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

    Hey Tom, thanks for stepping in. I am here to learn and no I am not ignoring, but I have a hard time with input that sounds like 'believe me it is better this way'.
    I am digesting the inputs and making mods to the rig.

    There are people like me, that are not satisfied with 'it's tried and tested, it is better' etc, you know, the nutcase type. Let's discuss the luff for example. First, there is no mainsheet per se, no mainsail, the boom on the aft sail acts like a Hoyt boom, I don't believe sheeting it in a bit (A BIT) harder will slacken the luff. You also don't get that on jibs with Hoyt booms. So, to me it is not that obvious that it is a flaw. There is no need to sheet it that tight to slacken the luff. It is not a mainsail to flatten out.

    I was very eager to know some real numbers from people that might have engineered Aframe rigs. I really get that people love the sloop rigs, I do too, but I hate its drawbacks. And there are some considerable ones, therefore I try to see if there are viable alternatives. For now, nobody gave any precise feedback on loads and weights, which I was interested in. However it was briefly recognised that the 3 stay rig has lower loads and lower rigging complexity, making it cheaper, then more expensive options were recommended. I don't need more expensive furlers, even if they are serviced on the moon.
    But as I said, I am not ignoring the input. Thanks again for all your input. I currently have to hoist a 19sqm main on my mono, I still think it is a pain to hoist compared to a jib, and people are recommending me an even bigger main. Lots of points haven't been addressed. Another example for safety - the 3 stay rig carries more sail area with a lower CoE for a constant lenght of mast compared to a sloop rig. But I guess this is again unimportant for some...
  2. RHough
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    RHough Retro Dude

    What is your list of considerable drawbacks for a sloop rig on a 35ft boat? I can't think of any that would make me consider any other rig.

    Do you own Skene's? The loads on the rig are very simple to calculate. They are derived from the righting moment of the hull(s). Once the loads are known the weight is also simple to calculate using the engineering data of mast material.

    The loads are a function of the righting moment of the boat. It is very difficult to support a claim that the rig type reduces load and can therefore be lighter and less complex. I've only heard the claim of lower loads and simpler rigging from people that don't design and build boats or sailing rigs.

    The bottom line for simple and cheap is production boats. The price of the boat is not going to change much. The incentive to use simple, cheap rigs and gear to increase profit is great. When no production boat and no design with a good reputation for seakeeping and passage making uses less complex, cheaper rig you have to wonder why.

    A 19^2m main that is a struggle is not the fault of the rig type. It is in the details of the install. If you can't raise, reef, or lower the main single-handed on any point of sail you need a better rigger not a different sailplan. I have to question your logic when you condemn the proven sail handling safety of a furler system and claim you have no need for "expensive" furlers. Safe offshore boats are expensive.

    Not unimportant. Just wrong. What is it about three stays that changes geometry? Why is a lower CoE safer? Unless you have too much sail up and the heeling moment threatens to exceed righting moment how is CoE a concern? When the time comes to reduce sail area what systems will you use to make your three stay rig as easy and safe to reef as a fractional sloop with a roller furling headsail?

    There are very good reasons for the simple sloop rigs used on 95% of cruising boats in the up to 45ft and 30,000# size. 100's of years of experience has not produced a better, lighter, simpler, cheaper alternative.

    How long are you going to avoid the very real question of lack of safety due to poor sailing performance? You must have rig efficiency to make an offshore boat safe. Address that first.

    Just so I don't get accused of being closed minded:

    To make the case for a non-sloop rig you can start with the weight of the structure needed to support the loads and your requirement for a demountable design. Consider a bi-plane rig with self tacking wishbone booms. The Hughes 36' Cat2Fold is might be just what you are looking for. I know one of these that has been a successful coastal cruiser on Mexico's west coast. The only thing that needs to be changed is the mast height/sail area. The design brief called for masts no longer than the huls so they are too short to carry enough sail area in light breeze. I think a free-standing two-part mast would be a valuable improvement. The boat goes upwind and down surprisingly well. It suffers in reaching conditions where more area would help.


