14ft Racing Dinghy - Comments Please

Discussion in 'Sailboats' started by PI Design, Feb 13, 2010.

  1. PI Design
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    PI Design Senior Member

    Hi all,

    I've been spending my evenings working on a 14ft dinghy which is a two person, hiking boat (designed for approx 145kg crew, 75kg all up boat weight). It will have 11.7sqm of white sail and an assy kite of 12sqm (quite small, in keeping with aiming at man/woman team), so is not intended to be the very fastest thing on the water. However, I would like it to punch well above its weight and perform well in all conditions - a little quicker than a 470.

    I'd be very grateful for any feedback on the hull and knowledge you all have - in particular about the rocker, waterline beam etc.
     

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  2. PI Design
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    PI Design Senior Member

    And the lines plan:
     

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  3. PI Design
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    PI Design Senior Member

    And other views:
     

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  4. mcollins07
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    mcollins07 Senior Member

    The lines look nice. What material will you use?

    The transverse metacentric height looks a little low to me, but perhaps someone more knoweldgeable regarding that will comment.

    ~Michael
     
  5. PI Design
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    PI Design Senior Member

    Thanks. GRP epoxy foam sandwich.
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Its very good looking! Maybe a bit tippy? Not too much for the type,though.
    When its not planing perhaps the area forward might slow down tacking?
    Have you done any analysis of the planing charateristics of the boat?
    I'd like to see the sail plan-will it be fairly conventional?
    Are you planning on the boat being produced-or is it just for yourself?
    Good job-thanks for showing it to us!
     
  7. wet feet
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    wet feet Senior Member

    Did you ever see a Shelley Moth?The similarity of the sections is quite close and if the weight can be kept down,the performance might relate.
     
  8. Tim B
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    Tim B Senior Member

    PI,

    You look to have a very steep run aft for this sort of boat. This gives you a lot of forefoot, which could cause handling problems at speed. You also have fairly V'd sections well aft, where you could probably carry flatter sections, and give yourself more planing area. You could consider a moving the chine lower, and reducing the fore-foot significantly. Whatever you do, keep the sharp bow. It makes a really major difference in waves.

    Also, do a weight estimate, and find out where the CG is. I think you'll find it's further aft than the centroid of your sectional area curve (CB). Hence why the forward sections can be much finer on a dinghy than they can on a full-size yacht. You might also want to look at using Savitsky's planing algorithms to find out what sort of forward force you'll need from the sails. Hazen presented an aerodynamic model for sails, which you can look at to give you some idea of your sail performance. Please remember, though, that these methods are not perfectly accurate.

    Hope this helps,

    Tim B.
     
  9. Paul B

    Paul B Previous Member

    It looks to be too deep and too narrow on the WL.

    Here are a couple of examples from The International Fourteen 1928 - 1989 book. I think a shallower, wider form with a harder turn might be better for your purpose.

    I would also think about integrating the wings in a more elegant way.
     

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  10. Zed
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    Zed Senior Member

  11. Typhoon
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    Typhoon Senior Member

    Looks a lot like a B14 too.

    Regards, Andrew.
     
  12. PI Design
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    PI Design Senior Member

    Thanks for all the feedback. The Ns14 was actually the inspiration for the hull, so if it looks like one I'm happy with that!
    Taking on board some of the suggestions I have tweaked it a bit to broaden the waterline beam (96cm rather 88cm) and KM has increased to 0.85m. I am happy for this to be at the tippy end of the spectrum, but don't want to be introduce new levels of instabilty that only a Moth sailor could master. I have also made the run in more horizontal at the back, not much, about 0.7 degrees less, but it all counts, and reduced the forefoot a smidge.
    Thanks for posting the I14 hulls Paul, it is nice to have something to compare to. Compared to them my top speed will be much less, and I want the hull to work as well in light winds as blasting in 15kts downwind, so I am deliberately not going as flat and skiff-like, but its food for thought.
    Any pictures of a Shelley moth, Wet Feet?
    Rig will be fully battened square top main, on an over rotating wing mast (NS14 style) with an 8.7/3.0 main/jib split. Kite will be smaller than an MG14 one, but hull is about 10kg lighter. For me, benchmark performance is RS400, but in a boat essentially RS200 size.

    Updated hull:
     

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  13. PI Design
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    PI Design Senior Member

    P.S. Any thoughts on a decent/normal/acceptable amount of transom submaergence in the design static condition?

    Also, I read somewhere that Julian Bethwaite designs his hulls to displace about 20kg less than the expected all up weight, on the premise that dynamic lift will create about 20kg. Is this normal practise?
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Keep in mind the "design static condition" is easy to change bow down in light air etc. I always design with a small amount of transom submergence for the design waterline,static. One thing-in Bethwaites first book he suggested an angle of 20 degrees each side of CL for the bow waterlines-improves performance in a chop-for reference a Finn is 30 degrees...

    This is WRONG-brain fade-sorry-it is 22 degrees TOTAL(11 degrees each side and 30 total for the Finn-15 degrees each side.
     

  15. Tim B
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    Tim B Senior Member

    You can still go flatter on the aft sections and the run. You might do this reducing the draught amidships, and reducing the forefoot a little. This will increase planing area and allow you to carry more weight aft (which you will find you have). A flat run will also give you more stabilty in yaw on the plane, and this will make it much nicer to sail.

    From a simple weight estimate... The CG of the hull may be 5' 6" from FPP, but the crew will be 7' and 11' from FPP and are a lot heavier. Taking 75kg Hull and 80Kg crew (standard values), that puts the CG at 7' 10" aft of FPP. I'm taking FPP to be the bow, and these are very approximate figures. But even from that, you can see that the CG is about 6% aft of midships. Where is the revised CB?

    Keep the half-angle of entry as small as you can. You may find that the bow digs in when gybing (the Lark does horribly, for exactly that reason), but being a small boat, you should be able to roll it to cure that problem. The bow sections can also be quite fine, as their main purpose in a planing skiff is dealing with waves.

    You might also want to have a wander around some dinghy parks.

    Which bit of the UK are you in?

    Hope this helps,

    Tim B.
     
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