Mast from a Tree or from metals ?

  • 01 Mar 2019 08:44
    Reply # 7193031 on 7184461

    Reuel Parker's The New Cold-molded Boatbuilding has a good chapter on building six staved unstayed masts, and he goes for barrel tapered staves - hardly more difficult to do than straight staves.


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  • 01 Mar 2019 00:19
    Reply # 7192653 on 7184461

    I had a barrel tapered mast that had very good strength and stiffness through the middle but, unlike a more traditionally built barrel tapered mast as Skeene would have analyzed, my mast was unusually light, I believe, in the upper few feet.  It was built lightly enough that the very top of the mast flexed under halyard and sail forces which I guess that would be consistent with  Arne’s “case 1”. 

    My first junk rig mast was barrel tapered and designed for me by Phil Bolger in 1994.  At the time he wrote “I have no personal experience with Chinese lugs.” This was before he drew his hybrid Chinese gaff rig.  (Which incidentally powered Jim Melcher’s  Alert and which is now owned by a friend and has been at one of our north east junkets albeit with a different rig) The mast was in keeping with Phil’s amateur friendly philosophy. The 27 foot O.A., 24 ft above the tabernacle, 4 3/4 inch square tabernacled mast was for a 3200 lb. boat, an O’day 23.  Two of the scarfed staves were 1 1/2 “  X  4 3/4” X 27 feet. Two were 1 1/2” X  1 3/4” X 27 feet. When glued up the hollow center of this 27 ft box was 1 3/4” throughout its length.  The barrel shape was sawn from this and the corners radiused. 

    The barrel shape, of course, allowed for wider sections. For example at 25% above the partners (tabernacle) the mast was 96% of the 4 3/4 inches and at mid point it was still at 87% .  In this design wall  thickness was reduced as well. At midpoint it went from 1 1/2 inches to 1 3/16 inches. Six feet from the top, the walls were three quarters of an inch thick and at a station three feet below the top the wall thickness was just one half inch.  

    The mast head was at 42% but the walls in the last three feet were quite thin and, despite a little possible help from a long tapered mast plug, the top of the mast flexed. Despite that I was very happy with the mast and its tabernacle. I’m sure it would be better for a junk sail if this amateur buildable mast it could be designed with a with a stiffer top section, perhaps with top closer to 50%. Being square with well radiused corners worked fine in all respects and matched my building skills and the tabernacle was the utmost in simplicity. Of course it was highlight for me to meet and converse with a most gracious Mr. Bolger. 




  • 28 Feb 2019 13:58
    Reply # 7191572 on 7184461
    Anonymous member (Administrator)

    Distribution of moment and strength

    Now I have tried to sort out my thoughts about the distribution of bending moment on a free-standing, wooden JR mast.

    ‘Case JR’ has to sit somewhere between these two extremes:

    Case 1) The sail is only working at one point; the mast top.
    Case 2) The sail distributes the pressure perfectly evenly, up along the mast.

    On Fig. 1, the resulting moment curves along a mast have been drawn up. Case 1 results in a curve which is a straight line, while Case 2 results in a parable curve.

    Then I have calculated the strength distribution of a PJR-style cone-tapered mast, and then superimposed it onto the same diagram. As can be seen, such a mast would have proven to be fine if the JR lived in a perfect Case 2 situation. However, practical experience has shown that such masts tend to bend quite a lot around their middle, so I suspect that Case 1 is a better and safer model  for designing a mast for JR.

    On Fig. 2, I have removed the Case 2 curve, but have introduced two more masts, and calculated their strength curves. The barrel-tapered mast, nicked from “Skene’s Elements of Yacht Design”, fits in very well with the Case 1 load distribution. However, it will be a challenge for an amateur to build. I have therefore suggested a modified cone-tapered mast, which keeps full diameter up to 20% above partners, and then tapers to 45% thickness at the mast top. This will also have a good strength distribution, and will be much easier to build, although a bit heavier than Skene's mast.
    I would anyway stay away from the PJR-style mast, from now on, in particular since my Johanna-style sails put some loads 'up there'.

    Figure 3 shows the actual distribution of diameters of the three masts.

    Fig 1


    Fig 2


    Fig 3



    Last modified: 28 Feb 2019 23:22 | Anonymous member (Administrator)
  • 26 Feb 2019 10:46
    Reply # 7187820 on 7184461

    I agree, David & Arne. A mast that bends evenly is highly desirable. The "elongated half-a-barrel", or entasis, shape (thanks for teaching me a new word, David!) gets nearest to that ideal. The tapered aluminium flagpole (usually parallel over its bottom half) and the hybrid aluminium tube + wooden topmast are both good approximations. My current mast has a top diameter of 46% of the diameter at partners, and that's as small as I'd want it to be.