  3. isvflorin
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    isvflorin Junior Member

    Thanks for your patience Randy. I am considering the biplane actually, I am considering all rigs and trying to see the pros and cons and what would fit me better. What I don't like about the bi-plane is that it seems to me it is not easy to balance the sail trim in order to float atrim while sailing, I have looked at them how they sail and the bows either go down or up, upwind/downwind. With the sail area spread out a bit you can adjust that balance, at least I assume so.
    I like its simplicity though, I have drawn one already for the boat and still looking at it's pros and cons. I also heard from people that it might have a weird motion as one sail always pushes more then the other and they don't work in tune very well.

    About the sloop rig, I think people got used to these already and claim these are so normal and good to have, but I don't like at all: swept spreaders having high loads on the shrouds - high point loads, large sails compared to rigs with smaller sails for same mast height, high loaded side stays to keep forestay tension=high compression on mainbeam, relying on diamonds for mast integrity, heavy mast as it is required to take side loads from mainsail and point loads from forestay (if fractional), mast has to take S bending if fractional, high point loads need beefier fittings=more expensive, high loads(sheets, shrouds, mast needing to take S bending)=heavier. On a cat, you have the added point loads in bows needed to support forestay with a sprit, loaded bridles etc, it's associated extra rigging wires, its weight, more stuff that can brake.

    I am old school, I will not use furlers unless I really have to, they can jam, or release a lot of SA accidentally, are expensive, heavy and rattle. I will use staysails with hooks on a wire instead of those heavy things. If you need to drop SA immediately just release halyard, and hook on a smaller stay sail.

    I am also looking at a masthead cutter variation, with sail area distributed more or less evenly on the 2 staysails and main with a tacking inner stay on a track.
  4. Skyak
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    Skyak Senior Member

    Sorry I have taken so long to respond.

    The reason I recommend going from two staysails to one main is reduced complexity and easier handling -particularly under duress. Each stay requires high force to keep the luff straight -but the whole idea is to save beam weight. A large roller furling main is much easier to manage than two staysails and its luff can be held close to the mast -minimizing the structure to hold tension. Aerodynamically the best place to put the main is behind a mast for minimal drag, and downwind one large lower aspect sail is better than two higher aspect sails half size. The advantages are more obvious when you consider different points of sail and wind speed. Different ways of getting sail area are not equivalent. The aft portion of any sail rig always wants to be flat and in an exact position.

    I figured your nice graphic question would stir up very loud doubters. The tough point to beat is that catamarans need substantial beams just for wave forces so the weight added for centered mast compression is only a dolphin striker -not much.

    I maintain that my proposed sail setup would be very manageable and very effective, costing less than conventional sloop and much less than the biplane it is most similar to. If you are game we could add detail and press the experts that have assembled.
  5. RHough
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    RHough Retro Dude

    I think you will find that multis are much less sensitive to sail balance than monos. I'm not sure if you are talking about weather/lee helm or up/down hull pitch.

    Weather/lee helm is a function of getting the sail area placed correctly for and aft. Once that is done you want it to change as little as possible when you change sail area.

    As far a pitch goes this is a holy war subject ...

    All boats trim down by the bow when under sail. They must. The drive from the sails acts above the water. The largest drag is in the water. There is a net bow down pitch in every sail driven vessel. (let the flames begin).

    My friend Brian that owns Cat2Fold has never complained about sail balance problems. If you would like to get his opinion after cruising his boat for three seasons in Mexico I can introduce you.

    The loads don't change. They are concentrated and distributed differently in different rig types. But the total load has nothing to do with the rig. The total load is from heeling and drive forces resisted by righting moment and drag.

