Removeable mast compression columns ?

Discussion in 'Sailboats' started by seasquirt, Jun 26, 2022.

  1. seasquirt
    Joined: Dec 2015
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    seasquirt Senior Member

    Is this the answer for trailer sailers and slightly bigger boats with internal restrictions caused by these structurally needed, but inconvenient items ? I haven't seen much mention of the idea anywhere else.

    How could it be a bad idea if all the force is vertical / linear load bearing, and virtually no other forces applied, since the deck / cabin roof, mast step, frames, and scantlings would take all other mast base loads, and transfer most driving force through the roof and topsides to frames and hull.

    My idea: in the cabin, under the mast step, well attached to the roof panel, is a metal truncated cone receptacle to locate the top of the removeable column. The top of the column has a metal or plastic wear pad which locates centrally easily in the cone. A bit lower on the column is a threaded adjuster (bottle jack), for height adjustment. Down the sides of the column are all your badges, stickers, and small awards. At the bottom of the column is another metal or plastic wear pad, with a ball detent in the bottom centre, or a barrel bolt type central locking lifting pin, which drops into the metal bottom locator plate centrally. A pair of flanges could be used at the bottom to help align the column centrally, and drop 2 lock pins in at the edge, or wrap with velcro or electrical tape, as backup centralisation and security.
    As long as the cabin roof doesn't flex and hammer, or lift the cone off the post top, (unlikely while sailing, possible if turtling), the setup should function as standard. Or why not ?
    Not useful for keel stepped yachts, unless everything is suitably reinforced.

    When you want a ball room in the cabin, attach the removed column(s) to the uphaul or anchor system, so you remember to re-install it before hoisting sails or motoring off. Unless a cabin roof is completely unsound, it should be able to take a bare stick forever, unless running before the roaring forties.

    Why isn't this a common item for cabin roof stepped yachts ? If solid green water wiped out the cabin it could be an issue, amongst many others at the time.
    In the hundreds of years of sailing, why isn't it popular ?
    So many otherwise awkward TS cabins could be opened up for island double beds, big dining tables, maintenance operations, playing twister, endless options for 'new' uses.
    Just remember to put the column back before powering the hull, or if extreme weather approaches.
     
  2. TANSL
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    TANSL Senior Member

    You have just invented the "stepped deck" mast.
    Any structure, including a boat deck, will support the normal forces induced by a mast, if you size it correctly.
     
  3. seasquirt
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    seasquirt Senior Member

    Thanks Tansl, but I'm talking retro-fitting existing cabins with a removable king post, or hinging it up out of the way, with no additional 'sizing it correctly', just replace the fixed post with a removable one with the same compressional rating, in exactly the same position and height, the only real change being the use of load bearing plates to assist the removal / insert process without post end damage, and with positive location. Looking at keel stepped vs deck stepped arguments, there is little difference in their operational advantages.
    Few yachts don't have a king post, or a mast base in the way in the cabin, and it isn't needed when not underway, so what stops manufacturers from making a removable one for the deck stepped models ?

    I can only think of insurance lawyers vs idiots' lawyers as the main reason. Any mechanical reasons ? Any old wives' tales reasons ?
    What are the possible dangers if I make a removable king post for eg. a Hartley TS 16, or a Careel 18, or any sized trailer sailer with a king post ? Princess 18 doesn't need one. The wood and aluminium posts I have seen so far are just posts, held top and bottom, maybe a boss or small flange at the top, with no cross bracing or reinforcement for anything other than for vertical compressive forces. They're nothing special, and don't add anything substantial to all the other sideways forces acting on a mast step.
    Who has tried it and saw it fail ?
     
  4. TANSL
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    TANSL Senior Member

    I think I don't understand you very well. Under what circumstances would you remove the removable king post?
    How do you think insurance lawyers, idiots or not, influence the calculation of structures?
    As I said before, you can do whatever you want as long as you prepare the structure to withstand the loads it will be subjected to. And, even if you don't see it, the structures that support the masts are prepared to support lateral loads, in addition to those of compression.
     
  5. kapnD
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    kapnD Senior Member

    If the post is in the way, the load can be easily spread over a bulkhead or a bridge of some sort.
    A 16-18’ sailboat hardly has enough cabin to warrant additional structures of limited usefulness.
     
  6. Rumars
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    Rumars Senior Member

    It's called a compression post because it is under compression from the shrouds. It's role is to keep the deck from buckling and transfer the load to the keel. If the deck is suitably reinforced (ringframe, bulkhead, etc.) there is no need for a separate post. Therefore the answer to your question is obvious, nobody makes it removable because if you can remove it and the mast doesn't shoot trough the deck there is no need for one.
    Compression posts are much cheaper and lighter then stiffening the deck, so they become the preferred solution when a bulkhead can not be placed directly under the mast.
     
  7. seasquirt
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    seasquirt Senior Member

    The idea is that when NOT under way, and having visitors over for dinner / drinks, whatever, or planning on sleeping in a decent sized / shaped bed, the post is pulled out for instant un-encumbered space, and the cabin set up for whatever activity you need space for; and then before moving off afterwards, the post is replaced to take the loads it was designed for. You only take it out when you need the free space.

