Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. InetRoadkill
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    InetRoadkill Junior Member

    Thanks for your reply. I know there is no magic formula which is why I was asking what the industry has settled on for a typical hull thickness that provides reasonable resistance to unfortunate encounters. I was considering a solid hull below the waterline, but the method I envisioned for construction won't allow it (foam over forms). So the portion below the waterline would likely get some extra layers. (kevlar maybe?) Fortunately, most of the interior floor is above the waterline so bulkheads and watertight compartments are no big deal. The quoted weight of 14K lbs is a first order guess. Hopefully, I'll be able to do a better weight estimate when the hull is further along. (Going to need more napkins.)

    The loads on the akas are pretty much the same whether they fold or not. The hinges are going to be the trouble spots since they are high stress areas. My napkin load calculations are showing compressive/tensile loads on the order of 90K lbs and bending moments peaking around 80K ft-lbs. Those are certainly quite large. Throw in a safety factor and it's in the neighborhood of a half million pounds. Certainly an engineering challenge there.
  2. redreuben
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    redreuben redreuben

    There are a couple of ways to make foam hulls more bashable, the first would be use a higher density foam under the waterline and the second is to use fabrics that handle compression loads better, carbon Kevlar et al are great in tension but for bashable use Sglass or basalt.
  3. InetRoadkill
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    InetRoadkill Junior Member

    I was thinking of kevlar on the outer layers for abrasion resistance, at least on the underside of the hull. Though, this might bring up thermal expansion issues between the kevlar and s-glass acting as a source of delamination. Will need to do some research on that.
  4. fallguy
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    fallguy Senior Member

    Kevlar is not really the best. The best way to deal with bashing is to expect and plan for it.

    watertight compartments, foam compartments, extra fiberglass, sacrifcial timbers on beaching keels

    But developing the hulls to be bash proof requires an excess of glass.

    Kevlar, from my reading, is best to keep a grounded hull from holing. I used one piece 8" wide and it is buried in my beaching keel under a layer of regular glass and above 6 other layers. I am not familiar with how you would layup kevlar and expect it to perform better than e glass in an impact.

    I sure am grateful for this thread.

    Cactus Island is really a sexy boat.

    I have one question about the biplane rigs.

    Would it be better for one sail to be forward of the other? I have a vision of one of the sails being in less wind than the other on a reach.
  5. oldmulti
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    oldmulti Senior Member

    Fallguy. Your thoughts about biplane rigs on reaches is the main issue. There are 2 types of biplane rig boats. Relatively light boats for there size with fairly efficient biplane rigs and heavier boats with inefficient rigs. Cactus Island, Ozone and Saphira are samples of light boats with good rigs. These boats by there very nature can go can fast and pull the apparent wind forward so the rig appears to be going "upwind" on some points of a pure reach. But there is always a point of sail on a reach where there is some "blanketing" of the lee sail. With light fast boats all you do is adjust your course by steering up or down EG 15 degrees to get both rigs working again.

    On relatively heavier boats biplane rigs still work but have to make bigger course adjustments to optimize the rigs efficiency. EG Thomas Firth Jones original Dandy cat had a biplane rig but he changed to a single rig after a season because the boat did not perform as expected. The real consideration on heavier slower boats is the rigs lateral separation (further apart the better) and the efficiency of the rig. Wing masts, rotating masts and good sails all help to get reasonable performance on biplane rigs on heavier boats.

    Fore and aft two mast rigs are better on a reach but unless they are very well designed they have issues going upwind and in some cases going downwind. BUT there is a few boats where a fore and aft schooner rig can be very good upwind. EG Shotover 2, Ross Turners 7 mtr cat, Gary Bagiant 20 foot monohull and even Wharram 31 foot cat. The common theme in these boats is the relatively close spacing between the 2 mainsails. The forward main appears to act like a "jib" feeding airflow onto to the second main.

    Again you make your choice. Biplanes in the right boat are very good. Fore aft 2 mast rigs if well designed are very good. have fun thinking and researching. The jpegs do not lead to book but Thomas Firth Jones book describes his conversion of Dandy from a biplane to a sloop and the reason why.

