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Cash prize of 250 GBP - Dinghy Design Competition

  • 15 Apr 2022 03:13
    Reply # 12709324 on 10211344
    Anonymous member (Administrator)

    Hello John, it is good to read your thoughtful and illuminating post on this forum, and congratulations on submitting the winning design.

    The Editor of the JRA magazine has asked me to write a series of articles, one for each of the dinghy design entries. I have thought it best to make each of these articles comprise the words of the individual designers themselves, and I can do this by editing the designers’ comments which they each submitted with their entries, together with comments they themselves have posted on the forum.

    The editor has suggested about 1,000 words and perhaps 5 or 6 illustrations. There is probably sufficient material already contributed by the designers to make this possible – the hard part will be deciding what to leave out, as each of these unique little vessels has many features of interest.

    In my mind, the purpose of the series will be to document permanently this rich portfolio of dinghy designs, for members’ future reference and use, on behalf of the designers. I Intend to keep any comments to a minimum (perhaps just in order to point out obvious good features which the designers may have omitted to mention, or were too modest to do so). If any of the designers would like to make further comments on this forum, now is the time. Also, in some cases, further photographs and drawings will be welcome. Where we do not have a photograph of the dinghy, I guess we can resort to a photo of a model, as a scale model for each has been made.

    If any of the designers would like to take on the job themselves, of writing up and illustrating their own design within about 1000 words, then by all means please do so. Otherwise I will do it, based on what the designers have already written.

    I think Arne’s specially-designed dinghy junk rig is worth an article in itself, either written by Arne or based on a compilation of what was written in this forum. In addition, there were a number of designs which were mooted, gave rise to discussion, but were not officially entered. I think they too are worth documenting, and with the designers’ permission I intend to do so.

    Thank you to each of the contributors.


    PS: as the series, at one dinghy per magazine issue, would stretch out over a period of more than a couple of years, I personally think it would be more useful to print a magazine supplement dedicated to the dinghy designs - with a copy sent to each of the members who subscribe to the paper magazine - and a digital copy to the other members, and to be added to the digital magazine archive. That, of course, is a matter for the Editor and the committee to decide. I have no idea what other members think of that idea.


    Last modified: 15 Apr 2022 04:48 | Anonymous member (Administrator)
  • 14 Apr 2022 23:23
    Reply # 12709095 on 10211344
    Deleted user

    I have just come across this discussion.   I was pleased to hear that I won the competition and I have transferred the prize money to Sailing Horizons,  a charity that offers a chance to try sailing to young people who might not otherwise have such opportunity.  

    A few comments:

    Since submitting my entry to the competition I had the thought that my design might be better with a single wheel at the bow rather than two wheels at the transom.  Pic attached shows how it would look with a single wheel at the front, the oars could be drawn back through holes in the transom to become handles like on a wheelbarrow.  I think this would have some advantages. A possible dissadvantage is that if you have to land on a beach under oars with any waves breaking I think you are best coming in stern first in which case strong wheels under the stern would give some protection to the hull.  Maybe not a big thing but if anyone wants to build my design and would prefer a wheel at the front I would be happy to modify the dxf files.  Alternatively the wheels could be left off althogther, its hardly worth altering any files for that since the changes are fairly obvious.  I would say though that we have wheels on a longer row boat that I built and we find those wheels very useful.


    There is a sugestion that the mast of my design should be further forward and that if the bow storage compartment should be smaller to make that possible.  I wonder why it is thought that the mast should be further forward - is it that the helm balance is wrong or is it something else?    The mast and centreboard positions as drawn are my guess at what is needed for good helm balance and having a pivoting centreboard does allow for some adjustment of helm balance.   The bow storage compartment may be larger than some people may need but I see very little gain in making it smaller - with the bow storage as drawn there is still room for three, or maybe four, people to sit in the boat, albeit that would be a heavy load for any boat of this size.

    There is a sugestion for a daggerboard rather than a pivoted board. (I will call it a 'pivoted board' rather than 'centre board' since it is not on the centreline).   I do think that a pivoted board is much better than the daggerboard arrangement and not really much harder to build - basically the difference for the builder is drilling a hole and fitting a bolt or a pin, with something to seal the hole.  I think it is a big advantage to be able to sail into shallow water with little risk of serious damage.  Also a pivoted board looks after its own storage when it is not in use. When a daggerboard is partially raised it is sticking up and very much in the way, when fully raised you have to find somewhere to put it.  I think there is only one reason why daggerboards are popular for the smallest sizes of sailing boat, this being to minimise the space taken up in the middle of the boat. However, with this design the pivoting board is built into one of the bouyancy tanks so takes up no usuable space in the centre of the boat.  So I sugest that a pivoting board is the best option for this design, but if a daggerboard is really preferred the changes to the files would be slight and are fairly obvious.

