Sail Loading on Rig, Rig Loading on Vessel

Discussion in 'Sailboats' started by brian eiland, Oct 14, 2003.

  1. Kiteship
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    Kiteship Senior Member

    Would enjoy it Stephen, but our commercial product is a spinnaker replacement kite. No upwind, and the race is around the cans. Current biz plans have us developing these for cargo vessels' I've no time to do fundamental R&D for the proposed class for an all-round kite. Not sure I'd want one, at any rate. State of the art has upwind masted sails (on rotating masts) beating upwind kites, for VMG. Until that advances...

    Then again, I believe Rob plans to carry an OutLeader as his offwind sail for that race. He'll certainly carry it for the SS Transpac.

    Dave
     
  2. Pericles
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    Pericles Senior Member

    I looked at a selection of tuna fishing towers at http://images.google.co.uk/images?u...-GB:official&q=tuna towers&btnG=Search Images

    I was struck by their apparent suitability to carry one or two forestays on which to rig foresails in the manner advocated by Brian Eiland for his aft mast rig. The fore deck space is huge and uncluttered. A get home upwind solution, if engine problems arise?

    Doubtless there are no calculations available for the forces acting on the tower if used in this manner, but the rearward stance of these towers should help facilitate the necessary luff tension. It might even be possible to rig a mizzen sail as illustrated at http://www.runningtideyachts.com/sail/.

    This begins to look more & more suitable for a cruising catamaran. A quad mast setup with steps to fix any halyard etc. problems, en route, plus a real flybridge. :D

    Well, it was just an idea! :( :(

    Pericles
     
  3. zerogara
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    zerogara build it and sail it

    You forgot the most significant part necessary to upwind; A keel, preferably a fin keel. Fishing towers are meant to have very little wind resistance, be as light as possible and carry the load of the fishermen and the equipment up there.
    Masts are designed so they can carry a longitudinal load that is higher than the displacement of the boat.
    For a 45' Bertram what would that be? 20tons? Imagine placing 20 tons on that flimsy tower :)

    Pericles (o poutsos mou o meraklhs?)
    Kozmas

     
  4. Pericles
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    Pericles Senior Member

    Thought experiment.

    zerogara

    I had in mind a substantially stronger 4 legged structure mounted onto a sailing catamaran cruiser. If a sport fisher needed to have recourse to sails such as I mentioned. a leeboard would need to be carried or a centreboard, similar to Mike Storer's designs, could be incorporated during building the vessel, but it's probably not realistic. Just carry a spare traction kite.

    http://www.storerboatplans.com/Venice/Venice.html

    The twin mast rigs being discussed in the forum just set me thinking about four masts. Three would not work as there needs to be a completely clear area between the legs, for the sails to be rigged on stays. Hard to describe at three in the morning.:D The crow's nest/flybridge stands above the sails like the deck of an oilrig. At this stage, I am not thinking about drag. The Cutty Sark was not the most streamlined ship.:D

    Kozmas, do you mean " o poutsos mou o meraklis "? It is not translated by the Internet. Greek Comedy?

    Regards,

    Pericles
     
  5. zerogara
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    zerogara build it and sail it

    Ok, now I see that you do see the principles so I will not waste more bandwidth. I'm still fasinated of the variations of design of the original single mast design. Only multihulls I believe have room for radical rigs like the ones you mention. Evolution in composite fabrication techniques may lend a hand in abandoned ideas that were simply prohibitive with aluminum and stainless wire.
    The system of one compressive loaded member and numberous in tension working against each other could evolve and be refined. In racing dinghies and skiffs rig design has become an art and science. Materials again are presenting limitations. On your average 14-18' going from what is readily available in the market in 2.5 and 3mm wire is a huge spectrum for fine tuning.
    With large boats their use can be more precise.
    In a single mast you use various points of attachment to maintain the shape you want. With a 4mast arrangement and no rigging how will you be able to control the shape of the masts without over-designing a heavy rig?
    Would a trimaran with 4 mast pyramid and variable foot positioning be a good example to analyze this?

    Pericles is a very Greek name, so if you didn't get the joke it can't be translated. You had to be there to get it. In secondary school we had various funny rhymes that attached to most common names. The one for Pericles was one of the funnier ones.

