Multihull Structure Thoughts

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

  1. Gary Baigent
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    Gary Baigent Senior Member

    Old Multi, I'm not convinced that a tall single rig is lighter than a lower double rig, especially so if set up on a catamaran, because as you say, a single has to have beefed up base and dolphin strikers for main and forward beam - whereas the double rig is mounted on simple ring frames with carbon in the secure and stiff main hulls - which is considerably lighter than spread metal/stainless/carbon dolphin striker setups. But major point is that the double rig carries considerably larger sail area than tall single rig, and it is lower so masts do not have to be reinforced to the same extreme. More power is good?
    Talking from experience with smaller scale 5.5 metre Cox's Bay Skimmer, (a wide and very light monohull with rudder and dagger lifting foils) the small chord wing masts weigh much lighter than you would think, can't remember exact figures at moment but I can mount and hoist them on my own - and they are probably lighter than a tall single rig, but not by much. The old argument that double rigs are less efficient to windward is incorrect - especially so if your double rig has aerodynamic and rotating masts. And again especially so if comparing to tall FIXED setups ... plus off wind the double rig is faster, large sail area carried lower. Common knowledge?
     

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  2. oldmulti
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    oldmulti Senior Member

    Compression molding has been developed by Russell Brown to create many small repeatable parts (eg rope guides) that may be needed on a boat. They can be made of any glass resin combination but Russell prefers carbon epoxy for weight savings. As you can see in jpeg one it is simply 2 strong strips of timber shaped the way you require the part. The inner faces are waxed and a glass (or Carbon) resin mix is placed in then you clamp the pieces together. Simpler than vacuum bagging for small parts. The following are Russell’s words.

    “It seems that for small parts such as these, the laminate stack can be resin rich in the center of the strips and drier toward the edges, because when it’s being compressed between the molds, the epoxy moves from the center toward the edges. We use PRO-SET LAM-125/ LAM-226 for all compression-molded parts, and we often warm the laminate with a heat gun (held well away from the surface) when wetting-out the layers, which quickens saturation of the fabric. Once we determine the necessary laminate thickness, it’s easy to figure out how many layers of a specific cloth are needed. The following rule of thumb works for vacuum-bagging laminate as well as compression-molding.

    Woven carbon and glass fabrics are measured in ounces per square yard and in grams per square meter; 6-oz cloth is roughly 200 gm². Each 100 gm² of cloth equals 0.1mm, so five layers of 200-gm² cloth equals 1 mm thickness. For the alignment clips, we needed about 4.5 mm of thickness (about 3/16″). Seven layers of 668-gm² cloth equals 4,676 gm², and that’s what we got for laminate thickness—just over 4.5 mm (0.18″). The amount of epoxy to saturate the cloth and the amount that squeezes out are worthy of study, which we haven’t done, but generally we wet the cloth as one would for wet-lam vacuum-bagging, and about 30% of what we mix (by weight) ends up being squeezed out. It seems right if epoxy starts to really pour out when the clamp pressure goes on. Sometimes there will be surface pinholes, but I have seen no evidence of air in the laminate, unlike with wet-lam vacuum-bagging. As a separating agent, we employ adhesive-backed PTFE tape (Standard Grade Tac-Tapes from Taconic), but when compression-molding these small parts, we choose a common mold-release wax instead.”
     

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  3. oldmulti
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    oldmulti Senior Member

