A frame masts

Discussion in 'Boat Design' started by rgbeuk, Jul 22, 2013.

  1. rgbeuk
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    rgbeuk New Member

    I have read about several sailboats with A-frame masts. One 40' motor sailor, Kolika, custom built in 1995, designed by NATHAN SMITH MARINE Architect and Siegmund Klimmek Structural Engineer. They analyzized the compression loads on a mast, and found that an A- Frame Mast will only see half the compressive load compared to a single mast. That allowed them to build two lighter masts (200lbs each) instead of a single wooden mast, which would weigh about 600 lbs. The windage of the two masts was reduced by foil shaping them. The benefit was simpler roller furling of the main, similar to a roller furling jib, and a wishbone boom. The original owner reportedly sailed Kolika from Alaska to Mexico, and all up and down the East coast and Caribbean Islands, until being sold in 2009.

    Back in the 80s, I recall reading a Sail Magazine article about a "Bi-Pole" rig by an engineer who also calculated that two masts would put half the compression load on deck, and considerably less weight aloft, and have the benefit of no boom, if a loose footed mainsail was used.

    An A-frame rig would simplify a cruising sailboat lowering its mast while underway, to get under bridges, such as on a Great Loop Cruise (which has 16' max under some bridges).

    Are there any sailboat builders or rigging hardware manufacturers doing any research in A-frame mast rigs?
     
  2. El_Guero

    El_Guero Previous Member

  3. alan white
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    alan white Senior Member

    I like the idea of an A frame. Only backstay and forestay to deal with but keeping the two masts in column means diamond stays or bigger mast diameters. The spreaders/diamond stays may interfere with overlapping jibs on the inside between the masts. Going with bigger mast diameters would help but now you have a lot of windage.
    The cat in the video has two cross-bars so it eliminates some staying complication. It seems that adding cross-bars is another source of windage though. My own thought is to make a shorter A frame that has a single bendy mast (inverted Y rig) so that the masts can be small sectioned and therefore have less windage both because of the lower wind shadow and a smaller one too. In addition, the upper mast could be lowered easily while leaving the rest of the rig up. To stiffen the rig even more (non-lowerting), a couple of shrouds could run from masthead to deck.
    There's a lot of potential there I think.
     
  4. PAR
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    PAR Yacht Designer/Builder

    This has been done and tested. What has been found is the two masts create more drag and weight, even with the clean luff, so . . .
     
  5. groper
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    groper Senior Member

    I agree with PAR... i think Tom speer did some analysis on this and worked out that 2 masts with a separate mainsail between them, creates considerably more drag than a single mast with sail attached. IIRC, the drag from a single mast is somewhat offset by the sail attached to it and the lift induced drag the sail generates. You dont get that with seperate sail, and of course you have 2 masts, then you have almost double the windage with no sail to offset the drag from either of them...

    Sure you get lower compression load, but at the end of the day, your boat is still less efficient...
     
  6. petethai
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    petethai Junior Member

  7. petethai
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    petethai Junior Member

  8. Petros
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    Petros Senior Member

    consider that all of the fastest sailboats, being either mono-hull or multi, and all of the passages records, were set with boats with relatively conventional sloop rigs. all of the improvements have been in materials technology, and refinements in various details of the basic sloop.

    Even the rigid wing sails of the America's cup, are really refinements of a sloop rig. Since those sails do not have to be reefed or stowed, the wing sail can hold its shape with rigid internal framing.
     
  9. rgbeuk
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    rgbeuk New Member

    According to the Kolika owner, the 2 A-Frame masts, on his 40 foot motor sailor, weighed 400 lbs. instead of one 600 lbs. single mast. Thus with 200 lbs. less weight aloft. And not being intended for racing, it was comfortably ocean sailed from 1995 to 2008. The other benefit was that the genoa could be sheeted in closer without the interference of spreaders on a single mast.

    For a potential Great Loop trip, a 30' to 35' sailboat with an A-Frame mast, that could be lowered and raised simply with a whisker pole and pulley system, to get under fixed bridges, would be an advantage. Others have completed the Great Loop in sailboats without masts, to take advantage of lower fuel costs than similar sized power boats or trawlers.
     
