Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    Bluebell. I have not seen a floating Streaker 23, I have only read about them (Xeno in Holland and Pterodactyl in Canada). The owners appear to be happy with them. Streakers were designed at a stage where Tennant was using light cross beams and hull build techniques which indicates they should be fast for there size. The boats were built in the early 90's and still looked in good condition. Xeno was built as a folding version (as the original design) which can be 8.2 foot wide for trailing and 13.1 foot wide for sailing. The hull shape looks similar to the 28 foot GBE which was a very good design.
     
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  2. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    After showing many catamaran crossbeam options I will finish this part of the thread with the relatively simple approach to bridgedeck catamaran crossbeams. The foam glass beam. Cat foam glass beams are still a lot of work but are easier and a lot more integrated (if well designed) than building a timber crossbeam structure. The 1984 design PDF gives you the idea. Materials and design has moved on but this will give a good idea of how its done. Again I reintegrate that the top and bottom unidirectional glass flanges should be done in epoxy over about 24 hours to prevent excessive heat as build up as layers cure. Also pretension the unidirectional glass in the flange slots prior to glassing. Do not under build foam glass beams, follow the designers specifications, as they depend apon an integrated structure to distribute the loads over the beams and hulls. Also use good quality foam like divinycell, airex etc. The latest foam glass beams are likely to use uni directional carbon fibre in the flanges and on the foam glass faces. Do not under any situation mix carbon fibre and unidirectional glass in a beam flange. Use either one or the other material. Carbon fibre and uni directional glass have different elongation characteristics (stretch) which means in a flange all the loads would go onto the carbon fibre first before the glass would be loaded.
     

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  3. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    When you build a multihull it is generally a series of parts which are then assembled into a boat. Each part is designed to meet its given function with materials that suit that function so we end up with a variety of materials of varying weights and thicknesses to put strength were it is needed. In the attached document is a sample of a Kurt Hughes 55 foot bridgedeck cat that highlights the differences. The cabin roof and under wing have 50 mm airex (or equivalent) core with Triaxal glass rovings either side. The hull has 25 mm airex (or equivalent) core with Triaxal glass rovings either side. The furniture can be as light as 12 mm foam and 350 gsm glass either side. The foam weights in furniture can be 60 kg/cubic meter to 100 kg/cubic meter in heavily loaded structural areas such as keels. The joining tape weights also vary according to structural requirements. I suspect the tapes indicated on the plan show the material used but not the detail of the number of layers used etc. Again the taping matches the structural requirements to pass the the loads from one part to another. Joining tapes can be made strong but have one problem they are difficult to fair into the surrounding surfaces to be smooth and visually look good. This is part of the reason production boats use molds as big as possible to minimize the number of joins in a boat which increases strength and reduces weight. Please do not underestimate the amount of joints and taping required in a boat. It literally can add 5% extra weight and time to build the boat.
     

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    Last edited: Jun 14, 2019 at 12:45 AM
  4. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Another really good post , thanks Oldmulti.
     
  5. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    The Kurt Hughes 23 daysailer trimaran is an example of a modern use of a combination of materials to suit a purpose. This boat is mainly timber and ply hull structures, but it uses glass and balsa and some flat decks with ply, balsa and glass. It will be a long lasting boat if built to plan. BUT it is a complex boat to build requiring many skills to finish the boat. It would be an excellent training build if you were planning a larger build later. As an example the float deck is combination of several layers of unidirectional glass, balsa core and 3 mm ply. Balsa needs to be effectively sealed to prevent any rot, multiple layers of unidirectional glass needs to be laid smoothly to prevent ripples and get a uniform strength over a surface. All this could be simplified to a single 6 mm ply deck covered with 200 gsm glass. The weight would be similar but the work to build it would be reduced. Don't get me wrong Hughes has designed an excellent boat that reportedly sails very well as designed. All I am trying to say sometimes simplification can achieve a very similar outcome with less work. There was a guy who built a full carbon fibre 16 foot long tri for fun. The boat took a long time to build and cost more than a new Hobie 21 that used to be able to sail past it. Keep it simple in boat building has its virtue.
     

