Multihull Structure Thoughts

A little basic maths which will help understanding of material choices. This example is about SOLID fiberglass laminates to emphasis general structural characteristics. There are 3 main structural properties needed for a hull. Stiffness, Flexural Strength and Shear Strength between the layers of glass fabic.

Stiffness is a function of thickness cubed, this means that if you double the hull thickness the stiffness of the hull skin is multiplied by (2x2x2) = 8 times as stiff.

Flexural strength, also known as modulus of rupture, or bend strength, or transverse rupture strength is a material property, defined as the stress in a material just before it yields in a flexure test. If you double the hull thickness the flexural strength is multiplied by (2x2) = 4 times the flexural strength.

Shear strength is the strength required to stop a shear load force that tends to produce a sliding failure on or between materials along a plane that is parallel to the direction of the force. Shear strength is a function of thickness directly, this means if you double the hull skin thickness the shear strength will double.

Now the fun. If you do resin infusion of a solid skin compared to a hand layup, the resin infused hull skin will be thinner because it has less resin in the infused layup making it thinner. Therefore the stiffness, flexural strength and shear strength will be reduced by the same factors if you use the same fiberglass materials.

Also I have not spoken about a cored layup. A cored layup increases stiffness dramatically BUT due to thinner skins on either side of the core the flexural strength and shear strength will be controlled by the fabrics and resins chosen. If foam glass materials are the same in a hull then it is doubled in thickness the same characteristics will apply.

 

You need to be careful with your oversimplification of SOLID laminates.

The terms you cite are referenced using conventional terminology to materials that are isotropic - same properties in all 3 direction.
Composites are not isotropic, they are either orthotropic or anisotropic.

The flexural strength is an example for a measure of such, as it is designed to produce a failure from a given load. Since the test determines the failure which ever comes first, the compression on the upper surface or the tensile, on the lower surface from a given load. It is a simple 3 point bending test. As such the quality control and direction of fibres greatly influences the result.

This is the fundamental difference between a "home" build and "commercial". Since a commercial build will be required to satisfies a set of established design rules.
These rules generally have a minimum reinforcement attached to the laminate - just as they do for metal structures. Thus, infusion may simply result in more unnecessary work - merely to pass the minimum reinforcement required, that a home build would not be measured against.

But the most impost design criterion is deflection - not strength - in low modulus materials.

 

Tara Vana was a unique catamaran designed by Crowther. Design 222. It was a sailing sports fishing catamaran that could place a fishing chair on its rear deck to catch marlin. Its biggest catch was a 440 lbs black marlin. It was built at Alwopast and sailed out of Bora Bora. Picture below give you the idea, notice the fly bridge which was advanced concept in 1994 on sailing cats. The boat is 50 x 27.5 foot and weighed 17000 lbs. It was designed to meet survey standards and be simple to build. As a result Crowther used a thick foam thin skin approach to the build. EG hulls were 1176 gsm triax glass 30 mm airex foam 705 gsm Kevlar on the inside. The under wing was outside 1176 gsm triax 40 mm airex foam 705 gsm triax glass inside. The watertight and strength bulkheads were 1176 gsm triax 25 mm airex foam 1176 gsm triax glass. All thick foam and single skin layups to simplify the build. Nice boat.

 

This is information only and is from GMT Composites. It contains issues about mast making and compares laminates where the final compaction is made under vacuum at one atmosphere with laminates that are cured at 5 atmospheres in an autoclave.

GMT manufactures spars by applying uni-directional, pre-pregs to either a male or female tool. The resin system in the pre-preg is specially designed and manufactured for a vacuum bag cure at 250F. This means that the viscosity profile with temperature and cure rate was designed so that the resin would exhibit excellent flow and fiber wet out under one atmosphere prior to curing. The result is a uniform laminate with very low void content (under 1/2 % in tests).

Good results also depend on laying down the carbon properly. The plies must be applied smoothly in the correct orientation. Fibers must be kept straight as no amount of compacting pressure will straighten out poorly aligned fibers. In addition, the best compaction and curing schedules must be developed. The part is allowed to dwell at lower temperatures for a number of hours. This allows the softened resin to flow and the laminate to compact. After the dwell, the oven temperature is raised to the cure temperature. The resin viscosity decreases further and completely wets out the fiber.

GMT has investigated the benefits of using autoclave compaction and found them to be minimal. A typical test result is shown below in the report from Emerson. The autoclaved part has a bit lower resin content. It is thinner. Mechanical properties are about the same. The autoclaved part is less than 3% stronger and stiffer in tension and compression prior to taking wall thickness into account, while the vacuum bagged part is better in shear.

