A-frame vs single mast rig, loads & weight on a cat

Discussion in 'Boat Design' started by isvflorin, Nov 25, 2014.

  1. isvflorin
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    isvflorin Junior Member

    In the process of choosing a rig for my future cat. Would like to get some input from engineers and owners.

    The cat is a 10.5m, demountable, with a central pod. Initially I was keen on a 3 stay rig like Pic #3. I need a safe rig with the sail area broken down into manageable areas and loads. The 3 stay rig was presented to me be an experienced multihull sailor and it seems the right fit for my needs. But I am wondering about the structural benefits of an A-frame mast instead of a single mast. My main concern is the compression in the middle of the forebeam, the added weight and rigging (dolphin striker) needed to take that compression. I want as simple rigging as possible and it looks like the A frame has benefits in that area. I will not discuss efficiency here, except option #1 vs option #2. The cat is light and carries sufficient sail area to be fast anyway. I am not looking for the most efficient rig, obviously. I want the easiest rig possible, and would even take the weight penalty of the A frame if it is easy to sail.

    I'm looking for professional and non professional opinions regarding the complete weight of the rig and it's weight influence of the rest of the platform. I am aware of the fact that the A-frame itself is heavier, but I'm more interested on the overall weight of the entire system and it's simplicity. I believe weight savings are possible with the A-frame on the rest of the structure, lack of side stays etc, furthermore, the A frame would allow me to put instrumentation on its connecting spars. An added short mast for a radar an windmill needs to be taken into consideration on the global weight for a single mast. I will most probably not use furlers, or maybe only on the foresails.

    So I'm after rigging simplicity, multifunction and curious about global weight of the rigs (all rigging, loads influencing the rest of the platform, auxiliaries like windmill or radar mast).

    If you look at the A-frame rig with the twin forestay option #1, it can't get any simpler than that. In any case spars will be carbon, and it is worth mentioning that the central pod might carry quite some weight (people+heatpump+outboard+anchor+chain). I strongly dislike dolphin strikers because of its point loads. There might be an option #4 with single mast and twin forestays. Total SA is around 55-60sqm with a 12m long mast.

    So, what is your take on it ?

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  2. Skyak
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    Skyak Senior Member

    This has come up a few times. I think in the past it has been referred to as a wishbone rig -you can search the threads. Your nice model pictures differ a little bit -you seem to have a shorter overall height with many stay sails. Unique to your design, there is a lot of load on each of those stays which will mitigate the structure and weight reduction from the twin masts. Another significant consideration is that a mast with no sail behind it has much greater drag than one with a sail and the intersection of the masts will be draggy.

    My recollection of the 'best' setup was twin masts located over the inside edges of the hulls, leaning in slightly with a spreader and stays between them. In this design the mast is on the hull, the jib is on the hull (no added structure for stay tension, and the jib can close off to the hull) the main is on the mast for low drag. Sailing upwind and reaching the windward rig would be a sloop and the downwind jib could be added in light air -the lee main would be shadowed. Downwind it can make a pretty fierce three element foil with the two jibs and the lee main for sailing deep angles. If it isn't the best it is the best in my opinion.

    If you take your option one, replace the two sails aft of the mast with a single roller furled main (about it's luff) that could be moved from mast to mast while furled. Similarly you could have a blade on a roller and a screacher on another forward -each that could be moved to either hull, you would have great options on any point of sail or wind speed -with just three sails that are always hoisted. Could be sweet.

    Good luck!
    Mark
     
  3. upchurchmr
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    upchurchmr Senior Member

    There have been some very interesting threads with actual boats, either here or on Woodenboat forum.

    Search and keep looking, the best ones were a couple of years ago.
    Sorry I don't remember the names.

    What is the pod for?

    It would be very strange to have the major cross beam be in the center of the boat. The hulls will twist relative to each other then the fwd and aft beams will provide the majority of any resistance to twisting. The center beam will never pick up significant loads.

    Dolphin strikers are one of the most efficient ways to take out a bending load on a beam. If you have such a beam, it will be significantly heavier without a dolphin striker and more flexible.
     
