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Old 30-01-2009, 22:55   #1
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Compression post repair

Compression post repair
Hi everyone I just got back from the boat today and tightened some of my wires on the mast and I noticed a slight depression in the cabin when I went below and checked the compression post base at keel it seems to be contracting downward (wooden square post with hollow center) My guess is I will have to redo foundation of post . My first question has anyone done this any advice would be welcomed. 2ed can I repair the post/foundation without removing the mast, like loosen all turn buckles have secondary temporary adjustable post to lift back to original position then adj. old post and re epoxy foot to new height. If I can do this with the mast still on the cabin then it would save me hulling the boat out of the water demast and pay while I am on the Hard doing the work etc.
42 foot sail boat deck steped mast 50'
This seems like a scary job any advise would be appreciated
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Old 31-01-2009, 02:00   #2
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Is the cabin top deflected or is it at the interior at the joint of the compression post? I am assuming the compression post is a wooden square post with hollow center.so glued up box construction. This post is then stepped on the keel. At the keel to compression post joint there is a base and this is deteriorated allowing the post to compress into the step? There are some large loads going on at these points. I don't think trnasfering these loads and removing them from the compression post is a good idea. By tensioning the stays you can in effect warp the boat and if the rig is not tuned you transfer loads to many different areas. When you tension the stays you are pulling in on the hull sides. Putting pressure against the deck to cabin joint. This can cause the cabin top to bow or flex. You also are driving the mast down against the keel. And then there is fore and aft hogging which was more preveant in older wooden boats but the point is these loads need to be balanced/tuned.
Might be worth getting a proffessional rigger to take a look.
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Old 31-01-2009, 05:44   #3
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Odd that it would be a box construction, but of course when you tighten the rigging the mast is compressed or pushed down and the hull is pulled up at the chain plates making it appear that the deck step is depressed / compressed by the extra mast loading.

The mast loads are transferred through the deck step to the compression post.

What you first have to do is asses the situation. I would slacken the rigging so that there is little tension on the mast (don't go sailing like this!). Do this at anchor or dockside. No take precise measurements around the compression post. If there is a bulkhead near w/ a door, look for the margin around the door when closed. Is the frame square and true? Since the joinery was done with no mast or hull tension you need to first try to get to the no load condtions and take measurements and observations. Use a digital camera to documents as many "areas" as you can.

Now properly tension the rig. There is a tutorial on the Selden site with a link found on this site. You need a two meter or so length of wood or metal and a micrometer and some tape. IIRC each millimeter that you stretch the rigging adds 5% of breaking strength loading. You want to tension the rig to 15% this means that the upper shrouds have"stretched" 3mm. This does not mean that the turnbuckles have been screwed 3mm. The 3mm change is measured using the change in OAL length with refernce to the fixed (taped on 2 meter stick).

Now go below and take your measurements, obsevations and photos and make an assessment of how much distortion has occured.

If the compression post is not strong enough it will begin to crush under the load. This means that the force is effectivelly shortening the OAL length of the compression post and allowing the hull to list at the sides making the mast step seem more and more depressed. If you were to over tighten (if that was possible) the step would get lower and lowe, the chainplates higher and the beam reduced.

You don't want any crushing of the post under the 15% or more sailiing loads. If this is the case then you need to re inforce or replace the post. If you can remove it you'll be faced with getting one in that fits tight and has no gap at the underside of the step or at the keel.

One approach is to you a jack to push the deck up enough to slide the post in. This is not likely with a large cross section post unless there is sufficent clearance to allow you to do this. You may have to build up the area where the post is to seat on the keel. This must be rigid and non compressible. With the new post AND the build up made to the precise OAL height you can slide the new post on place (with the deck jacked up a few MM) and then release the jack and the deck will now return to the design height. You should be able to now re tension to 15% without distortion. Use the right material and cross section for the post compressive strength. Make sure it does not sit in bilge water where rot can occur. Make sure you have perfect mating at the base and top of the post and lateral restraint as well.

If the post abuts a bulkhead secure the post to the bulkhead the entire length which will distribute loading to the huil as well as the keel and there will be less distortion at tensioning and sailing loads.

This is best done in the water as the hull shape changes from when on the hard.

On the hard the keel pushes UP distorting the hull making the compression post distance smaller. In the water it pulls down and this may lengthen the post distance!

Good luck.
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Old 31-01-2009, 07:56   #4
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Compression post

Well Gentlemen thank you for your welcome advice it is nice to see I am not a pioneer in this adventure
(Makes sailing easy compared to all the other factors) I never considered the loads on the chain plate and hull. All this seems scary and the normal thing would be to through my hands up and give the job to experts unfortunately I am not a millionaire and will have to work my way through this myself, with all the advice I can scrounge.
One thing that has become clear is the amount of force I am placing on the mast downward are mighty high. Last summer I replaced my front forestay and back stay I also adjusted all the rigging without using the 15% rule, it is possible I have been applying too much pressure? I will revisit and tune the tension first, before I go much further and then take measurement and pictures. Thanks again for the preview of what I might be up against. If I have to build up under post to Keel is Epoxy the right material?
Thanks again for your help
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Old 31-01-2009, 09:28   #5
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You may very well have over tensioned your rig. So before doing anything you need to establish the relaxed hull shape which includes the height for the compression post.

