Dinghy 13 ft with bi-convex sections option

Discussion in 'Sailboats' started by Dolfiman, Apr 22, 2019.

  1. Dolfiman
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    Dolfiman Senior Member

    I understand that it is usual for some designs to help start planning (RS Aero), but some other modern design like the Melges 14 have not such chine although good all-around performing dinghy. So I will stay simple without that kind of chine.
     
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  2. gggGuest
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    gggGuest ...

    Its more complicated than that. The sharp transition and the steep topside gives less wave drag and a number of other beneficial consequences. But those may well not be important or even useful for your design aims.
     
  3. Dolfiman
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    Dolfiman Senior Member

    Thanks for this another instructive link ! (for me), the one above and also this one a bit lower in the same web page, telling the story of the successive breakthroughs in Moth design :
    And now for something completely different - The International Moth story https://www.yachtsandyachting.com/news/215173/And-now-for-something-completely-different
    (to note some minor mistakes in the text about french sailors : Marie Faroux >>> it is Marie-Claude Fauroux, and idem his well known brother is Jacques Fauroux. He is now semi retired but his naval architect cabinet is still active, led by his son Nicolas : Constructeur bateaux à moteur à Mougins, voiliers, catamarans et régates à Cannes, SARL Groupe Fauroux http://fauroux-construction-bois-06.com/naval/constructeur-bateaux-a-moteur-mougins-voiliers-catamarans-regates-cannes.html#compétences)
     
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  4. Dolfiman
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    Dolfiman Senior Member

    Back to my initial objectives (as detailed in quote #1), I have eventually converge to 2 versions : Bi-convex soft and Convex-concav-hard chine, sharing (quasi) the same dimensions, inc. beam waterline Bwl (I am fixed on a Bwl ~ 1,00 m, in relation with the design payload 95 kg), hydrostatics, stability and righting moment features. On the performance side, I wonder what could be the best one, hoping at least that both could give fun to sail without demanding skills re. stability and physical/acrobatic posture re. RM. Probably that I have a preference for the Bi-convex soft one, it is more subjective than rational I confess.
    Versions like the Bump one are really another approach, where Bwl is deliberatly important (Bwl/Bhull > 0,8) to focus on the stability, and requiring a sailing at 10° to 15° to reduce the wetted surface.
     
  5. Dolfiman
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    Dolfiman Senior Member

    Well, I did it eventually … :) , i.e. introducing such lower hard chine as transition between the convex and the concav part of each sections. I converge to a Bwl of 1,12 m, a bit more than the 2 previous versions but the wetted surface at 10°-12° heel angles is the same. That leads to greater stability, both initially (+ 75% for the GM at 0° heel angle when the payload 95 kg is at the center) and for the dynamic reserve during manoeuvers (I compare the aera below the GZ curve for the 0°-25°, still for the payload at the center). As regard performance, … ?
     

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  6. gggGuest
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    gggGuest ...

    That's looking quite nice...
     
  7. revintage
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    revintage Senior Member

  8. Dolfiman
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    Dolfiman Senior Member

    Thanks Doug and gggGuest for your likes.
    I had also a look on the new (2018) First 14 Beneteau, a « convex-hard-concav » (to take my non-poetical typology) dinghy hull, 4,27 m (14ft) x 1,70 m, designed by Sam Manuard who comes from the Mini650 world :
    - the Bwl seems quite generous (not minimalist at least) to offer a pleasant stability, for one or two persons,
    - the originality is to offer one, two or three sails which can be operated like by an ocean singlehandler during his day sailing depending of the wind force or direction : main, main + jib, main + assymetrical spi, main + jib + assymetrical spi.
    - and to keep a good balance, two daggerboard wells are proposed, the second behind the first on the central line : with the mainsail only, you use the aft well, with the main + jib the fore one, or even with the two daggerboards back to back if necessary in some cases.
    Web site : First 14 | Beneteau https://www.beneteau.com/fr/first/first-14
    A light trolley design is also proposed :

    In planning mode :
     
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  9. CT 249
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    CT 249 Senior Member

    The 14 looks to have some nice touches, but a couple of points concern me. One is the trolley concept;. trying to get the screws (or whatever they are) in place in a chop and sandy water could be a major problem, but of course the bolt-on wheels could work on hard ground and flat water.

    I still don't understand the idea of having the BMAX at the transom, which is what the 14 seems to have like other Open-inspired boats. It's similar to the concept that was seen to be flawed years ago in development class dinghies. There are advantages but the development classes tended to move to having comparatively narrower sterns to allow the stern to sink low under crew weight and pop the bow out to prevent nosediving, as with the Merlins and NS14 below.

    images.jpg P2210443s.jpg

    2009-03-31_085925_flight24.jpg
     
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  10. OzFred
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    OzFred Senior Member

    Though in the planning video it does seem to keep the bow up nicely, and the gunwale is cut away near the transom so the crew isn't expected to sit on the back corner. I'd like to see it going downwind in +20kn and decent chop to see if it works though.

