Aluminum mast

<< First  < Prev   1   2   3   Next >  Last >> 
  • 10 Sep 2019 06:34
    Reply # 7872935 on 7872079
    Anonymous wrote:

    John

    I notice that Mark Hibdon's boat has a flat junksail. Is it your plan to make your sail flat as well?

    Arne

    Yes. I may at a later date make a sail with some shape in it. Mark feels his boat is no slower all around with the flat sail. I like the idea of a shaped sail but fear complicating it
  • 09 Sep 2019 21:23
    Reply # 7872079 on 7309942
    Anonymous member (Administrator)

    John

    I notice that Mark Hibdon's boat has a flat junksail. Is it your plan to make your sail flat as well?

    Arne

  • 09 Sep 2019 05:28
    Reply # 7870778 on 7869633
    Anonymous wrote:

    John

    Eleven inches (27.94cm) at the partners sound good to me. Now I ran it through my ‘modified Practical Junk Rig calculator’ (assuming LAP =10m), and came to 27.1cm with a hollow mast.

    Next step is to get the position of the CE right. Have you had a chance to sail a Rawson 30 with its original rig? This will give you a clue as to putting the CE of the JR in the same position, or forward of it.

    The integral rudders, as that of the Rawson, are not quite as efficient as free-standing rudders, and unlike the Bermuda rig, one cannot ‘reef the CE forward’ on a sloop JR (but some rig their junk sail so the whole thing can be swung a bit forward).
    I suggest you keep the chord of the sail moderate and rather increase the AR (unlike the sails of my Johanna and Ingeborg). This will keep the CE closer to the centreline when reaching and running.

    Since you are a carpenter, you can have a plan B ready: In case the original rudder struggles, add a new rudder on the transom. That will do wonders.

    Cheers,
    Arne

    PS: The maststep could be made the way I show here and here, on my Ingeborg. I have used the method on three boats in the 1.5 – 3tons range, and they seem to be plenty strong.


    Thanks for running the numbers. Yes I have sailed my rawson with the sloopmrig and she does have some weather helm. I’m using the same drawing as Mark Hibdon though my plans may not be as complete as his. If she weathervhelms too much I can move the sail forward so long as I rig it to do so. Is that correct? Marks boat is a success according to him. I can’t remember if I asked him if weatherhelm was corrected. Hmm I’ll look at my emails with him and ask. The boom and battens above it are 18’ long I’m unsure of the height she will have 554 SF in total. The sail must be about 30’ high to the peak I’ll find it on the drawing. 
  • 08 Sep 2019 10:23
    Reply # 7869633 on 7309942
    Anonymous member (Administrator)

    John

    Eleven inches (27.94cm) at the partners sound good to me. Now I ran it through my ‘modified Practical Junk Rig calculator’ (assuming LAP =10m), and came to 27.1cm with a hollow mast.

    Next step is to get the position of the CE right. Have you had a chance to sail a Rawson 30 with its original rig? This will give you a clue as to putting the CE of the JR in the same position, or forward of it.

    The integral rudders, as that of the Rawson, are not quite as efficient as free-standing rudders, and unlike the Bermuda rig, one cannot ‘reef the CE forward’ on a sloop JR (but some rig their junk sail so the whole thing can be swung a bit forward).
    I suggest you keep the chord of the sail moderate and rather increase the AR (unlike the sails of my Johanna and Ingeborg). This will keep the CE closer to the centreline when reaching and running.

    Since you are a carpenter, you can have a plan B ready: In case the original rudder struggles, add a new rudder on the transom. That will do wonders.

    Cheers,
    Arne

    PS: The maststep could be made the way I show here and here, on my Ingeborg. I have used the method on three boats in the 1.5 – 3tons range, and they seem to be plenty strong.


  • 07 Sep 2019 23:21
    Reply # 7869403 on 7866658
    Anonymous wrote:

    John,

    a hollow wooden mast with 20% wall thickness will have 13% lower breaking strength than a solid mast of the same outer diameter. However, the hollow mast will be 36% lighter, and since much of the stress in a sailboat’s mast comes from rocking and rolling, rather than from wind pressure, I would not be surprised if the hollow mast (of same diameter) will stand up just as well as the solid one. Moreover, by increasing the hollow mast’s diameter with only 4.7% over the solid one, the breaking strength of the two will be the same.

    This will raise the weight of the hollow mast to 1.0472 = 1.097 times the slimmer version. The beefed-up hollow mast will thus be about 10% heavier than before, but it will still be 30% lighter than the solid wooden mast.

    As for boom, I agree with David. Only on my last (5th) junkrig have I beefed up the boom a little. This has let me move the tackline (TL) aft so it looks like a kicking strap. That lets me hoist the full sail until the TL is taut, and this ensures that the leech of the lowest panel will not flutter in a rising breeze. The boom’s dimension is now the same as the upper sheeted batten.