    Compare two sails. One set on a mast and one set on a wire. When you calculate the structure needed to maintain luff tension in the free-standing sail there is no weight savings that anyone has ever shown with data. The mast section of a sloop does not have to be increased to handle any side loads from the sail. The structure required to keep the mast in column is where the weight is. The masts in an A-Frame will be very nearly as heavy or heavier than the mast and rigging of a sloop. A 40ft a-frame with no stays will require either too much weight or too large a diameter. This puts more weight aloft unless the rig is custom carbon fiber and very well engineered. That would negate any cost savings. You would still be left with a rig that does not sail well.

    This is not correct. The only way to safely handle a hanked on sail is with a large crew or with a proper downhaul system if short handed (these are much more failure prone than a modern furler). If there is no sheet tension the clew of the sail is a rattle snake waiting to foul deck gear and toss crew off the deck. If enough sheet tension is maintained to keep the clew from being deadly there is too much friction on the hanks to get the sail on the deck. The sail is out of control while not hanked to a stay. This means one out of control sail during *any* sail change. You don't just drop a halyard and hook on a smaller sail when the wind is 25+ and waves are washing you and your sails off the boat. I've been there and done that. It is not fun and certainly no where near as safe as a well maintained furler. You need at least four crew to safely change a hanked headsail. You need a driver (auto won't do the job). You need a person on the halyard to control the sail. You need a person at the tack to pull the hanks off as the sail comes down (you already have your new sail hanked on and lashed to the deck ready to raise). You need the 4th to control the loose sail on deck. My race boat has hanks. Makes the rig lighter and faster. The boat is always sailed fully crewed. Short-handed the furler goes back on ... always.

    You do not want a masthead rig in any way shape or form. The sailing dynamics of a masthead rig are 100% wrong. They are made worse with multiple headsails. They generate many of the high internal loads you describe. Modern fractional sloops do not.

    I get the feeling you don't understand the sailing dynamics of a modern fractional rig. Most old school guys (including me) had to unlearn things we took as gospel. Once you see the light you would not be considering lesser options. There is good reason that they are used for high performance dinghies and they are one of the big steps forward in rig design since WWII. When you consider the cost of production carbon fiber spars and synthetic rigging it is extremely hard to fault a fractional sloop on any boat sailed by 1-4 people.

    The free standing biplane rig has some of the advantages of a fractional. But none of the advantages of a sloop.

    The only thing I see as a challenge is the structure to handle mast compression on a catamaran that demounts for trailering. My guess is that total sailing weight will not change and extra mass in the main cross beam is a better place than in the ends of the hulls.

    IMO cruising should be relaxing, safe, and fun. In boats under 40ft or so my choice would be a tri rather than a cat for more usable volume. Over 40 ft a cat starts making sense. The flexibility of a modern fractional sloop rig fills the relaxing, safe and fun bill to a tee.
  6. Spiv
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    Spiv Ancient Mariner


    I have scanned through this thread, apologies if I have missed some comments.
    I know that my opinion is not shared by some much more active members, however I have been sailing more than 50y and been living aboard 4y and experienced most conditions.

    I am a declared mainsail hater, the reasons are many and I have mentioned it elsewere several times.

    I have studied all "A" frame rigs that I have seen both in the water and on the "Net" and of course one can find faults in any new rig or idea.

    From a 'cruising' point of view, I would definitely use furlers, they are reliable and you can handle big sails yourself, both upwind and downwind. Perhaps get 'one size up' if you are concerned.

    As far as your three options, I like No2 with 3 furlers.....
  7. JosephT
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    JosephT Senior Member

    I'm with Spiv. Some "reliable" furlers on a Bermuda rig go a long way for ease of use. It doesn't get much easier to reduce/add power to sails to ensure the rig doesn't get overloaded.

    The alternative: Change sails all the time in variable conditions.

    I've done the latter and can tell you furlers are the way to go.

    As for adding more, smaller sails, this only multiplies the amount of work, deck & mast rigging. To keep things simple go with a reliable mast, reliable furlers and perhaps consider routing the lines to the cockpit for easy access.
  8. JosephT
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    JosephT Senior Member

    Also agree with Randy on that Cat2Fold design. Looks quite simple to manage for a CAT. Only possible downsides:

    Cost: Twin masts & main sails would seem to be more expensive.

