Looking for free (but hopefully informed) advice on maximum reasonable boom length

Discussion in 'Sailboats' started by massandspace, Nov 15, 2017.

  1. massandspace
    Joined: Sep 2017
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    massandspace Junior Member

    What is the maximum boom length (in relation to luff) of a mainsail. If one desires reasonable upwind performance?

    I am building a custom smaller (27') sailing catamaran and am trying to keep the center of the sailplan low, for several reasons. Main will be fully battened, large roach.

    I post in this sub-forum instead of the multihull forum as I feel maybe a bigger readership here and the concept, I think, applies to all sailing vessels.

    Gracias
     
  2. TANSL
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    TANSL Senior Member

    You can put a boom as long as you want but, of course, you must calculate the necessary module so that the boom supports the efforts to which it is subjected.
    I do not know the reason for wanting to place the pressure center as low as possible, but you must take into account that the air velocity is greater the farther you are from the surface of the water. Therefore, the upper part of the sail is more effective, per square meter, than the lower part.
    These are only my opinions.
     
  3. massandspace
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    massandspace Junior Member

    I do not agree.

    A 50' long boom on a 30' long luff, I think, would not allow the sail to generate much lift. Airplane wings, and high aspect ratio underwater foils, are long and thin. There must be some rational ratio one must pay attention to?
     
    Last edited: Nov 15, 2017
  4. TANSL
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    TANSL Senior Member

    In what exactly do not you agree?.
    Put what you want, if possible something similar to similar boats, make calculations and check if your configuration is correct. You do not have to agree on anything, just calculate things, or get someone with knowledge to study them.
     
  5. PAR
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    PAR Yacht Designer/Builder

    There is no easy way, formula, rule of thumb or general guide for the answers you want. Simply put, the boom is physically dimensioned to the loads it must endure, including some safety margin. The length is also a set of calculations based on what you want from this portion of the area. Without a much better idea of what you're doing, answers will be hard to get. A general guide might be 2:1 for the boom, but this is grotesquely rough estimation.
     
  6. Blueknarr
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    Blueknarr Junior Member


    The Bird class boats racing on SF Bay have a 20' foot and 32" luff. Their up wind performance could hardly be described as stellar. It is considered bad form to do a 360 around them while racing.
     
  7. gggGuest
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    gggGuest ...

    It seems to me the question you are really wanting an answer to is "what should be the aspect ratio of my mainsail?"
    Discussion of physical strength of the boom seems beside the point.

    There's a school of thought that suggests that for upwind performance the main thing that really counts is the length of the mainsail luff, and beyond a certain point area added to the leech has relatively low effect. According to that school of thought then a low aspect ratio main simply gives you the performance of a much smaller rig, but with a lot more heeling moment. As presumably you want a low aspect ratio rig to minimise heeling moment this means you aren't getting the gains you think you are.

    There are no magic numbers I think. Its a compromise. The lower the aspect ratio the worse the performance upwind, but you may well get superior performance crosswind and especially deep downwind. I also think it depends on the rest of your rig. If you propose to fly some sort of spinnaker downwind all the time, and that will be your main downwind sail, you may as well optimise your rig for upwind sailing and rely on the big coloured rag to get you downhill. That would suggest a relatively small but high aspect ratio sail. If you won't have a kite, and will be prioritising downwind speed, then a low aspect ratio sail will make more sense, even though the back half of it may be contributing very little upwind.

    Thing is, there are no magic bullets, and there are no magic dimensions. Research boats as like yours in hull design and intended use as possible, figure out where the majority are, and go there yourself. You won't be far away. There's always the temptation to think - I could make a breakthrough: I could start a new trend. Well, such things do happen, but very rarely, and usually driven by new materials making things possible. The compromises in high AR/ low AR are pretty well understood over the years, and the chances of something radical succeeding are near enough zero.
     
  8. DCockey
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    DCockey Senior Member

    Increasing the length of the foot of the sail while keeping the length of the luff the same will increase the sail area and lift the sail generates. The aspect ratio will decrease and the lift coefficient, CL, which is the ratio of lift to sail area will decrease but not as fast as the sail area increases. Both the component of lift moving the boat forward and the heeling moment will increase.

