Cash prize of 250 GBP - Dinghy Design Competition

  • 15 Jul 2021 14:28
    Reply # 10753249 on 10211344
    Anonymous member (Administrator)

    Here's five more.

    Click to enlarge.








    Last modified: 17 Jul 2021 03:58 | Anonymous member (Administrator)
  • 14 Jul 2021 10:12
    Reply # 10750315 on 10211344

    Graeme, yes definitely still following it!!

    I think it only fair that all of the official entries in the design competition are represented in your fleet, so I will bring a model of my Webb 8 design down to you on Friday. I hope you will be in at around the same time as last week. See you Friday.

    David.

  • 14 Jul 2021 09:47
    Reply # 10750296 on 10211344
    Deleted user

    Graeme, I know that you are not going to be a judge in the competition, but after 'living ' with these dinghies you must of developed a personal favourite. Could you please put the name into a sealed envelope only to be opened once the winner has been decided. It would be interesting to see if the judges come to the same conclusion. :)

  • 14 Jul 2021 02:27
    Reply # 10749728 on 10211344
    Anonymous member (Administrator)

    I think now that David T was right to declare the "stability testing" with the rigid "passenger" invalid. I’ve started doing it all over again, with revised estimates for build weight, and a different way of allowing for the huge discrepancies in "over weight” of the models. I think we need to look at proportionality - and also, any additional weight should be added to the midship section, not perched on a rowing thwart.

    I am doing four static tests on each model.

    • 1.      A righting moment versus heeling angle (stability curve) for each dinghy model in its current over weight form. (Percentage overweight is stated in each case.) These will be represented by an orange line.
    • 2.      The same again, but with additional weight (a strip of lead flashing) spread around the midship section, to bring each dinghy up to the same level of over weight (250%). 
    • This (250%) might sound a lot, but it is the only way I can try to make a level playing field. Most dinghies improve their stability with extra hull weight, and it is not uncommon to load real dinghies up to 400% or more, of the empty designed displacement. These graphs will be represented by a grey line.
    • 3.      The same again, without the additional weight, but with the addition of a quantity of water in the hull equal to twice the weight of the hull, to look at the free surface effect on stability. There will be some surprises here. These graphs will be represented by a blue line.
    • 4.      A simple test in which a concentrated weight is applied near the gunnel, to see how much weight is required to bring the gunnel down to the waterline.

    Dinghies which do not have full buoyancy will lose stability when the gunnel is immersed, and the line graph quickly decays to zero.  Those with full buoyancy will have a wider range of stability.


    Here are the first four, in no particular order(click to enlarge):








    That's right - General Purpose Dinghy is more stable swamped than dry - much more - not that I can imagine how it could ever be flooded, as it retains stability beyond 80 degrees of heel, floats high on its side tanks and will recover virtually dry.

    above: GP Dinghy at 80 degrees, dry and still has stability

    above: Sibling Tender at 45 degrees, flooded but still retains some stability

    The whole project has got a bit out of hand and is taking “messing about with boats” to a rather ridiculous level, so please do not use these absolute figures to compare dinghies, rather it is just an interesting exercise to look at the effect on stability of weight, shape, beam, buoyancy tanks, free surface water, etc.

    Last modified: 17 Jul 2021 03:37 | Anonymous member (Administrator)
  • 10 Jul 2021 23:55
    Reply # 10743052 on 10211344
    Anonymous member (Administrator)

    Slieve: Lifting groceries from the dinghy (putting an asymmetric load on it) is just another way of heeling the dinghy, and finding its righting moment, isn't it?

    The answer should be buried somewhere in the existing "stability data" but off the top of my head I don't know how to find it. 

    I'm considering another common scenario, which might be amusing to video.  I want to simulate a 1/5 scale model "oaf" leaping into the dinghy from a fixed height and landing badly. The oaf might be a soft bag of wet sand. "Oaf tolerance" might be another useful parameter for the judges to consider, and at least it should provide some bath tub fun for my grand children (and me).





    Last modified: 13 Jul 2021 00:34 | Anonymous member (Administrator)
  • 10 Jul 2021 23:06
    Reply # 10742971 on 10211344

    I agree about the wide range of requirements Arne, but the judges have to somehow come up with an answer and I suspect they are not finding it easy.