  4. upchurchmr
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    upchurchmr Senior Member

  5. isvflorin
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    isvflorin Junior Member

    Thanks guys,
    I already read the other Aframe (wishbone) threads. I was hoping to get more input on the overall weight, maybe from someone than engineered such a setup. Most of the comments take the mast out of the context and say 2 masts are heavier than one, well that's obvious, but the problem shouldn't be looked that way, but globally.

    I don't care if the mid beam doesn't take much loads, the better, it is made as a huge box beam for handy stowage, helps distribute loads from the stays within the platform.

    Mark, why should I replaced the 2 aft stay sails with a single one ? I want smaller sail sizes, what is your reasoning for a single sail ? I'm not chasing performance but an easier rig with less loads in the sheets. Maybe there's something I'm missing about the single big sail (main) aft of the mast ?
    Don't you agree 2 stay sails are easier to manage there by small crew, or single handed ? Handling a 20sqm sail is much different than a 40sqm.
     
  6. gonzo
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    gonzo Senior Member

    You will increase the cost, weight and windage. It will decrease some of the loads. The overall performance will be less than a standard rig; particularly a wing mast. I think there is no obvious advantage. If high performance is not crucial, which you imply, bringing the shrouds all the way out and not using an overlapping jib makes a simpler and lighter rig. Also, it can be much cheaper because all the forces are reduced.
     
  7. isvflorin
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    isvflorin Junior Member

    I'm not sure what you mean by your second last sentence, maybe a drawing would help ?
    Also, your answer leaves a lot to guess, which version increases costs, weight, windage, decreasing loads, over which version or what type of rig are you comparing with ?

     
  8. sharpii2
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    sharpii2 Senior Member

    The forces are reduced because the stays base is wider. This means less tension on the stays, to hold the mast up, and less compression on the mast step.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Simply put, a bipod mast has considerably more windage and weight aloft, which hurts upward ability and the stability curve. A wishbone typically is a boom shape, usually a variant of the sprit boom. There's a half and a full wishbone, with the Freedom 40 being well known for it's full "bones" shown below

    [​IMG]

    Why some have called bipod masts, a wishbone is beside me, but some of the odder varants actually do look like an inverted wishbone, such as the aft mast monstrosity shown below.

    [​IMG]

    This is a half wishbone boom, with an obvious need for some slack in the topping lift.

    [​IMG]

    I've had good success with this arrangement on small craft, but my latest version of the gooseneck fitting has a built in boom stop, so it can't drop down into the deck (unless you want it too) and a purchase that keeps it's orientation with the mast, so no topping lift is needed. Eric Sponburg also has some success with this arrangement.
     
  10. RHough
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    RHough Retro Dude

    I think you have to compare any rig to a simple fractional sloop.

    It is a huge mistake to discount efficiency. Your safety may well depend on the sailing performance of the rig. All the options you present are heavier, more complex, and have lower performance than a simple fraction rigged sloop.

    Good luck, have fun and may god bless your effort to re-invent the wheel.

    :)

    Randy
     
  11. PAR
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    PAR Yacht Designer/Builder

    Agreed Randy, the various bipods and other contrivances haven't "panned out", to produce anything close to the more conventional approach of a Bermudian sloop. Cost, weight, performance, etc. are part of the process, when deciding what to employ.
     
  12. isvflorin
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    isvflorin Junior Member

    I'm not saying it is better, but I like to look at options.
    Performance - how good is it if you are unable to sail it properly and handle it safely due to it's high loads on the sheets, and hoisting the sails ?
    Is it easier to hoist a 35sqm mainsail or a 20sqm jib ? Is it safer to handle a 35sqm main or a 20sqm jib on hooks ?

    Loads on all the rigging, sheet diameter, block sizes, purchase, jammers - all that cheap on a high load sloop vs a lighter loaded 3 staysail ? A dumb 40mm block is around 30Eur, a cheap one, that is 1kg of carbon fiber. How much do your winches cost ?

