WishBone Sailing Rig

Discussion in 'Boat Design' started by brian eiland, Aug 17, 2003.

  1. brian eiland
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    brian eiland Senior Member

    I was just looking back thru a few of these discussions and though i might re-emphasize that link you made to the other subject thread discussing these two particular wishbone shaped rigs.

    BTW Angelique, its very interesting how you come up with some of the reference links and photos that you do. It's a pleasure having you on the forums. Your contributions are most often worthwhile.
    Brian

    PS: I had not seen this photo before you posted it:
     

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  2. T0x1c
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    T0x1c Junior Member

    A-frame rig with off-the-shelf glassfiber poles

    See http://www.ikarus342000.com/VOYAGERpage.htm

    Sold by the US designer B. Kohler: "Every effort was made to keep the costs low. This is the reason I designed the A-frame mast which can be build from glass fiber flag poles. This will lower the cost for the rig, one of the biggest posts, by 70 %."

    In EU, EN ISO 12215-7 will soon ask for rigs to be designed to take 100% righting moment.
    What is the rule for boat designers in the US?
     

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  3. T0x1c
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    T0x1c Junior Member

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  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi TOx1c, welcome to the forum. Here in the US, we do not have a standard design load for multihull rigs (or the rigs for any boat for that matter, really). Every designer and builder is on their own and they have to justify their designs accordingly, if anyone asks. There are no enforceable standards.

    That said, most sailboat rigs for monohulls are designed, just by common practice, to the maximum righting moment of the boat (either estimated or actually calculated) plus a safety factor. Depending on the design, that safety factor can vary, typically between 1.0 and 4.0. For multihulls, a lot of designers use about 60% of the maximum righting moment of the boat, give or take, because multihull righting moments are so high. Racing multihulls will reach maximum righting moments as soon as they fly a hull, so using the maximum righting moment (plus a safety factor) makes sense. For cruising multihulls, on the other hand, they rarely if ever fly a hull, so the loads just never get up to the maximum. For these boats, designing and building to the maximum righting moment would make the spars and rigging extremely heavy and expensive--two things that are absolutely anathema to multihull sailors.

    Being a consulting engineer for a fiberglass flagpole/lighting pole company, I feel qualified to comment on your example of the boat with the fiberglass flag pole A-frame rig. Not a good idea, in my opinion. They may be cheap, but they are not particularly strong or stiff, at least for their weight. If you want to go A-frame, aluminum makes a lot more sense. For the same strength and approximate weight, aluminum is 5 to 10 times stiffer than fiberglass, and so, structurally, it is a better material. If cost is no object (which I am sure it is in this example), carbon fiber would be the way to go.

    I hope that helps.

    Eric
     
  5. T0x1c
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    T0x1c Junior Member

    Thanks Eric. This is what I thought. Still I am surprised that a US designer can offer drawings for a yacht with a rig likely to fall at first breeze, knowing how easily you folks go to court.

    The cat is 4 tons disp. with 122 sqm sails, it could easily fly a hull ! Which is not uncommon for some cruiser cats (Schionning, Lerouge, Gunboat etc). You don't even feel them fly a hull, as the heeling angle can be as low as 8%.

    A CE-certified rig has to be designed with a safety coef. 3. I heard Lagoon was designing at 60% RM, but there is still something I don't understand: With a coef. 3, this will be 180% RM, so the cat is still likely to capsize before the rig breaks!?

    I would be interested to know what percentage of the righting moment Malcom Tennant has taken for the aluminium A-frame of Catbird Suite. Is the owner still around?
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    I think most naval architects and experienced yacht designers exercise great care in designing their boats and rigs and can justify every decision they make. And it is precisely that we live in a litigious society that they do take great care. It is up to the customer to accept or reject that justification, and if he/she doesn't like it, at least in a custom design, they are at liberty to ask for a higher load or factor of safety. This comes with consequences: heavier weight and higher cost. Take your pick.

    For production yachts, again, the builder should justify the loads and safety factors to the customer if the customer asks. It's a serious matter, but I wonder how many customers actually do ask. The customer may elect to NOT buy the boat if he/she is not happy with the answer.

    I have not read the CE rig standard for multihull rigs yet, but from what you say, the standard requires 100% righting moment for load, and then a factor of safety of 3.0 to that? That means that Lagoon is not complying with the standard if they are only at 60% Max RM. But yes, generally you want the boat to capsize before the rig breaks. The thing is with multihulls, if you calculate the 60% righting moment and work back to what the wind speed is to create that moment, its howling pretty badly outside, and you're likely to be under severely reduced sail or bare poles. It's just too scary to keep all the sail up. Therefore, the rig loads drop way down anyway by reduced sail area.