    Damaged/lost sail: What if you damage one of the twin sails and can't use it? You can limp along with one and perhaps add extra rudder to port/starboard to counteract the imbalanced sail plan.

    I would rather go with a single main & jib. A scaled up Hobie Cat if you will. It's a matter of preference though so you'll have to decide
  9. CT249
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    CT249 Senior Member

    For some reason your rig sketches don't show up for me. However, aren't there some major issues with the concept of twin forestays leading to each bow? For one, if your forestay is so close to the leeward gunwale, how do you get the sheet lead far enough out on a reach? If you're sheeting to a position only a couple of feet to leeward of the forestay, you can get the leach of the sail curling back in to windward which is pretty slow and increases heeling moment. And isn't the main more likely to interfere with the airflow over the jib downwind?

    Secondly, if the forestay is inclined to windward as in the twin-forestay rig, the force of gravity will tend to make the jibs collapse in light winds. OK, you may be motoring in such winds, but it may be a significant issue on those nice light wind days when cruising can be at its most relaxed.
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    isvflorin For a 35 feet catamaran there is no need to enter in complications. We have tried a lot of combinations in the 80ties on multihulls; the simpler, cheaper and more efficient solution is a sloop; fully battened main sail, automatic jib, gennaker (or asymmetric spi) on a furler. You can suppress the furler if you make an "avaleur" like that one used on the dinghies for the spinnaker but that takes a lot of place on the deck and asks for a lot of ropes.
    My main experience was on the racing F40 cats and tris at the end of the 80ties. A fully battened mainsail is a very well mannered sail. With the right hardware you can lift, reef, and control a 82 m2 mainsail alone with no problems. Pretty safe. With a self tacking jib, tacking upwind in a 30 knots breeze is a pleasure. No strength required, no thrills, just push the steer bar.
    The devil is a the quality of the hardware
    1 person likes this.
  11. isvflorin
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    isvflorin Junior Member


    Thanks guys for chipping in. Haven't updated this, but another rig will be on the boat. There is no point to agree or disagree with the aforementioned opinions as they are all valid or non valid in certain circumstances.

    I also hate the hardware aspect of mainsails, I love them otherwise. probably the most interesting thing about this cat is that any rig can be put on it, by the owner, it can be easily changed and it can provide several alternatives. I believe flexibility is very good to have. I haven't mentioned but the hardware will be produced in-house, blocks, tracks, etc. Although things can change again, the chosen rig is a cutter variation. Kinda like a child of a cutter and an aft mast rig. There will be a commercial brochure available in about a month or so, hopefully before JEC Europe in Paris. The cat carries around 65-68sqm of sail area split in 3 sails, most of the time, probably 90% of the time she will be sailed with just 2 sails, around 41sqm , max displacement is 2.7t. If anyone wants to race this boat, another rig can be placed on her. Otherwise she has plenty of sail area for cruising with easy reefing possibilities.

    Another important point to consider is usable space and how the position of the mast influences the creature comforts and accommodation. The chosen rig was a big part of the overall platform and design and not just an "engine" strapped to the boat. I find it displeasing cruising with a log in front of me and a boom that always have to take care about. Not to mention the height of the boom that will cut away quite some SA out of that sail.

    Anyway, I'm sure this cat will be a pleasure to sail relaxed while being able to enjoy the entire deck layout safely. The easy mode of sailing this rig will be just like a sloop more or less. If more SA is needed, one can deploy a foresail from a furler.
  12. zaca60
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    zaca60 Junior Member

    I think Tom.151, Rough and Par wrote what i also would have written; and here is how to do it cheap, with low windage, light and DIYable:www dot yacht-mast dot de
    I am in no way affiliated to those guys, but this would be MY way to go.
  13. Pericles
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    Pericles Senior Member

  14. sharpii2
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    sharpii2 Senior Member

    Compression and bending

    I can understand the appeal of such a scheme.

