Sailing Dinghy Design

Discussion in 'Sailboats' started by Tim B, Mar 12, 2003.

  1. Tim B
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    Tim B Senior Member

    tspeer,

    Are you sure that's the British Sailing philosophy? I thought it was never to get wet in the first place!!

    I'm not sure that the windage is particularly important (though less is better), there's a heck of a sail under a capsized I14. See renderings above. A solution that does work is to raise the gunwhale about two inches above the top of the bouyancy tanks. It's very easy to get on when inverted (with no additional inverted windage) and gives comfortable hiking. I'm not too worried about hiking, it is a double trapeze boat, so the chances are that you'd use the trapeze if you possibly could, depends on your sailing style though. Personally, I (the helm) will move out first, then my crew moves as necessary. I think what we should do, is suggest a version for each system, one with a false floor, and one without. I don't think it's going to be a huge performance question. It's a materials and stress question, and I think that either method is suitable and it just depends on how you want to sail.

    Cheers,

    Tim B
     
  2. tspeer
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    tspeer Senior Member

    I may be a Yank, but I spent three years in Merlin Rockets! (on the Thames at Cookham Reach) The difference in philosophy was one of the things that struck me when I moved to England. I thought the fear of a boat blowing away from the crew was a bit overstated.

    Personally, I think the floor is the way to go for a skiff. I've never sailed a trapeze, so I can't comment there. But I like what it does for the structure. Putting webs under the floor at +- 45 deg would probably help with torsional stiffness.
     
  3. b14maniac
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    b14maniac Junior Member

    tom is dead right. These days with modern carbon/foam construction, not having a false floor is a sin for any good small boat designer. It allows self-draining, stiffens the boat against torsional forces such as around the mast step/gate. Also, transoms have been made completely useless these days. All you need is to have some support in the form of some latitudinal bars across the stern. Personally, with the current trend towards narrow I14s with slab sides and small wings, I think that both of the I14 designs put up are still left somewhat lacking technology-wise, however the green one was a very nice rendering, which could be used to generate an all-conquering narrower, winged boat. I'll see if I can do anything in rhino worthy of putting up based around the green boat to correct it to how I think a modern I14 should be.
     
  4. Tim B
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    Tim B Senior Member

    Give it a shot, I'm working at the moment so my time to play with the design is limited to say the least.

    Tom,
    to be honest, inverted windage is not something that is worth designing for. After all, a good boat is fast to get up (30 seconds or less (down to 15 seconds for a Laser 13(dry capsize))) and should be designed and set up to let you ease off the power to virtually nothing, then bring it back on, fast. I find, even sailing the LARK, that the trick is to ease, level the boat, and then pull on the power (try it, it's a fairly fast method, if anyone has a faster method, do share). I've only capsized it once this season, and that was due to a drop in windspeed (thus going over to windward). I agree that it should be stiff enough, and I feel that there is no harm in offering both versions. It's down to personal choice and style as much as anything.

    Cheers,

    Tim B.
     
  5. tspeer
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    tspeer Senior Member

    I agree - I wouldn't give much attention capsize windage, either. Especially on a skiff where the racks in the water will provide a lot of resistance.

    I mainly mentioned it because it seemed to be a major point of contention among the conventional dinghy enthusiasts when I was in England. Personally, I'll take a boat any day that floats high and comes up dry over one that sits with the board at water level and comes up swamped. I thought it was one of those English eccentricities!
     
  6. Tim B
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    Tim B Senior Member

    Not as far as I know, then again, I'm south west England, not south-east!! How long ago were you sailing in britain anyway?

    Cheers,

    Tim B.
     
  7. tspeer
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    tspeer Senior Member

    1987 - 1990
     
  8. Tim B
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    Tim B Senior Member

    I think opinions change geographically in that case, which raises another question, what are we looking at sailing this thing in. An inshore water the size of say, Grafham, or the sea? It may make a small difference to the design, whether we are designing for speed through waves, or planing on flat water.

    I'll have a look at that hull this weekend, I'm too tired to do it now... Yawn.