    If someone failed to replace the post, and sailed in heavy weather, damage could occur, including possible major structural damage to roof, cabin sides, and hull, if flexing and hammering exceeded the structure's strength. Eg. roof gives way vertically a bit, standing rigging goes slack, mast top moves excessively, slapping sideways, chain plates / fore stay are stressed, damaged, wrenched out, and things fall down, maybe even piercing the hull. Then the negligent operator calls their insurance company to complain and make a claim, the builder gets hassled with poor reviews from the negligent *****, and his business may suffer as a result; that is how I think things would go, since humans are generally opportunistic, and its always someone else's fault. So builders would probably avoid that scenario, and also save a few dollars, by not making such a useful feature available to 'John Doe and Joe Bloggs'.

    A sound cabin should take all the lateral forces easily I would think, except for over powered yachts like super maxis, made to a weight limit for speed, and everything pared back to being just strong enough for the race conditions.
    I still don't see any engineering argument as to why it hasn't been done often already, apart from the dodgy human factor, (or dodgy cabin construction).
    Since there seems to be no real argument against it, when I get my next TS, I will try it for myself, unless I get something without a king post, or someone comes up with a logical engineering reason not to do it.
     
  8. seasquirt
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    seasquirt Senior Member

    Thanks Rumars, yes I also assumed costs would be a factor in the build, and a post is a cheap efficient way to support the mast, if the useful space remaining below is less of a consideration. But in smallish boats, we usually crave more space in the cabin, unless only ever racing.
     
  9. Rumars
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    Rumars Senior Member

    You are asuming that the stays only compress the mast when sailing, wich is not true. There is precompression when static and on most boats it's high enough to bend the mast at the dock while tuning the rig. Boats designed with compression posts normally don't have enough cabin top reinforcement to support this forces, it would be a waste of material.
    In theory you could design a cabin top to take the static loads and use a removable post for the dynamic ones, but it's a hard thing to do properly. To do it you either make the entire cabin very strong and therefore heavy, or use a ringframe where the beam and knees required will significantly cut headroom inside. You could of course put them on the outside, creating a big step across the boat.
     
  10. kapnD
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    kapnD Senior Member

    When I had a small sailboat, I always campaigned hard to get invited aboard nearby yachts for dinner, and discouraged visitors other than single females. My boat did indeed have a compression post right in the center, and it was darned handy for traction while mattress thrashing!
     
  11. seasquirt
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    seasquirt Senior Member

    LOL KapnD, good on you, I never had the use of a king post in that way, maybe I should re-consider. Where's all the sailor girls at ?

    Rumars, most of the TS's I have seen don't have highly stressed rigs, unless pulling down a back stay for sail shape, while sailing, and the masts usually have a little slack and aren't 'rigid' in their position while at rest, until the foresail luff is pulled tight and the fore stay hangs limp, or the main sail fills and tensions the fore and windward stays, while the lee shroud hangs limp. So the static force / weight of an un-tensioned medium sized TS mast and fittings wouldn't be much more than 50kg, and if the deck / roof was designed to be walked on by an average adult, say 70kg, there should be ample strength to support the mast's bare weight, and the standing rigging without a king post - if nothing else bad happens.

    I can see possible failure if the mast was struck sideways quickly up high, to punch down on the roof with great leverage, and no king post to support it; or if a weak cabin roof and side attachments, like a car's A, B, and C roof columns failed, allowing the roof or mast step to move, or shear away when under sail, in which case it is probably not seaworthy in the first place, and probably wouldn't rigidly support a crew member on it to start with. You could use the king post as a door blocker, so no one goes outside unless the post is re-installed, if paranoia rules. You shall not pass !
    I'm not convinced that it's a bad idea yet.

    On the same subject, to support a mast base without using a single vertical post, is a parabola the best shape to take the mast load and distribute it vertically to the frames and hull, other than a triangle shaped frame ? Or could a catinery type shape do as well, if the legs were ending near vertical, so as not to want to spread under load, then distorting and stressing the hull. My thinking is that a small parabolic frame would be too tall and narrow, inside or outside a cabin, but a wider load bearing shape would give more cabin space, which is the objective. I don't think an arch (too narrow), or a half circle (weak on top), would be suitable, but I'm no engineer, just guessing from experience. But I soak up all your collective info, so thanks everyone for so much knowledge on this Boat Design site. The arguments are entertaining too.
     
  12. ChrisVJ
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    ChrisVJ Junior Member

    In fine weather, as a pirate, you tie your victim to the mast. When it's cold and wet she complains too much and you need the king post below.
     
  13. rnlock
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    rnlock Senior Member

    I suspect most cabin structures, at least in fiberglass, can flex enough for the standing rigging to go slack, without actually damaging the structure. When sailing, the rigging will be under considerable tension and will drive the heel of the mast down hard. Something like a lally column would be great, and no need for a bottle jack. When set up, it would raise the top of the cabin a bit and put tension back into the rig. Just have to figure out an interlock so the boat can't be sailed without the column.
     

  14. gonzo
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    gonzo Senior Member

    I remember racing boats with a jack under the mast. The system allowed to release the stress on the rig and hull without affecting the tuning.
     
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