Cash prize of 250 GBP - Dinghy Design Competition

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  • 08 Jun 2021 12:56
    Reply # 10604549 on 10211344

    Here are five more: L to R Boxer, AD, Youyou, Medium Boy and a NZScow. 


    Arne's Medium Boy is not in the “competition” suite, but it should be I think, as it is a very nicely proportioned 3-plank  – from Norway, the home of the pram dinghy.

    (The NZScow I will discuss later, but it is out of the running other than as an item of personal interest to me, and as a possible comparator.)

    By the way, Boxer, the Chinese Sampan, has also turned out to have rather pleasing proportions – for sheltered waters it will probably make as good a tender as any. Most of the square bow designers have tried to keep the bow transom out of the water, but the extreme displacement required for an 8’ dinghy to carry 2-3 adults leads then, in some cases, to excessive beam, or excessive rocker (or both.) Sibling Tender has avoided these extremes by raking the bow transom sufficiently to make it immersible and part of the rocker. Boxer takes this refinement further, by providing an even more heavily-raked “intermediate bow transom” – requiring the rest of the (three-part) bottom to have only very moderate rocker, which may well result in something more easily driven than one might expect.


    The three-part bottom would have been better as a single curve, in my opinion (for looks, and possibly ease of construction) – but for a dinghy this size I am prepared to believe it would not make much difference to its performance.

    AD (whose designer has aptly described as “the Mack truck of 8’ dinghies”) has gone for maximum beam, large immersed bow transom with little rake - and very little rocker, instead, a shallow-vee bottom forward. (Sibling Tender’s square bow transom now pales into insignificance!)

    I am going to give this a few more days. I would like to have tried “General Purpose Dinghy” because after attempting David’s Sibling Tender, I have realised how easy and satisfying this “interlocking kitset” type of construction can be – and this one has a point of difference: full race-boat buoyancy tankage. To me, this somewhat disqualifies it as a tender, but it would have provided a useful benchmark for static “capsize testing”. Unfortunately, however, this entry as posted - although replete with a very impressive collection of illustrations and specifications – does not actually provide a lines drawing, or sections – or any other means of actually building one, and my attempts to contact the designer have failed.

    So, my thoughts have gone instead to one of David’s early concepts – the Baby Siblim – which I think should also have been in the “competition” suite (I don’t care about the actual competition) and would provide an interesting comparison to Arne’s Halibut and John’s General Purpose Dinghy – all being 5-plank. I started Baby Siblim yesterday.

    Searching back through the thread to find it, I stumbled upon a couple of other early, very basic prototype concepts: Arne’s Simplicity 8 and David’s Box Boat. My biggest fear is that the very small size of these models will make comparisons difficult – there may not be much difference between the worst and the best of them. So, I thought: why not instead see if anything can be made to perform worse – and these two box boats might just provide a “bottom end” for the spectrum – or at worst, enabling some idea of whether crude testing can actually determine anything. They will be quick to do, anyway, so back to the work bench, and these two will be the last.


    Last modified: 08 Jun 2021 12:59 | Anonymous member
  • 07 Jun 2021 00:45
    Reply # 10598838 on 10211344

    You could easily have a sit-on dinghy, if that was important.

    Indeed you could, David - an interesting observation. Not for me, though - I am of an age where sitting right down and getting up again is becoming increasingly difficult. Also I don't like sitting in a puddle (I have one of those sit-on kayaks)  so would need a raised thwart for a dry bottom and for comfortable rowing. 

    Maybe self-draining would be good for people who chronically forget to put in the bungs? I can't think of a seriously good reason for making such a compromise to a 8' tender - though it would certainly be nice to wake up in the morning after a night of heavy rain, and not to have to deal with the dinghy tied up behind, half full of rain water.

    Its worth noting Scott, or anyone who has a good reason for wanting self-bailing while empty (and there may well be a good reason) it looks to me that Sibling Tender might  be ideal for such a variation. All that would be required is a very slight change to the kitset and a bit of extra material for the "board and thwart" which would otherwise have been made from the cockpit cut-out.

    Perhaps this novel feature warrants an extra point.

    Surely every possible requirement has now been catered for, by this truly remarkable suite of dinghy designs.

    Last modified: 07 Jun 2021 05:23 | Anonymous member
  • 06 Jun 2021 09:52
    Reply # 10596949 on 10211344

    Graeme wrote:

    None of them will self-drain in the way Scott suggests, as this would require a false cockpit floor above the waterline, which takes us right out of the “tender” category

    But sit-on kayaks are self-draining, though holes in the cockpit floor, so it's feasible to make a dinghy this way too (at the expense of the extra weight of the double bottom). You could easily have a sit-on dinghy, if that was important. In racing dinghies, it is important.



    Last modified: 06 Jun 2021 09:52 | Anonymous member
  • 05 Jun 2021 03:13
    Reply # 10593080 on 10211344

    Thanks Dave. I'll have another look, and decide.

  • 05 Jun 2021 02:55
    Reply # 10593010 on 10211344

    Hi Graeme,

    If you want to build one of my designs then I would suggest AD. She is the Mack truck of 8-foot dinghies and is relatively easy to build, almost as easy as KISS.

