Closing halyard exit slots in aluminum masts

Discussion in 'Boat Design' started by Manateeman, Nov 12, 2019.

  1. gonzo
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    gonzo Senior Member

    I mean a simpler solution, an exterior aluminum sleeve. If you make an interference fit, there is no need for fasteners or welding. Heat the sleeve to below annealing temperature, freeze the mast with dry ice and slide it to position.
     
  2. waikikin
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    waikikin Senior Member

    Hi Manateeman,
    I think you're on the right track with most of this less the tig- the plexus or a rubber toughened epoxy where I go with it and slip and rivet a nice elliptical backing plate inside into the goo then use a couple of rivets & goo for your biscuit... then just keep looking forward to enjoying it..
    Jeff.
     
  3. Manateeman
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    Manateeman Junior Member

    Thank you. Just curious, monel or stainless rivets.
     
  4. Barry
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    Barry Senior Member

    And how would you actually build such a sleeve so accurately where the clearance between the hot and cold interfaces would be thousandths of an inch
    apart, press them together without galling. The mast will be an extrusion and even extrusions do not precisely match the die dimension due to contraction
    of various thicknesses. We owned several aluminum extrusion dies and I would offer my drawing but the die builder/extruder would make adjustments to deal
    with different thicknesses which could cool with an inherent lack precision to the extruding die. I would suggest it would almost be impossible
    to create a sleeve with normal tooling that would permit the process that you suggest though in theory it would work
     
  5. bajansailor
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    bajansailor Marine Surveyor

    To re-cap, we have a mast that has worked well in it's current set-up for x years while it was deck stepped.
    Mark now wants to keel step it on his new boat - this will reduce the effective height of the mast (and hence the sail area) considerably.
    The current halyard exit slots have not caused any failures so far, and one could say that with a more conservative rig planned for future use, the risk of the mast failing should now be much less.
    Hence maybe the best thing to do is to just leave them alone? Or if an improved cosmetic appearance is desirable, just glue / bond suitable cover panels over the slots, without worrying about rivets or welding?
     
  6. Manateeman
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    Manateeman Junior Member

    Hi Gozo. I’ve run your idea and the comment by Barry by engineering friends. The consensus: Barry is correct. It would be very difficult. There is no idea which should be dismissed without consideration. This is why it’s a great forum. Thank you both.
    I’ve come to the conclusion that riveting a small two part, aluminum, internal plate will be the easiest method. The internal piece will be 1/2” larger than the slot , well rounded corners, bent slightly to match the wall curve, and will have the same four holes as the original halyard exit plate. I will make a cookie to fit the mast opening as tight as possible. I’ll TIG this cookie to the backing plate so it will fill the slot OFF the spar. Thus, there is no debate about HAZ. Ill TIG a little handle on the cookie. Just a piece of rod. I’ll insert it and rivet it in place then cut off the handle from the cookie which will now be flush with the exterior mast wall.
    Two more questions : monel or stainless rivets. Should I 5200 the surface of the backing plate to prevent a water trap between the internal backing plate and the mast ? The slot is large enough so I can clean the interior of the mast and the edges of the slot. I thought about epoxy, but maybe it’s too rigid. I think a very tight fit of the cookie is the key.
    Again, thanks to all who responded.
    Mark the manatee.
     
  7. Barry
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    Barry Senior Member

    Consider an aluminum rivet with at least a stainless pin. Get the type of rivet that is waterproof. Many of the DIY type will leak water through them.
    Monel is too far away from aluminum on the cathodic chart as compared to active
    stainless. Find 316 stainless pins
     
    Last edited: Dec 1, 2019
  8. waikikin
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    waikikin Senior Member

    I'd be using aluminium blind rivets with an aluminium mandrel. occasionally I've seen them with a countersunk head also which would be neat if available.. similar here Marine Warehouse POP RIVETS - ALUMINIUM SEALED RIVET & Aluminium MANDREL https://www.marinewarehouse.com.au/pop-rivets-aluminium-sealed-rivet-aluminium-mandrel or Structural Rivets https://profast.com.au/structural-rivets/#HighStrengthRivets
    -if you rivet the biscuit in, those holes can be used to hold backing in position while you drill off the rest. If you go elliptical quite a nice internal footprint can be had- we used to do this if installing internal tapping plates for deck gear on composites.
    All the best with it.

    Jeff.
     
  9. gonzo
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    gonzo Senior Member

    Interference fitted parts are very common. In fact, the accuracy does not need to be so high, since the parts will deform and fit to each other. I explained in my post, that the external sleeve is heated, and the mast chilled. It is technique that has been is used for centuries. For example, train wheels are attached to the axles only by the friction of interference fit.
     
  10. Barry
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    Barry Senior Member

    I am not disputing the process. But the fit has to be very precise especially with aluminum which galls easy.

    My disagreement with your comment that this is an easy fix is this " the clearances have to be tight so that when cooling the outside piece contracts onto the
    inside piece to create enough friction at the interface as to carry shear flow stresses to kind of make the inner and outer components act as one.

    My question was this which you did not answer was "how would you build it" The only way without having a seam and weld two halves is to machine it from an almost solid block or an extremely thick walled tubing that would encapsulate the inside piece. For this you would need an equation representing the existing mast that you could put into a CAD program to generate a CAM tool path. Round, no issue, various oval, shapes- a lot more difficult

    Without seeing the cross section of the mast which may have internal stiffeners which would restrict a symmetrical shrinkage, I maintain that this is NOT a
    viable option. Could NASA do it, with enough money, certainly, could even a relatively sophisticated machine shop do it? Not likely

    If it is "easy" perhaps you can put the math on the thread to show how much the inside mast would contract and the outside sleeve would expand to guarantee a slip on fit. Press on, not likely to happen. With the extremely low annealing temperature and the high thermal conductivity coefficient, (ie when you begin to
    "press fit" the pieces together the cold piece will expand quickly and the hot piece will contract quickly so that any space between will close quickly

    And of course the idea when doing this process is that the outer sleeve will actually exist with a tensile hoop stress to keep a pressure at the surface interface. In order to do this, the clearances when the mast is cooled and the sleeve heated get actually less.
    Ie if the two parts were not assembled, at ambient temperatures, say the mast OD
    is 10 inches,(assuming round) then the sleeve would be maybe 9.980 inches.
    Which is the process that they do with the train wheel and axle.
     
    Last edited: Dec 2, 2019
  11. gonzo
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    gonzo Senior Member

    I never claimed it had to be built without a seam or weld. If the solution to pop rivet biscuits to cover the hole is acceptable, then shear flow stresses are not an issue. Further, you keep on mentioning pressing the sleeve on which is completely different from what I posted. When the outside sleeve gets heated, and mast cooled, the clearance increases; the opposite of what you are saying. A heated hoop gets an increase in ID.
     
  12. Barry
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    Barry Senior Member

    Gonzo
    You have not read the sentences accurately.
    I completely understand that the outside heated sleeve gets larger in diameter/dimensions and have stated this several times.
    My terminology which stated pressing can be replaced with slipped on, if you wish
    You did not ever suggest a two pieced, welded piece. With the low temperatures that you state, the varying cross sectional dimensions, quick cooling of the sleeve and quick heating to ambient of the mast, it is very unlikely that these pieces will slip on.
    And to reiterate, you provided the sleeve option as an easy option is not viable
     

  13. gonzo
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    gonzo Senior Member

    Pressing and slipping are two very different things. Perhaps things that are easy for me to do are not viable for others; no need to argue over it.
     
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