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Old 06-03-2017, 09:38   #1
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Compression post alignment

Hi

Looking for some input on a problem i am having with our compression post.

I took the post out and did some deck work on the mast foot area, replacing rotten wood etc, and glassed in the mast foot in the same place.

The foot has bolts that go through the deck to a solid plate the sits snug to the saloon roof underneath, again this plate is in the same place as before.

The above plate has a thread into which the post screws up into.

So far so good, except it appears that the internal roof plate has developed a kick upwards at the forward end ( only maybe 5mm), so that when the post is screwed home it misses its connecting plate in the bilge 2m below, by around 35mm ( missing forwards as the post is slanting from aft to forwards).

Now, i can wedge the post back into correct position at the bottom with a fair bit of effort , which also makes it plumb. But does put a fair bit of strain on the post i think.

What do you think? Is this a problem? The post will be plumb but will have been pulled back at the bottom to get it there. With a potential bend induced.

C.
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Old 08-03-2017, 05:58   #2
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Re: Compression post alignment

Are you talking about a compression post under a deck stepped mast that is metal(?)/or a metal flange and tapped on both ends(?) or a SS tie rod assembly that prevents the upper deck from distorting when the rig is tightened? If it is the former, any excessive force needed to reseat the post is evident of a problem that must be corrected before reassembly since the threaded bolts serve only to steady the installation not provide structural strength. The same would be true of a tie rod. Therefore, you must discover the real issue and correct it before re-installing the post. Hope this helps. Good luck and safe sailing. P.S. some pix are always helpful
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Old 08-03-2017, 07:19   #3
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Re: Compression post alignment

Ideally, the post works in vertical and we both know it.

Have you measured the induced pre-bend once you force the post into its working position?

If the post is very stiff and there is no noticeable pre-bend induced in the operation, you will be fine. Alternatively, if the post is a fine wall tube, you cannot induce to much side force on it without risking it buckle at a point. Etc. You know already all this too.

If you want to avoid a major mod, possibly a wedge of hard material could work as an in between padding, at the top, to remove any strain. However, this will only work if the holes at the top of the post plate allow for the studs of the bolts to be slightly mis-aligned.

I have only seen what I believe is what you may have on a Union. PLS send some pictures to better visualise the thing.

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Old 08-03-2017, 09:08   #4
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Re: Compression post alignment

Thanks for the input,

Boat is a hans christian 41 and built like a tank!

The compression post supports a deck step mast, the post is 5" diameter thick walled solid brass! ( maybe 1/4 " walls !).Like everthing on a hc the whole set up is over engineered which is all good as far as i am concerned.

I realise the set up i described is not perfect, but the post is vertical in its final position, the mast step is through bolted into a massive solid brass top piece that the post screws into on the saloon roof.

No noticeable bend in the post in its final position, its just the effort required to pull its bottom into position that concerned me.

C
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Old 08-03-2017, 09:50   #5
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Re: Compression post alignment

You can easily measure any induced bend by taping a thread at the top and then at the bottom of the post. Make the thread tight and watch it half way down. There would be zero gap on each of the four sides.

If the post is not exactly in line with the mast but is not pre bend (=it is 100% straight), there should be little if any risk involved. Most compression posts are always at some angle with the mast as most masts are raked anyway.

If the post is as solid as you say, you are unlikely to induce considerable bend by hand. It would take a tackle or a jack to push it out of line.

I am impressed the wall is 1/4 inch thick. Not easy to buy such material today. The overall diameter of the tube is not excessive. Never heard of any compression post issues on any Christian boat hence they did this part very well.

Cheers,
b.
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Old 08-03-2017, 10:09   #6
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Re: Compression post alignment

So if I understand correctly, once the mast is back on and rig is tuned, the deck will be flexed back to the right angle and there should be no more force trying to bend the post right? It would be nice to see a photo but it sounds like you won't be stressing the post too much to push it the 35mm before the weight is put back on.
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Old 08-03-2017, 10:45   #7
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Re: Compression post alignment

have you thought about pulling the plate out and having it straightened? You might be able to do it yourself with a table vice, and monkey wrenches (using plywood and/or rags to protect the finish)
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Old 08-03-2017, 13:29   #8
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Re: Compression post alignment

Click image for larger version

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ID:	142699
In the picture the post is wrapped in black shrink wrap to protect the mirror finish.

In this position there is no strain on the post, it is just screwed into top plate in the roof and hanging down, all be it at an angle from aft to front,. Actual distance is 25mm not 35mm required to pull it back as we moved the metal plate a little to the right after this shot. When it is pulled the 25mm it is vertical.

The wooden beam is massive and goes all the way down to the keel.

I am thinking the post has never been in totally correct position, as we have put everything back as it was in the roof area, i am a bit happier with it now having talked it through, would be great if it just landed plumb, but as i am learning during this refit, the hans christians are anything but regular!
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Old 08-03-2017, 15:00   #9
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Re: Compression post alignment

25 mm over say 200 mm of the pipe this misalignment is less than 1 degree (I think 0,74 of a degree, tan = 0.13).

I would be tempted to fit and forget. But DO cross-check against a more experienced source. I use boats a lot and fix them at times but never built one.

Cheers,
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Old 08-03-2017, 15:09   #10
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Re: Compression post alignment

Sounds like you needed to leave the post in place while you worked on the rotted deck/cabin top section. I would be tempted to undo the post from the top and attach the post at the bottom, then perhaps put a metal shim in place between the top plate and the underside of the deck/cabin top to make the pieces mate up properly. You don't want to go around bending stuff.
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Old 08-03-2017, 15:12   #11
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Re: Compression post alignment

Quote:
Originally Posted by Sulaire View Post
Attachment 142699
In the picture the post is wrapped in black shrink wrap to protect the mirror finish.

In this position there is no strain on the post, it is just screwed into top plate in the roof and hanging down, all be it at an angle from aft to front,. Actual distance is 25mm not 35mm required to pull it back as we moved the metal plate a little to the right after this shot. When it is pulled the 25mm it is vertical.

The wooden beam is massive and goes all the way down to the keel.

I am thinking the post has never been in totally correct position, as we have put everything back as it was in the roof area, i am a bit happier with it now having talked it through, would be great if it just landed plumb, but as i am learning during this refit, the hans christians are anything but regular!
I am actually more interested in why the post is above the base as much as it is. Does the deck have that much downward deflection once the mast is on? If I am seeing things right I'd be thinking I need to add a shim there perhaps. There can be a force approaching the displacement of the boat down on that, so it may have compressed the beam below over the years. Can you see how the beam is underneath? Also, how much room is there on the turnbuckles?
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Old 08-03-2017, 21:30   #12
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Re: Compression post alignment

There are 2 metal wedges measuring 80mm height when put together that make up the difference you see between the post and bottom plate.
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Old 08-03-2017, 21:41   #13
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Re: Compression post alignment

Quote:
Originally Posted by Sulaire View Post
There are 2 metal wedges measuring 80mm height when put together that make up the difference you see between the post and bottom plate.
Ah, ok. I am guessing that the two wedges are slid against each other to adjust to make the exact fit in thickness between the plate and post? What holds them together or do they fit between the bolts? There's 80mm of difference there? Didn't look like that much. In any case if the beam is not compressed, rotten or damaged, that is great!
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Old 08-03-2017, 21:52   #14
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Re: Compression post alignment

Can you add a shim on the roof side to deflect the post straight to the bottom plate?
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Old 08-03-2017, 22:10   #15
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Re: Compression post alignment

Have you re stepped the mast yet?
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