    Koz

     
  6. Spiv
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    Spiv Ancient Mariner

    The crows nest would be a great thing to have when negotiating lagoon entrances with shoal reef. On a sailing boat it would not need to be so substantial as you would only use it occasionally and for short periods, all you need is steering and motor remotes.
     
  7. zerogara
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    zerogara build it and sail it

    Although this all sounds like sacrilege to Poseidon followers, would someone calculate the radial velocity and G force exerted by the harness of the captain during a knock down or pitch pole?

    :)
    Koz
     
  8. brian eiland
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    brian eiland Senior Member

    Crows Nest


    I've included crow's nest on almost all of my designs (and plan on one for the SkySail one). They can be a significant addition to any ocean craft.

    But just as many sportfishing captains realize, you don't want to be high off the motion centers of the vessel in really rough conditions....and that includes their upper flybridges and crows nest. So go there at your own risk, or as a necessity such as when you must ascend your mast for some problems special up there.
     
  9. brian eiland
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    brian eiland Senior Member

    Mapping of Rig Forces

    ....back to the primary subject of this discussion thread

    Hi Spiv,
    If you were to go back to several postings on this tread including two of mine, # 38 & 45;
    http://www.boatdesign.net/forums/showpost.php?p=35689&postcount=45

    ...you will find my desire to employ the consultancy of this gentleman Chris Mitchell of NZ to help in this mapping of the rig forces in determining the optimum configuration for a particular new rig design. Granted it is more 'stactic oriented' rather than dynamic, but it could be an excellent indicator. If I remember correctly his prices were reasonable, and possibly he is less busy with the American's Cup no longer in NZ. I really liked the fact that some of college studies were based on this very subject of mapping the rig forces.
     
  10. brian eiland
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    brian eiland Senior Member

    Rigging Loads - or a tale of scientific progress?

    ...just the other day a gentleman sent me a private email to remind me of this reference web posting....interesting reading that I thought should be added to this overall discussion thread
    ..from
    http://www.classicmarine.co.uk/Articles/rigging_loads.htm



    Trying to work out the starting point for the design of rigs

    Those of you not already acquainted with the work of Douglas Phillips-Birt could do worse than visit your nearest second-hand bookshop to rectify the situation. Written some 40 odd years ago, his books have inevitably dated, but still have a remarkably high nugget to dross ratio. For example “Masts are tricky things. It is not for nothing that Lloyd’s, which is ready to specify the scantlings of nearly every other part of a yacht, washes its hands of them altogether and plants the responsibility for their size and shape squarely on the designer’s shoulders; then, as a happy afterthought, advises him to fit lightning conductors. The advice is good; but it leaves the part between the lightning conductor and the step open to various interpretations. The fact that Lloyd’s, with its vast collection of data on wooden yachts, feels like this about masts, suggests that masts are perhaps a little beyond rational analysis.1” As true now as 45 years ago, though it is yachts as a whole which defy complete analysis, not just their masts. The purpose of this article is to explore some of those “various interpretations”. Depending on your viewpoint, it is either a triumph of experience over science, or.....

    A study in ignorance

    As a working example, I’m going to look at a number of possible ways of sizing the mast and shrouds for a 30 foot gaff cutter which displaces 8 tonnes, with one cap and two lower shrouds. I’ll point out as we go the implicit or explicit assumptions being made. The crudest rule of thumb which I have come across - I don’t know its origin - is the idea that the shroud set should break at a load equal to the weight of the boat. So each shroud needs to be able to stand 2.66 tonnes, i.e. 8 tonnes displacement divided by 3 shrouds per side. As we saw in the last article, the maximum compression in the mast is in the lower section, and is roughly equal to the combined tension in the shrouds, so that means the lower panel of the mast needs to be able to stand 8 tonnes. For all its crudeness, this rule at least recognises that the strength of rigging relates more to the size of boat rather than the size of the rig. A steadying sail on a trawler may be no bigger than the mainsail of a 12 foot dinghy, but because of the relatively huge inertia of the trawler’s hull, the forces transmitted into the rigging and mast of the sail by a gust of wind will be very different. The rule also implies that factors such as ballast ratio, stability, rig shape and size, and beam/length ratio are “normal”. It might get you into the right area, but the limitations of such a guideline might be misleading.

