Placement of boom

Discussion in 'Stability' started by Bertil, Dec 1, 2007.

  1. Bertil
    Joined: Feb 2006
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    Bertil Junior Member

    I wonder why boat designers do not put the boom-end on the spar near deck, so it will be angled up from the spar about 75-80 degrees. I think it have following advantages:
    -Smaller area between deck and boom making better performance of the main. through less air flowing around the boom.
    -You can do without a wang at least on wider boats. The main sheet will work as vang.
    -The boom will be higher up in the cockpit avoiding headbangs.
    -Lower center of sail pressure from the main with the same area.
    -Same pressure from the bom on the spar, but lower on the spar.
    -The weight of the boom will be lower.
    -The only disadvantage I have noticed on my boat which have this solution is a bit of bad visuability.
    Is the only reason the rules of measuring the sailboat for racing?
    Bertil
     
  2. PAR
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    PAR Yacht Designer/Builder

    How much luff length are you willing to sacrifice to this concept? There are a current crop of "dipped" booms, but not so much as to be "self vanging" like a sprit boom. When the sheets are well freed the lower outer end of the boom easily "trips" which can be dangerous and hard on equipment. The CE doesn't lower very much, certainly not worth the complication this can boom type brings to the yacht.
     
  3. Bertil
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    Bertil Junior Member

    I think you misunderstand me. The luff length of the spar will be longer. The outer end of the boom sits higher than the end of the boom connected to the spar.
     
  4. PAR
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    PAR Yacht Designer/Builder

    The amount of "stive" in a boom (or bow sprit for that matter) is often a styling exercise, unless maximum sail area or rating rules present limitations.

    With the boom canted up at it's outer end, it isn't self vanging, it's quite the opposite, causing increased belly in the sail as the sheets are freed, which typically require a vang to control.

    The sail's CE will be slightly lower, but very slightly. The amount of area gained would also be quite small and possibly not within class rules for racers.

    The lose of considerable forward visibility is a hefty price to pay for marginal (if rule permitted) gains.

    This is also a portion of the sail that isn't as effective as area higher up, so small gains here aren't as big a consideration as those further up the rig.
     
  5. Bertil
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    Bertil Junior Member

    -You do not mention the gap between deck and the boom wich is very important. As you know on headsails you should keep the lower luff on deck to avoid the gap. Newer tests in windtunnel(Seahorse sep07?) is showing that mainsails give more driving force when the sailboat is leaning 15-20 degrees, than it gives when it do not lean at all. That depends on when leaning the hull side is covering a bit of the gap between deck and boom.
    -My mainsheet is functioning as wang on all wind directions because my 12m boat is 4,50 m wide. The spreaders are swept aft 25 degrees (no backstay).
    -Why do you think racing Open 60, racing cats and tris are using so solution we talking of.
    Bertil
     
  6. Fanie
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    Fanie Fanie

    Bertil, if you work out the area gained on your sail, the 90 deg corner of the triangle is the least productive wrt sailing area to increase. If you make a cad dwg of the area you'll see very little gain, but if you lengthen the boom a little bit the gain is substantially more.

    I think the boom height is usually higher than could be for convenience sake, not every one wants to race all the time.
     
  7. tuks
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    tuks Junior Member

    The open 60's all have wingmasts, I thought the boom on the deck allowed the mast to rotate. With a horizontal boom, alot of load is placed at the mast at the gooseneck due to the vang. This may be why many masts are sleeved at this level
     
  8. Bertil
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    Bertil Junior Member

    To Tuks: Not all Open 60 have wingmasts. You can also look at Class 40, for example, POGO 40: No vingmast, boomneck near deck and no vang. Winning Rhote du Rhum and more. Very popular, you have to wait a year to get one.
    To Fanie: If you leanghten the boom you get a sail with not so good effectivness, better to make the spar luff longer. If you fasten the boom near deck it will be higher up in the cockpit witch is nice for cruising(and racing).
    Another good thing is that you can make the spar lighter(and not so strong) when it does not must withstand the power from the vang.
    Bertil
     
  9. CT 249
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    CT 249 Senior Member

    "Why do you think racing Open 60, racing cats and tris are using so solution we talking of."

    Why do you think IACC boats, 18 Foot Skiffs, International 14s, IRC racers and TP 52s aren't using the solution we are talking of?

    Some boats have to be able to get a whole lot of crew across the deck each tack, and that's not practical with a low boom. Others don't want to have their sail area down low (in an area of lower windspeeds) when they could have it up high (more wind).

    Some classes much prefer having a vang, because you can adjust twist in the mainsail. This may be more important in boats that sail inshore in vary varied conditions. A wide traveller and no vang may be fine for beamy boats, but it's lousy for skinny ones, and skinny ones are better in some conditions and some classes. Skiffs have found that a low-mounted boom complicates mast tune, I think.

    I've got 3 boats with the boom like you describe (Moth, Taipan F16 type cat, International Canoe), and three with a normal boom. Strangely enough, the low boom works well on the boats with a low boom, and the high boom works well on boats with a high boom - because they all have different needs. No real mystery.
     
  10. Doug Lord

    Doug Lord Guest

    Main low forward

    Bertil, I agree with your idea but have gone about it somewhat differently. See the rough sketch below that shows the main on an experimental boat I am building. I use a modified version of a wishbone boom than can use dual vangs or a mainsheet on a traveler a bit farther forward than is shown(an athwhartship carbon tube takes the mainsheet and is tied in to both sides of the wishbone). The wishbone allows the forward third of the main to be brought right down to the deck. Visibilty is taken care of by clear panels/windows in the lower portion of the main.
    -------
    X21
    Address:http://www.monofoiler.com/images/x21/x21-3.html Changed:11:35 AM on Friday, September 21, 2007
     
  11. Bertil
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    Bertil Junior Member

    Thanks for the answers. I think the topic are ready-discussed. I can also mention that I do not have any traveller for the main sheet, but I have another solution that is simple, light and cheap and gives the possibility to place the sheetingpoint of the boom whereever you want. I try to explain how it works:
    -From the winch on BB to BB boatside a line going to up to the sheetingpoint, going under the boom to the spar down to deck and back to the SB winsch.
    -One more line the same way but starting on SB going the same way but ending on BB-winch.
    You can sit on any side and trim the main(61 kvm) and have full control. Another good things is that the solution works as a control of gibes.
    Bertil
     
  12. CT 249
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    CT 249 Senior Member

    How precisely and instantly can you adjust the twist, independently of adjusting the horizontal angle of the boom?
     
  13. Bertil
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    Bertil Junior Member

    In the same way as if I head mainsheet and a traveller. Thats because I can place the after end of the boom whereever I want.
     
  14. Brent Swain
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    Brent Swain Member

    Boom height

    tTo high or too low a boom makes reefing difficult. Too low obstructs visibility. To0 high increases heeling moment, and reduces sail area for a given size mast.
    Brent
     

  15. Bertil
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    Bertil Junior Member

    I use 1-line reefing, so reefing is no problem.
     
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