I recommend that if you really need to do this with the boat floating on her bottom, not on the hard as noted in my previous comments, the hull can and often does change shape when resting ON the keel as opposed to floating and supported by the hull.

If you are on the hard your post will be under additional compression so you really want to do this assessment and repair with the hull in its design profile. If you must do it on the hard better to take the mast down which will remove compression from the mast and rigging.

You want to be sure that your post has a cross sectional area and sufficient compressive strength to support the loads of the rig under sail and the rig tensioned on the hard. The fact is that GRP hull are not as stiff and rigid as we want to think they are - they do change form / shape under loading and the compression post is where the rig loads are concentrated so this area is subject to a fair amount of distortion.

You want the post step to also be of strong non compressive material and large enough to distribute the loads. You can use a steel or bronze plate glassed into the hull with vertical flanges to prevent lateral movement. The mating surface of the post and the step need to provide solid contact.

This is something you can do yourself with proper planning and analysis/surveys. But note that if you have a hull which easily distorts because it is too thin or missing bulkheads or proper internal stiffeners you will have a hard time to properly tension the rig without introducing distortion and damage at the chain plate areas of the hull.

You might want to speak with a naval architect familiar with the construction of the boat.
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Old 31-01-2009, 12:05   #6
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compression post


DEFJEF thanks again for sharing your valuable insight. The boat is in the water.
The boat is 28 years young and has enjoyed a good reputation here in western Canada unfortunately the architect has passed away and the boat has been out of production for quite some time. My first step will be to relive the rig I figured I should be able to mark all wires at the 2 Meter from compression fitting and when I slacken the tension the wire should contract so I will also know if I was over the 15% mark
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Old 31-01-2009, 13:15   #7
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Go online and down load the Selden Rig tuning manual. it's simple and anyone can do it right without guessing.

The age of the boat is not an issue and any knowledgeable architect or marine surveyor should be able to give you a some advice.

Plan plan plan and document everything with photos as you work.
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Old 31-01-2009, 13:50   #8
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Rot and failure of the base for the compression post is a very common problem in many older boats. Quite easy to fix, just remove all rotted wood and replace, using epoxy to protect the base. Then replace the compression post. Try to make the area self draining so that when water gets into the area it can drain out.

IMO this is not a job to do with the mast stepped.

There are many references to this problem on the web. I remember one excellent article with great pictures of the repair procedure, but could not find it today. I believe it was a repair on a Tartan 34, but may be mistaken on that.
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Old 31-01-2009, 15:06   #9
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Here is a link to a deck mast step repair. You should probably check the area around your mast step very carefully while you have the mast out. If the deck area has been depressed for a while it is likely that the core material has gotten wet. Maybe not, but worthwhile checking.

http://home.comcast.net/~independence31/core/looksbad.html
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Old 31-01-2009, 19:18   #10
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Great sight DeepFrz
If you ever find the link to repair on a Tartan 34 please post. The link to the deck repair was great.
Thanks again. For the positive words as it looks like there is hope after all. One statement that seemed strange is the author made a thick layer of epoxy then he added 4 layers of fibreglass?? I thought fibreglass does not stick to epoxy so I will assume he meant 4 layers of fibreglass cloth saturated in epoxy.
DEFJEF I have purchased a book with step by step tensioning of wire similar to the “Selden Rig tuning manual”
In French we have a saying (I will try to translate ) goes something like this “a man who is warned with a preview is worth 2 men” With the previews I have received today I feel more prepared to face the enemy Thanks
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Old 31-01-2009, 20:08   #11
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West System has some great info on using epoxy.

WEST SYSTEM | Use Guides

I'll keep looking for that site with the compression post repair.
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Old 31-01-2009, 22:03   #12
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You can do the repair with the mast still up (but slacked) using a lolly column or other jack and post. (Lolly column being inexpensive the big steel screw jack used in basements to support floor beams above.) The trick is, to make Real Damn Sure it is set securely and adequately braced to take the load from the compression post without slipping out of position.

I wouldn't be surprised by a hollow column, after all masts and spars have been made from planking in that same manner for hundreds of years. If the diameter is sufficient and the glue/joinery is well done, they work. When metal pipe is used--it usually is pipe, not a post, for the same reason of saving cost and weight while providing adequate support.

Folks have have custom alumnium shoes, impregnated hardwoord, UHDPE or Delrin plates under the old post, all after trimming off the rotted end, if the post has rotted. They all work, if the job is done carefully. Of course if the deck itself has compressed--you'll need to pull the mast and repair the deck.
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Old 01-02-2009, 18:29   #13
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Tension on 4stay

Hi One more question ok more than one
I downloaded “Selden Rig tuning manual” one of the best I have seen one issue puzzles me when tensioning wire using the 2 meter approach seems good but how do you do this to the 4stay when it is surrounded by roller furling, you can reach the turnbuckle but not 2 meters of wire??? Also I have 2 5/16 backstays do I tension each 1/2 the 15% as there is only one 3/8" forstay?
Also a separate question when I raise the mainsail I usually run out of rope halyard and begin 2 wraps of wire on winch which I suspect is not good should I shorten wire and by how much( when I say wire my rope halyard is spliced into wire for the last approx. 50 feet)?
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