What is the importance of a compression sleeve on spreader bolts?

Discussion in 'Boat Design' started by bntii, Aug 12, 2008.

  1. bntii
    Joined: Jun 2006
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    bntii Senior Member

    I was speaking with a rigger who cautioned that I should be aware of deterioration of the thin walled compression sleeves on the bolts attaching the spreader bases to my mast.
    I sighed the deep sigh of the boat owner who has just been warned to pay special attention to a absolutely essential component of his rig when in fact my rig does not have these little widgets....
    The aluminum spar on my 40 foot sailboat is fitted with 'C' bracket type spreader bases which allow the spreaders to fold up along the mast when the spar is off the boat. The brackets are held on by two bolts through the spar which also hold the normal plates for the lowers. As the spar in joined at the spreaders, there is double wall thickness in this area.
    I have recently had the spreader bases off the spar so I know that these sleeves are not fitted to the bolts.
    My question is if these sleeves are in fact fitted in all cases and further how is the load to the mast wall in this area addressed. I don't fully understand the role of the lowers in offsetting the compression of the spreaders, and how the leeward spreader acts against this load as well.
    Thanks all
  2. diwebb
    Joined: Jun 2008
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    diwebb Senior Member

    The compression sleeves are inside the mast and stop the bolts from crushing the mast sides together when tightened. The only way to see them is to remove the bolts and then poke a thin rod in to the hole. If there is no sleeve then the rod will slop around to an angle of 45 degrees or more. If sleeves are fitted then it will retain the rod in the line of the bolt as if it was a hole through solid material. A second option is to use threaded rod with nuts and washers inside the mast against the inner wall each side. this is really difficult to install but serves the same purpose.
  3. bntii
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    bntii Senior Member

    Hello David

    Thanks- Am I to understand that some sort of compression strut is installed in all masts in this area as a means to counter the crushing force of the spreaders? Or is this solely a requirement in those installations which have the spreader bases attached via bolts through the spar? I have seen many base types such as those welded on and the cast types which have no bolts.
    I am just wondering about how essential this component is in the spar and the exact nature of the force it is designed to counter.
    To provide some bald speculation on my installation it seems that the sleeves are required so as to be able to tighten the brackets sufficiently so that the do not drift about. If this is the sole reason then I can see that the a strut/sleeve across the spar is not required in those cases where the spreader base is welded or otherwise mechanically fastened to the spar wall in a tight fashion. Again- bald speculation. Knowing that I don't have these sleeves in place just leaves me somewhat uneasy about loading the spar if the intent is to counter rig forces...
    Thanks again
  4. kenJ
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    kenJ Senior Member

    The compression sleeve is primarily designed to offset the crushing force of the bolts that hold the spreader brackets in place. You said your mast is "joined" at the spreaders. By that I think you mean two sections of mast are spliced together. With the added thickness of the splice, the compression sleeves may not be required. Asking the maker of your mast/boat may be the simplest way to put your mind at rest.
  5. bntii
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    bntii Senior Member

    Thanks Ken-

    MY mind at rest?? :)

    This gal is so old that the whole gang involved in its creation have passed from this world.

    The spar is joined at just above the spreaders. As far as I can tell there is a full section of spar used as a joiner which is long enough so as to double the wall thickness in the area of the spreader bases. The spar is quite thick but in assembling the bases to the spar I had to make a judgment as to how tight I was going with the bolts as I could sense the potential to crush the spar somewhat with the large bolts used. This is just one of those cases where I should have taken pause and asked one of the riggers about this. I am now left with perhaps pulling the rig to sort this out if I deem it necessary.
  6. rwatson
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    rwatson Senior Member

    bntii - are you able to post a picture of the spreader arrangement. The idea of foldable spreaders is new to me, and I think it would be handy on my current project.

    Hope its not too tricky for you, but it would be appreciated

    good luck.

  7. Gilbert
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    Gilbert Senior Member

    bntii- To find out if tubes are in fact fitted using the test suggested by diwebb you would not need to dismount the mast- just slack the rigging a bit and hoist yourself (or someone else) up to the spreaders and remove one of the bolts to perform the test. If there are sleeves or tubes and they are failing or disintegrating you will no doubt need to dismount the mast to replace them. If there is a sleeved section or doubler inside the mast as described then I believe the tubes should be and perhaps were installed in the sleeve, not through both the mast extrusion and the sleeve, as otherwise you likely would have noticed them. I have always heard these called tube bolts and their function is as kenj described and has little if anything to do with transmitting rigging loads from one spreader to the other. It also serves as a sort of bushing for the bolt.
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