Single main sail with unstayed mast on cruising catamaran?

Discussion in 'Multihulls' started by valery gaulin, Jan 5, 2017.

  1. Ilan Voyager
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    Ilan Voyager Senior Member

    Yes, the problem resides with wings masts, stayed and unstayed.
    Round or similar masts have no portance or lift so no problem.
    But if you go to all the efforts, pain and money for unstayed masts on a multi you'll end with wing masts, it's too tempting...
    On a catamaran logic dictates twin masts as the lone serious place to install them are the hulls, so you'll have twice the trouble and twice the cost. As each mast must stand the whole stresses induced by the boat (unless you want to see the two masts fall down successively, some have seen it) you'll have almost twice the weight and twice the parasitic effects on the stability, longitudinal and lateral plus the mooring problem of a wing mast.
    If after all that pain you have the fastest boat of the west of La Trinité, the terror of the regattas of the Morbihan's gulf, so the federation has to calculate a new handicap or to ban the twin masts... ok, victory is the best reward.
    Because the worst of all the affair is that the results are not outstanding. The concept has been studied by able guys like Gilles Gahinet and Gilles Olliers, the well known NA. That gave the catamaran Edonil in 1980-81 which never showed any real good potential and which is now a cruising cat with one mast...
    I love unstayed masts, but on monos because you can get good results, I love wing masts (I made 2) on racers, but they have little to do on a cruiser.
    Applying the Occam's razor principle on the study of the rig of a 30 feet cruising catamaran, you see immediately that a simple sloop rig, alu mast, fully battened mainsail, solent jib and a gennaker with the right hardware will do an excellent job for the best price without pain. Results and performances guaranteed 100%.
    That works also on much bigger boats...
     
  2. UpOnStands
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    UpOnStands Senior Member

    sounds like a classic strawman argument.
    OP wants a single unstayed mast. No mention of wing.
    Lets imagine a single unstayed mast, regular sail arrangement works? well, poorly, not at all. Why?
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    The strawman is a retired naval engineer, worked on fast patrol boats and frigates for the French navy, also fishing boats in wood and composites until 160 feet and closer to sail boats worked on the procedures of the definition of representative norms for naval composites. Thus "he engineered" for some NA who designed a big bunch of racing multihulls of the French Flying Circus. He was part also in the technical team of two Class C and also has been consultant in the aerospace field.
    And some decades ago before resuming studies for being engineer, the strawman was a naval carpenter, who earned his master from the Compagnons Du Devoir et Liberté after 4 years touring in Europe in six different shipyards.
    So the strawman is beginning now after a few years of zen meditation during his retirement to have a global idea how works a boat, specially multis, field he had worked in for 18 years at the highest level.

    Take care of what you write, there are a few guys in the forum who are more qualified than I am. Some did the America's Cup and not once, some are very experienced builders. Some are excellent NA with many successful designs. At least 2 are Phd engineers. Beware they could be less gentle than me.

    Please think twice before writing something like your last post. But I'll be a nice old daddy and I'm going to give in simple words the factual reasons why an unstayed mast it's a false good idea on a catamaran, so you will learn something.

    I'll write it for the the third time in this thread. Catamarans have a peculiar curve of stability. The max stability is when one hull just barely leaves the water. Something like a meager 5 degrees of heel. After the righting moment diminishes sharply and you can say that at 35 degrees the cata will capsize on flat sea. More the rig is tall and heavy, the angle of capsize will be smaller. So the stability starts maximal, that the important point. Add the fact that multis have fast reactions. Keep that in mind.

    Monos and tris have very different curves. Monohulls are "tender", and the max righting moment is very grossly at 90 degrees on a old fashioned mono. Note that the curve is very soft, gradual, gently rounded.
    So you can calculate for a mono an unstayed rig rigid enough for example until 20 degrees, and having such a bend that he will empty the sail at 25 degrees for example to stay in a comfortable zone of sailing. Monos have slow reactions thus being more forgiving for the sleepy skipper.

    Inevitably you'll need a very rigid mast on a cata to take advantage of its initial stability, unless you sacrifice badly the performances of the boat by spilling out the wind from the sail each time the cata tries to accelerate. You'll get a floating pontoon not a sailing cata.
    Rigid means heavy even with carbon fiber. And this rigidity is useless as security valve, the righting moment is already going down fast, but the weight of the mast hastens the capsize. the angle of no return is smaller.

    In short words you have lost and performance and security. Not a good bargain.
    The true security on a catamaran is a fast release of the main sheet. To be more subtle you can work on the angle of the sail and the tension on the sheet. That needs a beefy hardware, blocks on ball bearings, big track with double chariot on rollers even on a modest 30 feet.
    With this good investment in hardware, you can command fast the main sail and open it within 2 seconds. That's efficient, you have the power and the security.

    For me it's just enough to discard an unstayed mast on a catamaran, but there are more complications, a genuine Mexican telenovela.

