Aluminium gaff rig construction

Discussion in 'Materials' started by Avoid Rocks, Apr 25, 2020.

  1. Avoid Rocks
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    Avoid Rocks Junior Member

    How would you go about constructing the gaff and the boom in aluminium? Would you weld flat bar bent to a U-shape onto a round tube or do you have better ideas?
     
  2. Rumars
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    Rumars Senior Member

    What do you want to do? What should be welded to what and why?
    Gaff: jaws or saddle?
    Boom: jaws or gooseneck?
    The main body of the gaff and boom is normally an extrusion of suitable dimensions.
     
  3. Avoid Rocks
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    Avoid Rocks Junior Member

    Yeah I don't think anything else than a round extrusion would make sense for the gaff and the boom. What I'm curious about is whether there are better ways (strength and weight wise) to construct the gaff jaws than simply bending a flat bar into an U-shape and welding it to the tube?
     
  4. Avoid Rocks
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    Avoid Rocks Junior Member

    And functionally it might be better to use a bent round tube instead for the jaws, to not have a sharp edge chugging into the mast?
     
  5. Avoid Rocks
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    Avoid Rocks Junior Member

    Pardon my crude drawing but here are the two options mentioned and I'm now thinking using a tubular section (the bottom one) would be better. Should be less in weight for the same strength (I think?) and wouldn't be so tough (no sharp edges) on the mast.

    [​IMG]
     
  6. Avoid Rocks
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    Avoid Rocks Junior Member

    Ok so this is what I've come up with:

    [​IMG]

    So the gaff is a round tube, the jaw is bent round tube (prob needs to be thicker than in the drawing, haven't done any calculations yet), and then there are two "knees" made out of flat bar for reinforcement.

    Thoughts?
     
  7. Rumars
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    Rumars Senior Member

    There is an articulated piece between the jaws that rides on the mast. The saddle is articulated by itself.
     
  8. Rumars
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    Rumars Senior Member

  9. Avoid Rocks
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    Avoid Rocks Junior Member

    Apologies, it seems like it might've been better to explain my actual scenario from the start here. I had figured that asking for advice for gaff sizing would be easier than explaining the whole concept. What I didn't know about the gaff rig is that most larger boats use fittings like you posted above.

    So, I'm designing a soft wing sail based on the junk rig. Here's a drawing showing a bit more of the concept:

    [​IMG]
    The luff former there is subject to change, I need to study foil profiles a bit first before deciding on the exact shape.

    The sail would be comprised of 10 of these battens, so keeping weight down is important (good thing though is that the weight moves down as you reef). But also, because of spreading the forces over multiple battens the stress for any individual batten is relatively low. Planning for a 35-37 sqm sail. The battens (except yard and top batten) are hinged (see sketch below) and the cloth is doubled over the forward part (connected to the luff former) with the aft part being a single cloth (attached by batten pockets to the aft batten and to the front part of the doubled cloth with a zipper). The luff former ahead of the mast here will be GRP since it's not structural and there will also be similarly shaped luff formers in between the battens. There will also be a parrel connected between the jaws ahead of the mast (although gravity will mostly hold it in place). In the above drawing the batten is 50 mm (probably 1.5 mm wall thickness) while the jaw is 25 mm (2 mm wall thickness?) tube. The jaws will lie against the mast at roughly 86 degrees, i.e. the battens will sit slightly higher aft, like most junk sails.

    Here's a rough sketch of the concept that I did in 2014:

    [​IMG]
    Partially due to time restraints I just ended up building a flat hinged junk sail instead. Another reason I didn't go through with the project back then was that I didn't really have a satisfactory solution for engineering the nose pieces, which changed recently when I realized that welding them out of aluminium would be great. Now I'm just trying to hone the details and come up with a good light but strong solution.

    Any thoughts are welcome.
     

  10. Rumars
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    Rumars Senior Member

    For your application I think you need UHMWPE. Cut the jaws out of it and bolt them on. You need a spacer into the Al tube so as to not crush it, this you also make out of UHMWPE. If you do not need a tumbler the whole thing, the two jaws and the spacer can be one piece, you only cut two slots for the pipe and the outside shape.
    Here is a page with drawings: The Gaff Rig Detail Pages - The Gaff https://www.frankhagan.com/weekender/gaffgaff.htm
    You can also google pictures of "gaff rig jaws" to see examples with and without tumblers.
    If you fit parrels you can just fit the nosecaps on the jaws, then you can take the battens off the mast. Nosecaps can be Al, drill and thread for screws or use snapcaps.
     
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