14ft Racing Dinghy - Comments Please

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

  1. gggGuest
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    gggGuest ...

    trouble is at that period the 14s were a good fifteen to twenty years off the state of the art...
    I don't know that the "design static" is ever a very useful design reference in an unballasted dinghy, because the boat will never ever be sailed to those marks (well not by a competent crew anyway). I would think its better to look at what you estimate the waterlines and buttocks will look like in real sailing conditions. As for normal practice: well the same applies really, especially in the development classes were designs are iterative on what went before, working back from what's established to work well.
     
  2. Paul B

    Paul B Previous Member

    I think closer to 25 years off the current state of the art.

    Which is precisely why I chose to show those examples. What was being proposed would be closer to an old rules boat than to the current generation of narrow skiff hulls.
     
  3. PI Design
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    PI Design Senior Member

    Thanks again for all your thoughts.
    I could go flatter at the back, but don't really want to. All design is a compromise and too little rocker/sectional curve is as bad as too much. The aft run and buttocks look okay to me compared to those old I14s and Merlin Rockets (see the class homepage and Keith Callagher's webs site). The sections aren't as flat as some skiffs, but again look reasonable compared to a RS200/400, Dead Cat Bounce N12 (although that has a cruciform rudder which changes everything) etc.

    GGG, I take your point about not sailing to the static condition, but if you don't know it there, when will you ever?
    I have looked at the water lines with various pitch angles and reduced displacements to check the waterlines at planing speeds and they seem good.
    There's been a couple of comments on the bow half-angle which have suprised me. The angle is at least as fine as pretty much any other monohull I know (compare to Bethwaite book for example) - I certainly wouldn't consider it bluff. To get any narrower would require significant hollows in the waterline, which I wouldn't want.
    I can't see that there's a weight/buoyancy balance problem. The crew can shift themselves to balance, and the LCF is within the cockpit area so this shouldn't cause a problem.
    The wetted surface area is about 0.5m^2 less than a Tasar and boat weight about 25kg less (although about 10kg heavier design crew weight).
     
  4. PI Design
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    PI Design Senior Member

    I have attached a spreadsheet that may be of interest to some of you. Whilst I am aware that designing by numbers is not the be all and end all, it is a starting point and a reasonable indication of certain performance charecteristics.

    I have calculated (based upon estimated values or data from books), the "Bethwaite ratios" of SCP: Disp, SA: WSA and SA: Disp for a number of boats, including the Tasar X trial boat that Bethwaite tried and my design (Volant). I have also estimated design wind speed, the ability of a boat to carry its kite (i.e. how powered up it is on a reach).

    Finally, I have calculated the Warp Factor, which is related to the SCP: Disp ratio and was first used (to my knowledge by Ian Hannay in an AYRS paper some years back - will try to find reference later). It is based upon the premise that, all other things being equal, potential top speed of a boat is related to righting moment. A high WF score indicates basic parameters that should allow a boat to be fast.

    Very finally, in a true stretch of imagination, I have compared the WF against boats Portsmouth Yardstick (UK) to give some indication of how well a boat performs in reality compared to its potential top speed. What this shows, kinda, sort of, is how much a boat is optimised to certain conditions or is a true all rounder. For example, a boat with a high WF (therefore potentially high top speed) but high PY (therefore actually slow across a range of wind speeds and directions) is not making the most of its inherant performance charecteristics - perhaps a high wind blasting machine that is slow in light airs. At the other end some boats have a low PY despite a low WF - therefore thay are sailing to their max potential more of the time. The 400 is a good example of this, and this seems to bear out in reality where the 400 does consistently well what ever the conditions.

    As you can see, the Volant does fairly well across all parameters and should be a fairly lively, good all round boat, but still easily sailable. At least, that is the hope!

    Please note, I have had to guess many values (and make up yardsticks for some boats) but I reckon I'm not so far out on the critical and senstitive ones.

    I have had to downsave the file to Excel 97-2003 so some of the formatting make have gone wrong. Please let me know if this is so!