    One level up, and despite his quote above, Phillips-Birt offers an empirical rule which runs as follows:

    Mast Scantling Criteria = B + M + H + 2cos4A. B is the ballast ratio, here used as a measure of stability in conjunction with M, which is the metacentric height as a proportion of the waterline beam. H is the foretriangle height as a proportion of the upper half of the mast(!), and A is the shroud angle. You can see that these factors will bear on the issue, but the more you look at it, the less you can understand why they are combined in the way they are. Why not B times M, what has the cosine of four times the shroud angle got to do with it, and so on. With a method so opaque in its assumptions, you never know what the range of validity is in terms of rig type, or arrangement of stays. It might have been a useful guide for yachts 40 years ago, but you can’t tell whether it might apply to yours. Besides it gives no clue to shroud sizing.

    At least the assumptions in Skene are more overt, which means that they are there to disagree with if you wish! He offers two approaches to mast and shroud sizing. The “short method” is based on the righting moment of the hull at 30 degrees of heel - fudge factor 1, why 30?. If you don’t happen to have that information, he provides a chart to give you a good guess - fudge factor 1a, to what sort of yachts does this reasonably apply? The mainmast maximum compressive load is 2.78 times the moment divided by the half beam - fudge factor 2, why 2.78? You then derive the panel sections by doing a sum about buckling, adding 50% more for a deck-stepped mast. By reference to a table apportioning the shroud loads depending on the rig, number of spreaders and lowers, you then work out the shroud loads and add a further factor of safety of between 1.5 and 3, depending on which wire you are considering - fudge factors 3 & 4.

    Well that is a bit more scientific; at least here there is some recognition of the staying geometry.

    The “long method” is based on sail loading. Good Heavens! That is the first time it has been mentioned, which considering that it is the sails which load the rig must be an improvement. Don’t get too excited though. “How much is the mast loaded and where? The answer is that nobody really knows. There are theories on the subject, however, and the best of them seem to be these: For a mainmast, assume a wind pressure of one pound per square foot of sail area...It seems to work well if we assume that the load (of the mainsail) is evenly distributed along the mast, regardless of the fact that most sails are triangular.2” Fudge factors 5 & 6. Now triangulate for shroud tensions, and add a factor of safety of 4 - fudge factor 7, it was 1.5 to 3 last time! Then resolve back for mast loadings, add some extra for weight of sail, boom, tension of halyards, and multiply by a factor between 2.7 and 4. I think I’ve lost count. Anyway it is time to bring ourselves up to date with one of the most recent books on the subject - “Principles of Yacht Design”, first printed in 1994.

    Turning to the chapter headed Rig Construction, we find that Lloyd’s (and ABS) continue to wash their hands of masts and spars, but the Nordic Boat Standard does provide a design guide. “The starting point when dimensioning the rig is to calculate the righting moment. It is commonly agreed that a heel angle of 30o is a good design angle. This corresponds to a reasonably high wind strength with the sails still generating high loads and the boat making good speed through the water. Letting the boat heel over more....in reality means a slower boat owing to increased resistance, with a correspondingly smaller dynamic force.3” So that is why 30o is a good angle. Hrrrumph. Sorry about this, but it is time for a digression. Calculation of the righting moment is based on a stationary boat in still water. It has precious little, nothing actually, to do with boatspeed, dynamic forces, wind strength, resistance or anything except a calculation, verifiable by experiment in suitable conditions, of the stability of a yacht. It provides, then, not a real life start point for some rigorous analysis, but a common assumption which can be used to compare craft with each other and/or with empirical data. You could start with the righting moment at 1, 5, 10, 17.386 or any other number of degrees and get to the same answers by changing the various factors applied to the moment. To try to rationalise an assumption like this is at best pretentious - an attempt to ennoble guess-work, at worst dangerous - someone might believe it.

