Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    We will get back to the 50 foot cat details in the next item but in the meantime we will discuss the Iroquois catamaran which was designed and first built in 1965 with over 70 of the mark 1 being built. It was a good cat from Rod Macalpine Downie with one winning the Round Great Britain race in 1968. The 30 x 13 foot cat displaced between 4500 lbs to 6800 lbs depending on what document you read and carried 495 square foot of sail. They have swing centreboards in each hull and were quite a fast boat in their day. But their speed was relative to the boats of the day, these cats had a relatively big fractional rig and a narrow beam. They needed to be treated carefully even in cruising mode let alone racing. The Iroquois had several versions growing in length, beam, displacement, sail area, masthead rig (Mark 2 onwards) and becoming more sophisticated in construction. The final run of all the versions topped 300 boats. A lot of them are still sailing today which says a lot about their structural integrity. Some have crossed oceans but they depended on good conservative sailors.

    The Mark 1 hulls were solid glass starting from the outside 300 gram csm, 3 layers 450 gram csm, 800 gsm woven roving overlapped for 1.25 foot at the keel line. The underwing has the same layup as the hull but is stiffened by underwing stringers. All this was done in a female mould. The cabin roof was done in a second female mould and was 300 gsm CSM, 450 gsm CSM, 12 mm balsa, 450 gram CSM. All in polyester resin. The hulls, underwing and cabin/decks were all rounded to help stiffen the structure. The mast support structure were 50 x 50 mm square metal tubes with 4 mm thick walls. They tubes were built into a triangular shape 6 foot long, 300 mm high in the centre tapering to 50 mm high at each inner gunnel. There was a centre 50 x 50 mm tube with 2.7 mm wall acting as the mast support base. This is under the table in the interior jpeg. This was a very advanced boat for its time in both basic design and structure and a tribute to Rod Macalpine Downie. The attached jpegs give the lines of the boat and a few sailing shots.
     

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  2. oldmulti
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    oldmulti Senior Member

    Sorry for deferring the 50 foot plan spec again but I had another request about a Buccaneer 24 detail of cross beam structures. Attached are the build instructions for the Buccaneer 24 and the PDF details the crossbeam structure.
     

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  3. oldmulti
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    oldmulti Senior Member

    Further on the 50 foot catamaran. A cat 50 x 27 foot weight 20,000 lbs displacing 30,000 lbs carrying 1500 square foot of sail. We will discuss crossbeams and decks.

    The main mast crossbeam is a foam glass I beam. The web bulkhead is 1000 mm high and has 850 gsm quadax (o 90 45 45), 751 gsm 45/45 or triax, 25 mm pvc foam minimum 4.7 lbs/cubic foot, 751 gsm 45/45 or triax, 850 gsm quadax (o 90 45 45). The top flange is 24 layers 1000 gsm unidirectional 100 mm wide, laid on the underside of the deck, either side of the mast web bulkhead. Each layer of the top flange is 200 mm shorter than the previous layer. The web bulkhead is attached to the flange(s) by 4 layers of 600 gsm 45/45. The lower flange(s) is 18 layers of 1000 gsm unidirectional 100 mm wide either side if the web bulkhead on the wingdeck. It is also attached with 4 layers of 600 gsm 45/45. All the structure is in e glass and at least vinylester resin with epoxy preferred.

    The rear beam web bulkhead is 750 mm high and has 850 gsm quadax (o 90 45 45), 751 gsm 45/45 or triax, 25 mm pvc foam minimum 4.7 lbs/cubic foot, 751 gsm 45/45 or triax, 850 gsm quadax (o 90 45 45). The top flange is 9 layers of 825 gsm unidirectional 100 mm wide. The bottom flange is 6 layers of 825 gsm unidirectional 100 mm wide. Again, attaching the flange to the deck or wingdeck is 4 layers of 600 gsm 45/45.

