What to look for when designing a fast sailing dinghy

Discussion in 'Boat Design' started by 1gerry, Jul 27, 2013.

  1. 1gerry
    Joined: Jul 2013
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    1gerry Junior Member

    Hello all,

    I am looking at starting my onw dinghy hull.
    I am thinking of a 12-14 ft sailing dinghy, fast and sporty.

    I have started drawing my first few lines, but I wanted to gather some basics from all the forum's knowledge first.

    I am thinking it should have a flat bottom for early planing. But what else?

    Please help me out with some basic guidelines. I am reading some of the recommended books in the meantime, but your help would be most appreciated as my readings don't seem very easy and clear just yet.

    Also, I have only found drawings for the javelin type boat. Where can I find other references to guide my development?

    What would a 29er line plan hull look like?

    Thanks to all!
     
  2. gonzo
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    gonzo Senior Member

    Do you want fast off the wind mainly or a boat that points high? Also, what kind of conditions are you planing on sailing in?
     
  3. michael pierzga
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    michael pierzga Senior Member

  4. 1gerry
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    1gerry Junior Member

    Hello Gonzo,

    I will be sailing in big lakes, with moderate waves.
    Would like a boat that performs well all round. Sacrifices will have to be made, so if speed off wind needs to suffer in order to have a boat that can sail well upwind, so be it.

    I am looking at something with similar performaces to a 29er... and for two people ( forgot to mention that before)

    thanks
     
  5. messabout
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    messabout Senior Member

    Flat bottom offwind if you want it to plane. Rather complex design details if you want upwind performance. Such a a boat wil sail on the wind at a modest heel angle and will not do as well unless heeled at perhaps 10 to 15 degrees. In so doing you lose some sail power.

    The quarter beam buttock run angle is to be minimized if you hope to get the boat to plane readily. A lot of fiddling around with the design drawings will be needed. Fun maybe, laborious for sure, worth the trouble, yes if you are serious. The flat bottom might lose a bit of potential in very light airs. Wetted surface don't you know? Light air sailing invites deliberate heel to reduce wetted surface which is dependent on your initial design. Sticky wicket here. Yes it can be done.
     
  6. JRD
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    JRD Senior Member

    hi gerry,

    There isnt much in the way of books about the therory of racing dinghy design specifically. Most designs are developments within a class and therefore have relatively small numbers, or the designer/builders interest is in commercial production, so they closely guard their IP.

    You should read "High Perfomance Sailing" by Fank Bethwaite for some ideas and design history. Also have a look at the UK Cherub website as the members have placed alot of information on design and build techniques there.
    Paul Beiker has notes about the design theories of his international 14s on his website and also there is much good discussion on the Int 14 web site forums.

    You may also benefit from reading Larsson & Eliasson, Principles of Yacht Design. While it focuses on keel boats, alot of the theory contained applies equally to a dinghy and many of the calculations are worked through in straight forward maths.
    There is not alot of data or guidance on planing design characteristics for racing dinghies, and this is a complex subject as there are direct tradeoffs between low speed displacement drag and planing. There is theory for planing power boats, but what I have read did not seem to cross over well for sail boat design.

    As I think you have seen, there is some info on the NZ Javelin website. I think a couple have been built in the US and there is a paper written about building with ply. The Javelin is a splendid race boat for an experienced heavier crew, but would overwealm anyone not used to trapeze sailing in anything but light winds.

    The 29er will be a hard act to follow for its size, as many years of development with 49ers preced its design. If you want 29er like performance the easy solution is pretty obvious.

    Perhaps you could elaborate a little on what boats you have sailed and what exisitng yachts are in your local racing fleet?
    Also you might like to explain what you want to achieve, why do you want to design your own?
    You won't save money or time, and what you build will probably have no resale value in spite of what you spend on building it. Not a criticism, just a fact to consider.

    From my own experience (and that of a couple of others who use these forums), going it alone to design some kind of one-off can be interesting and rewarding, but ultimately you never have a fleet to race against.

    In my case the hull from an existing class was modified to accept a different rig. While I have had plenty of people interested in what I have done and it suits the kind of sailing I like, no one has shown any inclination to do the same. When I'm done with it, it will have no resale value and will be worth little more then scrap.

    This aside there is alot of fun to be had drawing boat and if you actually build and sail your creation, you will be one of quite a small club.

    Good luck
    Jeff
     
  7. Munter
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    Munter Amateur

    My suggestion is that you don't try to reinvent the wheel. The design of development classes like the NS14 has been refined over years of trial and error. Look for lessons from existing designs and only deviate from these when you have a very good reason.

    That isn't to say that you shouldn't try anything new, just that you would want to have a pretty good basis for doing anything too radical. Remember that its pretty easy to kick design ideas around on a forum but you can't fool the wind and the waves with spin and hope.

    Good luck with your project.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Dinghy Design

    Gerry, as you learn more and begin to focus these references may help:
     

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  9. 1gerry
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    1gerry Junior Member

    Hello All,

    Thanks for all the feedback.

    So here is where I am at, after playing with my 3D model over the weekend:

    I realized that this is not going to be easy, and in order to make a super sailing boat I'd need to go back more than 20 years, when I decided that naval architecture was not for me, and took industrial design instead.
    Keeping that in mind and knowing that I want to make my own design, I started playing with the lines of the Javelin (the only free plans for a similar boat that I found).
    I don't want to completely redesign it, just make it a bit more modern looking, so I made it wider at the transom and a bit less deep (V shape bottom), hoping that this would help in planing mode.