    Attached Files:

  6. oldmulti
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    oldmulti Senior Member

    The following document is provided by the Food and Agricultural Organisation FAO FISHERIES TECHNICAL PAPER 321. This document is a 2.9 meg PDF of 121 pages that describes how to build a fiberglass fishing boat but it covers most aspects of fiberglass construction including sandwich construction. It is aimed at developing countries. This makes the document useful for backyard builders as it does not assume high technology is always available. The following is the table of contents:
    1.1 What is FRP?
    1.2 Equipment and Materials
    1.3 Advantages and Disadvantages of Boats Built in FRP
    1.3.1 Advantages
    1.3.2 Disadvantages
    1.4 Comparison with Other Boatbuilding Materials

    2.1 Glass Reinforcement
    2.1.1 Chopped strand mat (CSM)
    2.1.2 Continuous roving
    2.1.3 Woven roving (WR)
    2.1.4 Unidirectional roving
    2.1.5 Glass cloth
    2.1.6 Surface tissue
    2.2 Resins
    2.2.1 Lay-up of laminating resins
    2.2.2 Gelcoat resin
    2.3 Resin Putty
    2.4 Catalysts and Accelerators

    3.1 Fire Hazard
    3.2 Health Hazard
    3.3 Delivery and Storage
    3.3.1 Resins
    3.3.2 Reinforcement

    4.1 General Conditions
    4.2 The Building
    4.2.1 Construction
    4.2.2 Insulation, air conditioning and humidity control
    4.2.3 Ventilation and dust extraction
    4.2.4 Lighting
    4.2.5 Electrical and compressed air systems
    4.2.6 Access to the workshop
    4.2.7 Cleaning
    4.3 Tools and Equipment

    5.1 Mixing Resin and Gelcoat
    5.2 Application
    5.3 Preparing Reinforcement
    5.4 Laminating
    5.5 Stages of Cure
    5.6 Using Tools on FRP Mouldings

    6.1 Single Skin Laminate
    6.2 Double or Sandwich Skin Construction
    6.3 Mouldless Construction
    6.4 High Technology Developments in FRP
    6.4.1 Materials
    6.4.2 Advanced boat building methods
    6.5 Production Methods
    6.5.1 Production line
    6.5.2 Mass production

    7.1 FRP Applied to Core Material
    7.2 Making a Simple Plug and Mould
    7.3 FRP Practice Panel

    8.1 Plugs and Moulds
    8.1.1 Hull plug and mould building
    8.1.2 Deck and interior
    8.2 Plug and Mould Details
    8.2.1 Water/air inlets
    8.2.2 Split moulds
    8.2.3 Care and use of moulds
    8.2.4 Variations to a standard product
    8.2.5 Component assembly
    8.3 Construction Details
    8.3.1 Keels and areas requiring extra stiffening
    8.3.2 Framing and stiffening sections
    8.3.3 Bulkheads
    8.3.4 Deck and superstructures
    8.3.5 Deck to hull connection
    8.3.6 Fuel and water tanks
    8.3.7 Connections and fastenings
    8.3.8 Bonding to materials other than FRP

    9.1 Choosing the Right Design
    9.2 Choice of Construction Method
    9.3 Structural and Design Requirements

    10.1 Heat and Humidity
    10.2 Appropriate Technology
    10.3 Workforce, Skill Levels and Training

    11.1 Arrangements and Obligations
    11.2 Maintenance
    11.3 Gelcoat Repairs
    11.4 Repairs to FRP Structures
    11.4.1 Tools

    The attached jpegs are samples they do not lead to the PDF. The PDF can be downloaded from

    Or use the following is the FOA web site version of the FOA technical paper 321. Fishing boat construction: 2 Building a fibreglass fishing boat

    Attached Files:

  7. oldmulti
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    oldmulti Senior Member

    Gleda Tiki 38 builder owner was advised of a potential weakness with the foremast case on the Tiki 38. The advice came from Daniel Ganz on ‘Marabu’. He left the West Indies enroute to the Azores and was some 500 miles out when he noticed a crack in the aft end of the mast case. See the photos below. Fearful that complete failure might result in loss of the foremast and deck he lashed everything together, turned the boat around and thankfully made it back to a safe harbour without further mishap.