    Cabin sides are 751 gsm 45/45 or triax, 20 mm pvc foam minimum 4.7 lbs/cubic foot, 751 gsm 45/45 or triax. The decks are 1125 gsm 45/45 or triax 25 mm pvc foam minimum 4.7 lbs/cubic foot, 751 gsm 45/45 or triax. As with all the information I am passing to you this is the macro of the structure. There are many inserts, attachment points and additional cloth layups around things like windows, turning block points etc. Detail is the reason you purchase plans from designers. They generally have learnt by bitter experience of the area’s that need additional reinforcement of structure. The jpegs are just a sample of the type of cat. Next thing will be some internal details.
     

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  4. Skullduggerydave
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    Skullduggerydave Junior Member

    Thank you
     
  5. oldmulti
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    oldmulti Senior Member

    The Buccaneer 24 was a very popular 1970’s design from Lock Crowther. The trimaran is 24 x 19 foot displacing 2000 lbs (that is a very flexible number dependent entirely on the builder) and on a 30 foot mast carried a masthead rig of 320 square foot of sail for cruising or 402 square foot racing. The tri came with a daggerboard but a later version had a centreboard option. Now the fun part. There have been many owner modified versions of this boat to “improve” performance. The amount of sail area put on some Buccaneer 24 tris has been amazing. In NZ there is one with a wing mast, full headed main and huge spinnakers that had solid ply timber cross beams conversion that even today can beat much larger multihulls boat for boat. The Buccaneer 24 even in standard format is a very fast boat for its size and plans for the boat were still being sold in 2010. There are several web sites over 100 pages long devoted to the design.

    The attached PDF’s are part of the plans focusing on the hull lines and frames. There will be another post with the final plan information I have. The previous post had the cross beam plan and building instructions. There are a few jpegs of the standard build and the hot NZ Buccaneer 24.
     

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  6. Tony.Ellen
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    Tony.Ellen New Member

    oldmulit great work on this thread.
    I've attached plans in their entirety I purchased and downloaded from the WWW some time ago now there are a few more to follow in next post, I will not comment re the Trimaran you are much better versed at that than most but I felt the full plan set should be available in one place instead of stringing readers along. There was a mast and ply beem plans done also I purchased at the same time. If we were all to share our historic plan portfolios as you have done the home builder world would be a better place I am sure. Cheers Tony
     

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  7. Tony.Ellen
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    Tony.Ellen New Member

     

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  8. Skullduggerydave
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    Skullduggerydave Junior Member

    Designer unknown at the Moment but did pop up on the small trimaran website claiming to be designer and builder a few years and owners ago which all adds up . I’ve started a thread on this site Kangaroo Island trimarans design pop over there and have a look . She should be in the water by Christmas
     

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  9. oldmulti
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    oldmulti Senior Member

    Tony, thank you. The intention of this thread was to share design, structural and material information about multihulls. All contributions welcome.
     
  10. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    Welcome to the forum. The only thing is please be very careful before posting any plans on the forum here that you have permission to post them and that doing so doesn't violate anyone's copyright or rights on them.
     
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  11. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The final in the 50 x 27 foot cat series. One item that is often overlooked by most home builders is the importance of internal furniture etc to the overall design. Designers have 2 approaches to designing a multihull. Build a strong shell with appropriate bulkheads and strong crossbeam points. Then leave the builder to put in the furniture where they want. The second method used by EG Grainger, Schoining, Shuttleworth etc is design the furniture as an integral component of the structure providing additional support to the hull and/or wing structure.

    The cat we are talking about requires internal structural support. The cabin soles and bunk tops which form lateral stiffeners are 751 gsm e glass, 20 mm PVC foam minimum of 4.7 lbs/cubic foot, 751 gsm e glass. Furniture and interior shelves, cupboards etc are a minimum of 600 gsm e glass biax, 12 mm PVC foam minimum 4.7 lbs/cubic foot, 600 gsm e glass biax. All preferably in vinylester resin. Solid inserts for EG hatches in soles o eg attachment points for WC’s are generally in plywood. The attachment to hull sides or wingdecks of furniture or interior etc should have a fillet of bog under the edge before being built in. The glass layup to connect the furniture etc should be a minimum of the thickest skin on the eg furniture. If it’s a 600 gsm skin the tape should be 600 gsm. The tape should be on either face being at least 4 times the width of the core. IE a 12 mm core means at least 48 mm width of glass on either face being attached to, if there is a well radiused fillet.