These small differences in properties are insignificant when it comes to the structural integrity of a mast. The main design parameter for a mast is the compressive stiffness of the column. This is because a mast will fail globally by buckling under the compressive loads from the rigging. Resistance to buckling is proportional to stiffness, E (compressive modulus) x I (moment of inertia). The compressive modulus of an autoclaved laminate is 1 to 3% higher than for a vacuum bagged laminate. However, for a given weight of laminate, the wall thickness is lower resulting in a lower moment of inertia. When the higher E is multiplied by the lower I, the resulting overall mast stiffness from quality laminates manufactured by either method is essentially the same.

Masts can not only fail from global buckling but also from local failure of the tube. Impacts can cause delamination that may lead to failure. Recent scientific studies have shown that autoclave produced parts are more subject to delamination than oven cured. One of these studies cited below was done by Baral, Davies, Baley and Bigourdan; a group from French Universities and Industry. The work was done for the Groupama 110’ trimaran project as they investigated the material to use in their huge mast. It was published in Composites Science and Technology, March 2008. This group had samples made from five different batches of carbon that had previously been used to build masts. The samples were actually made by the mast builder. One batch was made in their autoclave and the other in their oven using a vacuum bag. A conclusion of this study is that, “Oven manufacture results in more delamination-resistant composites than autoclave fabrication.” From the study it shows that it takes more energy to produce a crack of a given length in an oven cured part. The authors feel this is due to the fact that vacuum bagged parts have a less distinct crack plane. “This makes fiber bridging easier as the crack plane is not flat.” They then conclude that, “This is an important observation for damage tolerance of this type of structure.” References 3, 4 and 5 from the same study also show that vacuum bagged parts are superior.

Local failures are often traceable to splices between tube sections or to changes in wall thickness in the perimeter. Most masts are built from two or more tube sections as the length of the tool or oven/autoclave limits the length of a section. GMT splices sections together by using a pre-preg scarf splice. This splice is constructed and cured using the same techniques that were used to build the sections. The result is a joint that is the same stiffness and strength as the rest of the mast. No hard spots or stress concentrations are produced with this type of splice. Manufacturers who autoclave their tube sections cannot construct the splice in the autoclave. They can either use a vacuum bag cured scarf splice or a sleeve splice. Either will work well if properly engineered.

It is interesting that even aerospace companies are moving away from the autoclave. An article published in Composites World, June 2009, reported on some work being done by Boeing and Bristol Aerospace. Comparing test data shows that any differences are trivial for out of autoclave and autoclave curing are two different ways to get to the same result: a void free, properly cured spar that will perform as intended for its designed usage. Once you separate the facts from the fiction, it is clear that a high-quality mast can be built by either method.

 

For those who real want to race hard and don’t want any comfort here is the boat for you. Azulao 11 is a Dick Newick designed Atlantic Proa designed for the OSTAR in the early 80’s. The proa is 42 x 20 foot weighed 3000 lbs and carried 630 square foot of sail in a schooner rig. As you can see in the photo’s there is not much of this boat but its structure was strong for its weight. It had to go mainly upwind across the Atlantic. The hulls were constructed from the outside 330 gsm glass cloth 2 layers of 400 gsm glass unidirectional at 45/45 15 mm airex foam 2 layers of 400 gsm glass unidirectionals. The glass is doubled at the keel line outside and inside. The freestanding masts were 11 meters tall aluminium tubes 150 mm outside diameter at the deck with 4.8 mm walls. The masts were spun tapered to 75 mm at the tip. The sails had an aerofoil pocket at the leading edge. The booms were 130 mm outside diameter tubes with 3.2 mm walls. Mainsails were 7 oz dacron. The stabilizing tube, if the boat was caught on the wrong tack, was a nylon air tube. This boat was fast in trials but had limited success in racing. You would need courage to sort out the jib in strong winds with no safety lines etc.

 

I want to design a catamaran for graduation project .. and i want methodical series for catamarans to construct offset table..thank you

 