    A lot of trim issues could be solved, or at least attenuated, with a horizontal foil on the rudder, a la I14. I really don't understand the reluctance of classes, particularly new ones, to adopt it. Not one seems to think it's a good idea, even though it's well proven in I14s and, to a lesser extent, Moths ("lesser" because they transitioned to full foiling before they optimised their rudder foils, IMHO of course).
     
  11. Dolfiman
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    Dolfiman Senior Member

    Thanks CT249 and OzFred for your interest and comments. Ok , I have explored a more forward Bmax version inspired by the picture of the NS14 hull (>> V7 attached). However, for the fore waterlines, I prefer keeping straight lines (without inflexion) and a bit of roundness at bow very end, it is supposed to be better at « Froude 0,4 » speeds (the bow wave is more forward >> longer effective Lwl) and downwind at high speed (more volume to prevent nosediving occurrence). These are arguments of NAs for Open's formulas (since ~ 2010) and I suppose it is also valid for dinghy design up to a certain point, likely with less roundness than seen for the Open's last generation. I have also reduced a bit the hard chine upper volume and accentuated the sections concavity for a better water release at high speed, that does not change significantly the stability features / V6.
     

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  12. gggGuest
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    gggGuest ...

    The NS looks to be a rule limited shape though - seems like a mid length rise of floor rule shape to me rather than something purely for hull behaviour. There is a rise of floor measurement at 2.5m from the stem in the NS rules. When the new canoe rules were written they specifically allowed the rise of floor limit to float fore and aft, and the point of max chine beam is well aft of mid length in all the designs I've looked at, even with the Canoe stern
    Exactly parallel chines so the stern was the same as max chine width was popular for a few years then abandoned in the Cherubs mid 70s/early 80s. It made the boats too binary I think. Not least is the extent to which the corner digs in when heeled.
     
  13. OzFred
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    OzFred Senior Member

    Yes, there's a minimum beam I guess to prevent the evolution of ever narrower boats, exemplified by skiff Moths (and some 12' skiffs).

    There's also a max beam measurement to prevent excessive flare or wings. But given that development classes continue to make the transom narrower than max beam, it seems like it's a good idea. Even skiff Moths at around 30cm wide are narrower at the transom than their max beam, the foilers are too.

    It just occurred to me that a real test would be a development class with a minimum beam that doesn't specify where it is. If that ends up being at the transom then likely it's faster than having it further forward. :)

    Bladerider sm.jpg
    Bladerider Moth.
     
    Last edited: Jun 29, 2019
  14. Dolfiman
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    Dolfiman Senior Member

    Well, I am discovering all these interesting info about NS14, thanks !

    Here is the Ns14 class rules, the dimension constraints are in chapter 4. :
    https://ns14.org/-/ns14/lib/pdfs/NS14ClassRules_September2017.pdf

    And there, Ns14 history and design, inc. the isometric drawing of Tequila 96 model : following the black lines, is an hard chine existed for the transition convex / concav ? (it seems not) :
    NS14 Association of Australia - Class Information https://ns14.org/classinformation#morehistory
     

  15. Dolfiman
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    Dolfiman Senior Member

    When looking at the Moth design stories over 50 years, it is clear that the evolution mainstream is based on both a reduction of hull stability (more narrowness at waterline) and an increase of the righting moment (more acrobatic eccentrical hiking) with the bet that helmsmen skills can evolve in parallel and get it (through more training though). That evident consideration just to underline that judging the proper quality of a dinghy hull shape has meaning only under conditions of equivalent stability and hiking posture.
    I am afraid that should make loss the role of this rule to assure an equivalent stability for all designs, and likely "open the bar" for extreme designs. The beam requirement rule 4.3, i.e. to be > 1,22 m at section 2,50 m from stem ( = ~ 40% Lwl from aft) and at height 200 mm / keel line (so about 100 mm above the usual waterline when payload is 130 kg) + the additionnal in rule 4.5 (no hollow of this hull section below these reference points) are a way to assure an equivalent stability upright and with heel angles (the beam requirement being above the usual waterline). Another version of your suggestion would be to rule such beam minimum at 2 sections anywhere provided that these two sections are spaced from (say) 1.2 m, then we will see what will be the sections choice, at the aft or still in the middle.
    Here attached, I have explored various hull sections, including the bi-convex ones with soft or hard transition (my initial goal to add more capsize prevention at heel angles > 10° without degrading too much the performance), for a 14 ft dinghy in the NS14 rules, confirming that the current rule is efficient to lead to equivalent stability features (as I approached them with the ref. payload 130 kg at center and height 65 cm, and computing the GMo and the GZ curve) , with just some second order differences.
     

    Attached Files:

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