    I dont recall the rules of the competition requiring a junk rig but I chose a rig that from a distance looks a bit like a junk rig.  I expect the JRA will know what is and what is not a junk rig - is it to do with the number of battens or the sheeting being taken to the ends of the battens?    Certainly the design could have another batten or two and a pole could project behind the transom to allow sheeting to the batten ends. It might even sail better that way but I think it would be a bit fiddley for this size of boat.

    It seems that some people would like a boat design to include a table of offsets.  If someone actually wants to build this boat and are sure they would prefer a table of offsets then I can make one. Since it is a chined hull the offsets presumably would be measured from the chines to the centre plane and from the chines to a nominal waterplane. So for this '5 plank' hull that means 3 pairs of numbers for each cross section - not too hard.  How many cross sections? I think 10 cross sections is fairly normal but may be more than necessary for such a short boat.  But I would say that I would much prefer to build a plywood boat from .dxf files defining the shape of every piece of plywood rather from just a table of offsets.  Ideally one would start by getting all the plywood parts cut out automatically from the dxf files, although this is not the only way.  This adds cost certainly, but it also bypasses much of the more tedious parts of boat building, leaving the builder with the more enjoyable work and reaching the launching point much quicker.  As I think someone said, if you havent tried it this way you may not appreciate the advantage it offers.  I fairly recently built a 15 foot plywood rowing boat from dxf files and I got a price quoted for waterjet cutting and for cnc routing - the waterjet cutting turned out fine and was about half the cost of cnc routing but perhaps another firm would price these options differently.

    When I sent in my design I did include all the dxf files and I dont mind these being published.  If they are not going to be published I can email them to anyone who wants to use them for building. I cannot gaurantee that all this data is actually correct - to the best of my knowledge it is but I dont think anyone can be certain of that without building a complete boat.  Do let me know if you find any mistakes so that I can make corrections for any future builders.  And if you view or print out the files and think anything looks wrong ask me about it before you cut out material.  I can say that I fairly recently produced a set of .dxf files for our 15 foot row boat using a similar design procedure and that boat all fitted together as intended with no problems. In that case, since I knew just what fittings I was going to have on the boat, I included every fastener hole in the dxf files - nice not to have to do any marking out at all - just bolt the fittings on with bolt holes already cut out by waterjet.

    At the present time the dxf files for this design are not 'nested' - they are at somewhat random orientations.  If automatic cutting is envisaged, or even plotting at full size, the next step would be 'nesting' the shapes so that they can be cut out with minimal wastage.  I think it is unlikely that there will be any great demand for this design, there are so many small boat designs already available, but if there are a few people wanting to build to this design then maybe I could do some nested files.  For our 15 foot rowing boat I did this with software called 'mynesting' which you pay for each time you use it, not too expensive though, at least not compared with overal boat building costs.

    Talking about boat building costs, I dont think you should build your own boat to save money - if you just want a boat as cheaply as possible get a second hand one, if you look around I expect you will find a good one for less then the cost of materials, or even for free from someone who just needs to clear storage space.  I think the only justifiable reason to build your own small boat is for the satisfaction of doing it, which I have found can make it very worthwhile.    









     


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  • 07 Apr 2022 19:18
    Reply # 12698915 on 10211344

    As an alternative to a printed layout, offsets from a plywood sheet corner or centre line allow for less waste and can be more accurate than some print processes. It is important when springing curves to mark the stave with the length of mating curves to ensure they are the same length. But raw hull offsets? No thank you. Doable? sure, easy? no.

  • 07 Apr 2022 18:59
    Reply # 12698891 on 12694121
    Anonymous wrote:

    As a builder of a small boat like this, if I had just hull offsets, the only way I can think of to actually get plywood panels to build it would be to plug those offsets into some program to obtain plywood sheet layouts so that I could cut them out and build it... maybe I am just too amateur...

    Len, you mean you couldn't build it without a computer?

    I think maybe Annie has a point.

    Big grin! Not only that, I am not one of those young people... well at least to me after 6 decades, I am feeling my age. While I could find a way to do it without a computer, it would seem wasteful of both my time and energy as well as materials. The process of cut to close enough to be able to hold in place then rough cut, then fine cut just seems wasteful when I can layout the pieces on the plywood sheets ahead of time for best fit. Time is short and therefore valuable. I am finding energy is shorter than when I was younger as well. Anything I can do to save these things, including using computers, is worth while.
  • 06 Apr 2022 16:05
    Reply # 12696664 on 10211344

    Yes, I've used several sheets of A4 to plot the curved components of my wingsail battens, but when it's the length of a dinghy we're thinking of, then it's time for fewer, larger sheets.

    Back in my early days in a drawing office in the '70s, we had a plan copier that worked with sensitised paper. The original, on tracing paper or film, had to be laid on the copy paper, and then both were passed around light source in a roller. The slight difference between the radius of the original and the radius of the copy paper as they went around the roller resulted in an error in the scale on one axis but not the other. Then the copy had to be passed through an ammonia bath to develop and fix the image (ugh), which caused more distortion. I wouldn't be surprised if the pram design's paper print was of that vintage. 