    A mast than needs to take side loads (bending) from a mainsail is lighter than a mast that doesn't have to take side loads from a mainsail ?
    How much do your spreaders cost, the shrouds, all that hardware needed to keep a sloop rig up ? How heavy are they ? Is this part of your weight "estimate" ?

    Is it safe/smart to rely on diamonds for the integrity of your mast ? Are you really happy with that boom swinging over your head ?

    Is this cost and weight you are talking about an actual estimate you made on paper with numbers or is it just eyeballing it ?

    Would you really recommend a 57kg, 169cm person to single hand a 60sqm sloop rig with possibly a 40sqm main instead of 3 jibs (plenty of hoisting configurations possible, balancing), because the sloop is more efficient, "safer" , lighter and "cheaper" ? Offshore ?

    Out of all, I'd like an answer for the last question.

     
  13. PAR
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    PAR Yacht Designer/Builder

    I don't see a question in the posted quote above.

    You seem to be worrying about things unnecessarily. You don't have to use a conventional boom, you could use a sprit or a wishbone, both of which have very low sheeting loads and eliminate the need for a vang too.

    What we're basically are saying is these types of rigs have been tried before and found lacking in performance, have higher weight, an unacceptable amount windage and of course higher cost. This isn't speculation.

    Again, I think you're worrying about things unnecessarily. The sails you're talking about aren't very big, particularly using modern handling systems. The physical dimensions for the rig's fittings aren't very big and if cost is really a concern, than the odd rig, with twice the parts and weight of a conventional rig, the fractional sloop is the only real choice.
     
  14. RHough
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    RHough Retro Dude

    I wouldn't recommend that anyone take a home built 10.5M multi offshore if they think the sail loads on a little boat like that are too high to be manageable or that the mast and rigging for the boat would be too expensive.

    You aren't designing a 200ft square rigged vessel. It is a 35ft boat. You don't need complexity. You need simple and reliable.

    The basic rule I see people violate all the time is trying to outfit a bigger boat than they can easily afford. They end up with everything just a bit to small/cheap/weak and are plagued by failures. Smaller boats with slightly oversize top of the line equipment that is well sorted out don't have these issues.

    My sail inventory for a 35ft multi would be. Main with wishbone boom(for all the reasons PAR suggests) and 3 reefs. A good 100% self tacking jib with Pro-Furl roller furling (Pro-Furl's are serviced world wide). A fractional hoist asymmetric spinnaker in 1.5oz Nylon, a masthead hoist asymmetric spinnaker in .75oz. The 1.5oz fractional spinnaker can be used instead of a genoa close reaching in light air. In breeze over 12 true the full main and 100% jib will be easy to handle and easy to reef.

    To maintain quality and economy I'll bet you can find a complete mast and spreaders that has been professionally designed used. This will be a fraction of the cost of designing and building your own carbon fiber spars etc. The first set of sails can be used too.

    Out cruising the simple rig that everyone is familiar with will be cheaper and easier to live with. The will always be used sails and local sail makers that will be happy to work with the familiar. The farther from that you go the more you must rely on your own skills.

    I think a 35ft multi is too small to consider ocean passages. It is even a bit small for coastal cruising unless you enjoy a very spartan (backpacking) lifestyle. Hulls that are fast will be very weight sensitive and larger volume hulls will kill performance. Adding an unconventional rig will not solve any design challenge and will reduce any resale value.

    Cheers,
    Randy
     
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  15. Tom.151
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    Tom.151 Senior Member

    ((Well guys, I tried not to bite on this one, mea culpa.))

    Okay isvflorin,

    You asked for input.

    In this short thread you have the benefit of some very wise and experienced (except me) contributors - just research their other contributions.

    Then when you get good input you seem to ignore it and keep on touting your own preferences. If that's because you don't understand, then you should ask for clarification instead of regurgitate your preferences.

    BTW, the #3 rig you sketched has such obvious functional flaws that you do need to find input better than your "experienced multihull sailor". Just imagine what happens to the luff of the aft sail (and the trim of the middle sail) when you crank on the mainsheet tension. Arrrgh!

    Ta!
     
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