Multihull Structure Thoughts

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

  1. oldmulti
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    oldmulti Senior Member

    Weta 4.4 is a small day sailing trimaran of 14.4 x 11.5 foot (folded 5.7 foot) that weighs 220 lbs and carries on an 18 foot carbon fibre mast a 100 square foot main, 34 square foot jib and 86 square foot screacher. The trimarans sailplan is of a main, jib and easy-handling, furling screacher. The boomless mainsail, made by windsurfer sailmaker Gaastra, incorporates five full tubular battens that control draft and allow the mainsheet to be attached to a well-reinforced clew rather than a conventional boom. Its sculpted foot allows a crew to easily duck under during tacks but still keeps sail area maximized. Gaastra transferred quite a bit of its "go fast" sail shaping into the sailplan, including light tubular batten technology and X-ply and monofilm sail material. The high-tech Gaastra sail package, like the Harken hardware and carbon sprit and mast are all standard, and the result is a package primed for performance.


    The Weta 4.4 is capable of double-digit reaches in 10 knot winds, accelerating in puffs, and ghosting through lulls. Top speed so far is 16.5 knots. Designed by New Zealand father-and-son team Roger and Chris Kitchen, the Weta has a carbon-fiber and E-glass tubular cross beams connecting the outer amas to the main hull spreading rig and hull loads. The mast and bow spirit are made of carbon fiber. The light stiff hull structure, daggerboard and rudder are built using less-costly E-glass and Divinycell foam. Weta’s key to fabrication success stems from the builder’s attention to detail. Vacuum-bagging and strict resin control results in the elimination of excess resin offering excellent weight control. The 220-pound boat retains the strength and stiffness needed in a high-performance sailboat. These boats are built in China.

    There is quite a global fleet with many very happy owners. This boat can outperform many larger boats and is just plan fun if sailed solo although it will handle a few extra if “cruising”.
     

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  2. Banzai
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    Banzai Junior Member

    Weta hull form actually designed by Tim Clissold for which he received little or no credit
     
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  3. oldmulti
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    oldmulti Senior Member

    Albert Sedlmayer is an interesting designer. He originally came to notice for designing a 30 foot “self righting” catamaran. He designed several sailing models then went on to larger designs EG 177 foot model. The design we will look at is Alpha Omega, a 44.3 x 25 foot that weighs 10800lbs and displaces 15400 lbs. The sail area is 840 square foot. The length to beam ratio of the hulls is 15:1. The displacement and narrow hulls would indicate reasonable performance with the modest rig. I do not know if one has been built, and to the weight so reality may be different.

    For an early 80’s design the cat had several advanced features. The vertical bows and forward cockpit were the start but things like aft positioning of the rig and the centre of weight aft were also ahead of their time.

    The hulls have a high step at deck level and full width cabin structures which is part of the capsize minimisation (although this boat may not be self righting). The design can be home built using a plywood balsa sandwich construction which means there will be minimal fairing involved. There is an alternate foam glass option.

    The boat accommodation layout and structure make it an interesting boat that has performance potential. His web address is Vessels in Service – SEDLMAYER ASSOCIATES PTY LTD http://www.sednav.com/vessels-in-service/
     

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  4. Bruce Woods
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    Bruce Woods Senior Member

    Around 20 years back there was one in the D'Abora marina in Port Stephens. It was rough. The forward cockpit piqued my interest.
    From memory there had been a write-up in one of the Aus magazines at the time the design was released. The design looked futuristic when drawn, disappointed in reality (builder under financed??).
     
  5. redreuben
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    redreuben redreuben

    Bruce,
    I remember that write up the name Alpha Omega, except I remembered it as a Tri for some reason but the designers name is familiar too.
    Any info on the 30’ that’s more my size, I like those hull sections, very performance cruiser. RR
     
  6. oldmulti
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    oldmulti Senior Member

    Redreuben give me a day I will find the info on a smaller version about 30 x 16.5 foot. But now we talk about one approach to making a mast cross beam for a 40 foot cruising catamaran. This is what I will call a basic approach but very effective and strong, but it weighs a bit more than a full foam glass beam. First select good quality 7 ply 9 mm ply. Cut to the 8 x4 foot panels to required shape. As the full bulkhead will be 18 mm thick cut a second set of ply panels but at different start and finish points to offset any joints in the ply panels. Obtain a pile of weights and glue the 2 layers of 9 mm ply together. After the glue has cured and the bulkhead has been cleaned up, lay a layer of 750 gsm triax on both faces of the full bulkhead.