    What is particularly concerning about this failure is that ‘Marabu’ was only launched in 2008/2009 and so the plywood was sound without any signs of rot. Daniel e-mailed Hanneke at JWD to tell her of the problem and to seek her advice. She replied with some recommendations which I’ll get to shortly however her e-mail also revealed that a few months previously there had been a similar failure on another Tiki 38 called “Jumpa Lagi”. “Jumpa Lagi” failure was attributed to rot in the mast case.

    Although of differing ages both ‘Marabu’ and “Jumpa Lagi” have been sailed extensively and, as Wharram’s Hanneke Boon pointed out in her e-mail; “The mast case is under considerable strain in steep seas sailing to windward”.

    Her recommendations to Daniel were as follows;

    “I think you will need to dismantle the cracked wood parts and replace them with new good quality plywood as per the original drawings. I would then suggest that the whole mastcase is strengthened with two hardwood longitudinals along the outside upper edges of the mast case, i.e. hardwood timbers of 80mm x 25mm placed vertical. These should be full length, so they sit on top of the crossbeams at front and back. In both cases the maststep has worked well for many years before failing, so the break is due to a fatigue problem. Many Tiki 38s have sailed many ocean miles without showing a problem so far. I think this strengthening is sufficient to avoid any further problems’. Hanneke modified the plans and sent them out to all of the Tiki 38 builders and owners that they know about.

    Wharram has the following design package available on their web site for those who have not been advised of the problem.

    Design Improvements Package – Classic designs

    Item Details:

    This package is based on the Tiki 38 centre deck layout and is suitable for most Classic designs (35' Tangaroa or larger). It offers a small deck pod, boarding ramp and engine boxes to fit Yamaha 4-stroke 9.9 hp outboards. It shows alternative fixing of slatted decks and the main mast step.

    Attached Files:

  8. oldmulti
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    oldmulti Senior Member

    Zephyr is a three person, small light weight trimaran, that is 15 x 8 foot with what looked like a hang-gliding rig on top of it. Standing next to it was Tony Smith, retired creator of the Gemini catamaran and the Telstar trimaran. Smith built this prototype in his garage and is seeking an investor interested in putting this unique vessel into production.

    The rig works on the same principle as Paul Larsen’s Vestas Sailrocket, the world sail speed record holder of over 60 knots. A highly efficient sail foil that is canted to windward an angle (as on a windsurfer, for example) to offset the heeling force it generates, with the direction of its driving force perfectly opposed to a foil under the hull. The boat’s sail, instead of being set on a vertical mast, is canted to windward at an angle to offset the heeling force it generates, with its driving force perfectly opposed to a daggerboard foil underneath the trimaran’s opposite ama. The sail on the Zephyr can be tacked back and forth. This is accomplished via a pair of clever manually driven interconnected hydraulic pistons that hold the sail aloft. When the ram in the windward piston is pulled down, the ram in the leeward piston is automatically extended, which allows the boat’s skipper to easily cant the sail from side to side. One great advantage to the rig is that the sail, as it moves through the wind when tacking, loses none of its driving force. The boat’s controls are also simple and intuitive. You steer with your feet, which leaves your hands free to manage lines that controls the sail’s attack angle and its orientation to the hull. When tacking the sail is briefly in a horizontal position over the hull, just like a hang-glider. The Zephyr has a large wing that enables it to sail well in light airs. In heavy airs the wing can be lowered moving to leeward whilst tipping to windward eliminating the heeling moment. All the wind force is generating lift and speed all while the crew is simply sitting facing forward.

    The boat has planning hulls and a helmsman seat with foot pedal control to the rudder. In front of the helmsman is a table with the rope controls and jam-cleats. Just like a hang glider the power comes from a single surface wing on a carbon frame with pre-formed battens. The wing is attached in two positions on its underside to the hull with telescopic tubes. When one telescopic tube is fully out and the other is fully in the wing is vertical and the boat sails like a normal trimaran.

    The jpegs give a better idea. This has only a recently finalized in 2019 and won a special award from Sail magazine for innovation. This rig has been applied on many boats before but this is the most advanced for a normal sailor. A little bit of fun even if it is a little complex.

