Sailing Dinghy Design

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

  1. astevo
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    astevo Junior Member

    oh also Phil S in a previous post that the flat side flares are there to lean over when hunting for height.
    from what i have hear from the skiffies they wash off some speed to bring the apparent wind back when trying to make it up high.
    really they loose power (righting moment) when leaning over so the lean to leward is really just to balance the helm when you get hit by a big gust.
     
  2. Andy P
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    Andy P Junior Member

    The UK Cherub rules don’t require solid wings/flares, I have built one boat without, and it has tube wings, but with a wide gap to the hull.
    I worked out it was heavier and more complex to make tubes, + tramps and the fixings needed on the hull than it is was for solid ( 8mm thick) foam wings, - the wings are just an extension of the hull shell, and are like solid tramps.
    The freeboard of the design is actually quite high, so the wings are above the waves most of the time, and in fact the boat goes better upwind in waves than the shallow-bow type ‘one-design’ skiffs eg 29er, rs700.
    By the time the lee wings are catching waves downwind, it means you are too heeled and therefore slow for other reasons. The boat does not go at all well if heeled over, - if you are too low for a mark, then you can’t heel it and try to go high... it just stops and you fall in. It’s better to drop the kite and sail high 2-sail. Another way is to flap the kite, then the apparent wind reduces and goes aft as the boat slows, so you can then point higher.

    Downwind when it’s very windy and wavy it can be a problem keeping the boat down and under control, ( but it’s not very often. ), but I can see why the more veed transom could help here. The transom width at ~700mm chine to chine is less than the norm for UK cherubs, which usually have more parallel chines aft.
     
  3. TaSSie_deVil
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    TaSSie_deVil Resident Boataholic

    Has this fascinating thread taken a sudden nosedive into obscurity? Come on people... whatever happened to the Int. 14 design that was going on in here and the work on designing a singlehanded skiff for those with a larger than average mass?

    While the Hydrofoil/Rediculously narrow moth/cherub chat has been valid and interesting, the name of the game is "Sailing Dinghy Design"... anyone care to keep going on with designing a new sailing dinghy?

    Cheers,
    taSSie
    (formerly b14maniac)
     
  4. Doug Lord

    Doug Lord Guest

    Sailing Dinghy ?

    Should design of a sailing dinghy in the year2004 be limitedto water stuck versions only oy should foilers larger than a Moth be given consideration as well?
    Rohan Veal with his historic win in the Aussie Moth Natinals has shown the potential of monofoilers by winning 8 straight races on fois upwind and downwind!
    Seems to me the 14 guys should be thinking lots of air.......
     
  5. TaSSie_deVil
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    TaSSie_deVil Resident Boataholic

    Foiler Anomalies

    Doug,

    I wholeheartedly agree that foiling is the way to go (to the point of doing minor experiments with foils on a couple of models to investigate the effect of using curved foils in aft-placed cases)... however, not every class supports them (such as the i14s). That is why there is now a "Foilers" thread.

    I also agree that Rohan's success in White Knuckle Express is a historical win, and I also think that Rohan should have won the world titles, but other things (which Rohan may/may not have been able to control) intervened.

    But, a hydrofoiler isn't neccessarily a dinghy in the traditional sense of the word, but the culmination of a combination of hydrodynamic theories, mounted upon a dinghy as a platform. This is why I think that the foilers have a little way to go before they become accepted in mainstream sailing. Look at Rohan's moth as an example. If the entire idea of the rudder foil was initially to keep the boat from nosediving, what is the point of them on foilers? If the bow is physically forced above the traditional waterline, there is little point in having the extra drag of the rudder foil on the back. This is why I back Wardy's experimental monofoiler (the scow with the winged centreboard and no other lifting foils) over John and Garth Illet's flying machines, as it has to be more efficient in lift/drag terms, as there is physically less submerged wetted area.

    Besides, the original point of this thread was to design a dinghy, not simply a floating platform for some sort of crazy flying machine!

    Cheers,
    taSSie
     
  6. Doug Lord

    Doug Lord Guest

    A sailing dinghy that doesn't foil?

    What? My God! You MUST be kidding! Alright, call it an ISD:
    "An Integrated Sailing Device capable of sailing fast in all conditions with minimum contact with the water and maximum contact with fun"
     
  7. BrettM
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    BrettM Senior Member

    Lorsail,
    Sounds like you need to get into kite surfing :D

    Tas,

    Not at AMC are you?