    Construct furniture as you would the rest of the boat, well. Alternative materials like plywood can be substituted but understand the function of the furniture and build it to the same relative strength. Shuttleworth material lists earlier in this thread give an idea of some plywood substitutions. Page 7 and 8 this thread.
     

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    Last edited: Nov 29, 2019
  12. oldmulti
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    oldmulti Senior Member

    Tony your contribution is OK. Brett Crowther told Oldsailor he could "sell" the Buccaneer 24 plans, then Ezifold took over the distribution job after Oldsailor retired from the task. Ezifold then disappeared and as a final act they placed the full plans on the web for a few years. That Dropbox site and Ezifold site has now gone as far as I know. I fully support the moderators comments. The reason you get some of the thread information from an unnamed source is because I am protecting the designer rights and income.
     
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  13. Tony.Ellen
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    Tony.Ellen New Member

    Oldmulti
    Again great posts thoroughly enjoyed hence posted what I had thinking it was helping. To the best of my knowledge from correspondence Ezifold sold up and are enjoying sailing, you can't fault people for doing that. When I purchased the mast and beam plans the new owners informed me they have exited the small boat market and discontinued pro bono plan restoration work. I am looking forward to learning more from you as you expand this delightfull thread. Thanks again Tony
     
  14. oldmulti
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    oldmulti Senior Member

    The Buccaneer 24 was a very popular 1970’s design from Lock Crowther. The trimaran is 24 x 19 foot displacing 2000 lbs (that is a very flexible number dependant entirely on the builder) and on a 30 foot mast carried a masthead rig of 320 square foot of sail for cruising or 402 square foot racing. So we have a fast little tri that satisfies most people. I have sailed on 3 Buc 24’s in bay conditions and can assure you they are good allrounders that especially perform well in light airs up wind and can top 16 knots on a reach. But like everything in life people want to beat the next guy, so we will discuss 3 variants of the standard Buc 24. First is Miranda, a Buc 24 that had a 3 foot stern extension to smooth the water flow off the main hull. Also dagger boards were put in the floats. This improved the boats speed in most conditions. There were a few tweaks to the rig which also helped. I am sure the yellow paint job improved the speed as well.

    Next boat is Capricorn a NZ modified Buccaneer 24 that rates at 1.0001, this is 17% faster than a Corsair 24 which rates at 0.8478. Give an idea of the performance potential of a modified Bucc 24? So, what was modified on Capricorn. First it was built with good 4 mm ply, not 4.8 mm ply. The original aluminium cross beams were replaced with wooden cross beams. They are 200 x 200 mm 4 mm plywood boxes with a 25 x 40 mm timber in each corner. The top flange has an additional 4 mm ply layer on it. The beams are all covered with a 330 gsm cloth in epoxy. The mast was next being 35 foot high aluminium section of 160 x 95 mm. This was to be replaced by a 305 x 120 mm ply wing mast which had a 4 mm ply web and 4 mm ply bulkheads covered with 3 mm ply. Softwood nose and tail pieces were inserted. All was covered with a light glass cloth. 6 layers of carbon fibre was wrapped around the mast at the hounds and lowers. Sail area was over 500 square foot and the boat weighed 1000 lbs. Capricorn was painted green to intimidate its opposition and later got a name of “Deathtrap” because of the way it was sailed. No need for stern extensions as the main hull was rarely in the water.

