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  1. #1
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    Default Mast \"A\" frame construction ....

    25ft motor sailer ..... about 8m mast ... moderately heavy, deck-stepped on "Blade". Can be erected by 4 people without extra aids. J figure is near 3m

    I want to construct an A frame to assist in this act.

    So I have 2 lengths of 6m long 22mm diam. galvanized pipe. Wall thickness is 2mm.

    Now I am thinking that 3 - 3.5m length pipe - 2 lengths formed into an inverted V by bolting together tops .... bottom of legs slotted and drilled to take bolts to slip over U bolt chain-plate fittings on deck.
    Halyard from mast top to top of inverted V and then main-sheets and blocks from there to stem. Hauling on main-sheets to raise mast already connected and pivoting on deck-blade.

    Now the question is - Is the pipe strong enough to do this job ? If not what size pipe is suitable ?

    There will be at least 2 people steadying the mast...

  2. #2
    sarabande's Avatar
    sarabande is online now Registered User
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    Default Re: Mast \"A\" frame construction ....

    Without the tech spec for the modulus of elasticity, yield strength, and a few other bits of info, it would be difficult to perform a finite stress analysis of the A frame and the pipework. I guess you could show to yourself how relatively weak a 2mm wall on a 22mm dia pipe is by placing the ends on two chairs and sitting in the middle. It's guaranteed to buckle.

    Why not use a couple of old ally spinnaker poles ? They are designed for tension and compression, and the shape resists bending.
    I think, therefore I am. I am, therefore I sail.

  3. #3
    Lakesailor's Avatar
    Lakesailor is online now Registered User
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    Default Re: Mast \"A\" frame construction ....

    Interestingly I have just built a launch trolley for my trimaran from 1" steel pipe with 2mm wall thicknes (I know, mixed units) It isn't as strong as I would have supposed. The boat is balanced slightly forward of it's pivot point and the two lengths of tube which go from the handle to the axle support the bow. It is slightly springy when I put the trolley to the floor. The boat weighs 65Kgs But that is a test of beam strength and you will be exerting mainly compression loads.
    I would imagine it would do the job. A cross piece halfway up would help prevent bowing or pinching of the frame.

  4. #4
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    Default Re: Mast \"A\" frame construction ....

    I think the pipe you have is 3/4 id which could be a bit light even though all the load is in compression. You could stiffen it by cutting some timber to hammer up it or probably 1 inch id would be better.

    I have been gathering the bits and bobs needed to do something similar for my boat. I am thinking of some poles to provide sideways support as well ,

    This will require a collar around the mast that can slide up and down as the mast raises /lowers.

    Im thinking of a large steel pipe bracket for this ,the type that are made like two semicircles that bolt together. The bolt flanges could become the fixing points for the top of the poles and it needs to be loose enough to wrap the mast in carpet first. Hope this makes sense?

  5. #5
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    Default Re: Mast \"A\" frame construction ....

    The problem is getting hold of two old spinny poles ........ I only have one new one and any other such items - being an ex Soviet state - get repaired / scrounged / taken before yours truly even gets a look in !!

    I could of course cut each 6m length in 2 and double up each side ? But then it starts to become awkward to handle .. moreso than a larger bore piping ...

    Mmmmmmm maybe it's back to the 4x4 joists I have ... that take a lot of lifting !!

  6. #6
    Stemar's Avatar
    Stemar is offline Registered User
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    Default Re: Mast \"A\" frame construction ....

    I'm no engineer, but a back of envelope triangle of forces and rusty A level physics suggests that the poles sound a bit light to me.

    Could you lay your hands on some ally scaffolding pole? You'd have no worries about strength then/
    Steve
    Never believe anything until it has been officially denied.

  7. #7
    Guest

    Default Re: Mast \"A\" frame construction ....

    It's why I posted actually .... over the phone all sounded good... when you actually see them .... you start to wonder.

  8. #8
    VicS is offline Registered User
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    Default Re: Mast \"A\" frame construction ....

    They sound a bit light to me as well. They are only going to be subject to compression loads but if they bow a bit failure will follow quickly. Lakey's suggestion to fit a cross brace would only prevent bowing laterally they could still bow the other way, fore and aft, with the same results.

    FWIW worth I have a wooden A frame. The legs are less than 2X2 (not sure exactly what without going and measuring) but for a much lighter mast.

    I am fortunate in having a beam across the cabin top that the feet of the frame rest against. It is sized so that the apex reaches just short of the stem head fitting. In use I attactch all the shrouds and backstays and stand the bottle screws up with lengths of shock cord tied to the guard rails (otherwise they can snag under the U bolt fittings). I attach the forestay (complete with furler) to a eye one side of the apex of the frame and the main sheet between another eye, on the other side, and the stem head and haul it up with that. A spare halyard holds it up while the forestay is transferred to the stem head.

    It does need a bit of steadying too keep it all straight although with such a small rig I find I can do that at the same time as hauling in the main sheet. Another helper is useful because the shrouds and especially the back stays will catch on things you would not imagine they could. if necessary the operation can be puased and the mainsheet cleated.

    It is very important to realise that in the inital stages of the lift that there is a tremendous shearing load on the bolts/screws securing the mast step to the deck. I cant remeber if you have a simple pivot like mine or a proper tabernacle but either way you must be absolutely certain of the integrity of those fastenings. It helps of course to support the mast as high as possible before starting the lift.

    One day I must photograph my system in use. Other small boat owners might like to copy it.

  9. #9
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    Default Re: Mast \"A\" frame construction ....

    Nigel,having done this a few times using whatever is around ,I would suggest you need 2 poles 4metres long(actually 3m would do with someone hanging on the mast foot to counter its top heavyness as you swing it vertical..... can you get a couple of 4x4 posts?Lash/bolt em together at the top,rig a block and tackle to a strop around the mast just BELOW the spreaders,and tie the post feet to the shroud points on the deck.
    We once pulled the mast out of a 35 footoer by sandwiching it between 2 boats and using both of the outlying boats' main halliards,again led to a strop below the spreaders and tied at its tail down to the mast winch to reduce the load on the spreader feet.Worked a treat
    Why argue with a nautical wall? I just read the graffiti these days.

  10. #10
    Mudplugger is offline Registered User
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    Default Re: Mast \"A\" frame construction ....

    Nigel, whilst undoubtably an A frame is a useful piece of kit, it shouldn't be essential for the size of mast that you refer too. In the past regularly lifted and dropped my 33' mast on my 28' Atlanta. done with just using Spinnaker pole guyed in the vertical position to the chain plates either side,and attached to the mast in the horizontal posn. Cap shrouds & lowers loosely attached, Jib Halliard to spinny pole end, Main Sheet tackle from Stem Head to spinny pole end same as jib halliard, and haul away! don't forget to put the Backstay on tho!! Provided spinny pole is FIRMLY attached to the mast and centred, all is straightforward, and Robert is your mothers brother. hth
    Tony W.

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