Idea: Simple, cheap bi/triplane rig conversion using a 'yard' - would it work?

Discussion in 'Multihulls' started by chris14679, Jan 7, 2010.

  1. chris14679
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    chris14679 Junior Member

    Hello everyone!

    I've seen interesting designs for biplane catamarans with an unstayed mast on each hull and just a mainsail on each mast, apparently these perform very well.

    My idea would (hopefully) similarly enhance a standard catamaran (or trimaran) with the mast in the centre.

    EDIT: See diagram in next post

    Firstly, rig a 'yard' hung between a pair of halyards going to the top of the mast, braced square across the boat at right angles and all under tension so there is no movement and no danger of pushing on shrouds, stays or the mast. The 'yard' would be equal in length to the beam of the catamaran and would go up between the forestay(s) and the mast, with some padding to protect the mast. (a long spinnaker pole could serve as a yard)

    In the most simple form of the rig, two large genoas would be set with the heads of the sails attached to the ends of the yard, the tacks to the bows of each hull and the clews to the sterns of the hulls. This would be the basic biplane rig. You could optionally set the original jib and mainsail on the mast and centre forestay to make it a triplane.

    Sailing downwind you could set a huge four cornered spinnaker-like sail on the yard in light winds or a traditional coarse (square sail) in stronger winds.

    Any comments on this idea? Seems like it might improve performance and reduce aspect ratio of an existing catamaran or trimaran without much/any cost (if you already have two or three jibs you could perhaps set the biggest on the downwind side and the smallest on the windward side, rather than making new sails specially.)

    Should the genoas be the same on each side or should they ideally be assymetrical (the upwind one be a different shape/size from the downwind one)?

    The enhanced version...

    On a bigger boat, to get the 'slot' effect on all three rigs of the triplane, you could set both a jib to the bow and 'mainsail' from each end of the yard instead of one big genoa. The 'mainsails' (Perhaps 'trisails' would be a better word) would have a vertical loose-luff going down to the foot of the shrouds. Although they could be set on booms it would seem simpler to have them loose-footed. This rig would have the exact same shaped sails as the conventional jib and mainsail but without a mast getting in the way of the airflow (except of course on the centre rig if set up as a triplane). With this setup, you could use your spare/old main (reefed) and/or trisails from the conventional rig.

    Anyone with a good spinnaker pole, plenty of spare rope and a spare mainsail/trisail and jib (two spare sets for triplane) could set up this rig and see how it performs with no modifications to the boat! If anyone fancies trying it for a fun experiment I would love to see photos and comments on the performance (I don't have a catamaran to try it on, yet).

    Potential cons:
    Extra wind-drag from the yard and braces.
    More complex to handle and trim the sails correctly.
    More weight at the top of the mast.
    Lots of jib sheets to change for a tack on the more complex version.

    Potential Pros:
    Lower aspect ratio for the equivalent sail area.
    Genoa-only version would be self tacking.
    Much higher maximum sail area to windward in light airs.
    More, smaller sails are easier to handle.
    In strong winds, setting only the windward genoa would move the position of the leverage so that the windward hull would never be lifted, only the leeward hull would be pushed down.
    Reduced risk of broach/capsize because one rig will blanket the other as you go broadside on to the wind.
    Option of a square sail is a big advantage for running/trade wind passages.
    More fun stuff to play with!

    Any comments on this idea?

    Chris
     
  2. chris14679
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    chris14679 Junior Member

    Picture beats a thousand words:

    [​IMG]

    Is there an obvious reason why this hasn't been done? As the diagrams show, the rigs don't blanket each other going to windward. The drag and weight of the mast+yard should be much less than having two masts.
     
  3. DaveJ
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    DaveJ Senior Member

    I can't see why its not possible, but one of the reasons why the aeroplanes went from bi/tri wings to single is that they where finding out (as technology grew) that the top wing was interfering with the lift from the bottom wing. The same might happen here. Look up the sailing books and they will talk about the sail shadow, the region where the wind is effected by the sails.
     