    When the mast bulkhead is to be installed into the cat first lay 8 layers of 150 mm wide 1000 gsm unidirectional and 4 layers 450 gsm 45/45 biax interleaved is laid across the bridgedeck and down into the hulls to the keel line that will be under the mast bulkhead. When the mast bulkhead is in position use 750 gsm 45/45 biax to tape the bulkhead on both faces to the wingdeck floor.

    Then the top of the mast bulkhead has a layer of strengthening glass laid on it. First round of the top of the bulkhead for the full width of the bulkhead. Next lay 8 layers of 1000 gsm unidirectional glass interleaved with a 45/45 biax every 2 layers for the full beam of the bulkhead starting with a 300 mm wide layer then reducing each layer by 10 mm. All of this is done in epoxy.

    Any openings such as doorways requires an 18 mm wide layer of unidirectional plus 2 layers of 45/45 biax around the edges of the door openings.
     

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  7. oldmulti
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    oldmulti Senior Member

    Redrueben asked for some information smaller versions of Albert Sedlmayer cats. I found information on 2 models. The Alpha Centauri is a 30 x 16.4 foot, displacing 5950 lbs with 388 square foot. The 30 footer is built mainly with plywood with glass taped seams. Due to the hull shape and slight curves everywhere it generally is 2 layers of ply using a minimum of solid timber eg it only has 2 stringers in the hulls. Bulkheads and furniture are used as stiffeners. I do not know any further detail but it sounds like a very good build approach which would be fast to build.

    The Saracen Alpha is a 36.5 x 19.2 foot, displacing 9520 lbs with 445 square foot of sail area. The design is shown in the jpegs, also is included is a launch report about the cat. The 36 footer is conventional ply timber stringer and bulkhead construction. The cabin roof is a ply balsa sandwich.

    Now we come to the interesting part. Both cats were designed to be self righting from a 100 degrees. Monohull racing rules generally require self righting from 115 degrees. The hull and cabin shape assist in the self righting capability. This is better than a lot of cats and was a popular idea in the 80’s. Now people understand that capsizes can happen in many directions with wave action and momentum playing a big part. Prudent sail control and parachutes anchors are now considered a better solution than trying to make a cat self right.

    Both cats also have central centreboards under the main bridgedeck. Kickup rudders are used. Both these cats use an aft rig placement. These are interesting designs. I hope someone knows people who have built or sailed these vessels so we can all learn a bit more.
     

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    Last edited: Feb 28, 2020
  8. jamez
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    jamez Senior Member

    I saw a boat remeniscent of the 30 foot on the haul out area at Thames (90 minute drive SE of Auckland) just before Xmas. Had keels.
     
  9. redreuben
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    redreuben redreuben

    Thanks Oldmulti, fascinating.
    It amazes me that these 40 year old boats still look so modern. To my eye at least. The only thing I would question is the bows/front end looks a little fine by modern standards.
    Definitely an underrated designer, any New Zealander’s know a bit more about why this might be ? Garry ?
     
  10. oldmulti
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    oldmulti Senior Member

    I have spoken about Marc Louis Rifflart before but this is to emphasis that smaller simpler boats can do very good things. The Catlife 825 is a 27.5 x 14.5 foot weighing 1430 lbs and carrying a 146 square foot main with a 165 square foot fore triangle. The cat was designed as a high performance cruiser racer for a family of 4. This is a basic boat that would be a fast build and has enough accommodation for an overnight stay. The hull length to beam is 12:1 and with its light weight it should perform well. As Richard Wood says a good daggerboard is better for a cat performance than converting a dory hull to a round bilge hull shape.

    The Catlife is a plywood timber stringer design that can be covered with light glass or if built with good quality ply painted with 2 pot polyurethane without glass. The design is simple would be fast to built and relatively cheap.

    The second series of jpegs are of Echo (variation of Pacifica model) which is a 32 x 15.4 foot cat displacing 8000 lbs and carrying on a 40 foot aluminium mast a 305 square foot main and 260 square foot genoa. This specific boat has crossed the south Atlantic from France to the USA and cruised the full length of eastern US seaboard with a family aboard. It is a plywood timber construction and was built in 1982 and is still around. The flat bottom dory hulls have performed well able to carry the loads and still have a high wing deck clearance.

    Marc Louise Rifflart is a proven designer of very good home built boats (mostly plywood). Enjoy.
     

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  11. oldmulti
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    oldmulti Senior Member

    Concerto was a prototype cruiser racer bridge deck catamaran designed and built in the early 80’s. It is 27.8 x 15.3 foot displacing 4200 lbs and carrying 484 square foot of sail. This boat is interesting in 2 regards. One the cat can be disassembled into 2 hulls and a bridge deck component for transport (requires a crane therefore transportable not a trailer sailor). The second is the study of build options which are listed below.

