Dinghy 13 ft with bi-convex sections option

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

  1. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    I have investigated this option in the idea to enlarge the beam overall (to maximize the righting moment from the helmsman in hiking position) while maintaining at minima the beam at waterline (to maintain low the drag) but with offering more managable stability (especially during tack or gybe manoeuvers by rough seas). For user with less kills and/or seaching a more easy but still fun dinghy.
    The convexity can be more or less accentuated, here attached I have chosen a medium shape, for a comparison with a convex-concav one (a more common design) built at same beam overall and with exactly the same immerged hull waterlines.
    The main result is about the stability when the "payload" (i.e. the helmsman) is temporarly in the center of the dinghy : the bi-convex option can offer a greater tolerance.
    The main unknowns are about the behaviour at speed on waves :
    - the bi-convex leeward side can act as a spray rail (less wetty for the crew), can add some dynamic lift when heel is 10° or over.
    - … but at the cost of some extra drag, of some slam occurrences,
    Thanks in advance for your comments.
     

    Attached Files:

  2. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    I worked again on this bi-convex option, being not satisfied by the sharp point transition between the two convexities, a probable source of extra drag and moreover not structurally good. So I introduce a complementary function able to generate a soft transition, as illustrated by the 2D presentation attached. With that tool, I can propose Bi-convex soft versions, one with the initial Bwl 0,92 m and one at Bwl 0,97 m to show also the influence of this data on the stability tolerance, the document attached gives a full comparison with the previous Bi-convex sharp and Convex-concav versions.

    The fun of such numerical tool is that it also works reversely, able to generate a bump instead of a concavity. So, I have not resisted to also investigate a Dinghy Bump version, but then of course for a very different objective : a dinghy with a lot of stability tolerance, offering an extra RM even at small heel angles, at the cost of more Bwl (1,20 m) and + 15% of wetted surface : original at least, can be interesting for a sailing school, or for a family or expedition programmes.
     

    Attached Files:

  3. Doug Lord
    Joined: May 2009
    Posts: 16,496
    Likes: 289, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Dolfiman, I love the "bump" version , most especially for Emilie's application. Of the others I like soft 2.
    But, in my opinion, the "bump" is by far the best: it is modern, very good looking and impressive. Hope you post these in Emilie's thread.
     
  4. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    To me, it looks as if rough water performance will be very problematic. Singlehanded dinghies suffer dramatically if they have much flare in the bow; the Laser may be something of an exception but good Laser sailors work furiously to ease the boat's passage through waves, and I believe that compared to contemporary boats the Laser had a fine bow angle that alleviated the issue.

    I would suspect that the disturbance to the water flow caused by the crease may increase drag more than the simpler alternative of increasing waterline beam.

    Personally I am very dubious about Open-style dinghies. They ignore the lessons of about 30-50 years of development in Moths, Int 14s, 18 Foot Skiffs, 12 Foot Skiffs, Merlins, National 12s etc. When dinghy designers created Open-style boats they did not ignore the 20 years of development in Opens that had gone before them, so why should the much longer and more active period of dinghy development be ignored? As a simple example, even the Australians largely abandoned wide stern dinghies in the 1980s, because they had more wetted surface and inferior handling qualities.

    Sorry to be negative, as I do like much of your work. My remarks may not apply in your area; some European designs perform poorly down here where the water tends to be choppier and I may therefore be putting too much emphasis on this area.
     
  5. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    Dear Doug and CT 249, Thanks for your interest and comments,
    That's the question, I have no underpinned answer on that point, but it is true that just a small increase of the waterline beam Bwl can give rapidly more stability margin : I investigate that here attached by comparing Bi-convex soft 2 (with Bwl 0,97 m) with others more classic versions (Convex-concav and Convex-straight) sized up to obtain similar GZ curve (for the "payload at center" case) >>> with Bwl of just 1,00 to 1,02 m for the classic ones, we can have such equivalence. That inclines to think that it does not worth to search a very low Bwl/Bhull by the bi-convex artifice if no other advantage revealed.
    Actually it is another approach of the dinghy, where a lot of stability tolerance is the key point, for another market segment : beginners, holidays clubs, family ballads, coastal expeditions, …, not obsessed by high speed surfs but just searching for pleasant sailing. The confusion is may be that such design are derived from Open style Imoca (like the Phileas 360 by Finot-Conq), suggesting high performance while actually this design style at dinghy scale is for other objectives, stability, RM through hiking without physical/acrobatic posture, capsize occurrences very reduced. The Bump version is on this line as well as the FC13 (attached) that I did also, directly inspired by Phileas 360. They should be sailed at an average 10° to 15° heel angle, which reduces the wetted surface and gives a bonus +20% to the initial RM provided by the hiking.
    European waters have indeed a broad variety of coastal sea states, with various typical winds and waves periods : e.g. it is well known that in Med sea, the wind is lighter in average than in Atlantic, the periods are shorter (which can makes upwind sailing troublesome), the swell being quasi inexistant, mostly just the waves due to the local wind.
    Phileas 360 : Phileas 360 http://www.finot.com/bateaux/batproduction/phileas/phileas360/phileas360_pres.htm
     

    Attached Files:

    Doug Lord likes this.
  6. gggGuest
    Joined: Feb 2005
    Posts: 803
    Likes: 16, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    Interesting concept. I do worry about waves though. I had an early 70s glass international Moth which had had the bottom cut off and replaced with wood, and when I replaced the add on bottom I ended up with just a ghost of your double curve towards max beam where I was balancing what I wanted with what I already had. There were no obvious issues, but the amount of hollow was minimal, just a few mm.