    Attached Files:

  9. myszek
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    myszek Junior Member

    My old Nietoperz (the same principle)
    nietoperz1.jpg nietoperz2.jpg nietoperz3.jpg
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  10. oldmulti
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    oldmulti Senior Member

    Myszek. What a good boat! Did it go to windward well? Did you take it out sailing in strong winds? Could you reef the sail or did you need to change to a smaller kite? So many questions, can you give us some more information please.
  11. myszek
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    myszek Junior Member

    Long story. Nietoperz (the Bat) was launched in 1999. It took me 3 years to learn, how to sail her. Sailed 5 seasons on Zegrze lake, once on Zatoka Pucka bay, on Baltic sea, then on Jamno lake near the sea, during Polish proa meetings. Abandoned in 2012; hulls probably do not exist now, but the rigging is waiting for a new boat.

    Both hulls are made of "tortured" 4mm ply. Vaka is 4.5 meters long, ama is short and fat (it was the test of technology, but the drag of the fat ama was the weakest point of the boat). All the structure weighed about 60 kilograms. A fixed daggerboard on the ama and two rudders on the bows provide side resistance and steering.

    The propulsion is a typical, full-sized paraglider. It is not a free-flying kite, but raised on two poles, made of old windsurfing masts. The most important part is a mast fitting, that can free rotate around a vertical axis and can be tilted on the right tack. The wing can be raised and lowered with halyards; after some time of learning we were able to lower it into the hull (not water), and raise again. That was the main idea - to make the inclined wing propulsion as usable as a normal sail.

    Nietoperz was not a good boat, due to a poor design of the hulls. The drag of the fat ama limited the speed. The canoe-shaped bows caused the waves to flood the cockpit too often. The position of the helmsman was not very comfortable, on the beams near the water. Sailing was very wet for both the crew.
    In theory, the wing requires operation only when shunting. In practice, turbulence often causes deformation of the wing, which had to be corrected using the lines. Thus, the crew of two is necessary. The problem was particularly strong at weak wind. Below force 2, the wing didn't work at all. Force 3-4 was optimal. Once or twice we sailed at force 5, but it was terrifing, with the small canoe under 28sq.m of canvas. However, Nietoperz capsized only once (my fault, I allowed the wing to move to the windward side of the masts) and unfortunately she never flew over the water.

    The windward ability was a bit worse than of an average sailboat of a similar size. Not a surprise, since the wing is inclined only about 35deg from horizontal. However, we are able to sail 50-60deg to the true wind. The funny effect is when you try to sail too close to the wind. The wing does not warn you with luffing, the boat just stops, sometimes she starts moving backward. The right thing to do is to gently turn the rig manually, which actually turns the boat around the rig, until the proper course.

    I hope to rebuild at last Nietoperz, based on all the experience. With better hulls, more seaworthy layout, maybe better wing.


  12. oldmulti
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    oldmulti Senior Member

    A small detour into Monohulls for a moment or should we say the rigs of monohulls. Yves Tanton is a very versatile designer who started with Dick Carter and built his own practice that designed over 500 boats. He is inventive and meets his clients wants as shown by the following example of an updated “Spray”.

    The boat is 33.5 x 12 foot displacing about 20,000 lbs with a rising ballast keel and a swing wing sail rig of two 330 square foot mainsails on 200 mm diameter carbon fibre 37.75 foot free standing masts. The righting moment of this boat would be about 35,000 foot lbs or about the equal to a Tiki 31 (31 x 17.5 foot displacement 5200 lbs) righting moment.

    So, the rig on the mono boat could be considered for a multi (after you have done appropriate calculations) if you would like a biplane rig for example. PHA a biplane swing rig Tiki 30 had wooden 200 mm freestanding masts.

    The masts are carbon fibre tubes with 90% carbon fibre running up/down and 10% running at +/- 45 degrees to the vertical. The walls appear to be about 10 mm which is more than the Eric Sponberg 3% of mast diameter “rule of thumb” which would be a 6 mm wall. Again, genuine calculation would provide a better answer. Also, the fibre layout in the mast has a greater % of vertical fibres than recommended by others.

    The other part of this rig is the carbon fibre battens with an aerofoil shape in the forward 65% and a single part batten in the final 35%. The aerofoil shape is specified in a plan.

    PHA used a very similar rig when sailing across the Atlantic and performed very well. When the rig was applied to Grand PHA (a Tiki 46) it worked well again but the battens structurally failed in stronger winds which resulted in a change of battens to stronger full aerofoils without the swing rig component. Grand PHA has sailed around the world since.