    I came up with something in the same neighborhood a year or so ago.

    The idea was to fly a balanced lug without the mast intersecting it on the bad tack (see attachments). You can see the evolution of the idea, to the point of adding an internal spreader, along with wire cross bracing, to make the unsupported mast panel as short as possible.

    The solution was to put a mast on either side of the sail, which would be hung from a sturdy cross piece.

    One of the duties of this cross piece was to keep the masts dead vertical, so they would not experience any bending loads. They had to be stiff enough to stay in column, without any stay bracing in between them.

    The argument was that the yard could be peeked up about 45 deg. allowing some Sail Area (SA) above the cross piece, so the mast could be somewhat shorter than on a more conventional rig.

    The problem is the math simply doesn't work. The two masts are significantly heavier than a single one, even without stays. And that's not even considering the weight of the cross piece, way up on top of the masts.

    The problem with an 'A' frame arrangement is that the masts are not only in compression, but are experiencing a good deal of bending loads too.

    And this is with a proposed rig which requires really tight luff (stay) tensions to work well.

    The two masts are likely to bend outward, like a bow-legged cowboy, loosening such luff tensions, spoiling the set of the sails. They could continue to bow outward until they fail. And with a multihull, this could happen with a sudden squall.

    Making them stiff enough to withstand this, with reasonable fore stay tension and a reasonable safety factor, is going to require both large section areas and massive wall thicknesses.

    To keep the weight down to a reasonable amount will probably require custom carbon fiber construction.

    Not saying it couldn't work. Just saying I agree with the pros here that the fractional sloop, with high end furling gear, is probably not only the best solution, but probably the most economical as well.

    Even a mast aft rig, with a pair of jibs would work better than what you propose. A masthead cutter rig may be better still, and would meet most of your design goals.

    IMHO, the main reason the fractional sloop rig is used on multihulls is to reduce bending loads on the cross beams (akas). The shrouds mainly hold the mast up, especially after the small jib is struck. This is much easier to do, from a structural stand point, than maintaining a decent luff tension on a stay-sail as mainsail.

    Basically, what they really have is a cat rig with a leading edge flap (the tiny jib). This is important, not just for structural reasons, but because of the higher speeds these boats sail at. These higher speeds create what is known as apparent wind effect. This is when the higher boat speed causes the wind to appear to shift forward of the boat, even on a reach. So, in order to work reasonably well, the sail has to have very good upwind ability. This is why such boats have huge mains, which resemble airplane wings.

    A mast aft, or even a masthead cutter, would be a poorer second cousin to the conventional fractional sloop, performance wise. This is because each added sail has to operate in the back wind of the sail in front of it. If the boat is going fast, the lead sail is already operating in an upwind situation. The one behind it must be able to operate even better up wind than it has to.

    But, for a multihull with more modest performance expectations, such may be acceptable.

    Attached Files:

    Last edited: Feb 5, 2015

  15. isvflorin
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    isvflorin Junior Member

    Probably no need to discuss further if the rig is taken out of context and the only remaining argument is efficiency. I agree with you on efficiency. The chosen rig as a masthead cutter, with the mast quite aft. The foresail prod swings to leeward to keep the staysail in clean air, the back staysail does the opposite to be in clean air too. For lazy sailing they are all centered. Reefing is ridiculously easy and safe just furl sails. Sails can be adapted anywhere in the world as they are jibs only, if needed. Second hand sails are everywhere. The stick will take compression almost no bending, therefore light. As an alternative the back staysail can be a main on a wingmast. But then it will stay centered only. Or both can be possible. There is nothing new here, just canvas kept up by a stick, the shapes and size are a matter of preference, practically, efficiency and cost. We are not discussing 100% efficiency vs 20%, the margins are low. For racing a fractional sloop might be the answer, but there are many aspects to be considered, not just efficiency - redundancy, practicality of deck layout, hardware loads, ability to improvise a sail or getting a used one to fit etc.
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