Yacht Balance, CLR, CE, etc...

Discussion in 'Sailboats' started by ivansalasj, Feb 6, 2007.

  1. ivansalasj
    Joined: Feb 2004
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    ivansalasj Junior Member

    Hi to all,

    I am currently designing a 12 m, L.O.A, classic sloop, very similar to the traditional 6 m class, www.fintra.fi.

    I would like to open a discussion in what I believe is one very important aspect when it comes down to designing a yacht, this being the balance.

    I am now facing a certain level of uncertainty when it comes down to calculating the centre of lateral resistance (dynamic and geometric) and similarly the centre of effort of the sail plan when actually sailing before i go ahead and decide where to position the mast and if my underwater form gives me a reasonable CLR.

    It would be nice to see what the comments are related towards deciding the above query when taking into account sailing conditions, manoeuvrability and assuring that the yacht will be well balanced early in the design and how do people go about it when calculating the position of the CLR and CE, specially for classic yachts which have a completely different underwater hull form when compared to modern yachts.

    Hope you find this thread interesting and I look fwd for your comments

  2. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    If you search on "CLR" you will find 99 threads ...

    The "lead" for classic yachts is between 0 and 17% of LWL.
  3. ivansalasj
    Joined: Feb 2004
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    Location: Glasgow

    ivansalasj Junior Member

    After reading through the ongoing threads, which are excellent!, I was looking for a bit more information regarding the type of yacht I am actually looking at. Sorry about the previous link, but the yacht I am looking at will be similar to the classic 6m such as www.fintra.de , the main concern that I have is in how do I actually go towards designing the hull form, which I have already started, in order to get a well balanced yacht. I can base myself on a CLR regarding a certain % of the WL but there is also the question of volume and area distribution of the actual underwater part of the hull.

    Also how do I go about choosing the right rudder area and very confusing also is the location of the bulb. The lead of these classic yachts was distributed along some distance and I would like to know why this? instead of a more central location? the answer I can think about, is due to structural problems regarding the support of the lead, but I was wondering what effect does this have on the performance of the yacht.

    Basically a more general discussion in the principles behind these classic yachts.


  4. Raggi_Thor
    Joined: Jan 2004
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    A simple solution is to use the same relative lead as Fife and Anker did :)

    I say again and again:
    In many discussions people seem to forget that sailboats heel!
    If you sail with large heel angles, have a high rig and a deep narrow boat, the you will need a lot of "lead" (meaning the distance between ce of the hull and sails watched from the side).

    Imagine your boat sailing to windward. Look at it from the aft. Where are the center of effort in the rig (sails)? Where are the drag? How long is the arm between this pair? Compare this with the lead when you look from the side. The two pairs have to balance :)
  5. Scott Carter
    Joined: Oct 2006
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    Location: Annapolis

    Scott Carter Senior Member

    As a novice designer but having already climbed the steep initial learning curve I'll offer that CLR is not dependent on hull volume. It's a 2 dimensional center of the profile of the hull below the water line. The argument can be made that lift and drag on certain below water line surfaces can influence the lateral resistance, but without hydrostatic modeling or testing you are left with using the geometric center of the sillouhette (how the eff do you spell that word?) of the hull, heeled or not. Most CAD programs will find the center for you, but if you haven't access to one then just e-mail me any image file of the hull profile you want (scale doesn't matter) and I'll send you back a drawing of the CLR, and if you include the proposed sail plan I'll show you the COE. It's straightforward geometry, but I like to let the ol' PC do the numbers.
    Re. lead, depending on heel the CE changes. So what works in light air can have a negative effect when the gunwhales are submerged, and vice versa. Everyone says "decide what kind of sailng you want to do and base your CE on that" but that's not reality. What kind of sailing you would like to do with your boat is the exception, so a happy balance is one that considers where the boat will be on average in terms of heel. Don't neglect the importance of using the profile of the rig when heeled in your CE calculations. This is when it will play an influential role; less so when you're becalmed or just schooning along. Again, any decent CAD program would let you rotate the rig to an angle and then measure the projected profile of the sail area. Heeled 10 to 30 degrees is very normal, and you'd be surprised how much that moves the CE. Try a 40 degree heel and you might wonder why the boat steers at all. One of the reasons is that just as the CE migrates with heel so too does the CLR. This is all dependent on sail and hull profile, and they don't necessarily parallel one another in the movements of their centers. If you can, then just do a few test calculations on the rig vs. hull center to see for yourself.
    Good on ya for just building (er...designing) it.

  6. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Silhouette...but I cheated and typed a phonetic into Word and let it tell me.

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