    Option 1. Build the hulls of 2 layers of 3 mm plywood on 23 x 11 mm stringers at 100 mm centrelines. Bulkheads are 7 mm plywood. The skin in this option was considered to only have 70% of the strength multiple layers of wood veneer due to the “incorrect angle” of centre plywood plies.

    Option 2. 4 layers of 3 mm WRC wood veneers with no stringers built a timber framed male moulds.

    Option 3. 2 layers of 1.2 mm Khaya either side of 6 mm balsa plank core build over timber framed male moulds.

    Option 4. Was a single skin grp (to heavy at a thickness required for hull stiffness even with stringer support)

    Option 5. A foam glass option where the skins were considered to thin on the 12 mm PVC foam core for puncture resistance. Structurally and weight were good.

    Option 6. Either a Kevlar or Carbon fibre skin a PVC foam core. Again excellent structural and weight characteristics but again knock resistance and costs were major concerns.

    The final build was for the hull bottoms 4 layers of WRC veneers with in the upper hulls 2 layers of 1.2 mm khaya veneers either side of 6 mm balsa. The decks are also 2 layers of 1.2 mm khaya veneers either side of 6 mm balsa. The underwing and cockpit are 4mm ply 6 mm balsa 4 mm plywood sandwich.

    This is a summary of a well researched design. The jpeg is of the final build. I cannot find any jpegs of the cat sailing.
     

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  12. oldmulti
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    oldmulti Senior Member

    The TRT 1200 is one of my favourite designs. It comes in either a semi wing deck or “cruiser” version with the same basic specs of 39.33 x 25 foot weighing 6300 lbs and displacing 9000 lbs in the GT racing form or displacing 11000 lbs in the CR form. The 56 foot carbon fibre wing mast weight 132 lbs and uses synthetic rigging throughout. It carries a 640 square foot main, 300 square foot jib and a 495 square foot genoa. The boat has an IOMR rating of 1.07, that is quick.

    The Norwegian-designed TRT 1200 catamaran is a performance-oriented 40-foot offshore catamaran, available in both an open bridgedeck version (TRT 1200 GT) and bridgedeck-cabin version (TRT 1200 CR). Both models offer high performance (20-plus knot speed potential) and the GT has hit speeds of 26 knots. It has accommodation for four or more depending on how friendly you are. The TRT have been successful in racing but some owners leave a reef permanently in the mainsail as the boat is a bit to fast for them. Another owner wanted more accommodation and had the factory build a larger bridge deck cabin with full headroom and reduced the cockpit size. The boat was still very fast. Some of the video's on youtube give an idea of its speed capabilities.

    The construction is from foam glass with vinylester resin. The hull is female moulded with solid glass below the waterline and a 100 csm tissue, 600 gsm triax 15 mm H80 divinycell, 100 csm tissue, 600 gsm triax in the topsides. If a heavier layup is required 750 gsm triax is substituted for the 600 gsm triax. The rudders are either spade or transom mounted lifting rudders. The boards and rudders are carbon fibre.

    This boat is for those who like a very fast cruiser and several are available on the second hand market. A very nice boat.
     

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  13. oldmulti
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    oldmulti Senior Member

    White Wings is a Newick trimaran design of 36 x 24 foot weighs 5000 lbs and displaces 6000 lbs carrying 460 square foot of sail on a ketch ljugstrom rig. The main is 345 square foot and the mizzen is 115 square foot. The main is double side and wraps around the mast and can be spread wing on wing down wind. The main reefs around the rotating mast. The masts are free standing carbon fibre, the main mast is 54 foot high and is able to rotate on needle roller bearing at deck level. The tri also can carry a 120 square foot ¾ oz staysail on the mizzen mast. The boat was built in 1988. The tri is narrower than normal but Newick said the rig is flexible with a lower centre of effort which allows it to carry sail in stronger winds therefore it can reduce its weight with less beam. The boat has an IOMR rating of 1.25 which is seriously fast.

    The boat was intended to be a 2 berth cruiser for a couple who are in there 70’s. The boat can day sail 6. The boat has sailed from Florida to the Azores and can sail at 8 knots in an 8-10 knot breeze’s. It sails at 15 knots often and has topped 20 knots. The original owners did not push the boat but daily averages were high.

    The boat is built from Baltech 12 mm duracore with 495 gsm 45/45 biax in epoxy on either side. The cross beams are timber with carbon fibre. Decks are ply and timber with a light glass layer.

    This is a light fast simple easily handled 2 person cruiser. I will have more plans/jpegs later.
     