  4. chris14679
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    chris14679 Junior Member

    Thanks dave,

    There are already plenty of successful biplane cats including production boats, but they all have two unstayed masts.

    I assume avoiding the interference issue is just a matter of having sufficient distance between the rigs. Perhaps my triplane concept would suffer from interference, but the biplane setup ought to be OK in that respect as the distance is the same or greater than a proven two masted setup.

    The idea of the asymetrical example is to further reduce the chance of interference.

    On an aeroplane the wings on each side balance each other so it doesn't matter if they are longer. A boat is like a plane on its side with only one wing which tries to tip it over. On a boat the heeling moment is much less with two short masts than one long mast. Therefore a biplane setup has a significant advantage on a boat which would not apply to an aeroplane. A biplane cat can carry much more canvas and much greater total foil length in any given wind strength than a conventional rig. This should more than compensate for any interference between the rigs.

    My instinct is that there ought to be a lot less interference between the rigs on a biplane than there would be on a schooner or ketch where the flow over the second mast is certainly disturbed.
     
  5. DaveJ
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    DaveJ Senior Member

    Yer, thats a very interesting concept, I think i could get an idea like this to work on my hobie 18, but with 2 hobie 16 rigs. Let me know how you go with it.
     
  6. gonzo
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    gonzo Senior Member

    The sails would interfere in the flow when you are from a close to a broad reach. Those are the preferred and most efficient points of sail.
     
  7. rayaldridge
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    rayaldridge Senior Member

    I'm not sure you can really say that the unstayed biplane rig is "proven." As I understand it, even the Schionning biplane cat has had troubles.

    One problem I see with a yard is that it puts a lot of weight aloft, where it could contribute to excessive pitching. The fine hulls of multis are not naturally very resistant to pitching. Another problem, at least with the first approach, is how do you get those forestays tight enough?

    Thomas Firth Jones built a biplane cat. After a couple of years, he went to a conventional rig, for a number of reasons. You might find it useful to read what he says about it.
     
  8. chris14679
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    chris14679 Junior Member


    A reach may be the preferred and most efficient point of sailing, but it is also a very unusual point of sailing. In reality, as the boat speeds up the apparent wind comes forward and you end up close hauled.

    With this rig you would only set one set of sails on a reach. You could either set the original sails on the mast or just the rig on the windward side.

    I found that a similar rig already exists, but without using a yard (the rigs come close together at the top of the mast). It's called a 'Hitch Hiker' rig after designer John Hitch and is apparently quite popular in Australia but I can't find any detailed description of the performance, except an anecdotal reference to the 52 footer pictured ('X-IT') doing over 20 knots and averaging 14.5 knots on a long passage, although there was no mention of what point of sailing this was on.

    [​IMG]
    http://www.boatdesign.net/forums/multihulls/modular-cruising-catamaran-28446-8.html#111
     
  9. chris14679
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    chris14679 Junior Member

    The yard is only under compression, there is no bending force so it need not be very thick. Think of it like an extra-wide set of spreaders (in fact you could do it like spreaders in two parts if you rigged it permanently, but my concept would allow you to drop it down on deck in heavy weather). I don't think such a lightweight spar would make much difference to pitching.

    There is no problem getting shrouds tight over spreaders normally, nor is there a problem getting braces tight on a square rigger, so I don't see a problem getting the 'forestays' tight, providing the mast and step can handle the added compression load.

    I looked up the passage from his book and this was a very early example of a biplane rig before the concept was properly refined. It sounds like he got a lot of things wrong. He said the pointing ability was much better than a normal rig and it tacked much better than other cats, but it was too slow. Reading between the lines it sounds like his rigs were not big enough.

    His main reason for converting back to a conventional rig was because he couldn't reef the leeward sail safely, because it used slab reefing and the boom hung out over the water. That is simply bad design, he could have fixed it with a different reefing arrangement. My rig has no booms and could use roller furlers, or just good old fashioned 'drop and swap' hanked-on sails.
     
  10. Autodafe
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    Autodafe Senior Member

    Very cool design, it looks both efficient and easy to handle.

