Cash prize of 250 GBP - Dinghy Design Competition

  • 07 Jul 2021 16:28
    Reply # 10734987 on 10211344

    Each of the dinghy models has now undergone a kind of “stability test”.

    Each dinghy is rigged with a simple, temporary mast (0.5m in length) from which it is hauled down into a heeled position, the hauling line attached to a tension gauge calibrated in Newtons. The position of the tension gauge is moved, between trials, to try to get the hauling line to remain roughly perpendicular to the mast, as the mast rotates from trial to trial.

    Each dinghy is restrained by two lines which go under the hull and prevent it from moving sideways when hauled down from the mast head.

    When making an "observation", the angle of heel is estimated by lining up the mast with a large “protractor” scribed on the far wall.

    The righting moment in Newton-metres, at a series of angles, is given by force x 0.5m

    There is a problem: the weight of the model has a big effect on its stability. I have tried to compensate for over-weight in the models by placing a “passenger”  (jar of sand) whose weight is reduced by the approximate over-weight of the dinghy model (see previous post, and see the following examples).

    This way, over-all weight is near as possible to what it should be for a real-life dinghy and one person rowing. However, this results in anomalies in the DISTRIBUTION of the weight. The weight of the "passenger" is placed, high, on a rowing thwart - instead of the extra weight just being built into the hull. The trouble is, the dinghies are all over weight by differing amounts, and thus carry "passengers" of differing weights. The dinghies which carry a heavy "passenger" (generally the smaller, lighter dinghies are the least "over weight") now have a higher centre of gravity than those which carry a lighter passenger, and are thus at a disadvantage when comparisons of righting moment are made.

    Lighter dinghies, such as YouYou, KISS and Sibling Tender are particularly disadvantaged in this respect. 

    I don't know what to do about this - I should have worked within a budget for weight, when making these models. As it is, the project is somewhat flawed.

    [Edit: because of this, if I can find a place to post a spreadsheet for all the results, I will group the dinghies according to the weight of their "passengers". It probably all means that although there might be some valid comparisons possible, the absolute numbers probably don't mean much, which is why I have not bothered to scale them up.]

    Here are a couple of the results:


    David’s Box Barge

    The stability curves all differ greatly – a wide range of results – and some of them with a very wide and some with rather narrow stability range.

    I will try to find a way of getting all of these posted on the website.

    Edit: with their decks up near the gunnel, and full fore-and-aft buoyancy, Simplicity 8, General Purpose Dinghy and the NZScow can be heeled to almost horizontal and still have some reserve stability. They will float on their side tanks and come up dry. Little, light, easily-propelled open dinghies such as KISS and YouYou will flood and capsize as soon as the gunnel is hauled down to the water line. Big, beamy, stiff dinghies such as Halibut, and AD maintain high stability until the gunnel goes under - then suddenly its "all over, Rover". Easily-driven, narrow-beam dinghies with lower-placed built-in buoyancy, such as the baby SIBLIM have their own special merit but do not carry such heavy loads. Oyster, with its inverse v-bottom and two bilges has an interesting stability curve, and, by the way, actually behaves quite well when partially swamped.

    don't believe there is a perfect combination of ease of driving, convenience for rowing, stability, load-carrying ability - every one of these attributes comes with a price. Especially in the range of 8', which inevitably means an extreme displacement vessel. "You gets what you pays for." No 8' dinghy will ever row worth a damn, yet they are probably all good enough for a tender - and they all seem to have at least one outstanding feature - at the expense of some other feature. Then there are the unmeasurables: the presentation of plan and instructions, suitability for a schoolroom project, ease of construction and use of materials.

    Well done, those who submitted entries! I'm just glad I am not one of the judges!

    Last modified: 08 Jul 2021 03:47 | Anonymous member
  • 01 Jul 2021 05:46
    Reply # 10716255 on 10211344

    AD is a Mack truck alright! We piled the equivalent of 375 kg into it and it still had plenty of reserve buoyancy for more. You can hear the little boy (my photographer) chuckling.

  • 01 Jul 2021 00:01
    Reply # 10715624 on 10211344

    Great work Graeme!!

    Very interesting seeing the models in real conditions. Pity that DD was not there for comparison, I may try and build a model to the same scale so that you can test her as well, also my Webb 8. I will try and call in on my next visit to Auckland.


