Rob Dennys' unstayed masts

Discussion in 'Multihulls' started by Bruce Woods, Feb 4, 2020.

  1. Bruce Woods
    Joined: Sep 2007
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    Bruce Woods Senior Member

    So as not to clutter the other thread lets go our hardest here instead.






    Joined: Feb 2005
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    rob denneySenior Member
    New
    The subject is unstayed masts for cruising boats, not bridges, 18' skiffs, AC boats or IMOCAS. Maybe drop the red herrings and reply to the reasons why unstayed masts are better for cruisers.

    The idea of breaking a stick shows that a column in compression requires a far greater force to break than if it is in simple beam or cantilever mode. Columns are strong. Even stronger when supported by stays to keep them in column.
    Wrong. See (or address) my previous post about the structure required to support the stays. Which, incidentally, is composed of 5 cantilevered unstayed beams (hulls, cross beams) on most stayed rig cruising multis.

    Putting slight pre-bend in a mast is a simple way to get the mast more rigid because you know the "eccentricity" of the mast and can design accordingly. Push it forward in the middle and then lock it there - really well shown by the skiff type rig - huge load on a small pre-bent and locked section.
    How many wires does this require, how often do they replace the rigging, check the tensions, sail dead down wind, reef in a squall, or hoist/lower the main on the water? ie How suitable is a skiff rig for a cruising boat?

    And yes simple beam bridges are most common, for small bridges over roads and railways, just like cantilever masts are more common on Lasers, OKs, Finns and many singlehander dinghies. When loads are low the cantilever or simple beam is a great thing. But when the loads get large, say in a long bridge with a high load, most bridges resolve compression and tension into different members. The stability of my 38 footer is 16 times the stability of my 19 footer
    Maybe, but it is several orders of magnitude less than the Golden Gate bridge.

    so the engineering may need a different approach. One that can handle this exponential increase in load. Engineers have to determine where this cross-over point is where a cantilever is more work than a stayed truss.
    The argument about whether unstayed rigs could handle the loads ended with Team Phillips, 20 years ago. This had more righting moment than any race cat or tri ever built, and fewer crew than the rest of the fleet. The hulls broke, the rig was excellent. Unstayed rig yachts like Matese Falcon, Yacht A and others further prove the point.

    But I do think that if unstayed masts were the best option for larger boats then would they not be used in open classes like IMOCAs or a deed of gift AC boat or in record setting sailing boats? They don't seem to be. A clean slate and the best in the business go with the stayed rig. Or the charter boats made just for ease of use.
    A relevant comment! Anyone got any deas why the charter boats don't use unstyaed rigs?

    I would argue that a stayed rig still fits the bill for most users.
    Go ahead and argue it, then. Tell us why it is so good for cruisers, starting with the reasons I gave showing it isn't.

    I don't argue that unstayed mast are silly for everyone
    You do, apart from Lasers and Finns.

    I should temper my words to reflect that.
    Yes, you should, but you haven't.
    ,

    I however, will sit this one out unless the boat is quite small.
    Writing negative posts about unstayed rigs and then trying to defend your views with red herring posts is hardly "sitting this one out".
     
  2. Alik
    Joined: Jul 2003
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    Alik Senior Member

    I would be interesting to take, say, a 40' cruising boat and compare the loads and weight of rig (and supporting structure), for stayed and unstayed options. Otherwise it will go to another bla-bla-bla.
    PS I race dinghies with unstayed masts and well know their limitations.
     
  3. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Rob,
    Are there any merits to a stayed mast on a cruising boat?
    Thanks
     
  4. peterbike
    Joined: Dec 2017
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    peterbike Junior Member

    I think what is being overlooked here is, personal choice.
    Some like fat chicks, some like skinny - there is no right or wrong, just what suits/turns you on.
    Both Rob D. & Catscetcher have put forward good points.
    it doesn't have to turn into a **** fight, with people from both sides of the fence getting on their high horses & slinging mud.
    Please try to behave like adults - not oversized children who look like adults.
    As the french would say ; "cest la vie", or "live & let live"
    You might actually learn something, instead of saying "no, I'm right & he is wrong !"
     