    Gary, thanks for the response. I accept smaller wing masts can be quite light. You are a master at extremely light structures that are still robust enough to survive reasonable conditions and I accept that you can have lighter framing in a hull than a connecting beam with a mast standing on it but (there is always a but) crossbeams on cats and tris generally have to be strong enough to handle the righting moments of the boat and this means there is only a small amount of additional structural reinforcement required on the beam itself. Yes a dolphin striker which will take a lot of the load will add weight but with modern carbon materials not an excessive amount. The efficiency of the rig I think is the main issue. 2 masts fore and aft if well designed will perform very well as you have proven with your monohull and several Australian cats (including Shotover 2 and the Jarcat 7). But side by side rigs have varying reports. Thomas Jones Dandy 26 foot cat had a biplane rig which performed poorly and he converted to a single sloop rig. Jones basically said he though the cat was to much of a cruiser and couldn’t exceed wind speed that often so could not use the full capability of the biplane. The 40 foot biplane cat (Ozone) shown earlier in this thread needed course adjustments whilst reaching to optimise its speed etc. But my final conclusion is that racing craft like C class cats etc have played with biplanes and quickly reverted to single rigs for many reasons, some for efficiency others for controllability. Hence may assumption for “more efficiency in a single rig”. Further discussion is welcome.
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Agreed, modern C Class foiling cats (and A Class) are the performance standards and they are all una rigged or in AC cats case, also carry a small headsail but World's fastest sailing craft is una rigged, Vestas Sail Rocket. As were/are past record setters Yellow Pages and MacQuarrie Innovations. However there is also athwartships double rigged catamaran Techniques Avancees that still retains a class record of 44 or so knots and not to forget another Ketterman biplane una rigged tri foiler, also of very high performance that set records too. The latter two craft are 2 or 3 decades old now.
    One thing I learned 3 decades ago when playing around with across craft biplane rigged trimaran is that on a true beam reach you have to head up or down a degree or two to keep the leeward rig from being partially blanketed, not a problem in reality because at high speeds the wind is always ahead and I found it a rare occurrence, in lighter winds, to alter steering course.
     

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    Last edited: Oct 23, 2019
  5. Burger
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    Burger Junior Member

    I've seen two biplane cats here over the last year, a Kelsall (I think) and the Schionning that had the masts shortened. Thinking of the extra engineering/design/carbon/time costs versus the possible advantages leaves me unconvinced.
    I wonder why the John Hitch X-it rig: no main, twin headsails to the bows, with a small central staysail, all roller furled, hasn't been tried by more people after a simple powerful cruising rig?
     
  6. oldmulti
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    oldmulti Senior Member

    Michael Schacht of Proafile is a talented designer who came up with this proa concept in 2007 as a cheap fun boat. I do not know if the design was completed or if one was built but it looks a simple interesting project. Samwise is a minimalist cruising proa. The only thing not minimal will be the performance. The basic idea is reliable and cozy camp cruising. Sam’s godfathers include Matt Layden’s Paradox and Rob Denney’s Harry, It features simple plywood construction, an enclosed sailing cockpit and plenty of deck space. The crew helms from an enclosed sailing cabin. Unlike tacking craft, proa crew have no need to make the trek to the other side with every tack.

    The leeward hull is the load bearer, so it stretches out to 24’ of lean, wake-cutting form. The windward hull is the ballast, which makes do with 18’ of LWL. Skilled sailors will fly the windward hull at every opportunity. Being flat-bottomed in sharpie style, the hope is that it will plane if given half the chance.

    The free-standing schooner rig makes for a low center of effort, while the standing lugs create a powerful, balanced, easily handled sail that requires no purchase on the sheets. The forward lug is rigged with the mast to windward (its most aerodynamically advantageous position) while the aft lug is set opposite, which moves the combined CE well forward (a desirable thing on a proa).

    Leeway is prevented by a central, pivoting leeboard mounted on the leeward hull, and steerage is provided by fore and aft “dagger” rudders. The hope is that in the unfortunate event of a grounding, the deep leeboard will hit first, pivoting gently out of harms way as it stops the boat, whilst leaving the delicate rudder intact. As usual with proa designs, the rudders are sketchy at best. The proa’s beam is retracted to trailer width via telescoping aluminum beams.

    The goal was to make the hull sides from straight ripped 6 mm ply - 2’ wide. The straight ply goal meant the crossbeams buried in the hulls had to go, so now they’re lashed or bolted to the deck in beach cat style. The cabin has comfortable sitting/laying about headroom, 2’ beam on the bottom.