    Later on, plan printers fed with computer output had actual pens, and then inkjets and lasers took over. None of them had any scaling issues, it was just the copies of hand drawn originals,

  • 06 Apr 2022 14:26
    Reply # 12696443 on 10211344
    Anonymous member (Administrator)

    Jan,
    I’ve used multi-prints of A4 sheets like you, but not quite as many as you (yet). One just needs some reference points or lines on each sheet to ensure accurate alignment with the next one. I used this method when I made several parachute drogues of different sizes. Since I had drawn the thing in my QCAD program, I could get accurate printouts by simply setting the print scale to 1:1.
    Below is a screen-dump from my QCAD drawing. The highlighted bit covers an A4 sheet so on this particular drogue I got away with two sheets to make the pattern for one parachute sector.

    I could probably use your method to print out and glue together the pattern for the one and only curve needed for lofting the 8-foot Medium Boy dinghy. That should be a good deal faster than traditional lofting.

    Et nova et vetera, for sure...

    Arne


    http://goo.gl/5vSKKE

    Last modified: 06 Apr 2022 14:57 | Anonymous member (Administrator)
  • 06 Apr 2022 12:28
    Reply # 12696330 on 12696214
    But I have to question your reasoning for not printing on A1 or A0, which would have saved that hassle. The whole point of PDF format is that it is an accurate representation of the document. If I print a PDF on my A4 home printer, I get choices: "Scale" and a box where if 100% is entered, I get exactly that - I've just tried it; and "Scale to fit" where I have two choices,  "Print entire image" or "Fill entire paper" - both of which would result in an unknown scale factor. I cannot believe that a print shop, given a request to print at 100%, would deliver prints at the wrong scale.


    The only large format printer I knew of at the time was an hour and a half's drive away and specialised in art/photography and would have been expensive if numerous attempts were needed to get the scale right, and there's 4 sheets. I doubt I spent much more time printing and sellotaping than I would have done driving. I may have been wrong but my own experience as a previously semi-pro photographer was that the pdf had not been created properly, I wish I could find it now to check. Whether or not anyone else built to these plans or had issues I couldn't get out of WoodenBoat Inc. This was a pdf. created from a scan of a set of 40"x30" drawn plans and there may have been an issue with the scaling in one axis, a file created purely from a CAD program would of course be reliable. 
  • 06 Apr 2022 09:28
    Reply # 12696232 on 10211344

    Now I've made six A0 PDFs of the curved and complicated components of 'Tender to SibLing':

    https://app.box.com/s/giaancenpq5f06tgohtfw1420v8xso1q 

    The rest of the components are simple quadrilaterals, so can easily be plotted on the plywood by hand.

  • 06 Apr 2022 08:46
    Reply # 12696214 on 10211344

    Jan,

    Huge congratulations on building such a pretty pram.

    Even huger respect for accurately aligning 16 sheets of A4 as you taped them together.

    But I have to question your reasoning for not printing on A1 or A0, which would have saved that hassle. The whole point of PDF format is that it is an accurate representation of the document. If I print a PDF on my A4 home printer, I get choices: "Scale" and a box where if 100% is entered, I get exactly that - I've just tried it; and "Scale to fit" where I have two choices,  "Print entire image" or "Fill entire paper" - both of which would result in an unknown scale factor. I cannot believe that a print shop, given a request to print at 100%, would deliver prints at the wrong scale.

    And yes, CNC cutting uses the same basic info and the output can be scaled; if you cut at 100%, 100% accuracy is what you'll get. It's just the same as printing, but with a cutting tool instead of ink.

    Sure, a CNC kit will cost more than a DIY kit; but as I said, it's for folks who are cash-rich but time-poor, or lack the space/facilities/skills/tools to loft and cut parts themselves. Getting some A0 prints of PDFs done at 100% scale, and pricking the shapes of components through them onto the plywood is a good, less expensive alternative. Thinking back to when Tystie was built in 1999/2000, I remember that David Thomas supplied exactly that, and the hull builders did exactly that. I note in the book Principles of Yacht Design that this is now the norm for professional builders. Why should amateur builders deprive themselves of the speed and accuracy of doing things this way?  

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  • 06 Apr 2022 08:41
    Reply # 12696213 on 10211344
    Anonymous

    Hi Jan, I agree with You that having the dxf files printed in a shop is too expensive. I thought that I could have the panels cut at my marine plywood supplier but he informed me now that 4 mm plywood is too thin for their CNC machine and some of the pieces are too small so they cannot do it. But I did get the PDF 1 : 5 scaled plan from David Tyler which I could print already on my A4 printer. But You mentioned free poster software where I could print it full size so I tackle the PDF files again. I can export  from librecad the DXF files to PDF and then I will play with the poster software. When I have succsessfully printed a full size plan I report back here again hopefully with a similar file attached which You posted from your lovely dinghy. 

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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