  10. aaytch
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    aaytch New Member

    Less compression, skinny masts and no shrouds or spreaders. Isn't that going to lead to less weight in the boat overall? I don't see why an A-frame mast would not provide more weight efficiency than a boat with single mast of equal height and capacity. As for the sail efficiency, isn't it a given that eliminating the profile of single stationary mast is a good thing?
     
  11. Petros
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    Petros Senior Member

    both of those are secondary issues related to performance. there might be other reasons for choosing an A-frame rig, but performance is not one of them.
     
  12. groper
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    groper Senior Member

    Not when you replace them with 2 stationary masts, which have a full drag profile. A mast with a sail attached to it creates less drag.

    We dont know where the weight figures came from on your 2 masts vs one as we dont know anything about the design and how they ended up with those weights, its all just heresay stuff - we didnt design it. 600lbs does sound excessive tho. If it were built from aluminium mast section, in the way a modern boat is riggged these days, it would weigh much less than that on a boat that size.

    You can build a tensioned ply rotating wing mast of 10m that weighs 40lbs - the plans are on this forum... or you can build a gigantic solid timber mast that weighs 600lbs, the choice is really that of intelligent design, rather than absolutes one way or the other...
     
  13. PAR
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    PAR Yacht Designer/Builder

    Rgbeuk, the Kolika owner doesn't have a clue about design, scantlings or the weights of similar area rigs on his boat. He's taking out his butt to convince himself and anyone that'll pay attention, that his choices are justified. Again, the math and the in the wet tests have been done, several times and the jury is in. This isn't a debate, unless you use screwy math and lie about how much things actually weigh, which is what the Kolika owner is up to. Lowering a single stick mast is a task preformed all the time and requires less HP then lowering an A frame rig.
     
  14. peterAustralia
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    peterAustralia Senior Member

    after following these threads in detail over several years, I have come to the conclusion that the A-frame mast is not going to be faster or lighter

    Does that mean its bad... not at all.

    You can have less stays and chainplates, and advantage in simplicity. You can easily adjust the a frame a bit forward or a bit aft, to adjust trim, much easier to do on an a-frame than a conventional mast

    Idea would be to use more genoas and jibs, and kinda forget about the mainsail.

    Will performance be less, yes. Will raising sails, reefing etc be easier, will all around simplicity be a plus. These seem to be the advantages, performance is going to be less. For a large mainsail, raising it single handed can be hard work, the a-frame can get around that.

    You look around at the price of many large cats, $400,000 US and upwards. Thats big money to me. I would hazard a guess that 99 percent of the 7 billion people on the planet, thats too expensive for them

    So, if your looking at lower cost, ease of use, doing away with the mainsail, you may have something. Yes conventional masts can be lowered, but not really at sea. Can their be a front spar, which is supported by the a-frame say 2/3 way up. From the front spar you hang a lateen rig with roller furling.

    Supporting the spar, you dont use a loop, you kinda use a horseshoe contraption, that keeps the spar in place, yes allows it to rotate for furling.

    This exact setup has been done on this boat

    here
    http://www.boatdesign.net/forums/boat-design/frame-mast-25696.html


    and here
    http://smalltrimarans.com/blog/?p=4502

    Looking at the 2 links here, seems a reasonable setup. I dont think they will win races, but if its simple and easy to use, then maybe thats a plus
     

  15. Skyak
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    Skyak Senior Member

    Peter, the examples you cite are A frame supported lanteens. I think this is a good direction for lanteen and even lug sails (you don't have to chose between balanced or dipping).

    I see the argument that two masts are heavier and drag more than one but what about all the rigging drag and weight (not to mention cost and reliability)?

    I am also not convinced that A-frame masts are without merit. The conclusion reached by pros above is that there is no obvious performance opportunity. This conclusion follows from the fact that there are two kinds of sailboat racing -handicap and open. Handicap simply rejects anything new. For all of those above that conclude that A-frame masts are lower performance, my question is this 'What rating would you give to an A-frame mast boat?'

    The open class simply favors the biggest stick. There are many excellent sail configurations that don't offer anything to top end pro sailing and yet they offer great options. Unstayed masts for example and soft wing-sails.
     
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