Catamaran composite beam design

There is a third way

Speaking as someone who does not earn his living from designing cats but as someone who has built a few I think there needs to be more than a bit of both the theoretical and practical approaches to small cat design.

In many instances the failures of high tech engineering are glaringly obvious. Witness the break ups of the 2002 RdR tris (engineers forgot about crack stopping in the core) , the debacle with Team Phillips (let's make extra thin cantilever bows and remove the forebeam) and the break up on its first cruise of a cat designed by two engineers from a fab design firm here in Australia about 15 years ago. All boats designed and engineered very well and they broke. We see it still happening with the Volvo monos - delaminating and losing masts. Obviously engineering a sailboat is a very inexact science. Otherwise the best designed boats would not fall apart.

I would love to sit with Dick Newick, Jim Brown or Wharram and ask them some simple questions
-What is the max load condition you design for?
-What material properties do you use?
-How do you avoid stress concentrations and the like?

The 20-50ft section of the multihull market was born from the efforts of the amateurs - people like Piver, Wharram, Brown and Nicol. None of these guys went to engineering school. Piver was a printer, Wharram an odd job man, Brown a Piver protege, Nicol a policeman, Lock Crowther was an electrical engineer, Chamberlin a cat builder and deliverer, Schionning a boatbuilder, Grainger a surfer, Pescott a boat builder, Newick a builder who failed first year naval architecture, Irens was a super sailor. What about Farrier? Comes up with the most brilliant idea and he is no engineer either. Then again Shutleworth is a real brain who can do very good number crunching.

So almost all the design aspects of the modern multi owes its heritage to the designers who probably couldn't do calculus and certainly didn't have FEA. Yet they did a fab job. Obviously they had simple rules of thumb that they applied with wisdom and knowledge of the boats around them to tinker away and evolove the boats we like.

I like the fact that a fab boatbuilder I worked for had designed a 42ft tri that had won the AMOC. I was persistent in asking him about trimaran engineering - How do I load it? What safety factor? In the end he told me the best thing was to work from the beams on a Kraken 40. Crowther's landmark design. When the cat designed by the two engineers broke up it was modified by a school teacher who had built heaps of boats and had sailed multis since his late teens.

So to say you can only engineer a multi by FEA and engineering first principles is just as big a mistake as to not be able to calculate a second moment of area of a beam. I worry about engineers who have never built a boat and seen how hard it can be to laminate upside down or test a sandwich panel against the specs that come out from the material suppliers.

Even the idea of asking the client about the intended use is problematic. What happens if they sell the boat and the next guy wants to go seriously offshore? Schionning mentioned this once and it is probably a good reason to design for the same structural integrity no matter what the intended application. The sad thing is that our boats are hard to reverse engineer. A three beam tube cat is easy but a bridgedeck design is tricky. That's probably why Crowther's early cats have three beams. My 12 year old design does too. Getting the cabin to do some work is probably very tricky. What happens if the next owner cuts a window in a load path. Even the dumbest owner knows better than to cut a hole in a beam so it is probably best to only take the box beams into account for loading.

So can Groper be given what he wants. Obviously he can - a simple rule of thumb. Like tie the hulls onto the wharf and let the tide go out. Wharran once said his designs could take a second cat's weight and still not break. Then go away and do the maths. A builder will know how to avoid problems as they have done in the past. The problem as I see it is that these rules of thumb are a probably shamefully rudimentary and yet very valuable so they are not given out freely.

So Groper - I am busting to do an article on John Hitch who half designed the Kraken 33 and Hitchiker cats. I can see if he will let us know a secret or two.

cheers

Phil

 

In the case of the designers mentioned I'd add Rudy Choy, they all developed their boats and scantlings over what worked and what needed to be strengthened. I talked to Dick Newick about some of these things and it seemed like they had a intuitive understanding from seatime how things worked and where the loads came from. He was excited at the time about his large Traveler design because the client had been involved with the Chaparral race cars. He was asked where he got his scantlings and replied this is how we've built them. The client had access to advanced composite engineering and was able to come up with a lighter structure. I think this ability to keep learning and applying the experience gained is what helped these guys develop their craft.

 

I'm going to add th quality of imagination to the above posting for designer requirements. I think all these people are/were able to visualize/imagine the loads from observation and create the structure to withstand them. A straight engineering approach may well miss loads that weren't specified or allowed for.

