Centre of Lateral Resistance, Centre of Effort and lead

Discussion in 'Sailboats' started by cookie munster, Jun 26, 2017.

  1. cookie munster
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    cookie munster Junior Member

    I was just reading Principles of Yacht Design and there it is said that lead, the (longitudinal) distance between Centre of lateral effort and centre of sail effort can be larger in boats with al larger beam. No further explanation is given for this and I would really like to understand why this is. A Google search doesn't provide much info either. My reasoning was that boats with a larger beam aft would sink their bow a bit when heeling and thereby tilting the mast a bit forward, which in my theory moves the CE a tad forward. However I also read on the internet that boats with more volume in the bow can do with even more lead. Ist there anyone who can explain this to me?

    [​IMG] image from goodoldboat.com
     
  2. PAR
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    PAR Yacht Designer/Builder

    There are a number of factors that can affect the prefered location of "lead", much of which is experience dependant. The rig choices, the boat's shapes, appendage choices, performance envelop, etc. can all have an effect of lead. The range of lead locations can run from below 10% to well over 17% - 18% depending on these variables. For example, a fractional sloop with fairly tall aspect in appendages and sail plan, might need quite a bit of lead, particularly if a modern hull form with plenty of displacement placed aft, while a well balanced hull form and divided, considerably lower aspect appendages and rig might need nearly half of what the sloop needs, to balance out.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    The lead is there to balance all the forces. So it depends not just on the rig but also on the heel and leeway. You need to draw (or visualize) all the forces acting on the boat in 3D to understand it properly, a 2 D picture isn't enough. An upright boat that makes no leeway will have no appreciable lead

    I suspect most designers go by past experience with a certain type

    Richard Woods
     
  4. TANSL
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    TANSL Senior Member

    Would any theoretical explanation of this interesting topic be possible? I know that, as in many other subjects, practical experience is very important, but there is always, at least I believe so, a theoretical explanation that will, of course, be improved by the experience in the real world. So, after reading your expert comments, is there any good technical article to read?. For some it would be helpful, not to know that it can vary between 10 and 18%, but to know why. Many thanks.
     
  5. wet feet
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    wet feet Senior Member

  6. TANSL
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    TANSL Senior Member

    Thank you, wet feet, I will carefully read both articles.
     
  7. cookie munster
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    cookie munster Junior Member

    Interesting documents, wet feet, but after reading the claughton/hiswa one, I came up with more questions :confused:

    The main surprise to me was that his tests show that clr moves forward if sideforce increases due to leeway angle increases at a given speed and heel angle. And that this forward moving effect is grater for long keelers


    Back to my original question, the exact quote from Larsson and Eliasson is as follows:

    The following will increase the lead: A large beam. The beamy hull gets more asymmetric under heel, thereby creating a moment to windward

    In my mind, the asymmetric shape tends to increase drag at the leeway side of the hull, thereby initiating a moment to leeward. Contrary to what L&E write. Am I wrong here??
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    It's not drag related. Think about the shapes of the hull and how it creates lift while moving in water. Most boats can be steered by heel..
     
  9. cookie munster
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    cookie munster Junior Member

    Whether it is drag related or lift related, probably both, does not change the fact that when you push one side of the boat down, the boat will turn in the direction that is pushed down, which is seems to contradict with what L&E state in relation to beamy hulls and lead. What they say can be interpreted as a beamy hull generating more momentum to windward when you push down the leeward side.
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Most boats do just the opposite turning away from the heel..
     
  11. cookie munster
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    cookie munster Junior Member

    Under sail, yes, but L&E are talking about forces on the hull, not via the sail/CE forces. Therfore I think of the way a surfboard turns when pressing down a side, it turns in the direction of the side which is pressed down.
    I haven't tried this experiment with my boat though and the underwater shape of a surfboard is slightly different.
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    Planning hull forms as surfboards are another story. Dynamic lift forces you see..
    Canoe bodies in displacement mode act as I said.
     
  13. cookie munster
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    cookie munster Junior Member

    Alright, that takes away some of the mystery, cheers.
     
  14. Earl Boebert
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    Earl Boebert Senior Member

    I've made something of a study of hull balance (decoupling of heel and yaw) in model yachts. My CSYS survey of the topic can be found here:

    http://www.rsyc.org.uk/Uploads/Documents/That Peculiar Property_Balance.pdf

    The "Rip Tide" design documented in the paper is quite remarkable in this regard. When one encounters a sharp gust, the boat heels rapidly and jumps forward with no change in course.

    There are plenty of mysteries in this topic to explore. It has been demonstrated on multiple model and full size yachts that following Turner's "Metacentric Moment" process yields a balanced hull. Turner's explanation of why this is the case can be charitably described as needing work :) It has also been demonstrated that what he called "concertinaing" (stretching or shortening a balanced hull by varying section spacing) will maintain the balance property. Again, I have seen no explanation why. And finally the big question is why a hydrostatic property should have a hydrodynamic effect.

    Some years I go I thought up, but never implemented, an experimental way to test hull balance in a model. The idea was to take a hull and propel it with a model airplane ducted fan, which would give an adjustable thrust vector without propwash in the water, and one that could be angled to generate leeway. One could then set up varying angles of heel with an off-center weight. Then you could "sail" the model some calm day and video the action. Maybe somebody reading this would like to give it a try.

    Cheers,

    Earl
     

  15. cookie munster
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    cookie munster Junior Member

    Thanks for the link to your article, I greatly enjoyed reading it.

    I dug a little more into this subject and to my surprise haven't found any better explanations. Most designers seem to take an empirical approach when it comes to hydrodynamics, hull stability and lead. Still I find it hard to believe that there are no basic principles to be found, even though matters are complex and interdependent as hydrodynamic forces vary with speed, angle of heel and leeway. And then there are waves to be added to the mix... I hope that someone performs some of the basic tests you describe. It shouldn't be too difficult I guess to do some basic comparisons between a slim hull with weak/round chines and a hard chined wider hull and come up with some useful data. I just like to know why thing work the way they do and I am sure that I am not the only one :)
     
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