    Thankfully I will not face the problem but I do have memories from many years ago of trying to judge model aeroplane aerobatic competitions. We wrote down a list of all the manoeuvres that were to be judged and gave each one a difficulty factor. A round loop might get factor of 10 and a square loop of 15 and so on. The judges simply had mark each manoeuvre out of 10 and later multiply the mark by the difficulty factor, and finally add up all the scores. The highest total score got first place. They do similar things in ice skating.

    It might be possible to draw up a table with the rules/requirements in the left hand column and their importance (difficulty factor) grading in the next column. Then list each design along the top of the rest of the columns and let the judges fill in the table with a mark out of 10 for each requirement. Complete the table as a spreadsheet and multiply the marks out of 10 by the difficulty factor and get the total for each dinghy. Sounds easy, but as I say, thankfully it's not my problem.

    The beauty of the competition is that it has exercised a few grey cells, caught members imagination (and taken Graeme back to having fun playing with models).

    Cheers, Slieve.


  • 10 Jul 2021 21:51
    Reply # 10742883 on 10741916
    Anonymous member (Administrator)
    Anonymous wrote:

    Hi Graeme,

    ....

    Regarding the judging of the competition, as we all know there is no 'perfect dinghy' as there are so many possible uses, so the judges have to judge how well the entrants agree with the parameters asked for in the competition announcement. Exam technique says that to get top marks you must accurately answer the questions asked, and no more.

    Cheers, Slieve.



    The problem here is that the questions asked were too inaccurate, so therefore there cannot be accurate answers.

    I am tempted to use the term David Ty. often uses; ‘horses for courses’:

    • ·         If the competition was about designing an 8-foot rental dinghy to be used on a pond in a park, then I would no doubt have entered with the ‘Fat Boy’ design, or with Halibut version B, both with generous internal side tanks and maybe even fender sausages around the rail.
    • ·         If the specs asked for an easy to row tender for 1-3 persons who were used to operate such things, I would have suggested the ‘Trim Boy’ , 'Medium Boy' or the Halibut version A.
    • ·         If I needed a steady 'harbour barge dinghy’, I think that a stoutly built (50-60kg) ‘Simplicity 8’, well fendered all around it, would be good.

    Yes, horses for courses...

    Arne

    Last modified: 10 Jul 2021 22:18 | Anonymous member (Administrator)
  • 10 Jul 2021 11:30
    Reply # 10741916 on 10211344

    Hi Graeme,

    Your model dinghy tests remind me of the many happy hours I spent as a child kneeling on the floor and 'testing' my home made model boats in the bath. The fisherman's anchor I cast in lead was useless as it slid along the bottom of the bath and didn't 'dig in'! Great fun.

    Your stability tests have produced conflicting comments, so I would like to add a few more to help muddy the water.

    I consider the round tail inflatable Avon Redcrest to be an excellent tender, and still own two of them. They are so statically stable that it is possible to walk around in them right to the edges, that is if you can walk on a water bed. You can also climb into them after swimming and not bruise your body nor capsize them on your head. On the other hand, even though I find them great for rowing I doubt if the sailing performance would be worth considering. Probably the least statically stable dinghy I have owned was the Merlin Rocket. 14 feet long with a 23 foot mast, when stationary you had to sit on the centreline. When sailing in very light winds the helmsman would on the windward side deck and the crew on the leeward one to heel the boat slight to leeward and force the sails to take their cambered shape. With a decent wind both crew would have their toes under the straps, knees over the angled side deck and their bodies as far out as their muscles would permit. Fast exciting sailing, but not something that would be simplel to perform stability tests on.

    So my tuppence worth says that there is one serious static stability test worth doing. To be a safe tender it must be possible for a single occupant to stand up and lift the shopping/ cargo/ outboard motor up and place it on the side deck of the mother ship. I think that means standing off centre and with their CG about 50 cm from the side and holding a 15 kg or thereabouts load some 20/30 cm outside the gunwale at about shoulder height or higher with modern cruisers. No doubt these numbers should be adjusted but you may see where I'm coming from.

    With a tender which might only weigh 30 kg and a single crew at 80 to 100 kg this could be a limiting factor. With two in the dinghy the second one would/should automatically counterbalance the one lifting the stores. The complication with your models is that their non-scale weight would somehow have to be corrected for.

    Anyway, what do you think?