    This rig does not need to be as complex as the mono version but it gives you some structural ideas. Jpegs of the original mono and the PHA’s.

    Attached Files:

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  13. oldmulti
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    oldmulti Senior Member

    The following web address will lead you to a story about plywood written by John C Harris from Chesapeake Light Craft in Annapolis, Maryland. Marine Plywood

    If the link doesn’t work the article is available WoodenBoat Magazine #256: "Marine Plywood: A Consumer's Guide"

    Or from WoodenBoat Magazine #256: 'Marine Plywood: A Consumer's Guide'

    John wrote this piece in 2017 and he thinks not much has changed. He explains how plywood is made and the materials (read wood and glue) used. He suggests some good products but also gives a warning at the end of the article. Basically, it says even good quality “branded plywood” can have the occasional bad batches. Even very good builders cannot always pick faulty plywood panels.

    Marine Plywood offers high resistance to water, fungus and damp. It is composed of durable face and core veneers and should be free of voids. Essentially you are paying for the time it will last when exposed to harsh environments. Some ply is guaranteed for 15 and 25 years depending on what you purchase. Marine ply uses a waterproof higher grade of glue in between the layers of veneer which also has less imperfections than standard plywood, and is marked with ‘BS 1088’ meaning:

    “BS1088 is the British Standard specification for marine plywood that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with Weather Boil-Proof (WBP) glue.”

    Many years ago I purchased a batch of branded BS 1088 plywood for a build, but after a few months had a panel fail between plies at a glue line and also had 2 panels with what I found was “buc buc”, a form of wood borer beetle in them. After tracking down the “manufacturer” of the ply, I found it was a company in Asia that’s had a brand name of BS 1088. The BS 1088 is a British standard that is only enforceable in Britain. Please be careful about the product you buy especially where it comes from. There are unethical companies out there who will falsely brand products. The jpegs show some issues.

    Attached Files:

  14. oldmulti
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    oldmulti Senior Member

    The Simpson 13.7 cat is a genuine performance cruiser that was designed by Roger Simpson in the 1980’s. It is 45 x 23.1 foot bridgedeck cat that weighs 15,500 lbs and displaces 19,500 lbs. The cat carries a cutter rig on an aluminium 50 foot mast with dual spreaders. The mainsail is 343 square foot with a 521 square foot number 1 genoa. The hulls length to beam is about 10 to 1 depending on load. There are options f low aspect ratio keels or hull based daggerboards.

    Why do I call it a performance cruiser? To quote one owner: “I have reached over 22 knots with Imagine. She was smooth, and stable. It was the conditions that made it so perfect. In 25 to 30 knot winds from aft with 2 reefs in the main. Had half the headsail out, and we were doing 12-15 knots steadily riding the waves. The boat was like a train on rails going fast, and steady. We’ve done it numerous times, and I just love the sensation of the movement of the boat.” But another owner provides a warning: “Had there been any upwind content the boat would struggle to sail to her rating. The boat has no daggerboards, has 14 foot long (fore to aft) very wide keels, and inefficient rudders like small barn doors.” Those owners with daggerboards have no complaints about upwind performance saying the 13.7 will tack through 95 degrees. This cat has several owners who sail the 13.7 with only 2 crew. One example Vagabond Tiger is a proven blue water catamaran having been sailed from Europe, across the Atlantic to Florida, throughout the Bahamas, and along the eastern coast of the US as far north as Nova Scotia. She is cutter rigged, easily handled by a crew of 2.

    Another advantage of this cat is its accommodation layout with a simple but effective interior with a separate galley on the bridgedeck, a navigation-cabin and a spacious saloon where a dinette and loose furniture can find a place. There are 4 roomy cabins with double and single bunks.

    Construction options include cedar epoxy or cold moulded hulls and plywood for the decks and bulkheads. There is a foam glass version. The main cross beam bulkheads are 12 mm ply on either side of timber framing. The low aspect ratio keels are plywood sides and timber top and bottom planks. Roger Simpson had designed many multihulls before the 13.7 and more importantly had over 100 built by home builders and professionals and actually sailing. This is a proven design that if it is well built and maintained would be a good buy. Plans still available at $1870 Australian from Boatcraft Pacific. 13.7m Catamaran : Boatcraft Pacific, the home of wooden boat building.