    I can't understand why people are so anti anything unusual in rigs, as in my experience the details of construction are far more significant to performance than the arrangement or number of the sails.

    I'd say it's a toss up here whether the improved efficiency over the hitchhiker is worth the extra weight aloft (either the cross beam has huge compression, or the mast has a lot of extra height) but aesthetically I prefer your design.
     
  11. CT 249
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    CT 249 Senior Member

    There's nowhere outboard of the forestay to sheet the leeward jib, so your sheeting angle is too far inboard on a reach or run.

    The proper athwartships sheet lead for the windward jib on a reach could well be right in the middle of the boat, where it may cause a significant obstruction.

    If you do move the jibleads athwartships, how do you do so quickly and easily enough to tack quickly down a channel?

    If you don't move them athwartships, how much efficiency do you lose?

    If you set the windward sheet up along the centreline of the boat and then want to tack, what happens to those near the flogging jibsheet as you go through the wind? Conventional booms allow sails to keep a reasonable shape when eased, while a centreline forestay automatically allows a jibsheet in a convenient site to work fairly well.

    Keeping decent forestay tension is an issue even without the inevitable stretch in the yard and it supports. Surely the compression load on the yard will be very high? Is this system going to be any lighter than a taller mast?

    Aren't there easier ways to put more low-aspect sail on a rig?

    Is it really easier to handle lots of small sails? The modern fractional rig short overlap sloop has only two or three sails and covers a very wide wind range with just a few tweaks on backstay and trim.

    "In strong winds, setting only the windward genoa would move the position of the leverage so that the windward hull would never be lifted, only the leeward hull would be pushed down."

    Sorry, but surely the heeling effect of a rig is independent of the lateral position? Isn't heeling a matter of force and its height? That's not effected by lateral position.

    "Reduced risk of broach/capsize because one rig will blanket the other as you go broadside on to the wind."

    What happens if you are using maximum sail on a 'broadside', with the leeward rig blanketed, and the apparent wind angle changes so the lee sail is unblanketed? Haven't you almost doubled your sail area instantly? Isn't that rather dangerous?

    Wouldn't the blanketing effect change regularly and considerably as the apparent wind angle responded to gusts and wouldn't that cause the heel angle etc to change radically?

    What happens in a sudden gust, when someone has to dump the windward jibsheet and it snaps, flogs and rattles along the centreline of the boat? A conventional rig keeps such nasty stuff out over the water and not on top of the crew and passengers.

    Have you seen the recent test results that show that an efficient mast does NOT 'get in the way of the luff' but actually improve the flow over the sail? From memory, some of the early test results that allegedly said that masts were terrible were unreliable, because the masts on the test rigs were massively out of proportion compared to the sails.

    In a way it's not surprising, because the most efficient sailing craft have masts and no jibs, therefore practise tells us that masts aren't that bad. The fastest small boat around a course (C Class cat) gave up jibs years ago, even before they went to wing sails. Boards held the speed record for decades, and they have masts rather than jibs. The fastest singlehanders (Formula boards, foiler Moths or A Class cats) all have masts and no jibs.

    Surely if masts were so bad, they wouldn't be seen on the fastest boats? If jib-only boats were better, wouldn't they go faster?

    I'm not saying jibs are all bad, because they can work tremendously well IN CONJUNCTION WITH A CONVENTIONAL MAINSAIL. But jib-only craft just don't seem to go, and the very critical flow over the headstay may be just one reason; gust response and the natural tendency of a headstay to sag in a gust, therefore making the sail fuller just when you want it flatter, are further issues.
     
  12. chris14679
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    chris14679 Junior Member

    Thanks Autodafe, glad you like the idea.

    My intention was that this would be a conversion for an existing rig, so the mast would already be higher than necessary for a biplane, this would give the extra height needed to hang the yard without an extreme angle on the halyards. Compression loading on the yard wouldn't be massively more than on a very wide set of spreaders.

    The leeward jib would not be set on a broad or beam reach. On a dead run the jibs could be backwards goosewinged (sheeted to the opposite bows) or a special downwind sail used as discussed above.

