this point. But I remember it to be a reasonably close fit. I also remember that he managed to do the whole thing quite quickly.

Good luck!

--Alan

Very clever! I can imagine fishing the line out of the mast with a boathook duct-taped to a 1x2, cutting the line, slipping on the tube, retying the line, and yanking the tube up into the hole.

Hmmmm

There are some ways of doing this right and a lot of ways of doing it wrong.

The idea of compression tubes is that they prevent the mast from being compressed at all. Not even a little bit. Each compression tube is created exactly for the place on that exact mast that it goes. Having them only be accessible internally is not going to work to do this as you cannot cut them with the precision that you need to make them worth anything.

The idea of having them be some thin wall aluminum tube is really not going to work well either.

Generally the hole drilled in the mast is of a diameter that will allow a thick walled Aluminum, or some spar manufactures made them of SS, tube all the way through the mast. The inside of the tube fits the bolt that goes through the mast, and the outside diameter is the diameter of the hole in the mast. The tube is cut when the mast is first rigged such that it is exactly flush with or possibly 1/1000 or so proud of the surface of the mast. So that when the fittings are tightened for the tangs any tension loads generated by tightening the tang bolts are transferred into compression of the tubes, and not compression of the mast walls.

The reason that they have to be made individually is that the mast is not a uniform thickness. Aluminum masts are extruded aluminum and will vary significantly in size over their length (Really, take two squares one on each side of your mast, and measure it over a 6 foot length.. .Most will be off by at least an 1/8th of an inch. Don't panic this is normal!) Carbon fiber masts are laid up and have even more variance.

If you are cutting a tube and inserting it in the mast with string, such that the tube is completely internal in the mast and not accessible from the outside of the mast, this is a waste of time. Getting the tube exactly into place to prevent compression of the mast section this way is unlikely to work well at all.

If you don't want to do all of this compression tube stuff: You could also make the threads on the bolts ONLY allow you to get the tangs within a couple of 1/1000's or so to the mast, which would also solve the problem.

* An additional benefit if the compression tubes are done correctly is that the tubes are the part of the tang system that wears. Only the tubes are exposed to dissimilar metal corrosion, and they can be easily replaced when the mast is down. If created this way they also prevent the mast holes for the tang mounts from elongating due to wear, and allow a greater surface area on the mast wall to take the load of the tangs without having a needlessly large and heavy bolt. Also the compression tubes can be made of G-10 tubing if the tubing is created with the outside measurement being the stable measurement. (G-10 tube is produced with either an ID or OD constraint). In addition to being compression tubes, and tang bolt bushings g-10 compression tubes will also act as an isolation component between the mast and the SS bolt holding the tangs on.


This would be easier with some photos, but I don't have any of this system... I will have to remember to take some next time I have one of the well done masts apart.
Guy

I am offended that you did not care for my half-assed, redneck, git-er-done solution.

But I am concluding that there is no way 99% of the boats in the world are doing anything close to The Right Thing regarding these fittings...and that a torque wrench, used judiciously, and Lock-Tite on those nuts is probably going to be as effective as what has been happening on my mast since the boat was built.

Here's how I did it. After cutting the heavy gauge aluminum tube to approximately the correct length I did several dry-fittings. I used the bench grinder to get the length dialed in precisely. As you can see, if the hole is the same diameter as the tube. When the tube is exactly the right size, it will stay just where it's supposed to stay. The walls of the mast keep it from falling over inside of the mast.

Roscoe

Nice, Roscoe, thank you.

As long as we're looking at McMaster-Carr's rich assortment of tubing, is stainless better for this application? Or perhaps it's best if the corrosion occurs between the fastener and the tube rather than between the tube and the mast.

I'm noticing a bit of play between the bolt and the tube. That doesn't cause the tangs to grind against the mast, or the head of the bolt to fatigue as the bolt is twanged up and down from tack to tack?

How come Catalina and old Pearson owners are the only people to write about this stuff online? (besides Guy of course, and rare mentions by Brion Toss)