Center of Lateral Reasistance

Discussion in 'Sailboats' started by Don Case, Oct 24, 2007.

  1. Don Case
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    Don Case Junior Member

    When determining the COLR(or whatever it's called) is anything taken into consideration other than the underwater profile? Beam? Displacement?
  2. PAR
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    PAR Yacht Designer/Builder

    Some refer to it as the CLR (Center of Lateral Resistance) though it more correct is CLP (Center of Lateral Plane).

    You've asked a well loaded question (read dear slugs, in a 12 gauge, at very close range), the simple answer to your question is yes. There are many possible variables that can come into play, when working out a desirable CLP on a specific design, not to mention experience based the guess work.

    What are you intrested in doing?
  3. Don Case
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    Don Case Junior Member

    Thanks for the answer. I've built a model sailing yacht that has more weather helm than I care for. My plan is to build a little more rocker into the back half with bondo as a test. That should decrease the weather helm some but it will also increase the displacement. If that improves the weather helm my plan was to build a narrower hull with the same rocker to get the displacement back down but with nicer helm.
  4. marshmat
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    marshmat Senior Member

    Generally in boat design the CLP (definition, see Par's post above) is used. Designers calculate it based on geometric properties of the design, but a fair bit of experience (more than I have, for sure) is needed to know exactly what needs to be included in the calculation for a particular style of boat, and with what correction factors for each component (rudder, etc).
    Strictly speaking the CLR (much like the "centre of effort" of a sail, as compared to the "centre of sail area") is a much more complicated calculation, as dynamic factors, flow separation, etc. can influence how the forces actually behave when the boat is moving in ways that cannot be predicted from geometry alone (expensive CFD simulation and a lot of experience is the only accurate way to find the true CLR). Some designers, though, do seem to use the terms interchangeably.
  5. Man Overboard
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    Man Overboard Tom Fugate

    Would a scaled model provide accurate enough data to determine the CLP on a full size vessel?
  6. Landlubber
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    Landlubber Senior Member

    CLR can easily be determined, especially for a model boat, by cutting out the underwater profile and balancing this piece of rigid cardboard onto a knife edge. Include the rudder and the keel of course, I am referring to the complete underwater profile of the vessel.
    the balance point is the CLR. It will be easy not to put this make back onto the profile drawing, do the Cof E for the sail plan, and then you can work out the lead of the vessel. Moving the Cof E back or forward changes the effect of the vessel to head to wind or away from it as the position is altered.

    If you do not understand all this, send me a message and I will explain in full.
  7. Don Case
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    Don Case Junior Member

    Thanks guys, Par answered my question when he said, "Yes."
  8. Petros
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    Petros Senior Member


    Hi, I just joined the forum yesterday and thought I would add my thoughts. I have seen the basic rule of thumb of taking the centroid of the underwater profile which is a pretty reliable simplification of the hull behavior, though the reality is a lot more complicated. Historically it work pretty good for most hulls. If I understand you properly, you seem to think that adding volume to the aft hull will change the displacement. The displacement will be the same, displacement is based on the weight; if your design weight has not changed than the displacement will not change. You will alter the location of the displacement, if you increase the displacement aft you will lose it somewhere else. It is possible you might make the weather helm worse by pitching the bow lower into the water, depending on the shape of the forward hull profile.

    It seems to me it would be simpler to either move the sail forward (by reducing the mail sail size, or moving the mast), move the keel aft (a little), or enlarge the rudder (or add a fin in front of the rudder hinge line). It is easier to control the location and size of the surfaces (sails, keel or rudders) than the shape of the hull.

    You could also balance your cargo load further aft, this will move the displacement deeper aft and will have the same effect as I think you were wanting. A more complicated change would be to make the hull narrower in the back, this will lower the keel line aft, increasing the profile below the water in the back. It is easier to move your CG aft if that is an option.

    Good luck
  9. PAR
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    PAR Yacht Designer/Builder

    Yep, a can of worms, though it's been a reasonable one so far, during this post.

    Many things can account for adjustments in the location of CLP. Correcting weather helm is a common association with the CLP's location.

    First, I'd determine if you actually have real weather helm and not another issue. More then 5 degrees of rudder deflection, close hauled in a healthy breeze would be real weather helm. If you have a heavy helm and less then this, you have another problem.

    If weather helm is established, the natural course is to rake the rig forward a touch to move the CE. Other modifications (some mentioned above) include moving lateral area aft, shortening the main area, usually along the foot, a bigger headsail, rudder modifications, etc. Trim can effect balance on a model much more so then a full size yacht.

    If the hull form/appendage(s) can afford the addition, place some more draft or drag in the appendage well aft.

    Sorting these types of issues out generally divide the novices from the pro's pretty quickly. Make one adjustment at a time and test it. This way you can be sure of what fixed it, rather then guessing.
  10. messabout
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    messabout Senior Member

    The CLR moves around some too. Different angles of heel also change the line connecting the section centroids which affect the helm in a signifigant way. If that's not enough then consider that the speed of the boat will cause things to change. As the boat approaches hull speed there s a big valley in the waterline and so the middle of the boat will have less area in contact with the water. Really shrewd designers sometimes figure all that stuff into their drawings. That is what separates excellent design from the rest of the wannabes like me.
  11. Earl Boebert
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    Earl Boebert Senior Member

    See my contribution to the "Model Yacht Balance" thread.

    The point about the wave form at hull speed is very interesting. Most of the balance analyses are static, i.e., assume a level plane of water when the boat heels. Determining the actual "wavy" waterline at speed may yield some interesting insights.


  12. percyff
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    percyff percyff

    The effect of flow on an underwater (or in air) foil is to move the point of action to the 25-35% length point. Long, full hulls, such as HMS Victory, will have a point of action forward of the geometric waterplane area centre, moving perhaps fifty feet forward. This can be seen by looking at the rig, which is significantly biased forward needing the bowsprit, whereas the spanker has no overlap at the stern. Each sail will also have an active point of action which moves forward, on Victory because there are so many sails, the net effect is smaller. On a single-sailed boat the effect is more marked.
    For a 'saucer' shaped dinghy, underwater only the centreboard and rudder will show this effect, and then only within the foil. All this is further compounded by the effect of heel.
  13. Malas Mañas
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    Malas Mañas Chalana 24'

    Is not easier move the rig aft?
  14. PAR
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    PAR Yacht Designer/Builder

    To ease the weather helm on your model, I'd first try more fore triangle area. You can accomplish this a few different ways, all relatively easily on a model.

  15. Pericles
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    Pericles Senior Member

    I have a model sailboat from 1943. It always headed up until a bowsprit was fitted and the boom shortened. The jib is slightly the larger of the two sails and the boat sails very well now. Adjusting the sails changes the heading, then it's a swift gallop around to the other side of the lake. :D

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