Understanding the relationship between CLP and CE in sailboat design.

Discussion in 'Sailboats' started by Caldera Boats, Dec 6, 2005.

  1. Caldera Boats
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    Caldera Boats Beer4Ballast......

    This thread started here:

    http://boatdesign.net/forums/showthread.php?t=9669

    I am relocating it to the proper area....
    ------------------------------------------------------------

    I have a few questions about CB location that I need to learn more about.
    Can anyone recommend a good book or reading on this subject?

    Is there a good standard theory for finding centerboard location?

    Normally, the sail plan C.E. is located ahead of the hull's CLP by a percentage of the boat's waterline length.

    What percentage for gaff verses other types of rigs?

    What others factors need to be taken into consideration?

    How can I ensure some (but not too much) wether helm?

    I understand that formulas will only get me close and the final tuning will need to done during the testing stage.

    Thank you for your input...

    R.M.Domico
     
  2. Caldera Boats
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    Caldera Boats Beer4Ballast......

    Which comes first? the hull plan or the sail plan?

    In the case of a centerboard sail/row boat, I would tend to think the hull shape is designed first, next the centerboard size and location, along with the sail plan?

    To put it more simply:

    What dictates what? Does the sailplan dictate centboard location or the the centerboard location dictate sail plan?

    ah, those burning questions.......
     
  3. PAR
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    PAR Yacht Designer/Builder

    When it comes to the "lead" which what we term the relationship between CLP and CE many attempts have been made to generate a graph, formula or basic average, but these efforts haven't worked out very well. There are just to many variables that can effect the balance. Ultimately it comes down to experience, comparison with know types, guess work and some luck. This may seem harsh, but it's the truth.

    The hull form is designed with a sail plan type and performance envelope in mind. The preliminary drawings generally fix locations (hull form, sails, appendages, accommodations, tankage, engine, etc.) but these will get adjusted, some possibly radically moved as the design evolves closer to a finalized form.
     
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  4. RHough
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    RHough Retro Dude

    "Leeway for Dummies" is a study of the relationships between Lateral Area and Lateral Force. I'm the Dummy. :D

    "Lead" is described as the distance on the fore and aft axis between the Sail Lateral Area and the Lateral Area below the water line.

    There is a relationship between Lead and the tendency for a sailboat to turn towards the wind (Weather Helm) or away from the wind (Lee Helm).

    Lets call the fore and aft axis of the boat the x axis, with higher values of x towards the bow.

    The boat balances at the CLA point on the x axis.

    The sail balances at the CE point on the x axis.

    The pressure of the sail acts on the boat at the CE point on the x axis.

    The Sail acts opposite to the Boat, the sail pushes to leeward, the boat pushes to windward.

    Imagine the boat as a teeter-totter balanced at the CLA. LeeHelm sits on the left end, WeatherHelm sits on the right end. Along comes Sail and sits on the teeter-totter. If Sail sits on the right side the teeter-totter no longer balances, LeeHelm goes higher and WeatherHelm goes lower.

    This means that any lead will cause lee helm. We know that boats have weather helm with lead, so there must be some force to get the teeter-totter to balance with weather helm that just the relationship between areas does not show.

    The area relationship assumes that the force is the same over the entire area. The only way we can have weather helm and lead at the same time is if the force is not the same over each area. To get balance, the sail area force must act opposite the boat area force.

    Since we know that to get weather helm the sail force must act behind the boat force on the x-axis we know that the centres of area and the centres of force are not the same.
     

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  5. Caldera Boats
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    Caldera Boats Beer4Ballast......

    Excellent Job! That is one of the clearest explanations I have heard yet. :)

    Looking over your other post
    I find it interesting that a flattie would use 0% lead, could you explain?


    Lead (CE forward of CLR)*
    From various designers:
    Shallow hulls with fins: 12-14% LWL
    Deep hulls: 10%
    Classic Cruiser: 8%
    Scow: 5-15%
    Centre Board Yacht: 7-11%
    Normal: 6%
    Schooner: 7-12%
    Ketch: 11-14%
    Yawl: 12-15%
    Sloop: 13-17%
    Flat Bottom: 0%
    Sloop: 7%


    R.M.Domico
     
  6. RHough
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    RHough Retro Dude

    I'm getting to it, I don't have an answer yet ...

    The average lead of the suggestions above is between 10 and 11% of LWL

    One designer says a sloop should have 7% lead, another says 13-17%

    I have no idea what a "Normal" yacht looks like, but the designer says it needs 6% lead.

