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Mast materials and Specifications

  • 01 Jun 2013 09:27
    Quote
    Reply # 1306725 on 1306051
    Anonymous member
    I appreciate all the information on this subject,  the unstayed mast being, potentially, the Achilles heal of the junk rig,  as most other parts are readily fixable. 
     
    A question for someone with more Engineering knowledge than me:
    Is it the amount of flex that induce fatigue or is it just stress?  I always thought that the more a mast moved the more it would fatigue, but it is just supposition.  If this is the case a tapered mast must always win over a slimmer straight mast of equal strength.
      
    I also need to correct my previous thought,  just considering bending stress, a straight tube would be actually twice as heavy as a tapered tube!

    That said, the straight tube with a tapered top has a lot going for it, and would be my choice for economy (unless my lottery ticket comes up, then it is carbon fibre).  I would strengthen the lower 1/3 with an outer sleeve.  Has anyone tried options for the tapered top, other than wood?  Aluminium flagpole, fibreglass flagpole.  Or perhaps even using the lighter aluminium tube for the top and timber for the bottom?  (I know I should look at the mast data base, but it is not to hand on this computer,  and I cannot find it in the website). 

      
  • 01 Jun 2013 00:08
    Quote
    Reply # 1306574 on 1306147
    Anonymous member
    Gary King wrote:Graham, the weight of your mast doesnt quite add up. 200mmx5mmx10.5m makes for a ~74kg mast, less if the top bit is tapered. So it aint so heavy and probably not an overkill after all.

    I understand your point about empirical data but free standing aluminium is all new - the material only came into common use last century. We know about free standing wood, the trick is to use numbers to get an aluminium mast with the same durability, and still not sure if we have it right. But the alu has to be *less* flexy than the wooden counterpart because wood doesnt suffer from fatigue. In fact it's the only engineering material on earth that doesn't.

    I'm still open to strengthening our foremast if needed, by sleeving in a 150mm tube internally. Should be easy to do. Thats why I'm interested in knowing the dimensions of Lexia's foremast.

    Interesting data about the weight Gary.  I have never weighed it but the delivery docket said 200kg.  It did come strapped to a long skinny pallet and it never occurred to me until now that the weight of the packaging would be included in the freight calculations.  Dummy!  It also had a 25mm thick base plate that I cut off.  However, I left the inner sleeve in place, which means the first 1.5m is 10mm thick, and the mast cap and running rigging probably weigh as much as the base did.  Whatever it really weighs, it is so heavy that I can barely pick up the heel when it is lying on the ground. I had thought I would be able to walk around with it on my shoulder. 

    My comments about empirical data were a bit tongue in cheek.  Anyway, hopefully it is strong enough, though I do have some pitting about 600 mm below the partners, where I wrapped some old jib sheets around it for aesthetic reasons.  The sheets must have been salty and in the wet season trapped moisture and caused some corrosion.  No sign of any elsewhere.
  • 31 May 2013 22:45
    Quote
    Reply # 1306512 on 1306176
    Deleted user
    Mark Thomasson wrote:

    LEXIA. I thought: what a lovely seaworthy looking boat, but also thought: what slim masts.  Sadly my thought was correct.  Looking again it appears that the masts are two part, the lower a straight tube.

    I am sure we all know, (but has not been mentioned?) is that bending stress (for an unstayed mast) increases, from nothing at the top, to maximum at the partners.  A straight tube, is therefore much heavier, probably by about 1/3rd.   The CoG is also higher, so a double whammy. 

    As mentioned fatigue is the killer.  The thinner and higher strength allows probably the worse.  Of course it is accepted with a stayed rig to replace the rigging every few year,  stainless steel especially.  So with an unstayed alloy mast, should it not be accepted that it has a limited life, the only problem being how long.  Unfortunately not much data in yachts to go on, though the aircraft industry knows it exactly (or at least I hope so).  That said any data that could be collated would help, another column for the mast details spread sheet?

    I heard of a mast failure, Virgo, on the Clyde,  the previous owner had drilled a very small hole for the mast head light, just above the partners, no guesses where it broke.

    I am not sure, but is not carbon fibre also free from fatigue?

    So there are two main killers, Mark - holes and fatigue. I didn't see any holes pre-sailing around deck level of Jonathan's masts; he was well aware of that problem. I'm sure we'll get a report from him soon.
    Last modified: 01 Jun 2013 01:06 | Deleted user
  • 31 May 2013 20:43
    Quote
    Reply # 1306409 on 1306349
    Anonymous member
    Rudolf van der Brug wrote:The nice thing thing of a carbon fibre mast is that in order to get it stiff enough you have to
    add more carbon. The amounts are such that fatigue (and partially less than ideal c onstruction methods, I state this with reservations,) and strenght are not an isue.
    It will be overengineered when stiff enough to suit most.