    Eric
     
  7. brian eiland
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    brian eiland Senior Member

    I would suggest you click on his name and send him a private message, and email. I think you would find he will respond.

    Brian
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member

    Malcom Tennant died 5 years ago. But apparently the design office still operates.
     
  9. T0x1c
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    T0x1c Junior Member

    You're right Brian, I was being lazy.
    I've sent the following PM this morning, hope to have some answers.

    Hi High Tacker,

    On my catamaran, I am fed up having to harness to go on deck, front of mast or on boom hence taking risks and time to reef.

    The A-frame on Catbird Suite seems to me the best designed, as supervised by Malcom Tennant. But the new standard ISO 12215-7 due end of year should impose the rig to resist a righting moment of 80% with a safety coef. 3, hence 240%, for cats between 12 and 15m length*.
    Do you know what RM x coef. ws taken for the design of the A-frame for Catbird Suite?

    Kind regards,
    T0x1c

    * and 100% x 3 below 12m; at least it is so in the draft version, but Beneteau and FP are part of the approval committee... Maybe they will veto this rule, or their cats will need huge masts.

    Edit: ISO 12215 is one easy way to comply CE certification
     
  10. sharpii2
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    sharpii2 Senior Member

    100% righting moment rig strength may be a bad idea for a multihull.

    Generally, if something is to break, you want it to do the least damage possible.

    If the choice is between being dissmasted or being turtled, I'd pick being turtled any day of the week.

    Since multis are so Beamy, there is less likelyhood of the mast ending up in the drink.

    after the gale or storm is over, a jury rig should be much easier to set up on a multi than a mono.

    If the multi capsizes, it's pretty much curtains until a rescue of some sort arrives.

    During the early years of multihull development, sheet release systems were tried, so if the rig became over burdened, the sheets would release, which would be the third worst thing to happen.
     
  11. T0x1c
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    T0x1c Junior Member

    Indeed, but at the end, I don't think that reducing the %RM would help?

    As written previously, If Lagoon take 60%RM for their cat, with SF (safety factor) of 3 it is still 180%. I know it is not that straightforward in dynamic mode, but the cat is still likely to capsize.

    So it rather looks to me like a way to reduce the SF from 3 to 1.8 !?
    The rig is subject to fatigue, so reducing the SF does not seem so good to me. Plus these cats are likely to be overloaded, which furthermore reduces the SF.

    With the right wind, any cat can fly a hull (yes, even a Lagoon 320!). But before arriving to max heeling moment and capsizing, it is a long route. I would prefer being able to fly a hull under a sudden black cloud stormy wind on an Atlantic passage, rather than losing the mast.
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    You can rationalize loads and safety factors any way you want, and in your suggested case, you end up at the same place by two different routes. So it doesn't matter which way you go, simply justify your thinking so that you can handle the boat appropriately. Don't let the rig get to maximum load. You'll see that squall coming in plenty of time if you are paying attention to the weather, so you'll likely have sail reduced, which greatly reduces the chances of the rig being overloaded. You can't design and build against ALL risks--the boat would be just too heavy and expensive. As I said, pick your philosophy of design and justify it.

    Eric
     
  13. brian eiland
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    brian eiland Senior Member

    Yes I knew Malcom passed,...such a shame. But he was asking about the owner of Catbird Suites who has participated quite a bit in the forums.
     
  14. sharpii2
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    sharpii2 Senior Member

    Most multi capsizes I've heard of have been caused by a combination of big wind and big waves.

    Your max righting moment assumes the boat is on a level sea. If you are on the face of a wave, especially one that is confuse or about to break, you may already be heeled quite a bit. Then add a gust of wind, and a breaker at the same time, and you'll wish you never thought of flying a hull.

    Another danger of flying a hull is that more wind can get under the wing deck structure and gleefully assist with the capsize.

    Ideally, IMHO, a multi should have a resilient rig which bends under excessive winds rather than breaking, or capsizing the boat.
     

  15. T0x1c
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    T0x1c Junior Member

    Draft ISO 12215-9

    Attached is an excerpt of Draft ISO 12215-9 - Sailing Boats - Appendages and Rig Attachements. I would appreciate to have your opinion on the mentionned design RM for rig calculation. Note: all units are ISO.

    The final ISO 12215-9:2012 does not deal anymore with rigs, only appendages. Which proves that the answer for rig calculation is not straightforward, as the corresponding CE standard is long overdue.

    Maybe I should open a new thread on this?
     

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