    Scows tend to have pretty flat bottoms and need a lead of 5-15%, while another designer says 0% for a flat bottoms.

    Either these guys have no clue or I'm missing something. Some mystery force "?" that acts to turn the boat into the wind.

    When we looked down on the boat, the balance teeter-totter can't work if CE and CLA are on the same line.

    Something else must be happening.

    I drew a 3-view of the x line and placed a "?" arrow where the force must act, and placed the CLA and CE force arrows above and below the x axis to see if anything looks different.

    The first thing that you see is that from the top the forces must turn the x-axis "boat" in the x,y plane (horizontal) and also tip it in the x,z plane (lateral).

    To keep from confusing my self:
    x,y plane is horizontal ... like the water, with x+ to the bow, y+ to the wind
    x,z plane is vertical with z+ up and x+ to the bow
    y,z plane is also vertical, but 90 degrees to the x,z plane X+ = up, y+ = to wind

    Now I can keep it straight in my head ... maybe.
    X+ values are to the front
    Y values are + into the wind and - away from the wind
    Z values are + acting up and - acting down

    I'm going to build a virtual boat with a sail whose CE leads the hull's CLA by 10% of LWL since that is close to the average of what the designers use. I'll make the LWL = 100ft so the match will be easier. That means the CE will be 10 feet forward of CLA.

    Hmmm ... CE is trying to tip the boat over ... I'll have to add something to make this right ... back with a moment :)
     

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  7. PAR
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    PAR Yacht Designer/Builder

    RHough, you are missing things and those figures shouldn't be trusted, but used as a very rough guide. The flat bottomed ketch I just designed has a 7% lead.

    Models like this have been attempted, but they get so large they become unwieldy. An Americas Cup boat would do well to model their boat this way, but cost estimates for an accurate model run in the several 10's of million dollars. This is why we see two and three hulls from a designer, all having lots of changes, before one is selected to race, it's more cost effective and the models can't account for all the variables. Such as how far outboard and forward does the CE move in a gust, accounting for fabric stretch, mast bend, hull flex, etc., besides the normal components of equilibrium. Even end of boom/boat sheeting can effect the helm in smaller craft.

    Pick up a copy of "The Design of Sailing Yachts" by Pierre Gutelle.
     
  8. RHough
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    RHough Retro Dude

    Don't ruin my fun :)

    Let's see, Ketch = 14% ...Flat bottom = 0% ... average = 7%, Your design has 7% lead? Sounds like "the Rules" work just fine. :)

    I know it's only one data point. That it falls exactly where the "very rough guide" said it would struck me as humorous.

    I think it will be possible to estimate the lead required for a given design within the ranges that have been proved to work. 3D lifting line theory did not exist when many of these numbers got cast in stone.

    My intuition is that a boat with a high CE that sails at high heel angles will need less lead than a boat with a lower CE that sails at low heel angles.

    When I can identify the thing or things that cause a boat to have weather helm, I should be able to change each one and get a feel for how much weather helm each creates.

    I was able to write a BASIC program 20 years ago that predicted the flight polars for RC gliders very closely. Designs built to the dimensions suggested by the program performed as expected.

    This isn't rocket science and I don't think for a minute that a simple to use lead estimator in Excel will give me an exact lead number. But I'm pretty sure that I can predict a weather helm moment for a given design at a given windspeed.

    The weather helm moment and "balance" are not the same. I haven't quite decided how I'm going to define "balance".

    My first shot is that "balance" should be stable static yaw moment and possibly positive dynamic stability below the displacement hull speed limit. Static yaw stability requires that the forward foil is at a higher incidence than the trailing foil. The forward foil has to operate at higher CL than the aft foil. The hull will want to rotate in the yaw plane around a neutral point that is the CLP (Not CLA, since CLA assumes equal unit pressure).

    After I have a first level estimate of the forces I can start looking at excursions from the design point (gusts etc.) and predict response.

    While I'm not going to try to write a VPP, my goal for "Leeway for Dummies" is a Lead Prediction Program. I have to work this out for a design I'm working on anyway so I'm sharing my thoughts along the way.

    When I get as far as I can, I'd like some people to plug numbers into it and see how close I've come. :)

    May I ask that you follow along and wait to see if I've missed anything along the way? I'm not at a solution yet, there are things that I am working on that are not added to the picture.

    I get the impression that you are a meticulous designer, I value your input.
     