    As a point of interest, the Volant can hit the magic 0.3 upwind planing ratio by increasing the beam to 2.0m, but that would take 5-10kg of the target crew weight, which isn't what I want.

    I have data for other boats as well, but have kept this comparison to 2 person hikers. If anyone has data for other classes, or more accurate data for the ones listed, please feel free to let me know and I will add it.
     

    Attached Files:

  5. Zed
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    Zed Senior Member

    The brief sounded about right for that :D
     
  6. gggGuest
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    gggGuest ...

    Its maybe a tad "shouldery" above the waterline. This is inevitable with a vertical bow and flared topsides. I think that's why most one designs have raked bows - you get better lines that way.

    Personally I'd probably draw more vertical topsides and have an inverted chine and flare outfor the extra beam. Less good for roll tacking and sailing heeled, but a much better slice through waves. If the flare disappears along the bow you get a hollow curve which can look quite stylish. Here's one approach: in this case the flare and curves have been juggled so there's a straight compression beam from the shrouds to the mast foot.

    [​IMG]

    A small tip is that its good to draw the bow with some rake even if you plan a straight stem. In practice the boat ought to be built so that the bow tip is thicker at the top for structural strength/impact damage, and if you draw the bow with a few degrees rake and then truncate it for the vertical stem its all a bit smoother in that area. Tip stolen from Iain Murray!
     
  7. PI Design
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    PI Design Senior Member

    Like the tip! I see where you're coming from with the veritcal sides, but as I say, all design is a compromise and I wouldn't want to lose the ability to roll tack as my boat (for me, personally) would be largely lake sailed.
    Is that an Bloodaxe Cherub?
    I shall look into raking the stem - it also allows a bit more deck space for longer jib foot etc.
     
  8. PI Design
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    PI Design Senior Member

    have had a quick play with more flair in the bow area (more vertical sides flaring out) and bow rake. The results look good - will tidy up and post pictures this evening.
     
  9. CT 249
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    CT 249 Senior Member

    It looks nice. After looking at Woofs, NS14s etc, one wonders if increasing the size of the planing flat down the centreline, particularly in the bow, might work.
    Of course, that may mean that the boat is more demanding to sail, in that you must stay as flat as possible.

    The Northies certainly go quicker than Tasars these days, as one would expect from something with less waterline beam and wetted surface, but they always look a lot more Canoe-like in terms of form stability. That's great if you want that, but not all of us want that in that sort of boat.

    What is the bow half angle? It looks fine enough; I suspect (and AFAIK it's borne out in theory and reality as you'd know) that finer half angles become more important as the boat gets quicker, and in boats without massive rigs and traps a slightly fuller and flatter bow (Northie-ish) can work well.
     
  10. PI Design
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    PI Design Senior Member

    The bow half angle is about 11 degrees. I have looked at photos of a Tiger NS hull which has very flat bow sections - a bit like a surfboard stuck to the bottom. I can see that would get you planing early, but as you say it must need sailing very flat. That of course is one difference between designing a development class boat and a one design. The deb boat has to put speed as it's first and last priority whereas the OD can compromise a bit of speed for ease.
     
  11. Zed
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    Zed Senior Member

    NS's are delightful to sail... FWIW.

    Have you seen the solo setup for NS's?
     
  12. PI Design
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    PI Design Senior Member

    How does it differ - apart from having no jib?
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ====================
    This is the corrected post! I apologize.....
     
  14. PI Design
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    PI Design Senior Member

    No worries Doug.

    As promised, pictures with a raked bow (same waterline length), more bow flare and reverse chine under the deck - like the green Cherub.

    Although the underwater shape is unchanged, I'm not totally convinced by this one. There are nice bits (the bow rake works well), but it doesn't look like a boat you'd want to roll tack any more. On the plus side the hull is now almost totally developable, so is probably easier to make.
     

    Attached Files:


  15. Zed
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    Zed Senior Member

    Sometimes the mast moves, I think it depends on the hull and the owners preferences. I would think stepping the mast forward would be better but I have never sailed one. Looks looks like an interesting option.
     
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