Unstayed carbon fiber mast on cruising catamaran?

Discussion in 'Multihulls' started by Nico Crispi, Jul 15, 2017.

  1. catsketcher
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    catsketcher Senior Member

    Nico you are wrong about traditionalism, there is no conspiracy.

    On my cat I have a wishbone boom, replaced the steering system, tried two different cabins, three different motor set ups and more but have never wanted to go stayless.

    I have been a state champion in Lasers so I know about unstayed rigs but I don't want one on my cats. You can get gust dynamics from sails design, square tops etc or you can understand that cats with their huge stability do not need gust response in the same manner as monos. Cats almost never get to their maximum righting moment and so gust response can be handled by the increasing stability of the cat. My laser had no better gust response than any stayed rig I sailed. Some sailboards do get great gust response but their sails are quite different at the head from a normal cut sail.

    Sail a heap on a large range of cats before thinking there may be a conspiracy. You will find that there are good reasons for different rigs.
     
  2. redreuben
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    redreuben redreuben

    Nico;
    You should check out the Cirrostratus trimarans by Robin Chamberlain, they come up for sale now and again here in Australia in fact I think there is one on Gumtree going cheap as it needs work.
     
  3. rob denney
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    rob denney Senior Member

    Big issue. The smallest bury we have used was 7% on a heavy 35' cat without a bridgedeck cabin. The mast at the deck bearing required a fair bit of extra laminate. Which is why it is better in the hull, if you can live with the looks and assymetry.

    Catsketcher,
    No one mentioned a conspiracy, just pointed out that there are a lot of things to fail on a stayed rig.
    I'm not sure whether a Laser rig or a windsurfer rig has less in common with unstayed rigs on cruising multis. Suffice to say that neither of them has enough to make a comparison worthwhile.
     
  4. redreuben
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    redreuben redreuben

  5. UpOnStands
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    UpOnStands Senior Member

    from the Wyliecat site
    "For those bent on performance - and it must be underscored that the exact point of boarding a Wyliecat is to go for a good sail - carbon fiber is the wave of the present and future. The all-up weight of the 30's freestanding (no spreaders, shrouds, or attendant fittings), all-carbon stick is a mere 130 pounds [59 kgs] . The wishbone, with carbon end fittings on 3-inch -diameter aluminum tubes, adds another 35 pounds, "
    That is for a 48 foot mast (possibly 8 ft of which is bury), 400 sq feet 100% main sail area, Light air main sail area 590 sq ft.
    Wyliecat Performance Yachts: One Cool Catboat http://www.wyliecat.com/about/about_cool_catboat.html?contentid=1057
     
    Last edited: Jul 20, 2017
  6. catsketcher
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    catsketcher Senior Member

    This is what I was referring to Rob
     
  7. UpOnStands
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    UpOnStands Senior Member

    For the design/engineering gurus, that 48 ft mast (8' bury) seems awful light at 59 kgs. But, consider a 32 ft cat with 16 ft BOA, weight of 5 tons. If that mast was able to withstand the full righting moment of the cat but with massive deflection, would the mast pass the generally accepted design rule?
     
  8. catsketcher
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    catsketcher Senior Member

    I am sure that a mast can be engineered to cope with the cats loads but they will be far higher. It is usual practice to design a mono mast for maximum righting moment - often at 30 degrees. The Wylie 30 will have far far less righting moment than an equivalent sized cruising cat. Therefore the cat's mast will have to be designed much more strongly. Max righting moment at windward hull lift. Interestingly, it gets hard to design a cat mast properly for above 40ft heavy cruising cats. In an article he wrote about mast design Chris Mitchell. who designed IACC masts, said that you start chasing huge loads if you design to lifting the windward hull with heavy large cruising cats. In this case some designers say that the sails will be shredded and design the mast to cope with this load.

    rig design hints http://www.aes.net.nz/info.html

    cheers

    Phil
     
  9. jorgepease
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    jorgepease Senior Member

    The argument I have read for unstayed is that you eliminate the weight of the extra rigging so even though the mast is heavier it's a wash. That from Rob Denney who personal experience with these on cats.