  9. Guillermo
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    Guillermo Ingeniero Naval

    RHough:
    As you know, relative positions for CE and CLA are only a mean of estimating the behaviour of a boat at the design process, and are based on designers' experience (And very variable in the figures, as we can see). But this doesn't mean at all the CE or CLA are really where we locate them in the drawings by simple geometric considerations. Those are "static" estimations for a level boat, just to guide ourselves.
    When sailing in real life, dynamics are on charge, and both CE and CLA move forwards, backwards and sideways, with speed, heel angle, hull forms, leeway, sails area & profile, boom angle, sheets tension, masts bending, etc, etc, etc.
    From my point of view, the only way to estimate these effects with some kind of accuracy, without the needing of expensive and time consuming real scale testing, is by performing CFD calculations, yet quite difficult and expensive for the average design.
    So, probably, the "oldie" way, some experience and a bit of luck, is all what is needed for 99% of the cases, as PAR states.
    But if you get a trustable Lead Prediction Program working on Excel or whatever, I will be most admired and glad to use it.
    Goog luck and enjoy it! :)
     
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  10. RHough
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    RHough Retro Dude

    Thanks, If I can get the range of CE to CLA lead for a configuration to less than the 0-17% LWL range and reliably predict witch end of the rage should be best, I've reduced a variable. That's my goal.

    I may end up having to "invent" new centres to use and then try to relate them backwards to traditional CE and CLA.

    If I call the 0-17% range a first level range of confidence, and I can cut that range to 5% LWL within the 0-17% range it will reduce the range that needs to be looked at with CFD, VPP's etc. thus reducing time and workload.

    At the very least I should be able to identify the features of a boat that make it more or less sensitive to geometric lead.

    I'm limiting my calculations to displacement speeds, since I have not the time or desire to try to analyze transition and planing forces on a hull. Besides which I don't have a target range of Lead for planing hulls to work with. I'm going to make a grand assumption that when planning off the wind the drag of the hull moves aft and the Lead increases. Preliminary numbers indicate that the lateral area of the hull has small effect compared to the lateral area of the fin and rudder, almost to the point that for shallow hulls only the fin and rudder need be considered to find a balanced Lead position.

    I may be re-inventing the wheel, but I haven't found an explanation of how to determine Lead that works for me. As I said, I'm the dummy. :)
     
  11. Stephen Ditmore
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    Stephen Ditmore Senior Member

  12. RHough
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    RHough Retro Dude

    Thanks! The idea of relating lead to heeling arm makes perfect sense! There are wildly different opinions on that. :) I've seen designs where the reduced heeling arm sail plan (reefed) has the lead the same, larger and smaller. Its the old reef the main and set a stays'l vs reef the main and set a yankee. Both are claimed to give the boat good balance, while one move the CE down and aft and the other moves the CE down and forward.

    Using c/4 of the keel also makes sense to a point. The c/4 point on chord of the keel is where the forces act (pretty close). For rectangular foils the measured chord is the same as the aerodynamic chord. For tapered foils the forces act at the c/4 point of the Mean chord, or MAC. The MAC is a bit longer than the measured chord. the position of the MAC is also effected by sweep in the c/4 line.

    I'm using a centre that is at c/4 on the MAC on the x-axis and at the centre of Area on the z-axis (plus the local hull draft).

    Once that model is built. I'll do the same with the sail plan.

    The first model will use a generic CDprofile for the hull and foils, with the option of plugging in data from another source. I'm looking for trends, not hard number predictions at this stage.

    I'm not going to try to optimize the sail plan. I'll leave that to the user.

    The "start with the mast here" concept will become "start with the CE here".

    Since lead is built into the design, and is fixed while the amount of y-axis yaw moment changes with conditions and trim, the lead position should allow balance with rudder trim within the range of conditions that the design is targeted for.

    For example in 0-12 knots with a design target of 10 knots, I want to predict the lead required for balance within that range. Above a certain wind speed the sail area will become too large and the CE will have to move for the hull to operate within it's envelope. The designer will be left to come up with a reduced area sail plan that will place CE where it need to be for each range of conditions.
     
  13. Caldera Boats
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    Caldera Boats Beer4Ballast......

    There are a couple of books I have been looking at to help better understand this stuff:

    Yacht Design Explained: A Sailor's Guide to the Principles and Practice of Design - Steve Killing

    Sail Performance : Techniques to Maximize Sail Power - C. A. Marchaj

    Has anyone read either of these? are they any good?

    Recommend any other good books worth reading????
     
  14. Guillermo
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    Guillermo Ingeniero Naval


  15. Caldera Boats
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    Caldera Boats Beer4Ballast......

    Thanks Guillermo,
    :)
    Thats a nice article on Helm control by Gerr.
     
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