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Masts for a large rig

  • 16 Jun 2011 11:54
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    Reply # 622851 on 595819
    Deleted user
    When building Easy Go I was always worried about the weight of the masts and used Aluminum flagpoles, yards and battens to reduce weight aloft. When the rig is all together the boat is stable and there is a slow roll that has been known to make even veteran sailors queasy. I've had one occasion that gave me total confidence in the rig and the masts when I climbed then with the halyard and a bosun's chair in Morocco to wax the sticks. With my 200+ lbs at the top of the mast the boat was a stable as can be, much better than my old sloop would have been.

    My thoughts on the flagpoles was that being totally sealed they would offer added flotation in the event of a knockdown leading to a rollover. Been knocked down once but not severly enough to test this theory out.
  • 15 Jun 2011 01:26
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    Reply # 621513 on 595819
    Anonymous member
    Gary wrote 'I have some trepidation about the weight of my mast but short of shaving 20mm off and throwing away $150 worth of fibreglass and polyurethane coating, I guess I will find out next year how the boat will handle'.

    This is one where the theory and the practice don't always seem to stack up - and I say this with apologies to all those who are much more clever than I am about the theory of the rig.  It seems that everyone who fits JR to their boat (me included) worry about the weight of the stick, but I don't remember meeting anyone who was seriously concerned about it after the event.  My boat is noticeably more stiff since putting on the mast, and has a 3 second roll.  A heavier mast would have slowed that down.  As far as I know, most of Arne's designs have a lightly-modified tree (:-}) for a mast and many of them are off-the-shelf fibreglass boats, made for modern, relatively lightweight Bermudian rigs.

    I suspect, Gary, that once you see that the boat doesn't flop on her side the moment she's launched and take her out sailing, you will forget all about worrying about the mast's weight.  But I am really looking forward to hearing about that first sail!

    It is interesting that, at least in her earlier days, Galway Blazer got through a distressing number of masts.  I wonder why that was?
  • 12 Jun 2011 08:22
    Quote
    Reply # 619397 on 619383
    Anonymous member
    Gary Pick wrote:I have some trepidation about the weight of my mast but short of shaving 20mm off and throwing away $150 worth of fibreglass and polyurethane coating, I guess I will find out next year how the boat will handle.
    Weight in the mast does have one beneficial effect - it slows the roll. 
    And depending on the personality of the wave and how fast it throws its punch and leaves you alone, on that day that you get knocked down, a little reluctance to roll can be useful.
  • 12 Jun 2011 08:00
    Quote
    Reply # 619392 on 595819
    Anonymous member
    I think an unstayed mast's experience is both more complicated and less demanding than the technical parts of this discussion suggest. 

    I brought up the 'moment to pitchpole' comment half in jest - our masts do work hardest in the fore-and-aft direction, but there isn't any means available to pitchpole our boat by her masts, gust as it likes. The toughest challenge unstayed masts face is more related to their inertia and the conflicting periods of oscillation of boat and mast as the sea gives its kicks to the system. (As long as the boat keeps itself upright...)

    If I were a well-chosen mast, I'd be happy to take any forces the sails and rig could put on me. I'd dread the prospect of motoring into just the wrong big sloppy sea at just the wrong speed.

    And I'd rather be that particular freestanding mast on a long heavy boat like Leto, than on a short lightweight multihull. I don't see displacement as a primary factor, any more than stability in any direction. Sail area, displacement, length above partners, righting moment, roll period, moment to pitchpole (I still like it) can all serve to place a certain boat among its peers.

    In Hasler & McLeod's mast design scheme, sail area tells the scale of the boat pretty well, then length above partners has the stronger effect on the design mast diameter. If I were to add only area to mehitabel's foresail, 100 sq.ft say, the mast diameter should gain about 3/4". But if I added the 7' of height to haul up those two new panels, the table would demand a diameter almost 2" greater. So I'd rather be a mast in a low-aspect rig, and live on a long and lovely boat like Leto.

    (In fact, I cut 9' off of that mast... now I trust it.)

    There must be other junk-rigged boats on the scale of Leto and Pacific Spray that have sailed open sea. Hasler & McLeod's formulas remain a very good start.

    I'm confident Leto can find her masts, and you'll love her as a junk, Peter!