    Catamarans are strange boats where the mast is placed at the worst place, structurally speaking, ie the middle of a beam.
    Happily a stayed mast works mainly in compression on its base, cats are wide so the rigging is largely open thus minimizing the compression on the mast. And as you have plenty of place between the cables you can use very large spreaders. You get something rather light, highly tunable and if well designed very reliable.
    As it's rather rigid (we won't talk of differential bendings controlled by the "arthur" lever of rotation, variable angulation of the spreaders etc... on a cruising boat) the shape of the sail is well defined and its fullness can be easily tuned. These masts are very easily tunable.
    You can even change the rake. With a little tweaking you'll get a rigging perfectly adapted to your cata, with a light and responsive helm and good tacking ability. They are not cheap, but within the possibilities of a 30 feet catamaran owner.

    It's not the case on a unstayed mast. Almost nothing can be changed easily. Any mistake of conception is almost impossible to fix unless spending a lot of money...He bends too much, buy a new one, The sail is too full, change it. You would like yo move a bit farther the center of the sail plan, it's a major refit of the base of the mast,

    But I have not finished. I told you that was a bad telenovela. It's indecently expensive for a 30 feet cruising catamaran, whose owner is not a billionaire.

    If you have read all the posts of the thread and if you have a some engineering knowledge, explain to me how to put a free standing mast in the middle of nowhere, ie a beam, or a roof designed to receive only the compression of a stayed mast. The means a major recalculation of the stresses induced by a free standing mast which has to be embedded in the cabin to minimize the shear efforts and most important to get a decent weight. But remain the torsional effects of such a formidable lever. Plus the materials and the work. That is pretty heavy, expensive and it's not 100% guaranteed...

    Add the cost of a custom made carbon mast ( do not forget that this mast must have a conic tapered shape, it's not a simple pipe) plus the "experimental" sail compared to an ali mast and its rigging...The result is frightening for most wallets.

    And the telenovela goes on inexorably into the worst. After all that, the catamaran is slower, less sure because of the weight of the mast, cannot be tweaked by simple means, but you'll always need the hardware for controlling the main sail because your life depends on.
    Nobody has never seen a unstayed mast catamaran with truly appreciable performances.
    And the only mean to know is to make the accurate VPP of the boat...I would like to see precise VPP and VMG polars. That must be sadder than a Spanish Crucifixion painting.

    If you have good rational arguments in favor of a single unstayed mast on a catamaran, I'll be happy to read it. Have a nice day from the strawman.

    PS Unstayed single and twin masts have been closely examined by serious NA and NE several times since 35 years. Not worth.
     
  4. joz
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    joz Senior Member

    Ilan

    With this discussion about unstayed masts does this have the same effect on boat under 6 metres mainly off the beach class sailing dinghies like the Hobies, NACRA, Kitty Cats, Mosquitoes etc in the multihull range as well as the Sabres, Impluses in the monohull range. (Sabres and Impluses are unstayed mast boats) or does it only effects boat over a certain lenght?
     
  5. valery gaulin
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    valery gaulin Junior Member

    Thank you for all the reply. It make sense to me now that it is mainly a feasabilitg issue. Aluminum stayed mast and a small solent to help tacking if the catamaran is stuck in Iron is hard to beat, cost wise, efficiency and simple design! It as bean done this way so many time, a proven design! But i cant help to think about a single unstayed mast on a catamaran! Like a love affair, not rational and it also always get you in trouble!!! LOL
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    You're welcome and thanks for the laugh...that reminds an old French song, My man. He treats me badly, he beats me, he takes my money but I loooooooove him.
    I have remarked that most sailors have strong masochist tendencies. Who with a rational mind would use the slowest, the most humid and the most expensive mean of transportation for going from a point A to a point B, being cold and seasick? I suspect that their private lives are with strong ladies in black leather outfits and high heels, with whips and harsh voices...
     
  7. UpOnStands
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    UpOnStands Senior Member

    relax, VG, very very few of us sail for purely logical reasons. Your preference is what makes you happy/satisfied.
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    On beach catamarans the problem is that a simple beam is totally inadequate and has not enough inertia to withstand the torsional stresses. Besides that become truly complicated for no gain. That will be a failure.
    On small beach tris, specially the smaller ones with no search of speed, but made for a gentle sure boat accessible to all, that can be very good and very safe. As you have a central hull and very moderate stresses it's an easy task. Before making a mast look for some used windsurfer rigs...
    On monohulls no problem, all who have been lucky to sail a Finn know all the possibilities of a good free standing mast.
    But the fact that simplifies the fabrication of the unstayed mast is that in the small sizes it can be made simply in wood. Some spruce or clear light pine, simple tools, some elbow grease and you can make it.
    With epoxy lamination is a breeze. Plus some carbon fiber UD and you can get a mast precisely suited for your needs, with a very affordable price at a decent weight. Just some work that a good careful amateur can do in a garage.
    If you like rustic on a simple small plywood skiff, you can use a good bamboo pole. That works.
     
  9. Ilan Voyager
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    Ilan Voyager Senior Member

    Politeness is a great social skill...when you learn that, life is simpler and easier. Lot a things can be obtained being polite So be careful with the words you use.

    A badly made choice is not a preference, it's a painful mistake.
     
  10. UpOnStands
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    UpOnStands Senior Member

  11. rob denney
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    rob denney Senior Member

    I have designed/built/sailed multihulls up to 70' with unstayed carbon masts.

