Some dinghy design questions

Discussion in 'Sailboats' started by sawmaster, May 22, 2012.

  1. sawmaster
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    sawmaster Senior Member

    If I understand the theory of lead correctly(having the center of effort of sails slightly forward of the center of lateral resistance),its purpose is to prevent excess weather helm due to the clr moving forward in relation to the combined center of effort of the sails as the boat heels.Would'nt this result in lee helm on a dinghy that can be sailed flat?Also,when calculating center of lateral resistance, would'nt the entire underwater area of the rudder have to be included(assuming neutral helm on a beam reach)?-Hoping for clarification,thanks
     
  2. gggGuest
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    gggGuest ...

    All that theory is largely irrelevant on modern racing dinghies with decent hbydrodynamic foils which are sailed with a percentage of the side load on the rudders. You'll find in practice rigs get racked backwards and forwards large amounts without making noticeable change to the balance. Whether its 80% on the dagger board 20% on the rudder or 60%/40% really makes very little difference. What people call weather helm is usually a failure to keep the boat flat, and the extent is *much* more to do with the hull shape than the position of the foils.

    That stuff is of the greatest relevance in old style long keel boats with the rudder attached to the keel.
     
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  3. PAR
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    PAR Yacht Designer/Builder

    The exact opposite of your post is true ggg. The old full length keel, built down hulls could have their CE moved all over the place and still find a reasonable balance with some tweaking. I agree, a larger percentage of "weather helm" complaints are folks with something other then actual weather helm, but shallow bodied hull forms, with slender appendages and tall rigs need the balance to be much closer then what the old full keel boats would tolerate.
     
  4. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Looking at dinghies:
    It would indeed lead to a lee helm, when we look only from there.
    But on a reach your sails are further out and so they also produce a momentum to windward because of the lift which compensates to a certain degree.
    You also adjust crew position and centre board (or dagger?) position and further the sail trim has much influence too.

    I peronally share your view of including the rudder into the calculation.
     
  5. sawmaster
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    sawmaster Senior Member

    RE dinghy design questions :thanks Par,ggg and hakimklunker.Par,do you concur that including the rudder as part of the lateral plane is reasonable? Also,since I prefer a little weather helm shouldnt I place the ce slightly aft of the clr,say5 %of the lwl
     
  6. DCockey
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    DCockey Senior Member

    Don't confuse the so-called CE, CLR and associated "rules" about their relative locations with the physics of what is actually going on when a boat is sailing.

    If the sails are flat and on the centerline of the boat then the center of the aerodynamic forces acting on the sails will be close to the center of the sail area when the wind is directly perpendicular to the centerline of the boat.

    But while boats are drawn with the sails on centerline that isn't how they are sailed. Ease the sail out so the boat will actually sail and the aerodynamic forces move with the sail, and the resultant aerodynamic force will generally not go through the so called "center of effort".

    For different reasons the resultant of the hydrodynamic forces acting on the hull will generally not go through the so called "center of lateral resistance". A relevant analogy is a symmetric airfoil at an angle of attack. The resultant aero dynamic force on the airfoil is very close to 1/4 chord from the leading edge, not the mid-chord of the airfoil.

    In a previous thread on this topic I said:

    "CoE" as the geometric center of the sail area is generally not the location the the net aerodynamic forces act through.

    "CLR" as the geometric center of the some or all of the underwater profile is generally not the location the the net aerodynamic forces act through.


    and

    What is commonly refered to as "CoE" is NOT where the aerodynamic forces act while sailing. Likewise for the "CLR". So trying to figure where CoE should be relative to CLR on a first principles physics basis doesn't work. Lots of boats have the CoE ahead of the CLR, and have a weather helm. However putting the CoE in a similar relation to the CLR as other similar boats known to be successful is a good pragmatic, emperical way to locate the sails. Fabio Fossati in "Aero-Hydrodynamics and the Performance of Sailing Yachts" in discussing the use of such methods has said "they [lead method] are only valid for producing a boat design that we can consider balanced with respect to a series of previously analysed projects; otherwise they are excessively disconnected from the fluid dynamics governing the problem, and from this point of view there is room for in-depth research, ..."
     