    Cheers,
    Kurt

    Last modified: 13 Jun 2011 10:03 | Anonymous member
  • 12 Jun 2011 07:28
    Quote
    Reply # 619383 on 595819
    Anonymous member
    I have some trepidation about the weight of my mast but short of shaving 20mm off and throwing away $150 worth of fibreglass and polyurethane coating, I guess I will find out next year how the boat will handle.
  • 12 Jun 2011 01:16
    Quote
    Reply # 618986 on 595819
    Anonymous member
    I agree with Kurt's "What impresses me most is Hasler & McLeod's standing on practical experience: "Masts designed to these rules have survived...". ". We must do such number-crunching as we are capable of, so that we don't do anything too silly, and then we must take a rig to sea, and ask the sea whether we got it right or not. As I seem to say quite often, there are very few theories and calculations that will survive being taken to sea.
    All I can say for sure, from my experience, is that designing my masts in the way that did seems to have yielded a rig that can stand up to going to sea.
  • 12 Jun 2011 00:09
    Quote
    Reply # 618971 on 595819
    Anonymous member (Administrator)

    Stavanger, Sunday, just

    Robert, I am aware of the fact that the righting moment of a boat varies with the forth power of the linear dimensions. However, one rarely sees a fifty-footer which is just a scaled-up 25-footer; the length-to-beam ratio is generally higher on big crafts.

    Finding a boat’s lateral stability is one thing, but finding the longitudinal stability seems a bit ambitious.

    I can only vaguely imagine all the brain- and computer work that is needed to design a jumbo-jet’s wing to get it just strong enough without being too heavy and with just the right flexibility without any hard "break here" spots. This junkrig world is for a big part for amateurs without super-computers available. Therefore I think it is best to record what is out there sailing, what has proven to work on this or that boat in such or such conditions – and then use this info for careful and conservative scaling up or down from the known boats.

    Maybe we should ask all members about their boats, masts, sailing conditions and experience with the masts and from this info build a decent database?

    Arne
  • 11 Jun 2011 15:53
    Quote
    Reply # 618789 on 598022
    Anonymous member
    Arne Kverneland wrote:The thing to remember is that the breaking strength of a mast varies with the cube of the masts diameter. If you want a mast that is twice as strong as another one, just multiply it with the cube root of 2 (=1.260). Likewise, if you want a 3 times as strong mast, the cube root scale factor gets 1.442.

    Now if we start with David’s "Tystie" at 8tons, your 20tons boat is likely to need a main mast that is 2-3 times stronger than Tystie’s main mast, at 8 5/8" (=21.9cm)

    Are you assuming that righting moment is proportional to displacement?  Assuming similar proportions, displacement scales with the third power of linear dimensions, but righting moment with the fourth power, because you have to factor in the lever arm of that weight.  If you use displacement as your measure of size, righting moment should scale with the 4/3 power of displacement.

    I agree that, for the purpose of deciding how strong a mast needs to be, righting moment should be measured or caluclated in the direction in which it is greatest.  For a beamy muiltihull that may be lateral, but for a monohull it is likely to be longitudinal.

    Regards

    Robert Biegler
  • 24 May 2011 21:39
    Quote
    Reply # 600397 on 599211
    Anonymous member
    My (vague) conclusion is that for a serious cruiser, I would let the mast strength more or less follow the displacement of the boat (and height of the mast), almost regardless of the boat's beam or ballast position. The different boat types would anyway have to take sudden squalls from behind from time to time and none of them would tip over – forwards.

    Arne

    Arne's comment points to fore-and-aft forces from sails and sea motion acting against the boat's fore-and-aft stability, as the greatest challenge for an unstayed mast. That rhymes with mehitabel's experience, when she tested her mainmast to near-destruction.

    It still might make sense to determine Leto's righting moment, to place her among comparable monohulls for mast requirements. Other variables may be useful, too. Hasler & McLeod's design table is entered with sail area and length above partners. How frighteningly simple! Maybe somebody's figured a way to measure moment to pitchpole... I'm not convinced that any of those are the best inputs.

    What impresses me most is Hasler & McLeod's standing on practical experience: "Masts designed to these rules have survived..."

    Translating timber scantlings into other materials, they give over to engineers. Finding the spars in the marketplace, and deciding on any "safety factor" are of course, left to us.

    Great to run into you here, Peter!

    Cheers,
    Kurt
  • 24 May 2011 08:12
    Quote
    Reply # 599850 on 595819
    Anonymous
    Thanks for the interest David ,Arne and Annie. Derek Van Loan gives scantling guidelines for solid wood masts and even mentions that he owns a forest of good straight firs which at the time of writing was available to aspiring JR builders to choose and fell their own spars.. Unfortunately the forest is in Northern California and Leto is at present in Asia where the only junk rig I have seen is  Zebedee. My attempts at asking about buying whole trees in Borneo, Malaysia and Thailand have so far been met with mirth ,derision and even warnings about the illegality of transporting lumber . There may well be suitable trees in this region but short of meeting a local who understands such matters I have filed this one in the "too hard basket " Thus the interest in Alloy and I was hoping that someone interested in the subject may have  tracked down manufacturers or suppliers of large tapered poles ex. the USA . Leto will be in South Africa next year and I think it may be possible to re -rig there as shipping to SA is straightforward. It may even be feasible to do the righting moment exercise there . 
       " ...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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