     DD is difficult at model scale because of finding the material that will take the bend of the sides, but she is relatively easy at full size using the specified thickness of ply.

    My Webb 8 design was done for a friend in San Francisco, for a short series production. She needs a building mould which takes quite a bit of time and effort to get right. Once built, however, construction is easy. I was working at the Wooden Boat Centre in Marina Del Rey in California and my boss and I could cut out all of the pieces with their correct bevels etc in a morning, and assemble them in the afternoon. Next morning, we would pop it off the mould and start the finishing which would take a couple more days, and then painting. The sailing accoutrements and rig would take a couple more days. We had a local sailmaker build us the simple balanced lug sails, which he gave us a very good price on as the same sail went on the 10-footer and two of them on the 14-footer.

    Anyway, I just want to say that your exercise in model building is very interesting and informative, I will have to come by and look at them all when I am in Auckland.

    David.


  • 05 Jun 2021 00:36
    Reply # 10592781 on 10211344

    Here’s the first three, cleaned off last evening, with KISS in the background.

    Already you can see how very different they are turning out to be.

    I will do my best to try to get some comparative numbers for stability, towing etc. Conjuring up a short chop will be a challenge, but it brings back memories of senior high school when we made a primitive ripple tank and did a bit of learning about wave interference. I suppose it could be done, if one had the time and the resources.

    Rudders, centreboards and the whole dimension of sailing is beyond my resources, but I think we can take it for granted that with a little bit of ingenuity any of these dinghies can be made to sail well enough, and as for comparing: the human factor would probably outweigh anything inherent in the boats themselves.

    …..

    In answer to the question raised by Curtis (knockdown, recovery etc) I think we need to remember that these boats are primarily tenders, which will almost of necessity have to be open or with only limited built-in buoyancy. If the challenge were to design a sail boat, that might be different. Of course, someone may well want to have a capsizable/recoverable tender – the one called “General Purpose Dinghy” probably answers that requirement – and also probably answers Scott’s question regarding the ability to drain while upright. These are not features one would normally expect in a 8’ tender – the great thing is, this design competition has even provided an option for someone who requires that. I would expect “General Purpose Dinghy” could sail away from a capsize and with a couple of venturis in the bottom, it might even self-drain in the process. None of them will self-drain in the way Scott suggests, as this would require a false cockpit floor above the waterline, which takes us right out of the “tender” category. (The little boat I had many years ago had an enclosed cockpit a little like that of General Purpose Dinghy and the side decks were wide enough that it could be righted from a capsize without shipping any water – but that feature alone would have ruled it out as a practical tender). The Design Competition brief, in its typically vague fashion, states only: “Buoyancy could be built in or not – buoyancy bags are cheap and lightweight” from which I would infer that the requirement, sensibly, is that if the tender is to be sailed, it should have the means to stay afloat in the event of a capsize. After that, for some people at least, some assistance may well be necessary.

    Anyway, the designers have collectively provided a suite which surely provides almost every option for every requirement  – every special feature coming at the cost of some other special feature.


    Last modified: 05 Jun 2021 00:38 | Anonymous member
  • 04 Jun 2021 09:00
    Reply # 10590423 on 10211344

    Graeme,

    If not an actual judge, certainly a supplier of information very necessary to do a good job of judging. I hope you can do a static loading test on each model, to determine what each one looks like with "2-3 people" aboard. I hope you can do a dynamic test in flowing water with those people aboard, with a spring balance telling you how easy each is to row. And if you could do that again in a bit of a chop, so much the better. R/C sailing models might be too big an ask, though!

  • 03 Jun 2021 22:57
    Reply # 10588842 on 10211344

    Graeme,

    yes DD is not that easy to build at model scale as you need plywood thickness to scale. The design approaches the limit for bending of the specified plywood so is not easy to replicate at model scale, steam bending may help. I built a half scale hull model before I built the prototype and had difficulty bending the sides as the face veneers on the 3mm plywood were very thin and often had cross grain in them. I broke several side pieces before I got them to bend properly. A 1mm wood veneer may be a better idea for the sides.

    David.

  • 03 Jun 2021 22:40
    Reply # 10588779 on 10211344

    I won’t be involved in any capacity of judge in the “competition”, for a number of reasons including that I have had far too much to say already, and am now well outside of my limited knowledge.

    On the other hand, I am so impressed with the variety and quality of the designs that I cannot help wanting to comment from the sidelines, and to make my own personal judgements.

    I think that each of these designs, each in their own way, is very good – and the JRA now has a rich resource available to members – thanks to those generous and talented contenders who actually "entered" and put their designs up for criticism. (This might sound platitudinous, but in fact it is from the heart). 

    For those who have lost the link, it is: 

    https://junkrigassociation.org/dinghy-competition


    I intend to continue with these models and make all but three of them, because it is an enjoyable learning experience, and because I am changing my personal judgement of these dinghies as a result.