    You do not want an unstayed mast if performance is your only goal and you have the ability and inclination to continuosuly tune and increase/decrease the sail area on your stayed rig. You have an unstayed mast because it is simpler to set up and sail with, safer in strong winds, and it can be lighter, cheaper and faster than a stayed rig.

    Simpler to setup: You put the mast in the hole and forget it. There is nothing to tune, nothing to maintain and nothing to check. The sailmaker should have no trouble with the mast bend as it can be checked and measured with the mast on blocks on the ground and a load applied to the end of the boom.

    Simpler to use: The sail can be raised, lowered and reefed on any pont of sail. This is particularly handy when running downwind and being hit by a squall.

    Safer: The rig can be totally depowered on any point of sail, in any wind strength by simply releasing the sheet, which is a big safety advantage on a shorthanded cat.

    Lighter: An alloy mast with stainless rigging will weigh about the same as a low tech (hand laid and vacuum bagged) unstayed carbon mast. However, if the boat is designed for the unstayed mast, it will be lighter (and cheaper) overall. No forebeam and striker, no headsails or furlers, (and their prodder, sheets, halyards and winches), no chainplates and their supporting bulkheads and no need to build the boat strong enough to withstand the rigging loads trying to bend it in half. An example of this is the 50' harrypra at https://www.youtube.com/watch?v=8chR6DAFjGA It's carbon mast weighs 120 kgs, the boat weighs 3 and a bit tonnes ready to cruise. Not all of this is due to the unstayed mast, but a significant part is. Mast and rigging on a cat with similar righting moment will weigh about the same.

    Cheaper: Carbon tubes are remarkably easy to build, either for an amatuer or a professional. 3 of us built 3 x 6m sections for a telescoping mast and 2 x 8m beams for a 40'ter in less than 2 weeks. http://harryproa.com/?p=424#more-424 No moulds or mandrels were used. Carbon has never been cheaper. Especially in tow and uni which is what makes up the bulk of the mast. The expensive stuff is the double bias for the off axis loads which can be replaced with glass at the cost of a few kgs. Even more expensive than the carbon is the track and cars. Unstayed rigs need neither.

    Faster: The ability to depower easily and safely means you can carry more sail in marginal conditions. eg at night (when most cruisers reduce sail significantly because it is too scary), downwind in squally conditions (no need to luff to reduce sail), and downwind if a gybe is necessary, where unstayed rigs can "granny" the main around the bow under total control rather than hauling in the mian and releasing the sheet for it to crash against the stays while hurtling down the face of a wave.

    Few points from other posts:
    1) Stability curve shapes are pretty meaningless for most cruisers as they never get close to flying a hull. Consequently, the unstayed rig is quite suitable. Not many multis with stayed rigs will sail at windspeed under working sails like the one in the video. Even fewer with as little interest from the crew. The second half of the video shows the top of the mast bending in stronger breezes which significantly reduces the height of the coe and the sail power.
    2) Most cruisers (and many racers) have trouble downwind, not up. Releasing the main is impossible as it hits the stays. Dropping the spinnaker in strong winds is far more work than simply easing enough sheet to provide the amount of power you need.
    3) Unstayed masts can go in the centre of a cat if there is more than 7% of the mast length available below the deck and not too many big holes in the cabin top. They can also go in the hull, which makes better sense structurally.
    4) Stayed masts have many individual pieces which need to be maintained and checked on a regular basis. If one of them fails, the mast falls down. Unstayed masts have nothing. If it meets it's design load (bench testing will demonstrate this) it does not need to be looked at again until it needs repainting.
    5) All wing masts have windage problems. Nothing to do with whether it is stayed or not. Wing masts were the "straw man", not the poster.
    6) If you had listened to the experts 70 years ago, you would still be using a wooden mast on your planked and caulked monohull. Techniques are improving all the time, and most NA's get their money from designing race boats, not from making cruisers cheaper. Listen to what they have to say, but weigh the theory against what has actually been done.
    7) Most catamaran designers state that their boats will tack under main alone. In my experience, if this is so, it is in perfect conditions. Therefore a jib, or properly designed rudders are handy.

    Happy to provide more information on any of the above if required.

    Let me know the specs of your cat and i will price/weight unstayed carbon mast options, both amateur and pro built.

    rob
    www.harryroa.com
     
  12. UpOnStands
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    UpOnStands Senior Member

    Rob, while you are here. What would be your opinion as to the max LOA for which a wishboom boom would make sense.
    OP is thinking of 30 ft or so.
     
  13. rob denney
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    rob denney Senior Member

    I can't think of any limit. Wishbone booms are almost as much of a boon to cruisers as unstayed masts. No traveller, multipart mainsheets, lazy jacks or struggling with too small boom bags. Plus the main sheet loads are reduced by about 90%. They can be a bit tricky to build, but infusing them in a simple box mould eliminates most of the hassle.
     
  14. UpOnStands
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    UpOnStands Senior Member

    ok, thanks for the input.
     

  15. joz
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    joz Senior Member

    Ilan

    Thanks for that
     
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