Reverse bows on cruising cats pros and cons

Sorry, I would have elaborated, but it is miles off topic. And, check my first sentence!!

As students we naively assumed that we could tow two almost identical boats and obtain completely different results. But the scatter of the results were bigger than the differences.

Several of us made models, not just me. Usually about 5ft long, that being sensibly the biggest the tank could handle. Even so the dynometer wasn't as sensitive as we had hoped. But that was in the mid 1970's, no doubt things have improved since.

Having said that, we all know of stories where tank test results bore no relation to the full size boat. The 12m Mariner being an obvious example, and to an extent also Australia 2. And the comments from the current AC designers show they need to compare full size boats before they can refine their prediction programs to give sensible results. eg

http://www.sail-world.com/UK/Americas-Cup:-We-will-keep-learning,-even-in-the-Match---Simmer/108153

Excerpt "...While the increase in computer power, together with improved performance prediction software, the process of has made performance analysis more accurate and extensive. Simmer is unconvinced that the computer has taken the place of the second boat on the water – particularly in the AC72. ..."

So, again, to those who haven't had any model experience, I still say, don't believe all you read about "tank test proven design' or "CFD modelled". You have to know what you are doing to interpret the results

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

http://www.boatdesign.net/forums/images/editor/attach.gif

Mike, I think I've been in that wrong place wrong time on a Duncanson 34, drove it right under............... there was a fair degree of operator error involved in carrying sail/spinnaker to long in a rising breeze & seas. All the best from Jeff.

 

This is because in model testing time should be scaled as square root of linear scale. This is essential modeling factor. Thus, adjust video speed, and one will get picture close to reality.

 

You quote a designer who is not a naval architect nor a hydrodynamicist. At the time Couldrey was working on his designs, Davidson was already testing small moldes of AC yachts for Stephens and made important progress with R12 class hulls and not only...

Yes, full scale testing is important source of data, but models are often the place to start. For number of designs we have done both model testing and extensive sea trails, having those results our hand we always know how to interpret results of model testing and where are the potential difficulties and problems.

 

Who cares about your silly and arrogant pigeon holing referring to Bill Couldrey; he came from working class New Zealand mid last Century when there were sweet FA "marine architects" around, just sailor/designers/builders ... and he was a natural, (a word perhaps anathema to you) - and his work considered here as works of art (this too is perhaps something alien to you). Also he had an influence on young Kiwi designers of the time ... which has continued to this day, the list is long; you might? know of some of them.

 

He might be a great artist, but I don't need opinion of a carpenter about nuclear physics.

Objectively, most of Your 'great designers' are just amateurs who influenced other amateurs. Some of them produced great stuff but most of them swim in their own misapprehension.

 

Nor Edison about light bulbs then.... He wasn't an electrical engineer after all

This says nothing of his ability to correlate model data to full size yachts. Being a natural drawing lines doesn't mean he has scientific mastery of tank testing!

 

One doesn't need to be electrical engineer to invent a bulb, but needs to be such to comment methods used in electrical engineering design.

 

Oh stop it children !

 

Alik you are going to get a lot of noses out of joint in many circles if you call non trained designers amateurs. That list includes some of the greatest ever

Bruce Farr, Nigel Irens, Dick Newick, Lock Crowther, Iain Murray, Ben Lexcen, John Spencer, Tony Grainger, Ian Farrier, Arthur Piver, Jim Brown, James Wharram, - need I go on?

To my mind the best designer is the one who has built a few of his own boats before he asks anyone to pay for one. This shows they have their money where their mouth is and that they have proven their designs in real life.

Martyn Fischer had an interview where he talked of the issues correlating the compensations for viscosity and Froude number in tank testing.

MF:Tank testing is still used, but I reckon that nowadays numerical simulations are more accurate than tank testing, at least for sail boats. The reason is the following: Let’s assume we have a model of a boat at a scale 1:4 (this is already a big and expensive model). In order to get the wave pattern in the tank right, we must reduce the speed of the model by a factor of square_root(4), hence by a factor of 2.
This ensure that the so called Froude number of the model and the real boat are the same. So far so good.
In order to get the viscous drag right, we must ensure that the Reynolds numbers of the model and the real boat are the same, and to do so we must increase the speed of the model by a factor of 4! And that’s the problem.
It is impossible to get the Froude number and the Reynolds number right at the same time. There are techniques to compensate for that, which result in correction factors, but these correction factors also exhibit errors.
To my understanding the errors of the correction factors are nowadays more important than the errors of numerical simulations, which is the reason why I have a strong preference for the simulations. Another advantage of a numerical simulation is that it provides detailed information. It is possible to study in detail why one shape is more draggy than another one.

I am happy with your thoughts on models - not so with any disrespect to those whose shoulders we stand on

 

Thanks Redruben and Catsketcher, I was starting to feel the same way.

Every thing has a use, but is only part of the eqaution, in a design.

 

Yes those designers are competitive in some niches due to talent, experience and lack of interest of NA's to play in that field. Many (if not most!) of them hire NAs to work for them. But do we have to fully rely on their opinion on testing methods? I know few of such designers around, they 'design' boats in CorelDraw but can not dimension a simple composite sandwich panel and of course have now clew on Reynolds number. But they surely have their 'opinion' on model testing

 

Better check your history, Alik, because you're really revealing your ignorance; this regarding Laurie Davidson doing AC stuff when Couldrey was around - not in the 1950's, the '60's or even the '70's before Davidson even considered the heavy (backward) designs.
In the '50's he was sailing/designing 18's like Envy and the '60's doing his early Half Tonners like Tramp and Blitzkrieg while in the '70's, light displacement big dinghy champions like Fun, Waverider, Pendragon.
Davidson, like numbers of other designer/sailors here, was self taught but helped by Couldrey, who made a point of encouraging young aspiring designers.
Just carpenters, you sneer, amateurs, not worthy of contemplation.
When it comes to "now clew" - wtf, we know you're referring to yourself.

 

This is not my ignorance but Yours - have You ever heard on Prof. Kenneth Davidson who worked with Olin Stephens testing his AC hulls? They developed testing techniques for yacht models in 1930-s and used it for decades...

You are very good sample of a Kiwi sitting there on the island where there is no towing tank available and no single school teaching Naval Architecture; kindergarten of self-confident amateurs who think they are top of the world in boat design...

 

And one doesn't need to be a Naval Architect to design a perfectly serviceable and seaworthy pleasure boat (by which I don't mean a super yacht) as has been proven by the many thousands of successful vessels designed by people with little, if any formal boat design qualifications.