Cash prize of 250 GBP - Dinghy Design Competition

  • 19 Jul 2021 10:08
    Reply # 10760851 on 10211344

    You are right Arne, it was way “over the top”.

    Originally I was just thinking about stepping into a dinghy and standing off-centre – which upsets some boats more than others, and is a matter to be considered. There are plenty of dinghies which I would consider too sensitive, and I think a tender should be fairly robust against clumsy handing, which is pretty commonly seen. (But I would certainly not expect anyone to jump down onto the gunnel of a dinghy, and of course that should not be a requirement).

    However, to make a crude experiment with limited equipment, I wanted a simple a procedure which discriminated between dinghies. It had to be a bit over the top.

    It wasn’t just a matter of built-in buoyancy,  You have missed something there. General Purpose Dinghy was a stand-out case of that – but Oyster, with no buoyancy, was robust. David’s box boat, with its full buoyancy tanks, flipped instantly. Simplicity8 didn’t flinch – and its buoyancy tanks never came into play.

    I think we can learn something – but I wouldn’t take any of it too seriously, as I have repeatedly stressed.

    Arne wrote: “Almost any of the shown models could be winners. It all depends on what one needs.

    Quite right. You summed it up perfectly.

    PS I thought Medium Boy's slow-mo struggle to survive against that unreasonable assault was quite heroic, don't you?

    Last modified: 19 Jul 2021 12:32 | Anonymous member
  • 19 Jul 2021 09:42
    Reply # 10760787 on 10211344
    Anonymous member (Administrator)

    frankly, I think this test was a little over the top. Most small traditional workboats in my country, like prams and færings would fail on this test. Stepping on the rail like that is simply no-no. Most children until recent years would learn to handle a small boat from early childhood, and would from then on do it right every time, just as they today learn to handle a bicycle or skateboard. These færings, with their flaring topsides, are still much more seaworthy than vertically-sided dinghies. Just look at the dories used on the Grand Banks.  They needed to keep the sea out, and to retain stability as they were loaded, exactly as the færings.

    If tolerating lack of boat-handling skills is a requirement, then a wide-beam box boat with side tanks is the answer. I once rented a pedal-driven boat for a trip on the canals in Amsterdam. It was perfectly rectangular, more like a float than a boat  -  and its hydrodynamic qualities could be likened with Buckingham Palace...

    Most of the shown models can be built with side tanks and thus survive this test. With some of the buoyancy at the ends, these side tanks need not be so wide that they steal much space inside.

    I surely am glad I am not the judge. Almost any of the shown models could be winners. It all depends on what one needs.


  • 18 Jul 2021 23:26
    Reply # 10760106 on 10211344

    One last test, a kind of dynamic stability test: A 800 gram soft weight dropping from a height of about 15cm onto the gunnel of each of the models.

    This is an attempt to simulate a 100 kg clumsy person jumping down from a height of about 0.75m onto the gunnel of the dinghy. A rather severe test.

    The models which were able to tolerate this assault, were subjected to the test again, this time with a quantity of water in the bilge. Anyone who has jumped into their tender, half full of water from over night rain, might be interested to see what happens with these models.


    The tests were recorded on video here.

    I am not sure if all this "tersting" is going to help the judging committee very much.

    For the judging committee's sake, it would probably be better if more people might chime in with their opinions about what makes a good or not-so-good tender.

    Last modified: 19 Jul 2021 08:20 | Anonymous member
  • 17 Jul 2021 12:09
    Reply # 10757672 on 10211344

    Hi Arne, I was planning to write a conclusion and you stole my words!

    Yes, if you want stability in a 2.4m dinghy - you get it with beam (almost regardess of shape)

    And you pay for it, as the next tests show. I did another attempt at measuring drag, this time streaming the models in the creek, on an ebbing tide. Problem was, it was a fairly sluggish tide and hardly any tension was registered on the tow lines. I added an extension to the arm of the tension gauge, and just did the best I could. Also there was a ight breeze in the same direction as the tide. Did I measure drag or did I measure windage? Or was it both? I don't think the measuring was very accurate - but the little light ones did seem to have less drag than the big beamy ones.

    Here are the results:

    Here is the video

    I should probably do these trials all over again, with all the models carrying a weight to the same percentage, or perhaps with the same weight. But I am just about over it.

    If I can find the time, I will do one more series of tests, "oaf tolerance", simulating dropping a weight onto the gunnel or side deck from a fixed height- a sort of dynamic stability test. That will be easy enough - and in each case the result will just be "pass" or "fail". Maybe tomorrow.

    PS Another thing Arne - yes it would be very informative if Fat Boy, Medium Boy and Slim Boy could each be built, to the same specifications, and compared - this would probably be more informative than the diverse group of dinghies I have been trying to compare. I can't do any more, but if someone else could make those three, it would be very interesting. I would suggest planking them with balsa wood and trying to keep the model weight down to about 250 - 350 grams. Anyone else like to give it a go?

    Last modified: 17 Jul 2021 14:27 | Anonymous member
  • 17 Jul 2021 09:57
    Reply # 10757566 on 10211344
    Anonymous member (Administrator)

    This is a very interesting series of tests, Graeme, thanks a lot. I hope the results will be edited into a PDF article afterwards.

    The results seem to go along these lines.

    • ·         The beam decides the maximum stability.
    • ·         The slope of the topsides decides the initial stability.
    • ·         Flotation along the sides  -  tanks or foam  -  greatly increases ultimate stability.

    Problem is; some of the steadiest designs are also the slowest ones in the towing tests. My ‘Simplicity 8’ would probably be best as a harbour barge of some sort.
    If Medium Boy’s two brothers had been built; the Fat Boy and the Skinny Boy, they would have underlined what beam does to stability.

    My conclusion is that adding some buoyancy along the sides will make any 8-footer a lot safer. This was why I was thinking of adding a buoyant ‘fender sausage’ around the gunnel. These would both act as buoyancy in the best position, as fenders, and as spray rails, all without taking up space on board.


  • 17 Jul 2021 00:54
    Reply # 10756984 on 10211344

    (Stuart - I do have some general opinions and I'll reply to your post later).

    Dave W turned up again with another model - the Webb 8. If it were to be a beauty contest, this little canard-body semi-clinker pram design might be the winner.

    This is probably getting tiresome, but for those still interested, here are the rest of the stability reports. Click to enlarge.


    Last modified: 17 Jul 2021 01:26 | Anonymous member
  • 15 Jul 2021 14:28
    Reply # 10753249 on 10211344

    Here's five more.

    Click to enlarge.

    Last modified: 17 Jul 2021 03:58 | Anonymous member
  • 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.


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

    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

    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
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