(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.

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.

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.

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.

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.