 

Yeah Cav - In his book Rudy calls it R and T - Rough and tumble instead of research and development. Aikane - wasn't she the best!

Probably a really important facet is that the early designers used wood and glass. Nice materials that show overloading by cracking in a very controlled and slow manner. I have seen a well designed cat develop small cracks in a rear beam corner a year after launch. Strips of uni solved the problem - no more cracking. Using this approach with heavily loaded thin carbon laminates may lead to incredibly fast catastrophic failure. A ply box beam is still pretty light - especially when reinforced cleverly (same EI) with glass.

Contrast this with the loss of some aluminium multis like Manureva. Even Newick likes wood still because of its ability to handle high loads and not fatigue.

cheers

Phil

 

Agreed, excellent, needed to be said, Catsketcher Phil, a dose of reality, practicality, creative minds and observation of nature.

 

Aikane is a great example as they went by their "hunches" from the beach cats and Waikiki surf and found the corner stone of their scantlings. Interesting observation about the material failure rates between wood and carbon,the tech guys need to get a carbon alternative that doesn't just go bang when the laminate is wrong.....For me the stuff grows on trees.....

 

great post catsketcher,

the "secret" methods of "guessing" the loads must be propriety as all 3 naval architects posted in this thread all dodged or ignored the true meaning of the questions asked....

there some anecdotal methods splashed around the forum by various NA but they all lack peices of information, probably intentionally so as if their methods were revealed in full, the door would be left open to law suits if anything should go wrong in the future... understandable from a professional perspective...

here's a shout out to all the pioneers that had the guts to wing it...

 

I have nothing but admiration for all the men mentioned by Phil, and no argument with the methodology used.
To those having a gentle dig at contemporary NA's, do not forget that they exist in a much more regulated environment dominated by modern mans pathological infatuation with throwing money at insurance companies.

 

There are different avenues to design:

You are describing the suck it and see methods, build it break it, beef it up and if it doesn’t break it might last a lifetime. Or then again it might fail again in 5 years or in the middle of a spot of weather and kill everyone.

A big problem with this is a real understanding of fatigue and properly considering load paths. There are myriads of failures from self taught intuitive designers, but they cut their teeth on sheltered coastal designs were often quite ignorant of structural issues and they often enough didn’t scale well into offshore vessels. How many of these earlier multihull designs broke or the designers were even killed by their designs at sea? I know of numerous cases of structural problems with most of the early designers excepting maybe James Wharram.

Wharram has never been shy of saying how he or the Polynesian voyagers reduced transferred moments in cross beams.

Class societies do produce design guides and scantling rules for multihulls. They investigate the failures and produce (and continually revise) scantling rules with other factors like fatigue or storm loads considered, they modify the rules as failures and analysis reveal deficiencies. Recent data acquisition has given much better understandings of global loads in multihull vessels.

You can engineer very reliably to whatever loads you assume. So if you can take recent data acquisition then the design can be very reliable. But here is the big problem with blue water racing boats. The assumed loads are dependant not only on sea states but on the operator too. But then you could make it bomb proof but then it’s not going to be competitive because weight and strength are on opposite sides of the coin.

The 1968 Aluminium cat Alan Colas was sailing isn’t a great example of anything, it was an old, well used and suffered what would now be an easily predicted fatigue failure. Modern fully engineered designs can fall short in the production facility because of less than perfect composite layup.

How many readers here understand and can use an S-N curve or how to design to it? Those who learned some engineering can, bt there is nothing intuitive in it. How well was aluminium marine design understood in the 60’s and it wasn’t just multihulls that failed. The need to ‘soften’ load paths with cutouts brackets and overlaps has only recently been hammered home. You can’t subject any vessel of any material indefinitely to large loads unless it’s specifically designed for infinite life and racing boasts are not.

 

Understood, Redreuben, it must be a frustrating hell in that uptight straitjacket.

 

Some of them were abysmal designers who had poor understanding and made very dubious claims. The deaths due to the designs of the early multi's was appalling. Has everyone forgotten that ?

It's not the insurance companies that drive anything, there has always been direct professional liability for the professionals (but not for amatuer self taught designers).
A NAME professional designer could not get away with a structural failure that they should have predicted they would get a prison sentence. That's the protection the end user enjoys.

