Cash prize of 250 GBP - Dinghy Design Competition

  • 10 Jul 2021 23:55
    Reply # 10743052 on 10211344

    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
  • 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...


    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

    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
  • 09 Jul 2021 20:20
    Reply # 10740731 on 10211344


    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:

    Boy Scout motto.

  • 09 Jul 2021 10:13
    Reply # 10739505 on 10211344

    Thanks for your feedback and comments David, I have been thinking about this a lot, and I partly agree with you.

    But not entirely.

    “The whole process of stability testing” is most certainly not invalid. I think it might be true that some of it is irrelevant, at least to most people, but that is not the same thing as invalid.

    I am concerned though, because I think the way I have been going about it is invalid, and I am at present trying to think of a better way to do it.

    I also think that looking at the results and trying to figure out which dinghy is “best” is invalid, and I think that might be what you are saying. If so, I agree. I am not really very interested in the “competition” and wish to have no part in the judging.

    I intend to continue, because I think the testing can potentially show that the various dinghy shapes are different, and perform different tasks differently. In the case of drag, none of them are all that very different (they are, after all, only 8’) but even that is something that is worth demonstrating, rather than just taking for granted. I think it is worth it, though I would not take the actual numbers too seriously, as I have stressed repeatedly. As for stability at various angles of heel – these different section shapes perform VERY differently, and while some people, such as yourself, have a very good idea of form stability and how it depends on shape – some of us are not so knowledgeable and I think these practical demonstrations can teach us a lot. I do, however, want to try to do it better, because I can see some real flaws in the way I went about the stability testing.

    I think you are probably right in saying that beyond 30 degrees of heel, stability curves probably don’t mean much in relation to dinghies. After all, we are not expecting “recovery from a knockdown”. Never-the-less, some of these dinghies do continue to remain stable right up to 80 degrees or more, and do still provide some of us with something to learn, on the subject of form stability. How the struggle to return to an upright position plays out, might be of interest to some people. (How many people in this world still believe that boat stability only comes from a lead weight slung beneath the body of a keel boat – most people, I would suggest – but not JRA members I hope!)

    The nice thing about these forums is that one can skip what is of no interest, and I imagine most people are probably now skipping this thread because I have had far too much to say on it. I would be very grateful though, if you would continue to monitor this thread, and continue to criticise. That is good, and probably necessary.

    By the way, the shape of a grand banks dory has little to do with its motion in a seaway. They evolved from a need for something that can be built quickly and cheaply from flat planks and can be stacked up one on top of the other and "nest", on the deck of the mother ship when being carried to the fishing grounds. Horses for courses. (I have seen factory-made moulded fibreglass “grand banks” dories, which completely misses the point, and shows little comprehension of the concept or its raison d’etre). For purely motion in a seaway, there are probably better models. I am sure you are right, however, in observing that the “section” which has evolved, (the angle of flare etc) is quite important in the way they behave at sea, and they seem to have proved to do the job well.

    (While I was doing this post, my dinner burned on the stove - and also Arne posted a reply).

    Arne, I just saw your post. Thanks, you have said in a few words what I was trying to say in a hundred. I want to do the stability testing all over again, and then I will try to find a way to post all the graphs, and how they relate to the various mid-section shapes. I am sure none of it will be a surprise to people like you and David, but for some of us it will be quite educational I think.

    Last modified: 10 Jul 2021 00:02 | Anonymous member
  • 09 Jul 2021 09:10
    Reply # 10739461 on 10211344
    Anonymous member (Administrator)

    I can see David’s point here. The absolute numbers will not be of that much value, since other factors play inn her, like the builder’s chosen weight (thickness) of the plywood, the sort of interior (tanks, bulkheads or not), and not least, the chosen height of the thwarts.

    However, the stability curves obtained so far are far from useless:
    The shape of the stability curves gives us a good idea of the character of the boats.

    • ·         For instance, the 5-plank versions have a slack slope, indicating initial tenderness compared to the ultimate stability.
    • ·         The 3-plank versions with flaring topsides have a steeper slope,
    • ·         while the box boats with vertical sides have the steepest stability curve.
    • ·         The ultimate stability is then just a function of the beam of all the tenders.

    I don’t think there is much use in going into deeper science with this, but the curves surely are interesting to watch, so I hope you let us see all of them, Graeme.


    Last modified: 09 Jul 2021 10:29 | Anonymous member (Administrator)
  • 09 Jul 2021 07:44
    Reply # 10739339 on 10211344


    Sorry, but I have to say that I think this whole process of stability testing is invalid. The crew is live weight, not dead weight, and in the event of a lurch to one side will instinctively try to correct, and then will finally slide or fall down to the lee side. In a kayak or small dinghy, one instinctively pivots at waist level to keep the upper body upright. I don’t see any realistic point in these tests, certainly not beyond 30˚ of heel. And anyway, an unstable, round bottomed boat is better in a seaway - eg greenland kayak - but a flat bottom boat is better for standing up and hoisting water jugs onto the side deck. It all depends on what kind of boat you want. Probably the 5-planks are the best all-rounders. Though a 3-plank can also be good in a seaway, if the section is right - eg grand banks fishing dory. 

  • 09 Jul 2021 03:14
    Reply # 10739065 on 10211344

    Dave W came to visit this morning and brought another model dinghy – this is his DD which is one of his three entries in the dinghy design competition.

    I was pleased to see this somewhat radical model – it is one that I had wanted to include, but thought it would be too difficult for me to make. It is an “extreme” design, which is good for testing, because these “outliers” provide good contrast.

    Here is Dave W.’s “DD”.

    Dave stole an advantage here, by clever use of the “8’ sheet of ply” constraint. The bottom is made in two parts (it’s a shallow vee-bottom) and the centre line comes from the diagonal of a plywood sheet, rather than the length. So, it is a 2.4m dinghy which is actually just under 2.7m. That will make a difference to its hull speed, and to some extent its stability. In addition, the model itself is very over weight, to scale – so that gives it extra stability over and above what it will obviously have from its shape and its beam – compared with the other models. (That “unfair advantage from over weight" is a flaw in the testing procedure I have used.)

    We put the mast on it, and put it in the tank.

    The result was – the stability curve went right off my scale!

    Here is the graph again, with its own special vertical axis

    Massive stability, right up to the point where the gunnel is immersed. A few more degrees of heel is possible because of the buoyancy tanks, but once the dinghy goes beyond about 45 degrees and floods, like most of the others, it continues to float, but with little or no stability.

    I have not yet been able to test these models in choppy conditions. Dave says that his full size prototype DD, with its fine entry, is good in a chop.

    I think I will have to test all the models again, and find some other way of compensating for differences in the over weight factor. Putting a discount-weighted “passenger” up on the rowing thwart has disadvantaged those little dinghies, when compared with big ones like this whose over-weight is built into the hull, and which do not need a “passenger” sitting up high, which affects the metacentric height.

    (Maybe I'll just put the "passengers" on the floorboards this time - which then confers a slight stability advantage - this time to the "little" ones.)

    Last modified: 09 Jul 2021 04:57 | Anonymous member
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