    Regarding the judging of the competition, as we all know there is no 'perfect dinghy' as there are so many possible uses, so the judges have to judge how well the entrants agree with the parameters asked for in the competition announcement. Exam technique says that to get top marks you must accurately answer the questions asked, and no more.

    Cheers, Slieve.


    Last modified: 10 Jul 2021 11:38 | Anonymous member
  • 09 Jul 2021 23:44
    Reply # 10741074 on 10211344
    Anonymous member (Administrator)

    That’s a very interesting post you made, Curtis, and I can’t resist yet another reply. (Sorry, the rest of you). Yes, it is science – or at least my humble attempt at it – which equates more closely to “doubt” and nowhere near as close to “technical knowledge” as many people believe. You mentioned Apollo – there were no computers then, as we know them today, and probably more computing power in the cell phone in your pocket than there was then in the whole of Cape Canaveral. I was studying physics at secondary school around the time, during the “cold war” when the Russians stole a march on the Western world by being the first to launch an artificial satellite. This created such a panic in the US that school curricula were rapidly revised, and the ripples were felt as far away as New Zealand with the introduction of the first PSSC courses. This was explained to us at the time. The emphasis suddenly shifted away from rote learning of complex experiments carried out by “experts” in remote laboratories – to hands-on, real science, with make-shift, back-yard equipment. This sort of approach used to appeal to New Zealanders (not so much these days, I am afraid). We measured the size of a molecule using talcum powder and the diameter of a drop of oil spread on a puddle of water, and the wave-length of light (I forget how we did that). We studied the behaviour of waves directly, with simple ripple tanks – and remember those “ticker timers” for learning about dynamics? I wish I had one of those now, for the dinghy testing! It was an exciting time, and all too brief. I suppose I am trying to re-live my childhood – or perhaps entering into a second version of it!

    I can’t do a drag test for DD until we get another spring tide up here on the mud bank, but I will see what I can do by trailing them in the river a little further downstream. In a miniature chop, if the conditions are favourable.

    Some people will agree with you about the value of fore-and-aft buoyancy tanks – like the mandatory use of life jackets, it requires a compromise and doesn’t suit everyone. The PD (in your post) is primarily a sail boat rather than a tender and the side tanks don’t work so well with a little stem dinghy. The offset dagger board as part of a side tank is ingenious and used to good effect on David T’s narrow beam but very safe and easily driven creations. But while I wholeheartedly agree with your Boy Scout motto, I am giving brownie points to no-one. The motto here is “horses for courses”. I’m studying form, but I’m not placing bets.

    PS The doubts expressed  by David T, as to the relevance of all of this, is required reading. Stepping badly into a dinghy. Dumping your groceries on the side deck. Dealing with an unexpected partial flooding - these sorts of things are what good designers think about, and my little back yard tests barely delve into the myriad of problems that a designer has to solve. While I don't consider these experiments irrelevant, I do appeal to all, not to take the numbers too seriously, or to draw over-simplified conclusions.


    Good you have stated your opinion, Curtis. The judging committee will want to know what sort of dinghy would suit most people.

    I wonder which type of dinghy most people like best?

    Last modified: 10 Jul 2021 05:25 | Anonymous member (Administrator)
  • 09 Jul 2021 20:20
    Reply # 10740731 on 10211344
    Deleted user

    Graeme,

    I find these tests interesting and at least a little informative, so I hope you continue. This is science, which is always useful, and I like seeing the ingenuity you bring to devising tests on a near-zero equipment budget. It helps to remember that within living memory, a great preponderance of our technology was calculated to three or four significant figures. The Apollo astronauts carried slide rules.

    I'm anticipating your towing test of Dave's DD (I do hope you intend to do that), and kudos to him for providing you with the model.

    Also, there seems to be a general lack of concern about the consequences of a dinghy being knocked down, but to me, safety should a major part of the design of any watercraft, in every way possible. If I had a say in judging these designs I'd give extra credit for built-in air tanks, and moreso if they're on the sides. I know that the objection is a loss of cargo volume, but what's a week's worth of groceries if you can't right the boat or bail it? This should be doubly important to a utilitarian craft that you may be compelled to use in less-than-ideal conditions; strictly recreational sailors can chose their weather and sea conditions.

    To me, then, the gold standard is this:

    https://pdracer.com/emergency-flotation/side-airboxes/

    Boy Scout motto.

       " ...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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