 

What is common between the ancient Polynesians and the likes of those mentioned above in their rose tinted glasses? Neither had any method of determining the actual loads and hence what structural arrangement was suitable.

They ‘both’ as Mike noted ‘suck it and see’. Design by trial and error. This is nothing new and has formed the basis of all modern day methods both empirically and mathematical. Except that the modern day methodology has far surpassed the ….sharp intake of breath through the teeth and “aahhh..well” type. In their day, papers like Dinsenbacher had little to go, just as the early designer had little or none, and also the usage of such vessels in his analysis is considerably different to that which are used today. Dinsenbacher assumed an acceleration of 0.4g…and his resulting formulae does not even include the accelerations, as is inherent in all calculations today.

So either you wish to bask in the glory of the “good old days” with their suck it and see and lack of engineering rigour, or, you can use the widely and freely available methods of today; Class rules for providing loads. I have mentioned several times which route to go, but appears to be falling on deaf ears.

Those excellent pioneers in the 50s and 60s etc had nowt to go on. So, they tried something. If it broke, increased the scantlings until it didn’t…sadly only their limited successes are noted. However, to suggest that those pioneers are as good as or better than any modern day methods just demonstrates a major lack of understanding in design and naval architecture and structural analysis. I am sure anyone of them would use without hesitation the methods which are available today, if they had them in their time.

As I noted in my first post, it is all about 1) design and 2) fabrication.

Design requires firstly loads, otherwise what is it your designing? Thus Class rules provide the loads. From which you can apply to your vessel. But, each vessel shall experience different load case scenarios based upon the nature of the vessel, its arrangement and its area of operation. That is for you the designer to establish.

That is where the structural analysis comes into it…ie, load paths fatigue etc. All this must be accounted for when applying those “loads” into the structural arrangement. Not forgetting the SOR…what is the boat designed to do…which brings in weight and structural efficiency to minimise the weight.

Once you have completed that aspect, the fabrication is next. You cannot design any structure if you have no idea how it is built. Thus the structural design is a reflection of one’s experience and knowledge of fabrication of the vessel in question. The disciplines are mutually connected in every way.

However in this day and age of FEA and Rhino and other fancy software programs, the emphasis appears to be on”..hey doesn’t this look good” rather than any understanding what one is drawing with the how’s and the why’s to build it. It is easier to discuss “oh in the old days they didn’t bother with XX or YY, why should we” coupled with debating pretty pictures than actually addressing and learning correct methodologies for proper structural analysis and logical sound engineering. Just takes time and cannot be learnt with the click of a button or mouse!

 

Well, the most high echelon of naval architecture that Ad Hoc and Mike Johns and a few other established NA's commenting here, may/might?? refer to, perhaps even genuflect to, Juan Kouyoumdjian, Bruce Farr and Marcelino Botin, these apotheosis of modern marine architects with the very latest computer design equipment and formulae and large advisory teams, have very recently been revealed, in the highest technology monohulls, the Volvo 70's, that if they hadn't been able to scuttle to nearby shelter, would all have gone to the bottom faster than a stone.
But I could be completely wrong here; the above NA's posting here may actually not think much of the said Juan K, Botin and Farr. Would like to hear their comments on same.
This is just an antidote to the slagging off at the brilliant, creative and decades-ahead-of-their-time, pioneering multiihull designers that we all know.
Would that Ad Hoc and M. Johns had a similar line of breakthrough designs - I very much doubt it.

 

With all respect, JK, Farr, etc. are designing for very small niche, and their creations are considered break-through by those following TV coverage of AC and VOR. They are just doing excellent job and accumulate research and expertise on their field, exactly same as other naval architects do in other fields! We as naval architects have to solve different tasks every day, and of some we can not even speak about (such as naval craft, etc.) - unfortunately (or luckily) this does not get much attention by 'sailing snobs'. Designing craft with unique characteristics of performance, economy, seakeeping, weapon systems, etc. is equally interesting.

 

Ahh....that's more like it. That is the GB I know with his usual ad hominem and polemic ripostes. Nothing concrete to contribute just airy fairy words to appeal to a niche reader. You have strong opinions, fine (we all do to an extent), but that is about all as was noted on numerous occasions here:
http://www.boatdesign.net/forums/sailboats/perils-edgy-design-offshore-38903-25.html#post498320

Indeed. Most NAs just get on with the job...it seems others like to shout about themselves.