Tony Grainger produced a series of bridge deck cats that had no bridge deck cabins. They were called the Alfresco 9.2 meter, Spoonbay 10.6 meter and Alfresco 430. Grainger stopped doing this style of boat when many customers added bridge deck cabins after building the boats. Grainger then either designed full race boats or full bridge deck cabin models. We will focus on the 9.2 meter cat. One was called called Rum Tum Tugger and had berths forward of the mast and galley etc in the hulls aft of the main beam. The 30 x 19 foot solid semi wing cruiser racer cat displaced 6000 lbs and had 565 square foot of sail. The hull has 330 gsm cloth inside 12 mm wrc 440 gsm double bias outside doubled over bottom to WL. The underwing was 330 gsm glass cloth 18 mm duracore 330gsm cloth with a 600 mm wide duracore doubler strip fore aft on the centreline covered with glass. Non-structural bulkheads were 12 mm duracore with 9 x 6 mm deep unidirectionals around the edges and in any door openings etc. This is an early design and the crossbeam structures are bullet proof. The main beam is a box 900 mm high by 800 mm wide. The vertical faces were either 12 mm plywood or 18 mm duracore panels. Either had 2 layers of 400 gsm double bias on each side of the web. If the webs were 18 mm duracore panels they had a 15 x 15 mm unidirectional strips on the top and bottom edges. Next came the flanges, 800 mm wide between the 2 webs. The top flange was 11 layers of 400 gsm unidirectionals, The bottom flange was 7 layers of 400 gsm unidirectionls. The rear beams were similar but had 7 layers of 400 gsm unidirectionals on the top flange and 4 layers of 400 gsm unidirectionals on the bottom flange. The boat used epoxy and E glass. The Duracore had 1.5 mm faces with a balsa core. This boat was fast, comfortable and very reliable.

 

Ehad Hossam. I am not a designer, I have just gathered a lot of information about multi's. So I can only make suggestions. Hugo Myers, a US designer, was one of the first designers to use mathematics to design hull shapes. AYRS publication number 54 page 58 gives "a mathematical equation" of how he produced the hull shape. The equation is Cx(squared) + Dx + Gy(squared) + Jz(cubed) + Lz + 1 = 0 Where x = beam, y = length, z = height. If this equation is solved it produces hull offsets. Sorry there is no more detail. Myers designed cats from 28 to 60 foot using mathematical hull shapes. See if you can find any of his original papers. There have been other articles about how hull software tools use various fairing software equations to create hull shapes etc.

 

Hope, this can be helpful: http://www.multihull.de/technik/catdimension.pdf .

Catamaran Design formulas http://www.catamaransite.com/catamaran_hull_design_formulas.html

How to dimension a sailing catamaran? https://www.boatdesign.net/threads/how-to-dimension-a-sailing-catamaran.22274/page-5#post-298484

Open Source 13-15m Catamaran Plans https://www.boatdesign.net/threads/open-source-13-15m-catamaran-plans.59670/

 

Malcom Tennant produced many interesting designs but one was very interesting. It was a motor sailor with two 150 horse power engines that could power the cat at 15 knots and peaked at over 20 knots under sail. The cat was a full bridgedeck cat 60 x 29 foot weighed 36000 lbs and carried 1600 square foot of sail. The hull is 1120 gsm triax eglass 20 mm airex foam 770 gsm triax inside all in vinylester resin. There is a Kevlar strip from the top of the bow down along the keel line. The underwing is 1120 gsm triax eglass 60 mm airex foam 770 gsm triax inside. The deck is 1120 gsm triax eglass 20 mm foam 770 gsm triax inside. The main cabin top is 1120 gsm triax eglass 30 mm foam 770 gsm triax inside. The cat had daggerboards have an internal timber beam structure with a skin of 770 gsm triax 20 mm foam 770 gsm triax. There is solid unidirectionals on the leading and trailing edge and on the sides at maximum width. The hull was one of the first iterations of the CS hull shape that was used in most of Tennants power cats. Last 2 photos give an idea of the hull shape.

 

thank you, but i searched for papers but i didn'f find anything useful
i need a methodical series or offset table for (Maxurf or Nav-cad) software
anyone can help me for this ??

 

The Firefly 8.5 is an older version of the Firefly 850 and the Spitzfire 28. These are fast 28 foot open bridgedeck cats from Mark Prescott designs. The cat is 28 x 19 foot weighs 2300 lbs and in cruising mode displaces 3600 lbs. The sail area is 500 square feet. The original boats were strip plank WRC later versions were foam glass. I will deal with the strip plank versions. The hull from outside is 315 gsm 45/45 biax 8 mm WRC 200 gsm unidirectionals at 90 degrees for full race boats OR 230 gsm undirectionals at 90 degrees 10 mm WRC 230 gsm unidirectionals at 90 degrees for racer cruisers. In both cases the glass is doubled over the bottom to the waterline with an additional 200 gsm plain weave over. The cockpit wing tray is 400 gsm biax 16 mm foam 300 gsm cloth. Bulkheads are 195 gsm carbon cloth 12 mm foam 195 gsm carbon cloth. The majority of the boat to this point is done in epoxy. Any interior furniture is 300 gsm glass cloth outside 10 mm foam 200 gsm glass cloth inside. You dont wear hard heeled shoes on that internal layup, socks or bare feet. The main and aft beams are 203 mm outside diameter with 3 mm walls. The main beam has a dolphin striker. The fore beam is 152 mm diameter with 3 mm walls with a dolphin striker. As the photos will show these boats can be sailed hard and have done coastal races with success.