    Unstayed depowers so if designed right it's not going to break, though for that same reason, it may not be able to fly a hull ... I don't think many cruisers are interested in flying hulls though.

    Main thing is it's been done and spoken well of so that speaks for itself.
     
  10. UpOnStands
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    UpOnStands Senior Member

    Let's look at the Wyliecat 30. Casual light air sailing with full main up. Sudden gust slams the boat. The boat has considerable inertia so I must assume that the 59 kg mast is capable of carrying the shock loads.
    Think I would design for the loads expected from the sails and depend on the mast bending to spill the mainsail and release loads. Lots less weight/cost etc.
     
  11. rob denney
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    rob denney Senior Member

    The cat has righting moment many times higher than the mono, so if the mono mast is built just for strength, I doubt it will be enough. If the mast was able to fly a hull, plus safety factors, it would pass the generally accepted design rule.

    Almost always, unstayed masts are stiffness dependant. That is, if they are stiff enough they are strong enough.

    Diameter is the critical dimension for stiffness. The mast on the harryproas in the earlier posts could have been 300 dia/109 kgs, 350mm dia /90 kgs or 250mm dia/129 kgs for the same stiffness. Not very big difference in actual weight or diameter, but large percentages. These boats have rm of 18,000 kgm. Your cat has 12,500 kgm. Roughly speaking, your cat mast will weigh about 2/3rds as much, less the height difference, which is not far from the Wylie cat mast weight. However, I think it would be pretty bendy, which would result in not enough power.

    The mast is forward of the cabin on the Wylie cat, I suspect the bury is closer to 5'/1.5m. The Wylie masts are built by Ted van Deusen, who is a very interesting and knowledgable bloke, using a braider which is an amazing bit of gear. He is on the short list to provide the masts for the C60 CRUISER 60 (revised) – Harryproa http://harryproa.com/?p=1747#more-1747 being built in Peru.

    Jorge,
    the weight of the mast is a wash vs stayed aluminium, but the cog is lower on the unstayed, which is arguably more important (pitching vs speed). If the cat with the unstayed rig was designed for it, the overall weight of the stayed rig would be appreciably lower.

    Catsketcher,
    An unstayed mast designed for less than the rm on a heavy cat will break at a higher windspeed than a stayed one designed to the same parameters. Why? Because the mast on a stayed cat has no 'give', whereas an unstayed one can flex and depower.
     
  12. UpOnStands
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    UpOnStands Senior Member

    The Whyliecat page states that the luff of the main is 40 ft and the photo appears to show the luff right at deck level. Then again 8 foot seems rather a lot for the Whyliecat 30. You could be right.
    But back to design, stiff enough for what? Is there a rule of thumb for stiffness?
     
  13. rob denney
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    rob denney Senior Member

    If you ask a sailmaker, the answer is always "as stiff as possible". The mast on the 15m harryproa in the videos was designed to deflect 300mm with 50 kgs hanging off the tip.

    At tip over load with full sail, deflection is 3m, 17%. At 20 knots of true breeze, 29 apparent, 1.85m, 20%
     
  14. UpOnStands
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    UpOnStands Senior Member

    for rob, thanks for the details. As always, you have the facts.
    so roughly one foot per gal.
    I imagine that seeing the tip deflected by 3m would be enough encourage reefing. :eek:
     

  15. groper
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    groper Senior Member

    is this really an issue?
    I would have thought that simply a slightly tighter sheeting angle on the lee sail would be all that's required?

    on this deep point of sail, the biplane rigs can be sheeted all the way out to near 90deg (further than a Bermudan main due to the shrouds getting in the way) and then trimmed out with very little twist so the entire sail has the correct AoA. The lee sail sheets slightly further in to account for the more forward apparent it sees from the sail preceding it. unless the sails are stalled - then there should not be be any "blanketing" and the lee sail would never be entirely useless...

    With a std bermudan stayed rig on a deep sailing angle - the main sail is pretty much stalled in the lower parts and only the top part of the sail flys properly as it can twist off further without the shrouds getting in the way. You don't have this problem with fully rotating mast and sails...
     
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