    Good luck!
    Arne

    PS:  Only on my first boat, the Albin Viggen, Malena did I sail  first with a solid pole mast and then with a hollow one (25% lighter). There was no doubt that the boat stood better up to rising winds with the lighter mast. However, your Rawson 30 is much heavier, so even a solid mast will feel much lighter on that boat. Try to compare the mast weight with the displacement of the boat. If the ratio is not much over 3.0%, you will be fine.

     


    The drawing I have shows a solid spruce mast 11” at the partners. I’m tempted to use a Douglas fir And scooping it out and shaping it with a skillsaw and a sander/ drill with a drum sander. 

      This is my thinking as of right now. I’m not good with the math you are using. I’m a carpenter by trade, we occasionally use hypotenuse of right triangle. I may have to go back to school to get caught up. 

      That gives me the winter to weld up my partners and build the step. I’m leaning towards steal on a plywood base glassed in to the bottom. If I can’t get my steal galvanized I will epoxy paint it I suppose. The step is the scariest part for me as I worry about it bonding well and being big enough.

      Thanks for everyone’s feedback. 

  • 06 Sep 2019 18:12
    Reply # 7867240 on 7309942

    I've said this often before, but I'll keep on repeating it as often as necessary:

    In a marine environment, never, ever [and I feel like writing that in 36pt, fluorescent orange, bold, italic, underlined text] put carbon next to aluminium. With salt water between them, you'll almost be able to see the aluminium fizzling away, they are so far apart in the galvanic table.

  • 06 Sep 2019 17:32
    Reply # 7867190 on 7309942

    Thanks David:

        That was more or less my conclusion regarding carbon just from the fact that it does not seem to be done........ a shame not to be able to use it to enhance other materials,  however I do know that carbon fiber over aluminum tubing is used in some applications.


                                                       H.W.


  • 06 Sep 2019 17:00
    Reply # 7867137 on 7309942

    "would one end up carrying all the load until that material failed?" - yes, indeed it would. It's bad practice to put two materials with different moduli of elasticity together, to resist a load in one direction. Even glass used as sheathing is inclined to split, if the underlying structure is at all flexible. That's why Dynel cloth is often preferred for sheathing, because it is more stretchy than the wood it covers.

  • 06 Sep 2019 16:09
    Reply # 7867039 on 7309942

    Reading Arne's comments about hollow versus solid wooden masts, brings me to mind of a though I had about using carbon tow vertically at or near the surface of a hollow wooden mast to stiffen and strengthen it, not sheathing it in carbon, but just a dozen or so shallow carbon "filled" grooves.  Tow is unidirectional fiber.... like thread, and if available in different sizes.  It is not horribly expensive.   This is assuming that the wooden mast has a fiberglass sheathing, which would help tie it all together.    I know almost nothing about how to engineer composite structures, and the thing that concerns me with this idea is the different flexing behaviors of the materials.  Would they compliment each other, or would one end up carrying all the load until that material failed.  In other words would the wood contribute without the failure of the carbon coming first?


                                       H.W.

  • 06 Sep 2019 10:27
    Reply # 7866658 on 7309942
    Anonymous member (Administrator)

    John,

    a hollow wooden mast with 20% wall thickness will have 13% lower breaking strength than a solid mast of the same outer diameter. However, the hollow mast will be 36% lighter, and since much of the stress in a sailboat’s mast comes from rocking and rolling, rather than from wind pressure, I would not be surprised if the hollow mast (of same diameter) will stand up just as well as the solid one. Moreover, by increasing the hollow mast’s diameter with only 4.7% over the solid one, the breaking strength of the two will be the same.

    This will raise the weight of the hollow mast to 1.0472 = 1.097 times the slimmer version. The beefed-up hollow mast will thus be about 10% heavier than before, but it will still be 30% lighter than the solid wooden mast.

    As for boom, I agree with David. Only on my last (5th) junkrig have I beefed up the boom a little. This has let me move the tackline (TL) aft so it looks like a kicking strap. That lets me hoist the full sail until the TL is taut, and this ensures that the leech of the lowest panel will not flutter in a rising breeze. The boom’s dimension is now the same as the upper sheeted batten.

    Good luck!
    Arne

    PS:  Only on my first boat, the Albin Viggen, Malena did I sail  first with a solid pole mast and then with a hollow one (25% lighter). There was no doubt that the boat stood better up to rising winds with the lighter mast. However, your Rawson 30 is much heavier, so even a solid mast will feel much lighter on that boat. Try to compare the mast weight with the displacement of the boat. If the ratio is not much over 3.0%, you will be fine.

     


    Last modified: 06 Sep 2019 10:51 | Anonymous member (Administrator)
<< First  < Prev   1   2   3   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software