    It might be interesting to compare those options with something like this. A lower freeboard and aggressively curved sidedecks giving similar reserve buoyancy at large angles of heel, but a more conventional shape. Its just a wild idea, I have't analysed it at all for the implications. Probably be a royal pain in the neck to figure out a deck/hull seam joint too.

    Graphic1.GIF
     
  7. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    Many thanks gggGuest for your comments and this relevant suggestion that I have try to translate in this Convex-concav-hard chine version attached : the concav upper end is then lowered to a hard chine line below the sheer one, generating this reserve buoyancy acting only beyond ~ 15° heel angle while high enough to preserve at the best the first 0°-10°. The obtained GZ curve, compared with the one of a simple Convex-concav, clearly shows the difference : the upper volume prevents the drop of the righting moment for 15° to 25° angles of heel. To note also that for a lighter payload (45 to 75 kg), the stability increases although the reserve buoyancy acts for higher heel angles >>> no need to change the hull shape. Reversely, for payload over 95 kg (e.g. 2 persons), the reserve buoyancy acts early with the counterpart that the hard chine line closer to the water can give extra drag : stability is still assured but we are no longer in a performance configuration. So, such stability issue could be approached through 2 complementary and relatively independant points : a sufficient Bwl for the stability at low angles, some upper volumes able to prevent/delay most of the capsizes occurrences.
     

    Attached Files:

  8. Doug Halsey
    Joined: Feb 2007
    Posts: 376
    Likes: 81, Points: 38, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    Your convex/concave shapes with hard chine are starting to resemble some of the Moths from the 1960's.

    Here's mine, designed in 1965:
     
    Dolfiman and Doug Lord like this.
  9. gggGuest
    Joined: Feb 2005
    Posts: 803
    Likes: 16, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    Another consideration for a recreational craft is shipping water over the gunwhales. Be good to ensure that the topside volume at angles of heel also reduces swamping, which is nearly as inconvenient as a capsize unless the boat is self draining.
     
  10. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    Great ! , with more rounded sections and a small Bwl (Bwl/Bhull ~ 0,50 !) , did these designs give satisfaction ?
    I derived another 13' version (Convex-concav hard chine 2 attached), Bwl = 0,81 m (/ 1,02 m for my previous drawing), compatible for a 80 kg helmsman as design maxi payload. The stability is of course rather low at small heel angles, but again, when the heel becomes > 15°, the upper eccentric volumes can contribute significantly to the righting moment and the GZ slope increase up to give similar values at 25° / version 1.
    Yes, to prevent spray and/or some green water from forward is also a concern. With that in mind, I did also (attached) a version 3 with another hard chine line ending down, with a curvature so that the leeward line is roughly horizontal at heel 10° : is this shape can better work for the spray rail function ?
     
    Doug Lord likes this.
  11. Doug Lord
    Joined: May 2009
    Posts: 16,496
    Likes: 289, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Dolfiman if you want to why not explore a shape with a "chine" like the ones circled in red below from gg's sketch:

    GG.GIF
     
  12. Doug Halsey
    Joined: Feb 2007
    Posts: 376
    Likes: 81, Points: 38, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    Well, I hate to brag, but since you asked: I won 2 U.S. National Championships & was runner-up in 2 World Championships with this boat.

    This was not a very extreme design by Moth standards, even for that era. Its Bwl of 30" was about the same as the V-bottom shapes that were most successful previously, and Bwl of later designs became much smaller. Bhull of 60" when first built grew to 72" within about a year, after I learned of the deck extensions ("wings") on the Swiss designs.

    By the standards of normal small dinghies though, this boat (& most other Moths) would be considered too tippy & difficult to sail, so I don't expect to see you using similar proportions.
     
    Doug Lord likes this.
  13. Dolfiman
    Joined: Aug 2017
    Posts: 811
    Likes: 376, Points: 63
    Location: NICE (France)

    Dolfiman Senior Member

    Thanks very much for these info. A good friend of mine oriented me towards this cool blog, from which I extracted the articles concerning Classic Moths, narrow waterline, deep Vee, … , I have not yet read it all , 202 posts !!! :
    Earwigoagin: Classic Moth http://earwigoagin.blogspot.com/search/label/Classic%20Moth
     
  14. Doug Halsey
    Joined: Feb 2007
    Posts: 376
    Likes: 81, Points: 38, Legacy Rep: 160
    Location: California, USA

    Doug Halsey Senior Member

    That's a very informative and entertaining blog, but you should keep in mind that "Classic Moth" used there & more generally in the U.S. refers to the version of the Moth as it existed in the middle 1960's.

    The International Moth class has continued to evolve since then, and now consists entirely of foilers. However, there is another organization called the "International Moth Lowriders" that allows the more recent developments, except for foils. These would be more indicative of the fastest dinghies of their size than the "Classic Moths."

    By coincidence, they recently completed their UK Championship as reported here:
    International Moth Lowrider National Championship at Carsington Sailing Club https://www.yachtsandyachting.com/news/217949/International-Moth-Lowrider-Nationals?fbclid=IwAR0jwL-OrWy2wVn5tmfjoOpf0Eeyj8k4dIruzSxGJdTKWJNNeOn37r4W24c
     
    Doug Lord likes this.

  15. Doug Lord
    Joined: May 2009
    Posts: 16,496
    Likes: 289, Points: 93, Legacy Rep: 1362
    Location: Melbourne, Florida

    Doug Lord Flight Ready

    Never heard of the "...Lowriders" -thanks for the info!
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.