    My intention was that on the basic cruising form of the rig, the sail would be sheeted on the centreline of the hull, therefore self-tacking. Moving the traveller forward or back would give a lot of adjustment.

    I have often sailed monohulls with a trisail which is sheeted to the centre of the boat and self-tacks. It works fine like that even on a very broad reach. The sail just needs to be shaped slightly differently for best performance with a centre sheet.

    For maximum tweakable performance I suppose you could have two sheets on each jib with different leads for each tack, like on a conventional rig, or the sheet lead could be mounted on an athwartships traveller forward of the cockpit and companionway. Many cats already have a traveller there for the mainsheet. This would cause no more obstruction than the original main boom and sheet.

    I've short-tacked up a narrow channel with the mainsheet on an athwartships traveller and moved it on every tack without problems. If you believe moving the jibsheets athwartships will make a big difference then it would be no more difficult or obstructive than a normal mainsheet traveller. Alternatively you could have two sheets on each jib and tack them, just like on a normal rig. Personally I wouldn't bother, I think for most purposes a centreline sheet lead will be a lot less hassle and will perform OK.

    I think this would only apply to racing boats. Most cruising boats just leave the main traveller in the middle and would probably do the same with a jib traveller if they had one. The only way to find out for sure is to try it (or ask someone who sailed a Hitchhiker rig which would be similar)

    Anyone standing forward of the cockpit during a tack risks a bump on the head, just like they would from conventional jib sheets, main sheet and boom. The advantage here is that without a main boom everyone is completely safe in the cockpit, no matter how tall they are!

    Surely it's the other way round - The shape of a normal main has been designed to work best with a boom, but a normal trisail is a different shape designed to work well without a boom.

    The shape of a normal jib has been designed to work best with a centreline forestay and the jib sheet on the side deck. If the different sheet leads affect performance I'm sure a good sailmaker could do something about it.

    I never saw this as a racing rig, it's a cruising rig conversion which should in theory outperform the original rig significantly, both to windward and downwind. If it doesn't perform so well on a broad/beam reach (assuming you are lucky enough to get the wind in the right direction for that rare beam reach), then you could just put the original sails back up!
     
  13. gonzo
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    gonzo Senior Member

    chris14679:A reach is not a very unusual point of sail but the preferred one. Seems like you are denying reality and common practices. It is very usual with people that can't take criticizm on their pet projects.
     
  14. chris14679
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    chris14679 Junior Member

    I see you edited your post and added more stuff.

    You are wrong. Simple experiment:

    Place an empty cube shaped box on the floor without a lid.
    Push HORIZONTALLY on the outside top edge. It will slide. Tipping it is impossible.
    Push horizontally on the INSIDE top edge on the opposite side. Now it will tip (depending how slippy the floor is).

    The heeling moment on a boat (or a box) causes a rotation about the bottom of the mast, or thereabouts.

    It is impossible to capsize a biplane cat if only the windward rig is set.

    The rant about masts and mains vs jibs isn't relevant to this topic. This concept can be set up with just mains/trisails on each rig, or just jibs, or both mains/trisails and jibs. It's an enhancement for an existing cat so the assumption is that the mast is already there and there's only one of it.
     

  15. chris14679
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    chris14679 Junior Member

    A reach is both a very unusual point of sailing AND the preferred one. They are not opposites.

    I did not disagree that a reach is the preferred point of sailing, but it is very rare to be able to sail on a reach for any length of time, except perhaps on some trade wind passages. Even they are much more likely to be downwind.

    I have sailed tens of thousands of miles and only one or two percent of those have been truly reaching (with the APPARENT wind on or just aft of the beam).

    If you sail on a reach because you prefer it, you might have some fun, but you will rarely actually be able to get where you want to go! Few of us have the luxury of being able and willing to go wherever the wind happens to take us on a beam reach. If you are one of those lucky few who only sail on a reach then probably nothing will beat a gaff rigged schooner (except perhaps a square rig)! There's a reason there are not many gaff rigged schooners around. Windward performance is far more important than reaching and biplane cats are known to have exceptionally good windward performance.
     
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