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  https://junkrigassociation.org/dinghy-competition 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.)

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.

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

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.

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.