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  6. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    A variation. Small cruising catamarans are interesting and vary enormously from the "what is that" to Atlantic crossing boats. In the "what is that" category is a 3 masted junk rig 24 foot cat to the 18 foot wisecrack cat from Canada. But the most interesting is Micromegas 5. A 16.3 x 12.8 foot tube cruising cat of 2240 lbs displacement with 200 sq foot of sail that crossed the Atlantic. The hulls have a length to beam of 7:1 are flat bottomed and basically a box shape. 1.2 meter high and 710 mm wide. The hulls are 9 mm ply with just gunnels and chines no stringers and minimal BH's. The cross beams are 50 x 250 mm timber with 6 layers of 300 gsm unidirectional glass top and bottom of the mid and aft beams. The mid and aft beams have dolphin strikers of 4 ropes of deema 6 mm (very low stretch) to help support the fore and aft cockpit which also supports the rig. The brothers who sailed the boat are EXCELLENT seamen and can build a good boat. The cat, when loaded, is heavy due to need for stores, gear etc. It may not be fast but it can cross oceans if you have the skill.
     

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    Last edited: Jun 16, 2019 at 12:17 AM
  7. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Junior Member

    Rudders come in many formats from transom hung to full under the hull spade of hung off a skeg. But all rudders are vital to the control of a boat. Rudders have to be built just strong enough to handle the loads without being barn doors that are heavy and inefficient. In smaller boats transom hung rudders that kick up work well but when boats get above 30 feet and sail offshore transom hung rudders have an annoying characteristic. They get hit with wave action on reaches and downwind. Unless the steering mechanism dampens the shock load there is continuous vibrations through the EG tiller as you sail. OK for few hours, bad for a day. So lets look initially at ways you can attach a rudder to a boat and still have it kick up if it hits something underwater. The Reynolds has the classic kickup rudder system, good for a small boat. Wildflower is a trailable cat that cruises Canada's west coast and has a central rudder on its wingdeck that kicks up. This is a good solution IF the rudder foil section is well designed and the blade is strong. If the rudder foil is badly designed the rudder ventilates and you lose control. Another option is to have a semi under slung rudder that kicks up as in the Streaker 23. This option is light, simple gives the advantages of spade rudder hung partially under the hull. Spade rudders are the most "efficient" type of rudders for steering but they are not bullet proof. I know one designer who designed his under hull spade rudders to break off if the rudders are turned hard over at 25 plus knots of boat speed. Why? Because if the rudders hit ground at lower speed he wanted them to bend or break away in preference to splitting the hull apart potentially sinking the boat. Rudders like everything else are a compromise. Also there is a shot of how one person "shapes" his rudder blade during construction. PS please minimize the use of plywood in rudder blades as 50% of the wood is not working in the right direction for you. Find some western red cedar, shape the blade and cover it with several layers of glass cloth and epoxy and you will get a relatively light and effective rudder.
     

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  8. oldmulti
    Joined: May 2019
    Posts: 48
    Likes: 36, Points: 18
    Location: australia

    oldmulti Junior Member

    The structure of rudders and associated steering equipment is a problem. Spade rudders are often built with a stainless steel or aluminum tube with the blade at one end and a steering head attached at the other. The tube should be reasonably light and small in dimension to fit a slim blade of between 10 and 15% thickness compared to its width. But a larger diameter thinner wall tube is best for stiffness, strength versus weight. Next the rudder blade has to be attached to the tube. The tube either has to be flattened or have small square plates welded to the tube to prevent the blade rotating on the tube. Glueing a blade to a tube only works in light airs. The rudder blade can be built of many materials from WRC glass covered to high strength foams fiberglassed over with strengthening spars etc in them. Whatever the rudder blades are built from make sure you can repair or replace them easily as I can assure you, you will break a rudder or 2 in your sailing life (personal score 7). Also if you can have a sacrificial foam glass tip on the bottom 25% of your rudder it may save a bigger repair job if you hit bottom. Modern design says you can build an integrated carbon fibre rudder and shaft for lightness and strength. I agree but I don't drive a Ferrari. Carbon fibre shatters, it does not bend, resulting in any knocks can break the entire structure needing a full replacement. Attaching a steering system to the head of the rudder shaft ranges from metal clamps to bolts through the rudder shaft to the tiller catchment. Monohulls often use solid shafts with keyways or machined square tops to attach tillers to. It costs money, is harder to repair and a solid shaft is a heavy item, a tube is lot lighter. The easiest way for me is a tube with a welded plate covered with WRC and glass at one end and a tiller attached by bolts through the rudder tube at the other end. Transom hung rudders is another topic but make sure the rudder box is strong with the hold down mechanism that really works and is able to be used, repaired or replaced whilst at sea. I have seen a big cat that required someone to go for a swim or sit in a dingy each time its rudder kicked up. Not viable in big waves.
     
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