The Spitzfire 28 is similar in dimensions but due to advances in structure of foam glass hulls etc is 440 lbs lighter and run 14:1 hulls instead of 13.5:1 for the Firefly 8.5.

 

An interesting aluminium cat design is the Kat’alu 42 by NG design. The cat is a bridgedeck cabin boat 41.3 x 22 foot weighing 16800 lbs with a 4400 lbs load carrying capacity. Sail area 1056 square foot. The hull shape is flat bottomed with a high chine. The T frames are a 1 meter apart with T shaped stringers at about 300 mm apart. The hull and decks are likely to be 3 to 4 mm skins. Aluminium is a fast build technique for those who have the welding skills and understand how the material develops while being welded.

The low aspect keel is very useful in cruising. A South African designer of the Robertson and Caine cats (Leopard cats), Alex Simminous, says he prefers low aspect ratio keels over daggerboards for cruising catamarans. He says cruising owners don’t pay enough attention to the course and sail settings which mean daggerboards are often stalled out not providing any worthwhile leeway prevention. He said low aspect keels provide lateral resistance under the majority of circumstances and keels have less tripping effect in strong wind, high wave conditions.

 

May be you can ask your question here: Software https://www.boatdesign.net/forums/design-software/
Example for questions asked there: Gene-Hull version for Catamaran https://www.boatdesign.net/threads/gene-hull-version-for-catamaran.61654/

 

The following was written by Kurt Hughes and is very relevant about resin choices.

There are many good reasons to use epoxy instead of polyester. First, stretch to failure. Glass fabric has about 6% stretch to failure. So does most room-temperature epoxy, and many vinylesters. Polyester stretch to failure is about 1%. So in tension, the glass is only loaded to about 17% of its strength before the resin matrix starts to break apart and become a necklace.

Polyester will be bonding with water throughout its life, and it will gain surprising weight from that water, while losing strength properties. If I may quote from a D570 water weight gain test, “The orthophathlic casting had more than a 2.5% weight gain after 4 days, then showed a weight loss on the 7th and 14th days. That means the polymer is being broken down, solubilized, leached out of the composite, and replaced with water.”

The biggest benefit of epoxy is that epoxy is more forgiving during construction. Get the mix ratio right and you are there. With polyester, or even vinylester, one has to vary the MEKP level and vary the N,N-DMA level, oppositely, as a function of temperature change. Or, also, vary the BPO level and again vary the N,N-DMA level oppositely, as a function of temperature. The catalyst/promotors have to be done exactly right to get a good degree of cure. The catalyst/promoters will have amounts of down to fractions of a percent. Those amounts must be very precise if good results are intended. Also, if the part is stored at less than 10C it may never cure fully, even with a later post cure. Using epoxy, you can improve the laminate properties with a post cure, almost always.

Interplastic Corp. has some great papers on this, which I am quoting from. Specifically, Proper Cure of Vinyl Ester Resins and A 15-Year Study of the Effective Use of Permeation Barriers in Marine Composites to Prevent Corrosion and Blistering. “The point is if the temperature was too cold when you did an epoxy laminate, you could bring it up to close to 100% cure later. If you, for example, do a polyester laminate at 15C with 1% BPO and 0.3% N,N-DMA, you will have only an 80% cure and it probably cannot ever be improved. That same formulation however will give 96% cure at 25C. Most room temperature cure epoxies post-cure at around 65C to 70C. Static properties increase with the cure, and so does toughness against impacts. Epoxy can accrue the benefits of a post cure but polyester and most vinylester will not be improved by a post-cure. You got what you got”.

Blisters are also a huge issue. With polyester, especially with orthophthalic resin, as Terry McCabe of Interplastic said, “Its not if it will form blisters, but when.” The useful paper again is “A 15 Year Study of the Effective Use of Permeation Barriers in Marine Composites to Prevent Corrosion and Blistering“. And again, Interplastic. Shelf life is another issue. The useful shelf life of epoxy resin is years. Hardner has a shorter shelf life though it is still good for years. Both vinylester and polyester components have a shelf live of just months. If your project gets interrupted, that can cost you if you did a bulk-buy to save money.

In conclusion, both epoxy and vinylester are much preferred to polyester. Structurally, epoxy and vinylester are close in properties. Epoxy however is much easier to work with, and is much more forgiving. Attached are 3 other files that reinforce Kurt Hughes views.