Designing the Structure of 38' Catamaran

Simon, frames construction is more weight saving in aluminium construction than bulkheads. There is to rapidly have an idea of the minimum thickness plating that you can use on your aluminium hull. The rule is that your frames spacing is 100 times the thickness. For example in this 38' catamaran we are using 6 mm on the hull that gave us 600 mm for the frames spacing. To do easy calculations on almost 12 meters you have 20 frames or bulkheads. Imagine the weight of 20 bulkheads.

Regarding the watertight bulkheads in wood or composite material bolted to the aluminium frame, is a good idea. In this 38' we have 4 watertight bulkheads. We are doing directly in aluminium plate of 6 mm like the frames thickness to simplify the construction.

The composite reinforcement is a more complex issue, but in Civil Engineer are using carbon stripes with epoxy laminated to concrete beams to fix a crack or to give more stiffness to the beam once the beam is on a finished building for example.

Cheers,

 

Ad Hoc:

Is not an answer to your request of proof, but it has been done.

Here is document about composite and metal in ships.

Composite Overlay for Fatigue Improvement of a Ship Structure

http://www.dsto.defence.gov.au/publications/4465/RINA Paper-Composite final10-mirror pages (2).pdf

The question is not that you can't solve the problem upgrading the scantling. But the question is, what are the requirements. If performance and weight are important, then there is a limit to beefing up.

If you do not care, use Strongall. http://www.prometa.fr/FR/PAGE_Plaisance.htm they make some nice and strong and heavy cats.


TYD:

6mm is a lot for a 38' catamaran. Looks like it is going to be quite heavy.

Crowther had designed the 48' #85 aluminium cat in mostly 3mm aluminium. ViteVite (http://www.vitevite.info) is on it's second round the world trip.

What are the requirements for the cat?


Simon

 

Simon

"...Is not an answer to your request of proof, but it has been done.."

Where did i say i cannot be done?...You're missing the point and degrading an methodical engineering approach to one of a layman's approach.

Every problem has a solution..just not always the same. Each must be reviewed in its own merits for each unique application. Engineering is not a carte blanche approach...unless of course you're saying every boat should be built out of ferrocement, why because its been done!

Any mechanism must first be understood before arriving at a solution. That way a proper quantitative analysis may be performed and results reviewed according to each solution. Throwing away comments like..yes i reckon this might be good, without any reasons other than its been done before, is an unprofessional approach.

The paper you cited is an extension of the IE product which is well known.

Concur your comments on the 38' catamaran. But what is the frame spacing and stiffener spacing of the Crowther boat, its displacement, draft etc? Without this information...thickness for panel sizes are just guesses that it may be appropriate...!

 

TYD:
could you tell us bit more about your plannend structure, like

-midship section
-mainbeams
-frontbeam
-bow structure with collision bulkkeads and crash-box?
-daggerboard or keels?
-rudder
-engines

Looks like you have already advanced and defined the structure.

Simon

 

Hello Enrique,
I am a Shipwright and have been building boats for years. I have built boats in wood, aluminium and glass fibre and ships in steel.
We lately had a company which built about 23 power catamarans.
Aluminum is a nice light material to build boats with, but the lightness is lost to a great degree when the boat is fitted out.
Aluminium corrodes along stress lines. So much so, that the New Zealand government was thinking of banning it's use for commercial boats years ago.
Catamarans have diagonal streses in the wing deck as the hulls try to go different ways.
Aluminium will survive these stresses only if the boat is over-engineered and so too heavy, or if the shape is unsuitable for efficient catamarans, but better for aluminium use.
We built the powercats with marine plywood with a ply/foam/ly wing deck. For weight/economy it is the best structure.
We then built the hulls in GRP as New Zealanders and other people think that it is better than wood. There is no way that glass/foam/glass is as good as ply/foam/ply in the wing deck unless carbon fibre or kevlar are used, so that construcion was used in all of our cats.
It is my pleasure to join this forum and I hope for great disussions and great reading.

Ernie Travers.

 

Hi Ernie

"I am a Shipwright and have been building boats for years..."
With respect, if you are building them you are not 'designing' them. As such design is a totally different discipline. There are good designs and bad designs, just as there are good shipwrights and bad ones. One bad one doesn't make them all bad...

Structural design requires experience and understanding of the mechanism involved, as well as the materials being used, their good and bad points; each has them.