    Brett
     
  8. TaSSie_deVil
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    TaSSie_deVil Resident Boataholic

    Brett,

    Not yet, but I have gained a position there for Naval Architecture starting in a month's time and looking forward to it. Until such time as I've started the course, I'm little more than an informed amateur with an addiction for going rediculously quick over water :D
     
  9. BrettM
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    BrettM Senior Member


    Speaking from experience, you will still be little more than an informed amatuer for the next five years at least. :) Oh and don't expect to see a boat in the course for at least two years...well maybe a "box shaped vessel" but thats about it.
     
  10. TaSSie_deVil
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    TaSSie_deVil Resident Boataholic

    Sounds like fun....

    Brett,

    Sounds like an amusing way to have Physical and Mathematical theory drilled into one's head, all vaguely relating somehow to the engineering aspects of Naval Vehicles :rolleyes:

    Thanks for the heads up. Until then, it's a busy process of packing and sorting out what to do with my current boats (ie. how to sell them quickly for a minimal loss...) and then doing the long haul down.

    Cheers,
    tassie
     
  11. ekerebel
    Joined: Dec 2003
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    ekerebel Junior Member

    Prismatic coeficient

    Hi all,

    I am currently designing a dinghy and I am wondering a bit on the prismatic coeficient it should reach.

    The characteristics of the dinghy are as follows :
    Waterline Length : 4.2 m
    Overall Width : 1.86 m
    Waterline width : 1.20 m
    Sail Area : Main 9.7 m²
    Jib 3.5 m²
    Spinnaker 17 m²
    Prismatic coeficient : 0.565
    Wetted area : 3.7 m²

    My goal is to design a boat with a similar program to the RS400.

    Here is a render of the boat :
    [​IMG]

    What do you guys think about that ?
     
  12. gggGuest

    gggGuest Guest

    prismatic

     
  13. mad engineer
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    mad engineer Junior Member

    COmparison

    Time now for lies, damn lies and statistics...

    I spent a bit of time looking at some of the higher performance boats around in terms of stats to see if there is anything that points to what boats are faster then you would expect, or slower as an indication of direction to develop in.

    For simplicity, performance is based on PY numbers from the RYA.

    I have compared that to the ratio of (Sail Area)/(Sailing Weight) against PY Number and also just Sail Area against PY Number.

    Naturally there is a bit of margin for error in working out the all up sailing weight but generally this is based on Crew at 75Kg each, 30Kg for rig, rudder and C/board where no other data exists.

    The sail area includes white sails plus 30% of the spinnaker area on the basis that downwind you probably sail twice as fast, but only 60% of the distance that you sail upwind. I don't think this is really significant since only the Tasar lacks a spinnaker in this group.

    Also I have only looked at two man boats - single handers

    Anyhow, the graph is attached (hopefully) so you can have a look.

    My supposition is that any boat above the line is slower than could be expected given the power to weight ratio, and those below the line are faster than expected.

    The boats below the line in both cases include the RS800, 29er, and the Laser 4000.

    The I14's are way up above the line in both cases - way slower than you would expect for their power to weight ratio and sail area!

    Now of course, the reason is to assess why certain boats are faster, whereas others are slower given the relative power available to each...
     

    Attached Files:

  14. astevo
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    astevo Junior Member

    mad, id be interested to see a graph which took into account of waterline beam and righting moment. but that would require some pretty serious statistical work so maybe look at one variable at a time like you are.

    in terms of speed as related to sail area this is a good way to determine efficiency though. we should go through all the classes and determine things like waterline length, hull form and beam righting moment. ideally we could do some calculations incorporating all there features but obviously this is too much and any model we make is not going to be accurate.

    a wise man once said "37% of statistics are complete crap and totally useless"
     

  15. gggGuest

    gggGuest Guest

    For my money an awful lot of it is do do with waterline length, which is very important in most conditions. Length and fineness (and long boats tend to be finer because they can be for the same displacement) has a big impact in all conditions below full on planing, and even in the full on planing case without much boat in the water it tends to be possible to push longer boats harder than short ones.

    In your graph the I14, B14 and Cherub are probably the strikingly short boats for other dimensions, the 800 and FD probably the longest for other dimensions, although 49ers and 29ers are both pretty long and thin too.
     
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