    Cheers,

    Tim B.
     
  9. tspeer
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    tspeer Senior Member

    It sure made a difference in the Merlins. When I was sailing them, the "modern" Merlins were more designed for the sea. But the older boats were still competitive on the inland waters.

    Probably a bigger factor is crew weight. One of the things I discovered about sailing in a restricted class was the variations in hull design actually made the competition closer. Heavy crews could sail a design like a Smoker's Satisfaction (more rounded, able to carry the weight without excessive wetted area) while lightweight crews could choose something like a Summer Wine (flatter, a bit stiffer, but with more wetted area). One design dinghies often typeform their crews, with only a narrow range of crew weights being competitive.
     
  10. mad engineer
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    mad engineer Junior Member

    Just to add to everything else on the topic of false floors...

    Water in these boats doesn't just come in when you capsize, also when you plough into the back of a wave and try to imitate a submarine - then it really helps to get the water out quickly. Also the boats are stable when they are moving, not when stopped so if you come to a rapid halt having ploughed into a wave and filled up with water then you will almost certainly fall over.

    out of interest though, the 29er has a false floor but also has a transom with two big holes in it to drain the water out. Allegedly the intent of this design was so that the boat filled with water when capsized. The water in the bottom then stabilised the boat long enough for the cerw to climb in and get sailing again. Sort of contradicts the above but that is what the designer said.

    By the way, if you all get fed up with the I14 rules restricting the mid-length section, how about looking at the UK Cherubs... These are more restricted than a 12 but still give you enough freedom to experiment. The site also has loads of dicsussion around building boats and detailing.

    Personally, I find the 14 rules on the mid section force the boat into the wrong shape, so you end up having to fight the rules to get something that works properly.

    BY the way - who is going to build this beast??
     
  11. Tim B
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    Tim B Senior Member

    I'm not giving up, I'm sure it can't be that hard. I've started an I14, and by the gods of sailing I'm going to finish it. This cash thing is a problem though, I don't have enough to fund the whole thing to the standard I'd like, but we'll sort that out when we've got a design. (and don't ask why I suggested it. This design is progressing somewhat faster than I thought it would.) Oh, and while I think about it, spray is also a problem. A 'dry' boat is comfortable, and has the added bonus of good visibility.

    Mad Engineer,
    Do you have any sketches or anything of your thoughts, you imply that you've done this kind of thing before.

    Cheers,

    Tim B.
     
  12. mad engineer
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    mad engineer Junior Member

    I am not suggesting you give up at all - just highlighting the bits I found frustrating.

    Anyway - I had a few goest at getting a hull shape I liked using Hullform and the files are attached if I can get that bit to work.

    I was basically trying to get a fine entry to the bow for getting through waves easily, straight parallel run aft so the boat goes in a straight line when heeled with a decent flat plaing area. I was trying to keep the whole boat as skinny as possible at the water line for light airs speed and low drag through waves.

    The rocker line is deep forward, flat aft - so maximum rocker is about 30% or so back from the bow and then running as straight as possible to the stern. This is working on the basis that if the underside of the planing area curves in the fore + aft plane to much it acts like the underside of a wing and sucks the boat into the water resulting in a defined top speed that was identified as a problem with older designs.

    What are your thoughts on the rudder hydrofoil that is now seen as the bees knees in the 14's and Moths??

    The rig design must be a big issue too - current Cherub thoughts are that the rigs are where the nest big gains are coming from rather than the hulls. I can't remember if the 14 rules let you have wing masts but this is something that must be worth a try. One of the 12s made a wingmast work (at the R class champs I think) so it is possible on this sort of boat
     

    Attached Files:

  13. mad engineer
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    mad engineer Junior Member

    ANd the next 14 design
     

    Attached Files:

  14. mad engineer
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    mad engineer Junior Member

    ANd the last one....
     

    Attached Files:


  15. mad engineer
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    mad engineer Junior Member

    Just been reading through all the posts again - what material / construction are you planning to build the I14 from??
     
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