    Alik and Ad Hoc like this.
  5. trip the light fandango
    Joined: Apr 2018
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    trip the light fandango Senior Member

    The use of carbon fibre and epoxy to make a lightweight, strong free standing mast that can effectively reef itself by spilling excess wind at the top, is a big step forward in efficiency and simplicity. The only issue is where do you hang your jib, gen, spin?...Otherwise it's just a bloody good idea ,there is no valid argument is there? Feel free to ignore this I'm a complete backyarder with a poofteenth of the rest of you[s, ha] experience
     
  6. PNW sailor
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    Location: Seattle

    PNW sailor Junior Member

  7. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

  8. PNW sailor
    Joined: Nov 2017
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    Location: Seattle

    PNW sailor Junior Member

    It looks like the mast prebend and the wind direction from the aft side coming together to get the mast flutter going.
    Do you know if the housing broke prior to flutter or because of it?

    Personally I'm a fan of unstayed carbon rigs, I did a lot of work on Freedom yachts back in the day.
    Scary to climb, but easy to tune
     
  9. trip the light fandango
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    trip the light fandango Senior Member

    I don't know, but considering its pair is fine it must have copped some force it wasn't designed for or the housing wasn't built correctly. I'm a fan of these masts too, longevity and simplicity. There are plenty of examples of broken stayed masts.
     
  10. Burger
    Joined: Sep 2017
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    Burger Junior Member

    If I was building a cruising cat, I'd be carefully comparing costs/build time/weight between a conventional fractional stayed rig and an unstayed Balestron rig.
    The ease of use, ability to spill wind gusts with mast bend or totally depower by releasing the sheet, low maintenance, better visibility fwd, and overall simplicity are all very compelling pluses for the Balestron rig.
     

  11. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    For sure. Handholds! And riggers, chandlers and sailmakers all love them as a source of ongoing income.

    Some (most?) cats need a jib to tack, especially in big seas when heavily reefed. Even then, they still get blown backwards, placing big strains on the rudders.

    As Phil pointed out, they are faster (not least because of the millions of hours and dollars spent refining them), assuming the crew know how to trim them and are doing it constantly. If this is your idea of cruising, then you probably get a kick out of being got out of bed to change a headsail in a rain squall at 3 am, so go for it. The unstayed sailor releases the sheet, goes below for a cuppa and waits for the squall to pass.

    Alik,
    It is difficult to calculate similar boats. Best I can do is the harryproa in this video. It is designed for max righting moment of 18,000 kgm, about the same as a 12m/40' cruising cat (5 tonnes weight, 7m beam). The proa has the same total length of hull, (15m+10m vs 12m+12m) and similar accommodation, though probably less fit out (outboards, not diesels in particular). The proa weighs 3 and a bit tonnes, plus payload of 1.7 tonnes. There is definitely less structure supporting the mast than if it was stayed. A couple of kgs of carbon tow around the deck bearing, an extra layer of 400 double bias glass between the beams and a metre of 100 x 50 cedar glassed in fore and aft of the bottom bearing. The carbon/glass mast tube weighed 120 kgs, about the same as an alloy stayed mast and rigging, but excluding the chainplates, forebeam, dolphin striker, traveller and all the fastenings and structure required to support these.

    In the second part of the video, the breeze picks up and the depowering flex in the mast can be clearly seen.

    Peter,
    I am only pointing out the advantages of one choice over another. You decide which is best for you, but base it on facts and reason, don't blindly follow the race boats (and bridge designers). Trying to keep the conversation on track is not slinging mud.

    Ttlf,
    In light air, extras are no problem. And/or a taller mast with more sail area so you don't need the extras.
     
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