  • 30 Jun 2021 16:49
    Reply # 10714487 on 10211344

    Testing for drag – part 1 towing in static water

    There are two ways we can test for drag. Here is the first – it was suggested by Marcus, who saw someone testing in this way.

    It comprises hoisting a weight to the top of a 10m (approx.) mast, then releasing it. The halyard runs back through a turning bock and thence to the model where it is attached to a towing point. When the weight falls, the model is towed down a runway alongside my shed.

    Here is the towing apparatus (53 grams hoisted to 10m) and the “tow path” at spring high tide.

    The idea, as I see it, is that a fixed amount of potential energy (mgh – the weight at its highest point) is released and imparted to the apparatus. After friction losses (which should be approximately constant between trials) the energy is converted into forward motion of the dinghy (kinetic energy) and wave motion. I don’t intend to attempt any calculations, hoping that timing the run will be a sufficient measure of the model’s drag. We will also be able to observe the dinghy’s behaviour under tow, and perhaps the waves it generates.

    Using a sail-maker’s thread for a halyard, and tiny blocks – and much trial and error – a 53 gram weight was found to be sufficient to tow each model through the water at a fairly constant speed. (The first metre, while accelerating, was ignored, but after inertia was overcome the rest of the run was at a kind of “terminal velocity” and that part of the run was timed). A crude assumption was made that the time of the run would be a measure of the dinghy’s drag.

    (I think this might be the nearest we can get to learning if there is any difference in how “easy” each dinghy would be to row).

    Here is Sibling Tender under tow with one “passenger” (a jar of sand weighing 600 grams, equivalent to a 75kg person) – less a factor allowing for the model’s over weight.

    Each model trails a retrieval line, adding a little more drag, but the same amount for each.

    Timing the runs using a “stop watch” app on a cell phone proved problematic – wet fingers etc – and eventually, after many foul-ups, much bad weather (and bad language)  I recruited a grandson as technical assistant, and he simply made a video of each run, enabling rough timing (to the nearest second) between two recorded events (a) passing the start point after initial acceleration – and (b) running into a stop at the end of the tow path. Not very precise – but probably consistent enough.

    The video timed-runs have been edited into a clip and placed on Youtube – here is the link

    This will allow some observations of wave creation and behaviour under tow, though wind and other conditions probably affected some of the runs.

    There were two stand-out features regarding behaviour, which can be observed in the video: (a) the steady course (directional stability) of Oyster and some others with deep or long skegs – and (b) the yawing behaviour of David T.’s three, none of which have skegs. These three were OK when lightly laden, but with two or three "passengers" would tow straight only when trimmed by the stern which probably then added more drag. Note however the “horses for courses” motto – these three were not designed for towing, they were designed to have minimum drag and maximum manoeuvrability for rowing – and as it turned out, they were among the fastest on the tow path. David has agreed that in order to test like-for-like we might add skegs to the models, for future trials, and if I have time, I will do so. KISS also has no skeg – but this dinghy has its centre of buoyancy much further aft, trims down by the stern anyway, and does not seem to need one.

    (As a digression, directional stability is the inverse of manoeuvrability, so you can’t expect lots of both. And it may not just be a matter of skeg or no skeg. I think that if the centre of buoyancy is further aft, with respect to the centre of mass, a vessel might tend to have more directional stability. [Edit: I have since realised that is nonsense. The distribution of weight and reserve buoyancy might, however, have some effect on directional stability when the dinghy is overloaded.] By design, baby SILBIM and particularly Sibing Tender both have full forward quarters – whereas KISS has her main body fullness aft).

    Here are the numbers for the first trials of “drag while under tow”.

    Some of the trials were repeated, in order to check consistency, and where there seemed to be a range of results they are shown here. The asterisk refers to tows in which the vessel yawed, and the trial was repeated with a down-by-the-stern trim.

    The next set of trials will be with the dinghies stationary, on a tow line, in moving water.

    Down at the launching ramp, where there is a proper pontoon. When the weather improves a bit.

    There will also be some static testing for stability, posted on this thread in due course.

    Last modified: 01 Jul 2021 21:51 | Anonymous member
  • 30 Jun 2021 07:15
    Reply # 10713075 on 10211344

    There you go, David. It still doesn't show the real disparity in the sizes of these things - in terms of volume there must be a range of nearly 100%.