    The next boat started as a Buccaneer 24 but as you will see this is the ultimate modification. I will let the owner Ric Stevens describe what he did. “Unfortunately I become addicted to multihulls so I bought a Bucc 24 main hull, rig and cross arms, no floats, sight unseen. I should have looked. The centerboard box and bottom were pretty well shot so cut it off at the waterline and built a new one with some left over cedar from a previous job. Took out the chine’s, plumbed the bow a bit and flattened the bottom abaft. The Bucc 24’s and 33’s I have sailed on have had weather helm so I moved the mast back a bit, the dagger board exits in front.

    I built the first float out of cedar strip, but decided it was too heavy so used it as a plug for foam, epoxy, 10 oz glass with Uni S Glass and carbon reinforcing underneath the cross arm mounts. They came out at 105 lbs ea. They are higher displacement than the originals, about 1’ shorter than the main and mount about 1’ forward.

    The cross arms were heavy aluminum tubes that had corroded badly so I traded some epoxy with a friend in the rigging business for some broken mast sections, got rid of about 200 lbs of stainless bar dolphin strikers and replaced it with about 25 lbs. of 6mm wire and turnbuckles. All the chain plates are shackles or D rings taped in with Uni S glass or Kevlar.

    The original rig was the 30’ cruising version, sail number 12. Luckily for me a friend got his C Cat wrecked but the nice light 39’ 3 point rig survived. Kept the running back stays from the original rig. Bought a used suit of sails off a Dragonfly, the main was about 5’ short but a definite improvement.

    By this time the main hull and bulkheads were getting scary as cracks and plain old rot was catching up to the original cheap plywood. Anyway cut the top off at the waterline and kept the 2 main bulkheads for mould stations, bent some battens around until the shape looked right, then stapled 9mm Corecell to the battens. Glassed with 12oz DB and epoxy, pulled the mould and installed foam bulkheads under the cross arms. Made the deck, cuddy, cockpit and floors out of 18mm Corecell and the same 12 oz DB. No head (keeps the wife off), interior or other amenities, strictly a day sailor, although I have slept in the nets. Total weight of the boat with rig is 850 lbs. As a concession to the original Buccaneer 24 I have kept a piece of plywood about 8” x 12” embedded in the transom and the running backstays. After all this I decided to blow the bundle on a decent (Dacron) main, jib and screecher as well as a carbon board.

    This made a noticeable improvement in performance. The boat sails very well in light winds because of low weight and large sail area, also goes like stink in a breeze flying the main hull. Limiting factor is the rudder coming out. Next project may be rudders in the floats. Have a hard time beating a F25 C and an F 25 R in moderate wind, think I need to break down on a really good set of sails.” The boats name is Toy and is the white tri.
     

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  15. oldmulti
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    oldmulti Senior Member

    The Cabsib 32 is a cruising trimaran sloop with overnight accommodations for two people. Cabsib 32 was designed in 1979 SIB associates and is 32 x 21.5 foot weighing 3200 lbs displacing 4200 lbs. It carries a 38 foot mast with a 290 square foot main and a 190 square foot genoa for a total sail area of 480 square foot. The main hull is 5.5 foot wide at the gunnel and 3.75 foot at the waterline of 29.8 foot giving a 7.95:1 waterline to length ratio. The design features a kick-up rudder and low aspect ratio fixed keel or a centre board option that allows shallow water moorage or beaching. The removable beams allow seasonal transport of separate components for storage, or dry storage on a trailer.

    John Marples was involved in SIB associates but his current Constant Camber 32 is probably a development of this boat but has a published narrower main hull and probably better build and crossbeam structures. John also has a constant camber “fast cruiser” 32 version which has more headroom and wing accommodation on the main hull.

    The structure is constant camber panels of 3 layers of 3 mm WRC or ply with 9 m plywood bulkheads and 9 mm ply decks. The wood box beams are 20 mm deep and 210 mm wide. Where crossbeam go through the main hull there are doublers that build the hull thickness up to 27 mm. This would be a relatively fast boat to build but it has one down side, accommodation. The Cabsib 32 headroom in the main hull is 4.5 foot. The compact cabin interior has a small galley, dinette and double berth. If you want a cheap simple cruiser a modern version of this would be good.
     

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