    Important: In the example above the luff has stayed the same length and the sail area has increased. Most discussions about high aspect ratio being better are based on the sail area remaining the same which means the length of the luff would decrease when the length of the foot increases.
     
  9. philSweet
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    philSweet Senior Member

    This is in direct conflict with keeping the center of effort low. It's what you do when you have mast height issues like ICW bridge clearance on a 45' catamaran. The planform shape that gives optimal performance in terms of L/D is elliptical. But the planform that gives optimum performance in terms of lift/heeling moment is triangular, or even a bit Swede cut (hollow roach). That's where you can exploit a long boom.
     
    Last edited: Nov 21, 2017
  10. sharpii2
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    sharpii2 Senior Member

    I certainly would not have the foot longer than the luff. I'd be inclined to go with a foot which is 4/5 ths the luff. I saw a 17 ft Wharram cat a long time ago, which had a main with similar proportions. It was a boardless cat, but was moving quite briskly, maybe in the low to mid teens in knots. An elderly man was sailing it with a boy on board, probably his grandson. As fast as the boat was moving, neither one seemed to be breaking a sweat, or all that worried about sudden gusts.
    No doubt that the nearest competition, a Hobie 16 (another boardless cat), would be faster. But it certainly would deliver a disproportionate amount of drama, for a casual sailor, for its added performance. After all, thrills and spills are part of its charm. But not all would be multi sailors are in to that sort of thing.
    Some might prefer a relaxing time on the water on a sailboat which still reaches speeds of a more traditional racing dinghy.
    With all the literature I have read about working sailboats, I have heard of very few which resorted to tall rigs. Low to moderate AR seems to have been the order of the day.
     
  11. DCockey
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    DCockey Senior Member

    Only true if span/luff length is constrained.

    If root bending moment/heeling moment is the constraint then a half bell shaped lift curve will have a higher L/D than an elliptical distribution.

    Also, the "D" in the above statements is inviscid induced drag and does not include the portion of drag due to viscous effects such as boundary layers and local separation.
     
  12. fastsailing
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    fastsailing Junior Member

    Might be, but it doesn't have to. It depends on the unwritten assumptions.
    1) If you keep both sail area and length of foot of the sail the same, a large roach results less luff height and a lower center of effort. Thus not in conflict.
    2) If you instead keep both sail area and length of luff of the sail the same, a large roach results less foot length and a higher center of effort. A direct conflict just as you indicated.
     
  13. fastsailing
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    fastsailing Junior Member

    That does happen if you keep angle of attack the same. If you instead trim for max lift coefficient, there is some aspect ratio that gives the highest lift coefficient. Changing AR away from that in either direction typically lowers Cl. Typically Ar for max Cl is close to one. See:
    [​IMG]
    Source: Aspect ratio of wings and blades https://www.heliciel.com/en/aerodynamique-hydrodynamique/Allongement%20pale%20aile.htm

    But it might depend on what Re number is and if it's kept constant or not. In general cases most mainsails have AR > 1, and therefore decreasing AR will increase CL_max, but require a higher angle of attack to achieve that. Also same sail area with a shorter luff have greater Re number in a given apparent wind, typically increasing max Cl.
     
  14. fastsailing
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    fastsailing Junior Member

    A 50' long boom on a 30' long luff, allows the sail to generate plenty of lift, more than most mainsails, if we are talking about main only rigs.
    If we are talking about sloops, you should calculate AR for the combined sailplan, not for main alone.
    Some airplane wings have a wide span and a short chord and thus high AR (sailplanes), others have not (fighter jets). Reasoning for that is not related to sails of sailboats by much.
     

  15. fastsailing
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    fastsailing Junior Member

    It depends on how you precisely define word reasonable in the context you used it. Without a precise definition, only sensible answer is: use a reasonable boom length, and I don't expect that to help you much, if at all. It is unfortunately not possible to give more precise answer without more precise definition for reasonable upwind performance. And using sloop or cat rig has a lot to do with the suitable boom length too. The less upwind performance you consider still being reasonable, the less the over all AR of the sailplan can (but doesn't have to) be.
     
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