    Back row LtoR

    Halibut, Sibling Tender, YouYou, baby SIBLIM, KISS, GP Dinghy, Medium Boy

    Front row

    AD, Oyster, Boxer, David's Box Barge, NZScow, Simplicity 8

    I'll try to post some numbers later this evening.

    Last modified: 30 Jun 2021 11:42 | Anonymous member
  • 29 Jun 2021 08:20
    Reply # 10709989 on 10211344

    Good work, Graeme! This is the way to evaluate all the designs.

    Could we see a photo from an upstairs window looking vertically down, to get a better idea of relative size and planforms?

  • 29 Jun 2021 03:42
    Reply # 10709426 on 10211344

    Down here in the mangrove creek we now have 13 model dinghies resulting from the JRA Dinghy Design Competition.

    In the back row we have, L to R,  the 5-plank models: Oyster (which has an inverted vee bottom), the slender baby SIBLIM, the capacious Halibut and the full-bodied General Purpose Dinghy.

    Next row are the 3-planks: the whopping AD, the moderate-size Medium Boy and the little Sibling Tender.

    Next the two “very little ones”: YouYou and KISS.

    Lastly the barges: the slim and lively David’s Box Barge, the moderate NZScow, the monstrous Simplicity 8 and the rather elegant Boxer.

    Eight of these are official entries, and the other five are concept designs which were discussed on this forum, in relation to the competition, but not actually entered. Details and drawings can be found for the competition entries at and for the non-entries: further back in this thread.

    There are two other design entries which I would like to have included, both from David Webb – one very radical and the other a classic beauty. But they proved to be a little more complex to build as models, at least with my limitations, so regrettably they are not included here.

    The purpose of the exercise was to learn something about the shapes, construction details and characteristics of this very diverse little group of dinghies. Having gone this far, it seems worth the effort to do some comparative testing and perhaps produce some numbers – in a light-hearted way – I would not place too much credibility on the numbers as the testing will be just for fun.

    Before making any comparisons, it is important to note a couple of things. Firstly, the competition criteria were vague regarding size, resulting in designers interpreting differently the “one sheet of plywood” requirement. The sizes thus vary between about 7’ and 9’ and the sheets of plywood required vary between 2 sheets and 3 sheets, with differing requirements regarding extra material such as stringers etc. What this means, in relation to comparative testing, is that some are “big” dinghies, which have a distinct advantage in respect of load-carrying ability and stability – while the “small” ones tend to be lighter, more economical in materials – and perhaps not so suitable for carrying more than 2 adults.

    In this very small size class, when it comes to performance: size is everything.

    Here are the sizes:

    The other point is: being built to scale, the models are all over-weight – and to varying degrees. I used whatever materials were convenient, the result is: when scaling the numbers to full size, we would be looking at 8’ dinghies ranging from 50kg – 100 kg!

    Weight also is very important in regard to performance, so in order to attempt a level playing field, when testing the models,  I have factored an estimate for “over weight” into the weight chosen for simulating the first “ passenger”. (“Passengers” here are loads of 75kg. If the dinghy model scales to an over-weight of, say, 30 kg – then the weight of the first “passenger” is accordingly reduced to 45 kg to try to compensate – a sort of “handicap” arrangement). Most people will not be interested in this degree of detail, so it is sufficient to understand that the weight of each individual model is not a true reflection of what it would be in real life, and to compensate, as we add “passengers” to the models, for testing, the first “passenger” is reduced in weight to approximately compensate for the over-weight of the model.

    The “passengers” are jars or tubs of sand, a full “passenger” being 600 grams.

    As the lineal scale factor is 1:5, the scale factor for weight is 1:125.

    600 grams thus scales to 75 kg to represent a real “passenger”.

    If people want to know the actual numbers, here they are (the estimates of “over weight” are pretty much just guesses):

    I think Arne's Simplicity 8 is going to make a good benchmark for the bottom end of the "performance" spectrum - and perhaps for the top end of the load-carrying and stability spectrum (though David W's AD will give it a close run there).  After building David T's Box Barge I believe this one might perform surprisingly well and might make an exciting little sailing dinghy - for someone younger and more agile than me. We shall see.

    The first test trials for "drag" have been done and I am crunching the numbers this afternoon (while the wind howls, trees shed branches and halyards rattle outside - its not a nice day here for being outside.)

    Last modified: 29 Jun 2021 10:08 | Anonymous member
  • 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
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