Dinghy LCB

Discussion in 'Sailboats' started by Men, Aug 17, 2019.

  1. philSweet
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    philSweet Senior Member

    I probably should have gone into more detail. I mostly just wanted the OP to check he had things right way 'round. The OP said in their second post -
    The DSYHS uses aft to fore measurements, and the series LCB range is 0 to -8.2% wrt midship. That's why I posted my concern. I agree that small boats are routinely described using all sorts of different coordinate systems.
  2. DCockey
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    DCockey Senior Member

    Very valid to suggest the OP should verify that they have the correct measurements for the data they are using.
    In the first post the OP mentions "Principles of Yacht Design" as the source of "it should be around 4% of LWL aft of midships". Fortunately PYD is explicit about how they are describing the LCB location. In other texts it is not alwasy as clear.
  3. Men
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    Men Junior Member

    Thank you for your replies. At the moment I am trying to gather lines plans from suitable designs (Wayfarer like) to model them and define more precisely their LCB, but as I am also finishing my dissertation, I have limited time. What I have so far is photos, that give me the impression that older designs have much more volume forward and thus my feel that they have LCB forward of midships, in other words they are cod shaped / with a bluff forebody (see attached pdfs). This is what puzzles me. Do the designers try to optimize for speeds lower than 0.3 Fn, or is there another reason?

    LWL as in PYD, I also define the full length of the WL.

    Attached Files:

  4. TANSL
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    TANSL Senior Member

    The final shapes of a hull are the result of trying to fulfill in the best possible way a multitude of commitments, not only the position of the C of B but many other things. In any case, when you try to optimize the LCB you must clearly define for which load condition you want to optimize it. What is optimal in some circumstance can be very unfavorable in another.
    Last edited: Aug 21, 2019
  5. Richard Woods
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    Richard Woods Woods Designs

    Neither of the two designs you posted can be considered "Wayfarer like" so I'm a bit confused.

    Traditional small boats were "cods head mackerel tail" shaped. Uffa Fox in the late 1920s was one of the first to move away from that approach. Jack Holt and Ian Proctor followed his lead in the 1950s. But all the designs from those three were full forwards with a deep forefoot. Very different from the FD 505 Fireball and later the Morrison/Bethwaite etc designs. All the latter are faster, like for like, and also less squirrelly offwind and easier to sail. Personally I have sailed FD, Fireball, Albacore, Firefly, Oday17, R200, R400, FF15, Wayfarer, Enterprise, Mirror, Miracle, Beaufort. And many more from those designers mentioned.

    In singlehanders the Sunfish and Laser have similar length, sailarea and weight, yet the Laser is a far faster boat. The Sunfish is another codshead mackerel tail hull, very rare to see in dinghies. the RS Aero is a lot faster than the Laser, easier to sail even though it is nearly half the sailing weight. Again I have sailed a lot in all three designs

    You don't want to get too hung up with one design feature. as we professionals have said many times here. You start with a SOR and then work round/through a design spiral to get the concept how you want it. Usually the linesplan will be one of the last details to work on. Certainly you don't want to draw that until you have a complete idea of what the design should be like

    Richard Woods
    Last edited: Aug 21, 2019
    philSweet and Ad Hoc like this.
  6. philSweet
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    philSweet Senior Member

    Yes, older designs had less volume aft. There are a small host of reasons for this, mostly fitting into two categories. Firstly, as a direct result of material strength issues. Secondly, as a result of improved average performance - largely a result of improved material properties.

    The max beam was often where the shrouds landed. This was to lower mast compression and lighten the masts. Modern masts are 1/4 as heavy as good solid wooden dinghy spar. It paid to flare the hull to widen the shroud base and reduce mast weight.

    Boats hulls were a lot heavier prior to WWII. Conventional keel, frame, and plank construction put a crimp on performance. In particular, the keels and skegs demanded a metacentric shelf method to manage heeled obliquity. The keels on those hulls were draggy when experiencing leeway. Getting rid of keels and skegs, such as the Jet 14s and FDs, opened up the possibility of more heeled obliquity without the drag penalty.

    Rigs became more developed; and improved shape control, along with the knowledge to use it, provided higher average sail power and lower aero drag. Foils also became a bit more refined in order to exploit the better rigs. This pushes the performance window to higher speed ratios. In particular, planing was now fairly common, and designs that minimized the resistance hump and transitioned smoothly were needed. You used to need a ton of forward volume to get over the hump and onto a plane. Better sails and foils lowered the bar and you no longer needed so much forward volume. Higher speeds reduced the wetted area when planing and allowed the planing waterline to be further aft.

    As boats got both lighter and faster, the canoe body draft became less compared to the height of nearhull surface waves. There is a fundamental hydrodynamic issue in this. The nonlinear waves near the hull offer less resistance if the entry angle at the surface is small and the waterline has low curvature. This contrasts with the farfield wake energy which is minimized with a particular displacement curve. The lighter the boat, the more the nearfield, near surface components factor in. This inevitably pushes the LCF aft, and as you look at deeper waterlines, you can only drag their centers forward so fast. With shallow boats, you just run out of draft before you get the displacement curve where you'd like it to be. So you end up trimming nose down in light air to improve the displacement curve, lower form stability, and perhaps lower wetted area. Note that lowering form stability reduces drag. That is something you can't do as well with a cod headed hull when trimming the bow down.

  7. gonzo
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    gonzo Senior Member

    I would disagree with that statement that boats of over a century ago were much heavier. Consider sailing canoes for example.
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