    LOA: 24’ Length lee hull: 24’ Length windward hull: 20’

    BOA: 12’ Beam lee hull: 1’-8” Beam windward hull: 2’-8” Interior headroom: 3’-6”

    SA: 220 sq. ft. Masts 17’ Rig Height: 22’-8”
     

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  7. oldmulti
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    oldmulti Senior Member

    Burger. John Hitches approach suits his boats needs. It evolved over time to "maximise" the performance of a cat to windward and reaching. A similar approach is used by Jarcat tacking forestay that runs on a track allowing the forestay to move from side to side of the foredeck. Yes it does increased performance in certain wind angles and strengths. In larger boats "tacking forestays" become much harder due to structural issues. John choose 2 forestays with roller furling gear etc. Good if you can afford it. The real issue here is the cost and the downside of dragging around additional weight and aerodynamic drag of the extra gear required for the arrangement. The weight and drag increases pitching, raises the centre of gravity of the rig and in strong winds create very unnecessary drag. Everything is a compromise.
     
  8. Gary Baigent
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    Gary Baigent Senior Member

    Simple answer is get rid of the archaic headsails, problem solved. Of course you would have to compensate with slightly larger mains. But mains are closer winded and also don't distort ... so again no problem?
     
  9. cavalier mk2
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    cavalier mk2 Senior Member

    Now Gary, simple is skip a mast, run the twin headstays and use jacklines on the jibs.

    For cruising I doubt the weight and windage of 2 furlers would have much impact, cost wise it is a cutter?

    Now up and down sizing 2 jibs with jacklines on the head and clew would keep you spry.

    I ran the Yankee this way on my mono and laced the pulpit to act as a catcher's mitt and it worked well. It was for light weather only anyway.
     
  10. oldmulti
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    oldmulti Senior Member

    The following information is going to be a little hard to follow but it is an aggregation of several articles. It is about a “semi wing” dual sail system developed for high performance boats. In this case a mini 6.5. But they are trying it on the new mono AC boats. The first article is about the working class wing in professional boat builder magazine 140 page 63. Professional BoatBuilder - 140 - Dec-Jan 2013 https://pbbackissues.advanced-pub.com/?issueID=140&pageID=63 The second input is from the companies web site. http://gre69.wixsite.com/advancedwingsystems

    The final input is from another article about the minitransat boat shown in the jpegs. The mast section is suitable for a Mini Transat extruded-alloy mast tapered above the hounds. Sail tracks are set at an angle to enable taper and minimize weight. Mast is extruded with the sail track intergral. The upper tapered section of the mast is joined with an angled section. It’s simple and cost effective, with no welding required, as the tracks are bonded and riveted into position.

    Untapered Section size
    · Fore and aft: 5.7″ (144mm)
    · Side to side: 7.1″ (180mm)
    · Wall thickness: 0.07″ (1.8mm)

    Linear Taper above hounds over 3.3′ (1m) to section size
    · Fore and aft: 2.8″ (70mm)
    · Side to side: 3.9″ (98mm)
    · Wall thickness: 0.07″ (1.8mm)
    · Estimated weight: 1.6 lb/ft (2.39 kg/m)

    The hound fittings jpeg shows what the hound fitting looks like.

    Hound fitting Details
    · Stainless steel 316 TIG-welded
    · Tang thickness: 0.16″ (4mm)
    · Curved base plate thickness: 0.08″ (2mm)
    · Sheave webs thickness: 0.08″ (2mm)
    · Spreader support tang thickness: 0.08″ x 0.08″ (2mm x 2mm)
    · Jib sheave size: 60×10
    · Fasteners: 1/4″ and 3/16″ Monel rivets
    · Alloy doubler backing-plate thickness: 0.07″ (1.8mm)
    · Tangs drilled for toggle pins

    There is a detail jpeg of the spreader hinge and detail below

    · Spreader Hinge Detail
    · Can pivot about the horizontal axis and is fixed about the vertical axix.
    upload_2019-10-24_17-16-1.png
    · Spreader attachment is with two pins to the circular stainless steel band. This sits on a plastic bearing surface with flanges at top and bottom to capture the stainless band. The band runs inside the sail tracks via reinforced slots in the mast. The spreaders are then supported by an upper wire that runs up to the hounds fitting. This arrangement allows the spreader to move with rig loading while keeping the band in a constant orientation to the mast.