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  7. PAR
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    PAR Yacht Designer/Builder

    It doesn't mater if the CLP or the CE are realistic or accurate or not, so long as the various things that affect their placement, are acknowledged and adjusted accordingly, they serve their usefulness.

    These particular centers, from a technical stand point aren't especially accurate as mentioned, but they're used for reference and comparative points only. This permits one to make generalities about them, for comparative purposes. For example you can generalize that a sloop of common proportions, might have it's lead between 13% and 17%, but this doesn't mean that all sloops would balance within this range.

    There are several factors that can affect lead percentage; hull form, entry, exit, sail plan, appendage shapes and configuration, even how the boat is sailed, etc. All of these considerations are examined in the lead percentage decision and even at this point, you'll be making a guess, so some adjustment possibility should be available come launch day.

    As to the specific question of including all or part of the rudder in the CLP calculation, the answer is, it depends. Typically on keel or skeg hung rudders you would include 1/2 the rudder area, but a spade probably not, though this isn't a hard and fast "rule" either. The other variables would enter the mix, pretty much forcing your hand as to the rudder's inclusion or not.
     
  8. sawmaster
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    sawmaster Senior Member

    Thanks again,I suspected that there would be a certain degree of "educated guesswork " involved in the decision ,was only looking for confirmation that my general understanding of the theory was correct.I intend to make arrangements to be able to rake the mast and also an alternate mast step position.Part of the fun will be in seeing if I can "guess right " the first time.--wish me luck !
     
  9. PAR
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    PAR Yacht Designer/Builder

    If we had a much better idea of the hull, appendages and rig, a better "guess" could be made.
     
  10. DCockey
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    DCockey Senior Member

    Back to the original question. Emphasis added.

    The effects of boat heeling part is a popular "theory" but is only part of the story, and generally a relatively small part. As mentioned above the need for "lead" starts with the fact that the CE and CLR do not represent the actual locations of the aerodynamic and hydrodynamic forces.
     
  11. sawmaster
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    sawmaster Senior Member

    thanks,Par Tapproxhe hull is a very simple flat bottomed affair,similar in appearance to Ted Gearrys flattie (Geary,18)--only scaled down to around 13.5 ft.--I know this is an ancient design but it was chosen primarily for ease of construction and light weight.(Just doesnt take much 1/4 ply to produce )Rig will be sqtop main of 80 sq ft and blade jib of 20. Exposed daggerboard area on preliminary design is 3600 sq in (10 x 36 )but this is not written in stone --rudder is 10x 30--again-open to sugguestions
     
  12. sawmaster
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    sawmaster Senior Member

    also thanks -to david cockey--accepting the fact ,as par stated That even though " the centers arent especially accurate as mentioned ,they can be used as reference and comparative points"--so no use to beat a dead horse--we are SO beyond that now,hopefully with the new info provided and maybe a little more, par can help me make a more educated guess
     
  13. philSweet
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    philSweet Senior Member

    The problem with including the rudder in CLR calcs is that if "properly" designed, you have nothing left to bare away with (drag spikes when baring off). The rudder should be designed to be lightly loaded unless this is some kind of speed trial course.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Certainly, some appendage configurations require the rudder to bear some burden, but the amount can vary pretty dramatically. Without a sail plan and hull lines, including appendages placement etc. is just speculation, without in necessary data. Can you post some drawings? Odds are if it's within the typical norms for it's configuration, a good guess at lead would be 15%. Less for lower rig aspect ratio, more for higher, plus the other variables.
     

  15. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Guess work: We recently had to 'estimate' a design decision that was similar.
    Not really feeling sure, we walked through the marinas and took relevant measures where possible and talked to the owners who kindly gave us their feed back about how they perceive their boat's performance.
    Back at the computer the found data where listed and sort of analized.
    This showed us a tendency and a range. Together with the statements we narrowed out our decision.


    Still building, this method needs to be proven of course. We may not have met the optimum, but cannot be too far off either. It serves our target.
     
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