    I really don't know what I am doing and how this would affect the boat performance. I want a fast boat, but I won't be racing it, nor I expect to beat the super fast class boats.

    I just want a fast, fun, lake sailing dinghy, made out of a plywood core.

    Would it be fair to ask everyone's help in guiding me on this project? Like a collaborative project...I also have some crazy ideas about using a rigid wing sail americas cup style, but let's be honest, I am really not there yet.

    So here is the result of my first few hours of work.

    Please feel free to comment, and if you will, guide me in this development.

    Thanks a lot.

    Gerry
     

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  10. messabout
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    messabout Senior Member

    Gerry; so far so good. This boat will need to sailed flat or the corners of the transom will drag. So OK figure on keeping it on its feet with a trapeze, hiking plank, or whatever.

    I would ponder the immersion of the forefoot. Your drawing has it pretty well stuffed into the wet stuff. It also seems to be pretty sharp, in plan, and the two characteristics may cause it to become a bit twitchy in a chop.

    This drawing looks pretty good so far. You must proceed with this project with the unwelcome understanding that you may be disappointed with the outcome. If you are willing to accept that, then go for it.

    If it was my project, I'd just go with Paper Tiger, 29er, or a similar hot rod dinghy that has already gotten past its growing pains. If all you want is a reasonably fast, comfortable, cheap, easy to build, fun boat that will not compete with true hot rods, then take a look at a GIS.
     
  11. JRD
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    JRD Senior Member

    Gerry,

    I race against Javelins here now and then, as a rule they would beat a 29er. If you are keen to build something in ply and improvise a bit I would suggest you use the lines from the Javelin website and stick closely to that. Its a few years old but well proven. As with most proven designs there is a reason for everything, so I'm not sure you are making improvements in the right areas for the right reasons.

    Using those lines you will still need to do a fair bit of designing to complete the structure and decks. It would take six months to a year for most mortals to complete the build in their spare time even if very committed.

    I agree with Messabout that something like a GIS (Goat Island Skiff) may end up being more fun if you ultimately want to sail around a lake with your familiy and friends. You havent elaborated on your sailing experience and I dont want to make assumptions. But if you dont have a background in sailing fast boats, or a group of experienced sailors in a racing / club situation to learn from, you may find some the sporty centreboarders a brutal introduction to performance sailing. Many of the modern trapeze boats designed for assymetric spinakers dont like being cruised, and some become unpleasant and even unsailable unless you are going for speed all the time. Check out the video links and you will see some of what I mean.

    http://www.youtube.com/watch?v=nHYZ8eDU848

    http://www.youtube.com/watch?v=SqToxomFx9E

    I do think you are off to a good start with your 3d models, but look at every picture of an undewater hull shape you can find and try lots of shapes. You will be amazed at what subtle changes will do. Also I would use Freeship or Hullform to make your lines and export those to Rhino to detail. This way you can input the hydrostatic details such as displacement and see your waterlines at different trims.
     
  12. 1gerry
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    1gerry Junior Member

    Hi again,

    Messabout:
    Thanks for your comments. As the situation evolves, and I come to understand the amount of work that I will need to do to get this going I am starting to think about just using the original Javelin lines like some of you seem to suggest.

    But in the meantime, I am still learning and would like to understand your comment about the forefoot: (sorry, I am french canadian, so the english sailing lingo is still hard to understand to me)
    You find the hull is too deep in the bow? -Did I get this right? The javelin lines show a much deeper bow. What are the implications and results?


    JRD: As I said, you got me thinking, and maybe I'll just settle for some minor aesthetic changes.

    So maybe we can look at this from a different angle, and start the opposite way around: If I were to make a Javelin skiff, are there any hull details that would benefit from being changed? Any improvements that would be worth making?

    Thanks to all again!

    Javelin and my hull (the really flat one) below for reference.
     

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  13. JRD
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    JRD Senior Member

    That would be for you as the designer to figure out :cool:

    If you are looking at a design for a certain class, look at all the pics you can find, watch all the videos, see how the boat moves through the water. Consider how waves will affect it when approaching, and following it. Does it look stable enough, or are you agile enough as a crew to make it narrower and maybe a bit lower drag.

    Sketch up the rig, the decks, work out how you will pass the rigging loads to the boat without bending it. Plenty to do.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Agreed, the fore foot looks too sharp and too deep. It'll "root" around a lot on certain points of sail. The Javelin has a wholly different entry, typical for it's era.

    Skip the pretty pictures for a while and start studying hull forms and hydrodynamics. Conversely, you could just pattern the hull after a known type and add some styling clues to make it your one off. Simply put, you really don't have the skill set yet to completely design a performance dinghy. Simple and quite basic issues elude you, so you're best advised to study up or use a known hull from with the performance envelop you desire.

    This isn't a personal dig at you. We see this all the time, pretty pictures that have no hope, of doing what you might think it will and imposable to build in the method you want. It takes some time and study to get a rudimentary feel for what you need and frankly a lifetime of study, to come up with pocket rockets like a Swift Solo. You may be successful designing a relatively mild mannered day sailor, but given the shapes you've shown so far, you want a lot more then this, which will require a much better grasp of design fundamentals.
     

  15. El_Guero

    El_Guero Previous Member

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