    Meanwhile, back to the Nordic Boat Standard. Starting with the righting moment, you add some correction for the crew sitting to windward. Old Gaffers might safely ignore that bit! The method is then based on the most severe of two load cases, the first under full working headsail only, the second under reefed main, using not sail area, but a function of the righting moment. The loads are apportioned to the masthead, hounds, and gooseneck, and with suitable application of trigonometry the shroud loads are determined before multiplying by 2.5 to 3. Similarly, the mast compression is based on the righting moment, with factors for keel or deck stepped masts and times by 1.5 “to handle the dynamic factors”.

    You may be getting the impression that this is not much advance on earlier efforts. To an extent that is true, though as we’ll see later, that can be a source of comfort. But the NBS is to my knowledge the most carefully codified approach to a variety of rigs - all Bermudan, of course - and for the first time begins to provide the flexibility to extend the analysis across different types of monohull, or a variety of materials for the rig and stays. And whatever the relation of the various assumptions to real life, they are at least quite clear.

    And the answer is.....

    Table 1 - Loads in mast and shrouds

    see attachment below

    1. Based on solid round Douglas Fir mast, converted where necessary by matching section inertias

    2. Because this method takes account of spreader length, it is possible to vary the shroud loads considerably by varying the spreader length. For example, omitting the spreader changes the loads to 2.9t in the lowers, and 6.8t in the caps. It is reassuring, compared with Skene, to note that with a spreader the lowers are more highly stressed than the caps, which is what you would expect.

    3. Interesting that the NBS method does not specifically relate mast loads to shroud tensions, but starts again with the righting moment. The mast diameter goes back up to about 160mm if derived by resolving forces, but then you never know how to interpret the factors of safety used in sizing the shrouds!




    Table 2 - Transverse loads


    see attachment below​


    Have a quick look at Table 1 which compares the results of the various analyses. Perhaps surprisingly, the answers, except for the lower shroud design loads, come out very similar. I think that is for two main reasons. First, the craft I am looking at is a “moderate” design, so the rules which make implicit assumptions about stability etc can reasonably apply. Try to run the same series of sums for a more extreme design like an Open 60, and you would quickly find the limits of all but the NBS rule. Secondly, however numerate the rules appear, in practice they are all founded on empirical data - how else could the safety factors be determined? - and the similarity of the results confirms that the forces of wind and sea have not changed too much. Besides, masts don’t often fall down, except when designers are at the very edge of technology, for example as they struggled with the huge Bermudan rigs and new mast materials for racing yachts of the early decades of this century, or carbon sticks during the last decade. In my view the difference lies not in the answers, which don’t vary much, but in the approach. As assumptions are made more clear, it becomes possible to extend the analysis to include either a wider range of boats, or a wider range of materials for masts and stays. Given the range of designs and materials now on offer, that is very useful. Also, see from Table 2, we are getting the beginnings of an analysis of transverse loads on the mast. As you can see they are, by comparison with the compressive loads, very small. Almost suspiciously small. The loads would indicate, for example that a gooseneck pin of about 6mm diameter would be adequate for my test boat; but I think I could predict the response of the owner if I proposed such an arrangement. It would seem that the design loads on a gooseneck come from other considerations, perhaps the shock load from a gybe, or the flogging of a boom whilst reefing. Or maybe Bill Tilman was right not to worry about worn gaff saddle bolts4. So we are a long way from a complete picture of the loads in a rig, particularly traditional rigs. Why? For a start, the more sophisticated approaches have developed during the age of the Bermudan rig. The usual assumption that shrouds can be analysed separately from fore/backstays probably holds better for Bermudan, with fixed backstays and forestays mounted inboard, than for gaffers, where there will be a complex interplay between peak halyard, runners, mainsheet, bowsprit shrouds and so on. I don’t think anyone has attempted to resolve this yet. Another by-product of the Bermudan based analysis, is the assumption that windward work induces the highest rigging loads. I am not utterly convinced that this is true of the gaff rig, but have no ready way of finding out one way or the other.

    Finally, and most significantly, none of the methods derive loads from the force of the sails, which is after all what is loading the rig. Such an analysis would be fiendishly complex, but with ever more powerful tools and computers, I think it is not an unrealistic thing to attempt. It is almost certainly done for Americas Cup boats, but the results are of course not published.