    The jpegs give an idea of the cat. A good size practical boat that can go far with limited crew.

    Attached Files:

  15. oldmulti
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    oldmulti Senior Member

    Rapido trimarans are designed by Morelli and Melvin and is being developed and built by Paul Koch and co. who used to own Corsair marine (read F 25, F27, F28, F31 etc). They wanted a series of modern fast foldable tris of 40, 50 and 60 foot length that could be put in series production at reasonable prices. We will focus on the Rapido 50 foot model.

    The Rapido 50 is 49.9 x 34 foot that can fold to 18.1 foot using a patented folding system (similar to farrier). The weight numbers vary but the bare shell is about 10,000 lbs, the launch weight is 14,500 lbs and the displacement is 18,500 lbs. The mast is 81 foot above the water with a 1023 square foot mainsail, a 581 square foot solent jib, a 194 square foot staysail, a 1345 square foot Reacher and a 1722 square foot Spinnaker. The length to beam of the main hull is 8 to 1. The floats have a length to beam of 13 to 1. The foils are raiseable daggerboards. The engine is a 53 horsepower desil.

    The number would indicate this tri will be fast and capable of 300 mile days. This is a very fast cruiser that has reasonable accommodation with two double berth cabins and other convertible dinette berths. This boat has less accommodation than a NEEL 50 tri but I suspect the Rapido will be a faster cruiser. I would like to see them go head to head to find out.

    Now we come to the production and build of the Rapido series of tri’s. These guys understand how to build boats and have the money to do it well. Let’s start with the plug for the Rapido 50 Trimaran main hull. Just go out a buy a Kuka 6 axis milling robot. The robot has a working area of 22.5m x 3.7m and comes with a 22.5m linear track (which gives it a 7th axis). We use the milling robot to prepare the plugs for building new moulds. The robot is much faster – and has a far higher degree of accuracy when compared with traditional methods of building molds.

    For smaller items have a CNC Machine (Computer Numerical Control Machine) to manufacture plugs or components for the tris.

    Next buy a 26 x 6.5 foot Autoclave that is used for prepreg to control a high pressure, high temperature, cure profile. The high modulus carbon beams for the eg Rapido 60 with a structural rating of over 100 tonnes are produced in the autoclave. Also the rudders and daggerboards for the tris are made in the autoclave as well.

    RTM (Resin Transfer Moulding) adds to our capabilities, RTM injects a measured dose of resin into a closed mould using pressure. The process is repeatable and faster than other processes. The key benefit, however, is that we can get a good quality surface on both sides of the part. Also they use resin infusion which sucks resin under a vacuum into the part whilst compressing all the plies tightly together. This process is used for large foam-sandwich core parts such as boat hulls. The process ensures there are no air voids and no inter laminar bonds - and sufficient time for staff to position the cloth and reinforcements before the vacuuming commences.

    We are talking millions of dollars of equipment that can be used on boats from 10 foot to 65 foot. All this equipment ensures more accurate and higher quality products that once in real series production ends up being cheaper than a “one off” approach.

    Construction is infused E-glass Divinycell in the hulls with carbon fiber reinforcements eg stringer lines. Cross arms and associated gear are mainly carbon fibre as are the foils. Many parts are RTM to simplify finishing. To give an idea of the complexity of the design lets look at the foam selection. A range of Divinycell H products, including Divinycell H80 and H100 are used for the hulls, Divinycell H130 for the daggerboard, and Divinycell H35 for the boat’s furniture. Divinycell H offers good strength-to-weight ratio and excellent fatigue resistance. In the autoclave beams Divinycell HP 80, which combines all the advantages of Divinycell H with high temperature resistance for prepreg processes, was used for the Rapido’s beam.

    The tri has multiple water-tight compartments. The build goal was to produce a strong, fast, offshore capable and unsinkable trimaran. Larger tris can be really good boats to sail on but generally take a lot of space to park. The folding solution helps but at 18 foot wide folded it still will not fit into a “standard” berth. But its better than 34 foot. A fast fun boat.

    Attached Files:

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