  • 26 Feb 2019 00:04
    Reply # 7187025 on 7184461
    Anonymous member (Administrator)

    The reason, if I remember correctly, why Hasler and McLeod went for cone-tapered masts in their PJR, was that the junk sail is supposed to deliver the forces more evenly up the mast, unlike on a western lugsail.
    My guess, after watching my masts under load, is that the force distribution in the sail of a JR is not as linear as one may wish. At least on my sails, where there is a big fan-shaped top-section working on the mast top, the load is quite high near the mast top. I noticed that the otherwise very stout mast of my Johanna (25cm/10cm), bent a bit in the top. Therefore, I made the second, 9cm thick mast for Broremann with 4cm diameter in the top (= 44% versus only 40% in PJR’s recommended masts.).

    Arne


  • 25 Feb 2019 21:07
    Reply # 7186701 on 7184461

    I think you are right Arne; I have noticed that cone tapered masts always bend more about 2/3 of the way up from the base, and this appears to be where they mostly break. I have found that entasis tapered masts seem to have a more even bend. The Greeks obviously got it right when building their columns!!

  • 25 Feb 2019 15:22
    Reply # 7185859 on 7184461
    Anonymous member (Administrator)

    I have had a few wooden masts. Most of them tended to be oversize, until I learned more.

    However, the first JR mast for my dinghy, Broremann, was undersize, at 70mm. The second mast was 90mm which should be 213% as strong as the first mast ( strength varies with the cube of the diameter).

    The photos below shows the two masts in action. This is the fine thing with unstayed masts; one can get an early warning from the bending mast. That first summer, I reefed more to protect the mast than to avoid capsizing. With the new 90mm mast (weight 10.0kg), the mast strength was no longer a limiting factor.

    I have tried to analyse the strength distribution of a cone-tapered wooden mast (which PJR recommends), and I have come to that this design leaves a weak spot near the middle of the mast. This fits in with how the thin mast of Broremann bends much just below the middle.

    If I were to make another all-wooden mast, I would therefore keep it at full diameter up about 15% of the LAP before making the cone-shaped taper above it. This would be easy to make and add strength at the middle without adding much top-weight to the mast.

    Cheers,
    Arne

     

    Last modified: 26 Feb 2019 09:37 | Anonymous member (Administrator)
  • 25 Feb 2019 10:33
    Reply # 7185532 on 7184461

    Thank you again David, and Len.  Perhaps some FAKE news :)

    And your discussions makes sens.


  • 25 Feb 2019 09:03
    Reply # 7185453 on 7184461

    With any given material, it's possible to design and build a structure that is too weak/just right/too strong. There have been wooden masts that have lasted a very long time and there have been some that didn't. There have been some aluminium masts that have lasted a very long time, and there have been some that didn't. There have been some carbon...

    You get the picture. Design it right, and build it right, and it'll be OK.

  • 24 Feb 2019 16:48
    Reply # 7184521 on 7184461
      Raymond Liljeros wrote:

    I read somewhere that a mast for a Junk Rigged sailboat

    made of a tree would be much stronger than from metal.

    It is true that I have rarely seen a tree broken in half,

    usually it is the roots that give away.  Then to be on the safe

    side, I suppose that it would be quite easy to change

    from a metal mast to a tree mast on any junk rigged sailboats ?


    Almost all the stories I have read of broken masts have been tree and wood masts. The only metal mast failure I have read about was not the mast but the deck structure. Certainly I have not read or heard of everything that goes on and metal stayed masts have been known to fail as well. I think in both cases, do the math and get to know your material before making a decision. A wooden mast (solid or hollow) will be of greater diameter and heavier for a given strength. So to change from metal to wood likely means making a bigger partner, using more interior room, changing any rigging to deal with a larger mast, possibly changing the location of ballast for trim, etc.

    Aside from the PJR Arne Kverneland also has some good documents on this for both wood https://junkrigassociation.org/Resources/Documents/Arne%20Kverneland's%20files/Chapter%206,%20THE%20(wooden)%20MAST.pdf and alloy masts https://junkrigassociation.org/resources/Documents/Arne%20Kverneland's%20files/Chapter%206b,%20The%20Hybrid%20Mast.pdf  

    Honestly, I think wood masts can be strong and long lasting. I do think that a lot of unstayed wooden masts are calculated by "That looks good enough to me" or "that is the best I can find". But many are just "It broke at a flaw in the wood that was not visible when I selected it". I think selecting a tree for a mast takes some talent and experience of knowing what to look for. I would not use a wood mast (even though they look nice) for this reason, I know I don't know...

    Last modified: 24 Feb 2019 16:50 | Anonymous member
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