    Making the models not only gives a better appreciation of what these things look like in reality – there have been some surprises for me. It also forces one to look closely at the drawings and instruction details and here too there is a wide variety of approaches – though regarding this matter, the quality varies considerably. I think each of these dinghies is fit for purpose (in some cases depending on the purpose) but not all of the entries are sufficiently well-developed to be fit for offering to the public as plans to build from – or for a school building project. Another thing which emerges is a sense of which builds are likely to be quick and easy, and which will be a little more difficult – and in this area also there have been some surprises, for me anyway.

    Curtis: As for testing them on the water – I believe some crude comparisons can be made, of SOME of the performance aspects. A little bit of static testing, for example -  and as I have a 2-3 knot flow of water alongside where I live, maybe some crude hydrodynamic comparisons, though whether of any real value I agree with you, and have my doubts. For my own personal benefit I am merely  hoping to find the time for a little “messing about”.

    [PS Slieve: in the meantime KISS has been thoroughly bath-tested by my grand daughter and found to be "cool", and I agree.

    I have no idea how the judges will reach any decision, other than choosing which of the entries is the most suitable for the school building project, and I suggest that might as well be the deciding factor. It won't be my decision anyway.

    Edit: I have just gone back and looked at what Slieve refers to as "characteristics" which might be the basis for an objective  judgement. Sorry Slieve, I disagree. I know a bit about "exam technique" too - and I think most of these "characteristics" are so vague as to be worthless for that purpose, having the sole valuable merit of producing a proliferation of ideas. For example: "Not too big, not too small"  - well, what size is that? and "as light as possible..." ??  "No longer than can be obtained from a sheet of plywood" - you can see the number of ways in which this has been interpreted - all of them valid according to the wording (although none could be built from a single sheet of plywood). And almost anything can be carried on a roof rack, and you can put a mast "in a variety of positions" into anything (though one position ought to be enough). "Simple and cheap to build" is a guide, though not exactly objective. "Capable of carrying 2-3 people" is helpful, though even that is a matter of judgement. On the basis of these "characteristics" it might might be possible to eliminate a couple of the entries, but that is all. Personally, I don't think it matters which one is chosen - what matters is that all are now available and worthy of consideration.

    [PS Arne: Halibut is nearly complete – a handsome little dinghy which I would be proud to own. I did not find it as easy to interpret the plans as I had expected – I expect the sailing season came along before Arne had quite finished tidying up the drawings. With five planks there is of course a bit more work and complexity. However, for anyone building this dinghy the result will be well worth the effort.  (I intend to have a go at building a model of Medium Boy, starting this morning – although not an "entry" - just working from the sections drawings which I can scale up on the computer and print on A4).

    PS David T– I am liking Siblim Tender very much now. It went together very easily and suddenly it was finished – seats and buoyancy tanks already done! To my surprise it is a rather dainty wee thing. Its the only one which has very moderate beam, and is "full forward and fine aft". All the others seem beamier, and to carry their weight in the aft quarters. Regarding the mast - given that when rowing with 2-up it may be necessary to shift weight and body forward, I would like to suggest that sailing it will only be practical 1-up, and in that case the mast would not be in the way, and could be unstayed, as it should be, IMHO. What's to be done about that in-your-face bow transom? If it works well, as  expect it will, then very soon it will look OK.

    And as David once remarked on another matter: "If you've got it, you might as well flaunt it!


    PS David W I am hoping to have a go at DD, but the construction method is giving me a headache as it does not suit the thin materials I am working with, and with my limited skills, and there are some parameters missing if building from first principles. I will do my best with it anyway.

    PS The closest to a Chinese sampan is Boxer - which is not a simple box as it first appears to be. I nearly didn't bother with it, but I am glad now that I did - actually it is worth a look. Why the bottom is made in three pieces is beyond me. If it were a simple curve on a single sheet it would be far easier to build and to some people, the boat would then be far more likely a potential contender. Let down not at all by its shape, but by the building instructions and the drawings, which would be worth re-doing from scratch - after actually building the prototype. ]



    Last modified: 04 Jun 2021 01:19 | Anonymous member
  • 03 Jun 2021 22:21
    Reply # 10588764 on 10211344

    This dinghy design competition has been most interesting and has everyone looking for the perfect dinghy.

    We all know that there is no such thing as the answer is, “Horses for courses”. Most situations that require a dinghy will probably want a dinghy somewhat different from the one available. This would seem to mean that the search on the forum is for the best all round compromise.

    On one hand you start to pity the judges when they are confronted with the wide range of views expressed by the readers, but those who presented the competition were very wise in the way that they did. Not only did they get the readership exercising their grey cells, but they also made it easy for the judges to make their selections. Good exam technique requires the student to answer the question asked, and not the one that the student thinks should be asked. This means that the person marking the answers simply has to be happy that the answers tick as many of the points asked for in the question as possible.

    By suggesting 9 characteristics in the competition rules the judges should not feel overloaded with information from the readership, but be able to see if a design fulfils the requirements asked for.

    No doubt the search for the prefect dinghy will continue into the future.

    Cheeers, Slieve.

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