    Rudolf 

    and still very light weigh.........
  • 31 May 2013 18:45
    Quote
    Reply # 1306349 on 1306051
    Anonymous member
    The nice thing thing of a carbon fibre mast is that in order to get it stiff enough you have to
    add more carbon. The amounts are such that fatigue (and partially less than ideal c onstruction methods, I state this with reservations,) and strenght are not an isue.
    It will be overengineered when stiff enough to suit most.

    Rudolf 
  • 31 May 2013 16:20
    Quote
    Reply # 1306272 on 1306176
    Deleted user
    Mark Thomasson wrote:


    I am not sure, but is not carbon fibre also free from fatigue?

    The stuff isnt perfect. I know the aerospace industry run a spectrometer, (or whatever its called - its been years since I was given the guided tour) over carbon fibre components to detect micro imperfections in the lay up, so they can reject the component. Something a mast maker probably doesnt do.
    Last modified: 31 May 2013 17:07 | Deleted user
  • 31 May 2013 14:02
    Quote
    Reply # 1306176 on 1306051
    Anonymous member

    LEXIA. I thought: what a lovely seaworthy looking boat, but also thought: what slim masts.  Sadly my thought was correct.  Looking again it appears that the masts are two part, the lower a straight tube.

    I am sure we all know, (but has not been mentioned?) is that bending stress (for an unstayed mast) increases, from nothing at the top, to maximum at the partners.  A straight tube, is therefore much heavier, probably by about 1/3rd.   The CoG is also higher, so a double whammy. 

    As mentioned fatigue is the killer.  The thinner and higher strength allows probably the worse.  Of course it is accepted with a stayed rig to replace the rigging every few year,  stainless steel especially.  So with an unstayed alloy mast, should it not be accepted that it has a limited life, the only problem being how long.  Unfortunately not much data in yachts to go on, though the aircraft industry knows it exactly (or at least I hope so).  That said any data that could be collated would help, another column for the mast details spread sheet?

    I heard of a mast failure, Virgo, on the Clyde,  the previous owner had drilled a very small hole for the mast head light, just above the partners, no guesses where it broke.

    I am not sure, but is not carbon fibre also free from fatigue?
  • 31 May 2013 12:49
    Quote
    Reply # 1306147 on 1306051
    Deleted user
    Graham, the weight of your mast doesnt quite add up. 200mmx5mmx10.5m makes for a ~74kg mast, less if the top bit is tapered. So it aint so heavy and probably not an overkill after all.

    I understand your point about empirical data but free standing aluminium is all new - the material only came into common use last century. We know about free standing wood, the trick is to use numbers to get an aluminium mast with the same durability, and still not sure if we have it right. But the alu has to be *less* flexy than the wooden counterpart because wood doesnt suffer from fatigue. In fact it's the only engineering material on earth that doesn't.

    I'm still open to strengthening our foremast if needed, by sleeving in a 150mm tube internally. Should be easy to do. Thats why I'm interested in knowing the dimensions of Lexia's foremast.
  • 31 May 2013 09:40
    Quote
    Reply # 1306092 on 1306051
    Deleted user
    Ok guys, over to you. 
    Lesley

  • 31 May 2013 09:37
    Quote
    Reply # 1306091 on 1306051
    Deleted user
    On 31/05/13 Arne Kverneland wrote:

    Paul Thompson wrote:
    What you do want to remember is that while righting moment is a good guide, the highest mast loadings are when running, not when beating as many assume. Hence the saying that the boat can handle more than the crew upwind but the crew more than the boat down wind. Then theres the stresses from knockdowns and (hopefully never happens) an inversion. The above being the case, I like to be very conservative in my calculations.

    This is a most interesting subject! I have done some comparisons of weight of aluminium versus spruce masts and the wooden masts seem to only be moderately heavier at the same breaking moment. In addition their centre of gravity will sit quite a bit lower. The lovely thing with the hybrid mast is that one may make them from off-the-shelf parts and that it is so quick to make. No need for critical scarf joints or waiting for a tree to season. The hybrid mast also comes out cheaper than both a "properly" made aluminium mast and a wooden mast.

    Paul, you may well be right in that a boat can take less than the crew downwind. However, it depends of the boat. A sloop-rigged junk with small or shallow rudder will soon call for a reef and thus be mast-friendly. Other boats, like for instance Edmond Dantes, with her deep, balanced rudder will let one keep on in increasing (down-)winds until something bends or breaks. Such boats must be reefed consciously to save the rig. My rule of thumb is that if I keep the same sail area downwind as upwind, the rig will be safe, the steering will be light - and if something is lost overboard, we are ready to head up and pick it up without delay.

    Arne
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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