To say "..Aluminium will survive these stresses only if the boat is over-engineered.." demonstrates a lack of understanding of structural principals and material science.

I was also shocked to read " Aluminium corrodes along stress lines. So much so, that the New Zealand government was thinking of banning it's use for commercial boats years ago..." Aluminium does not corrodes along stress lines any more or less than other materials. It will corrode owing to bimetallic corrosion or other similar mechanisms that initiate corrosion. Unless of course you're referring to stress corrosion cracking which is also a very detailed and complex mechanism that cannot be pulled out of hat as a cause without establishing the facts first. Sounds like there are many poor designs in NZ, and shame on the Govt for considering such a move if it is true.

Any boat can be built from any material. But as with everything in design, it is all a compromise. A structure designed in steel, ally, wood or GRP can all satisfy strength requirement under the same conditions. But depending upon the geometry and the loadings and hence the requirements, each yields a different solution. Some acceptable some not, either because of weight or cost or ease of construction etc. One cannot make a carte blanche statement about materials and design in this way.

 

Hello adhoc,
I did not expect such a neurotic reply so quick.
Name calling is so childish. Have you ever been to New Zealand?
I am a tradesman and do what works, not what does not work.
The technology we use is derived from the New Zealand Americas Cup successes.
Will be happy to explain my experiences, but be calm, settle down and think.

 

Some info for Ad Hoc.

We chose the best powercat designer we could find.
As our boats developed with our experience, we asked him for a new more fuel efficient hull. We had found that bigger motors were more efficient and the new hull was to be suitable for them.
He was not capable of the task so we spoke to five other designers and found that we had outgrown them.
The only solution was to design the boat ourselves. That was a daunting thought.
However, we did that and the next boat (9.75m long with big accommodation and flybridge) with 250hp Mercury four stoke outboards achieved one litre per motor at 20 knots, 2 litres total. It topped out at 48 knots on 35 knot propellers and, get this, at 3.5 litres/NM per motor - totally unexpected.
How many power catamarans have you designed or built?
If you have superior knowledge or experience than me I would like to benefit from it.
You now know emphatically that aluminium does corrode along stress lines without any other influence. It is also affected badly by metal fatigue.It is very susceptible to electrolysis.
I like to work with it very much, but it is the least suitable material for catamarans. Best is a composite of ply, foam and GRP. Then steel as they get bigger.
My son drove one of our cats with outboards from New Zealand to Noumea (about 950NM) in 52 hours travelling averaging 16.48 knots.
Last year one of our boats was washed onto rocks in a 100+knots gale and was there for two tides. The next day, my son pushed it off in winds still blowing 50 knots and drove it for one hour to a boat ramp.
If anyone would like to see reports of these events, please present you e-mail addresses

 

Ernie
I think you need to re read my post. It is not negative, I'm sorry that you feel it is, it is just a factual statement. The pith of it is that you have selected one or two designs which have had a bad experience with and then call all of them bad, this is not logical. To say one material is better or worse than another in a unique application just demonstrates that either the design is wrong or the level of knowledge in using said material in such an application.

I can cite endless applications of aluminium where the use of aluminium has been terrible. But I can also cite just as many where it has been advantageous. Every material has its good and bad points.

Would you design an aircraft using steel, no, why too heavy...this is one simplistic example of using the correct material for the design. Would you use bare copper piping in an aluminium hulled vessel, no, again another example, but this time the reason is different.

Every design has a solution. Every design may have more than one solution. Design and the whole design spiral/process is a compromise. What you compromise to obtain a solution may, when in service, appear to be incorrect, whereas another choice or selection may not be. There is no perfect design or solution. Just the right one at that time.

Apart from designing/fabricating with aluminium for some 20 years, i also occasionally lecture post graduate students, on how to design and fabricate with aluminium. I was recently invited by RINA in the UK to present a lecture on designing with aluminum with emphasis on welding/fatigue and practical experience/applications, based upon my experiences.

I'm not here to blow my own trumpet, there are plenty of others one here that do that. I am more than happy to give advice. Whether you wish to use or take the advice is purely your choice, I'm not forcing you.

So, when you say you 'designed' a boat your self. I assume you mean you drew up some lines and fitted some 'structure' which 'looked about right' from previous vessels etc etc..ie you copied what was done before. This is basically what any naval architect does, we call it a database of information. BUT, the difference is, can you demonstrate from first principals (scientifically) why certain structure/materials/geometry was selected..??