    So what are we left with? Well, I hope that I have shown that we can have some confidence in calculated shroud and mast loads. These can be used to size appropriate and consistent sets of equipment, i.e. shrouds, rigging screws or lashings, chainplates, tangs and so on. And I’ll bet you can’t guess what the next article will be covering.

    References
    1. Rigs & Rigging of Yachts D. Phillips-Birt Adlard Coles 1954
    2. Skene’s Elements of Yacht Design F.S.Kinney Dodd, Mead 1981
    3.Principles of Yacht Design Larsson & Eliasson Adlard Coles 1994
    4. Mischief Goes South H.W.Tilman various publishers
     

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  11. Robin Larsson
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    Robin Larsson Junior Member

    But, its not really the sails that load the rig, its the hull, or the RM. In an upwind situation, if you heel 30degrees you will always have the same total load. Doesn´t matter if it the big genua or the stormjib that does it, the load will be the same in the hounds. And therefore mast compression and shroudload will be the same.
    That is of course a bit simplefied, the mainsail will put its load differently if it reefed or not. And the dynamic forces will make it more complex, but companys like Seldén use 30degre RM as the dimensioninf force, and their rigs seems to work well.
    The longitudinal loads is not a closely coupled with the transverse RM, but again, those "rules of thumb" work well.

    Of course the sails load the mast in different ways, but the its still the boat that makes it possible for the sail to generate any power. Newtons laws you, extremely basic physics.

    On more traditional rigs it probably more complex, with more shrouds, peakhalyards and such. The gaffriggers I sail on seldomly have any aft stays, some have runners, but usually thats the more extreme boats. We have backswept shrouds.
    But again, there must be some rules of thumb that works good, we almost never break our rigging, and we do alot of rough wheather sailing:)

    www.msatene.com thats the site for the gaffer I mostly sail on, dont think there´s an english version yet though, but you can atleast se what kind of boat it is.

    Regards
    Robin Larsson
     
  12. brian eiland
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    brian eiland Senior Member

    Central Hull-Pod on Oracle Racing Tri, Alinghi Cat


    Commenting on the newly launched Oracle 'America's Cup' trimaran, Roy Mills noted;
    "Looking at that short small central hull it is obvious that she is meant to sail on the leeward ama at all times except during a tack. Really she is a catamaran with a godpod in the middle for forestay/foresail rigging purposes. Whatever, it will be impressive, probably frightening, to see two boats like this duking it out over a relatively short inshore course. Barcelona-Malta and back might be better."
     

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  13. TYD
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    TYD Junior Member

    I'm a little lost in this big Thread.....with a lot of interesting comments....

    My commet is about this last post of Brian, This ultralights cats experiment enormous loads because of his enormous righting moment. Remember that the righting moment on this boat is not regarding the weight (only) is more regarding the withspan.
     
  14. brian eiland
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    brian eiland Senior Member

    Shroud Loading vs Forestay Loading

    Remember the 'righting moment loads' are on the athwarthships rigging...not necessarily the forestays.

    Did you get a look at that attached PDF article?
     

  15. brian eiland
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    brian eiland Senior Member

    Another 'Structured' Cat




    The "SYZ & CO" is a revolutionary hydrofoil catamaran which could become the fastest multihull on Lake Geneva with a view to both beating Lake Geneva speed records and winning its main races, most notably the Bol d’Or Mirabaud.

    The hydrofoils are lifting foils placed under the hulls which allow them to rise so that only the foils remain in contact with the water. This substantially reduces the drag in the water, allowing the yacht to achieve far greater speeds than traditional boats.

    Contrary to other prototypes which aim solely to beat records in very specific wind speeds, wind angles and wave conditions, the "SYZ & CO" hydrofoil catamaran aims to win races. As a consequence, she must be able to sail fast in all situations.

    http://www.syzfoiler.com/projet/concept/index.lbl

    ..and look closely at the longitudanal supporting structure to maintain rig
    tension
    ..... http://www.syzfoiler.com/images/photo/index.lbl ..look under 3D VIEW
     

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