    And finally a jpeg of the mast base. The rigging details are Forestay: 1×19 1/4″ diameter, Lowers: 1×19 1/8″ diameter and Upper spreaders support: 1/8″ diameter (or larger if a forward baby stay is used
     

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  11. oldmulti
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    oldmulti Senior Member

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  12. Gary Baigent
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    Gary Baigent Senior Member

    No mast Cav? You mean, headstays that just levitate and stay there, no mast support, held aloft maybe by cooperative black backed gulls?
     
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  13. oldmulti
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    oldmulti Senior Member

    Kurt Hughes wrote the following document in response to many enquirers about affordable multihulls to quote Kurt “Or…. How to Save the Money that You Need to Build a Boat During a Time of Recession; Strategies and Substitutions to save Money on your Boat Construction.” The web site PostApocalyptic Boat Building | Multihull Design Blog http://multihullblog.com/postapocalyptic-boat-building/
    document contains many interesting tips and idea’s on how to build a cheaper boat that still is capable of high performance. He details the characteristics of many materials and how to best combine them for cheaper boat that will still satisfy your requirements.

    On 16 September 2019 on Kurts multihullblog showed a new 31 foot trimaran design which combines some of the features of what he was referring to in his postapocalyptic page. The boat is of plywood flat panel construction main with core as needed. Amas in CM developed plywood. Amas and tubes slide in for mooring in a marina. The jpegs give an idea of the boat.
     

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  14. cavalier mk2
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    cavalier mk2 Senior Member

    Ha Gary, single mast the headstays to each ama/ cat hull. That set up makes more sense for a tri though, you might convince me on a mast in each hull for a cat to lighten the beams needed.
    Gulls are too hard to train and mess up the deck.
     

  15. oldmulti
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    oldmulti Senior Member

    The Banuls MC2 is a 60 x 28.2 foot performance catamaran. MC2 weighs 21,000 lbs and displaces 26,000 lbs. It carries and 82 foot mast that is 110 square feet and a sail area of main 1270 sq ft, jib 600 sq ft, 1200 sq ft code 0 for a total of up to 2470 square foot. The hulls are 15 : 1 length to beam and the underwing clearance is a minimum of 3.6 foot. Translation of these numbers, the MC2 is a very fast cruiser racer. The boats build is suitably impressive. Hull and deck are E-glass / Epoxy foam (Corecell) sandwich construction. Hull and deck parts moulded from female tooling to reduce fairing and save weight. Vacuum bagged and infused to control resin content, fiber ratio and improve strength/weight ratio all post cured in an oven. The mainbeam, aft beam, daggerboards, main cabin roof, rudders, chainplates and selected reinforcements are all carbon fibre construction. If you can afford it the entire boat can be built in carbon fibre. There is no timber or plywood in this boat. Only high density Corecell cores where required. The reason they use Corecell is it tends to be more stable at higher temperatures than linear PVC foams, with no out-gassing properties, which suits boats that will be cruising in hot climates. The reason for using carbon everywhere bar decks and hull is “While Carbon is probably the best available fiber to achieve structural stiffness while reducing weight, it does not forgive localized impacts as well as E-glass. A dropped winch handle, the outboard engine from the dinghy, or the rough edge of a pontoon…. No matter how careful and a skilled sailor you are, your hull and deck will suffer from some form of impact. Therefore a minimum thickness of carbon would be necessary on the hulls and deck. The kg/m² of carbon used would be similar to that of E-glass - offering less weight savings than the structural properties of carbon fiber would suggest. Carbon also has an issue with noise propagation. Because of its stiffness thin carbon laminates resonate more than E-glass. When a gennaker sheet under tons of load is eased on the winch drum, or when the hulls get hit by wave the level of noise and vibrations might be acceptable on a stripped out racing multihull but not on a luxury catamaran.” An impressive boat.
     

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