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  14. oldmulti
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    oldmulti Senior Member

    The Tektron 50 was designed about 1987 by John Shuttleworth. The open bridge deck cat is 50 x 34.5 foot cat weighs 10000 lbs and carries 1970 square foot of sail on a 72 foot carbon fibre wing mast. The hulls length to beam ratio is 14.5:1 and has a Prismatic Coefficent of 0.562 with a centre of buoyancy 55% aft of the bow waterline. The underwing clearance is 3 foot.

    This design pushes the performance limits of a cruising boat and was 10% faster than the OSTAR record holder Fleury Michon VI and was probably the fastest cruising boat in the world at the time. The outstanding windward performance and a top speed of over 30 knots the Tektron 50 is an exciting boat to sail. The cat has four double state rooms, a saloon that seats eight people, a large galley, full sized chart table and workshop. This type of design makes sense to many people who would like to race and yet do not feel inclined to invest so much money in an all out racing boat that has a limited life span, and worse still, a low resale value. The Tektron 50 has good interior volume and clean aerodynamics has outstanding performance.

    But the real design achievement is the detail of things like the whole boat is designed with a very rounded form, and the cockpit and crossbeam structure areas as aerodynamically clean as possible. All the deck edges have generous curves and even the primary winches are set below deck level. Flare in the hull creates the interior volume needed for comfortable cruising, without increasing the frontal area at all. The designer was able to model and study the air flow across the hulls at the angle that the wind actually flows across the boat. This is of great significance, because the wind never strikes the boat from dead ahead when you are sailing - in fact it will be 25 degrees off the bow at best.

    The hull construction is all vacuum bagged epoxy with Kevlar carbon and "S" class laminates throughout, using a PVC foam core. The rig has deliberately been made extremely powerful to compensate for the added weight of the cruising accommodation and the carbon fibre wing mast, and carbon boom were designed to give maximum efficiency for minimum weight.

    After this design was done and several were built Shuttleworth designed a bridge deck cabin version that still performed very well as shown in the last 2 jpegs. But now the downside of such a aggressive design A Tektron 50 capsized of the coast of the US whilst being driven hard. All people were saved as was the boat but when you have a real performance cat you have be very aware of the consequences of hard driving. This is an excellent design and was ahead of its time. It was superseded by the Dogstar 50 cat which could sail even faster. Jpegs attached.
     

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  15. oldmulti
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    oldmulti Senior Member

    The Dogstar 50 was designed about 2000 by John Shuttleworth. The open bridge deck cat is 50 x 34.5 foot cat displacing 9000 lbs racing with crew and can carry and additional 5000 lbs for cruising. Dogstar 50 carries 1722 square foot of sail. The rig is a double diamond swept spreader 78 foot carbon wingmast of short chord. The jib, which is sheeted at 6 degrees is not self tacking because a small overlap is needed to obtain optimum effect from the slot. On a boat with such narrow sheeting angles, a large chord wingmast rotating near the forward end of the mast tends to close the slot between the jib and the mainsail. Keeping the chord small and rotating the mast aft of the centreline will keep the slot open and will improve sail power upwind. The hulls length to beam is 15.5:1. The underwing clearance is 3 foot in the centre section with 2.5 foot near the hull sides. This boat is a development of the Tektron 50 being lighter having finer hulls with slightly less sail on a taller mast.

    The Dogstar hulls have reduced freeboard with more rounded deck edges to minimise windage especially upwind to allow higher performance. The Dogstar also had a very good analysis of structural options. The all foam glass option would weigh 9400 lbs, a carbon foam option would weigh 9100 lbs and a Carbon Nomex version would weigh 8800 lbs. This was considered to heavy so Shuttleworth again redesigned the cat to minimised the surface area of the cat and removed any “unnecessary” structure, furniture etc and got a prepreg Carbon fibre Nomex honeycomb above the waterline version design got the weight down to 7950 lbs. The new hull shape saves significant weight because the reduced area of the hull not only saves weight, but all the bulkheads have less area as well. The lighter structure allows the rig weight, and the crossbeam structure to be reduced. This translates into even lower cost and less weight.

    Subsequently Shuttleworth redesigned and improved the Dogstar 50 with the Pulsar 50 designed in about 2010. The open bridge deck cat is 50 x 34.5 foot cat weighs 7800 lbs and carries 538 main square foot 290 square foot jib 670 square foot code 0. I have little detail on the design but it looks like a better accommodation layout, lighter displacement and again less sail.

    I have no doubt that the Dogstar and Pulsar would be faster than the Tektron 50 even though it has less sail. The 20% weight reduction from the Tektron to the Dogstar would more than compensate for the reduced sail area. Interesting designs.
     

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