Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Lets have a look at hull panel options for mid range multihulls. This is general and assumes the boats are cruiser racers of 30 to 40 foot of about 4500 to about 15000 lbs displacement. We will do a foam glass, Durakore and Strip plank cedar version of hull weight. There is a light weight or smaller boat layup (1) and a heavier weight larger boat layup version (2). There is also what I will call the cruiser layup for a boat that can carry heavier loads or take knocks (3). The foam is PVC Corecell, Divinycell, Airex etc. The glass is e glass in all cases. The resin with WRC or Duracore is epoxy. Foam is polyester or vinylester or epoxy.

    Foam glass. (1) from outside 440 gsm biax 12 mm foam 330 gsm (2) 750 gsm biax 20 mm foam 440 gsm biax

    (3) 1150 gsm triax 20 mm foam 750 gsm biax (this layup could be used on a light 50 foot boat).

    Duracore (1) 330 gsm DK 12.7 mm 270 gsm (2) 440 gsm biax DK 15 330 gsm

    Strip plank cedar (1) 330 gsm 12 mm WRC 270 gsm (2) 440 gsm biax 15 mm WRC 440 gsm biax

    (3) 750 gsm biax 18 mm WRC 600 gsm biax (this layup could be used on a light 50 foot boat).

    The lightest laminate is foam glass, the heaviest is Strip plank cedar. This is assuming that you have a relatively fair hull that did not require excess filler. As you can see the hull weight of a foam glass hull can be half of a strip plank cedar hull. In the real world it does not always work out that way due to the resin taken up by the foam and the weight of timber or glass inserts in foam glass hulls for local reinforcing etc that add weight. Also a hull is only part of the total weight of a boat. EG the mast, winches, ropes, internal gear etc will be the same weight in either hull medium.

    Structurally, the strongest is Duracore for each category but it is expensive. (Attached PDF gives some detail) Foam glass is hard to beat on a strength weight ratio if the right fabrics resins etc are used.

    Strip plank cedar can be made thinner EG down to 10 mm in a 40 foot boat and still have global strength but it will have less knock resistance. Also all hulls are assumed to be round bilge. Flat panel construction can require heavier panels or more internal stiffening according to the design.
     

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  2. oldmulti
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    oldmulti Senior Member

    Gary Lidgard is an Australian monohull and multihull designer of production and home built boats. His cats range from 39 foot to 73 foot with round bilge and one chine 50 foot panel cat. They are mainly foam glass boats. He does detailed plans which cover all aspects of the build process. Attached are a few sample sheets from his web site. Multihull catamarans by Lidgard Yacht Design Australia http://www.lidgarddesign.com/Multi_sail.html#top His cats I would describe as all rounders with good performance. His 50 x 25 foot flat panel chine cat displaces 18000 lbs at DWL and carries 1080 square foot of sail with 13.5 : 1 hulls. This boat will be capable of going fast. The study prints give details a rudder, centre case details, forebeam and rear beam details from different cats. Also is the cross section of the 50 foot chine cat and a photo of the finished boat. The 50 foot chine cat could have the panels made by an outsourced company specializing in EG Duflex (foam and glass on either side) for construction at your site or by a a specialist builder. Gary designed the “Fusion” cat, a 40 foot kit boat (similar to his 39 foot cat) that when delivered to your home address takes a professional “kit builder” 2 weeks to get from parts to a complete shell.
     

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    Last edited: Oct 14, 2019
  3. catsketcher
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    catsketcher Senior Member

    Thanks again for the posts. Just a clarification query. With the cedar laminate, you wrote Biax. I have always called 0 90 fabric biaxial. I was told to use double bias +45 -45 on cedar laminates. Did you really mean biax on the cedar laminate and if so, why reinforce the cedar along the grain? I have always used double bias on cedar and biax on foam. Cheers Phil
     
  4. oldmulti
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    oldmulti Senior Member

    Phil. Sorry for the confusion, I get caught out by my use of short cuts in language again. When I refer to biax in the WRC situation on the outside its 45/45. Depending on the designer inside can be 0/90 but I prefer 45/45 as it provides not only cross strength but some longitudinal strength. As Shawn Arbor said to me, every repair of WRC due to damage had the inside glass skin broken. Where as foam glass often only the outside skin was punctured.
     
  5. Qmaran
    Joined: Oct 2019
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    Qmaran Junior Member

    Hi, I started a new thread in the materials section about outrigger construction. Member Bajansailor suggested I would post here at the multihull forum as well to get the right people to see it.

    My construction idea is as follows:

    Sculpt the hull out of a very light foam, then cover with fiberglass cloth, then with a foam core sheet material and then fiber cloth again.

    If the foam sheet is perforated, will the inner layer be properly reached with vacuum infusion of resin? I explain more in this thread:

    Outrigger construction https://www.boatdesign.net/threads/outrigger-construction.62983/
     
  6. oldmulti
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    oldmulti Senior Member

    Qmaran. You can build a boat any way you want but there are really painful ways versus fast and effective approaches. Most professional designers try and find the compromise between what can be done at home versus what is done in a series production factory. The fastest way to build is to have a female mould but for a home builder it virtually means building 2 hull shells (the mould then the actual hull). Using a light foam insert mould is a reasonable thought but have you ever tried to sand EG cheap large polyurethane or styrofoam foams, it is dusty, annoying and slow. Then the surface of the foam has to be sealed before you can start to do the layup. If you intend to do vacuum infusion the light foam will need a glass layer over it, which will need to be faired, before you seal the surface to provide sufficient strength to handle the vacuum infusion process. All of this starts to get very painful.

    Alternatively if you are designing this boat yourself there are simpler ways of doing things. If the design has many areas of near flat surfaces design the boat to take advantage of panels that can be laid up on a flat table top or panels that can be purchased from commercial suppliers. Then only do the areas that need to be round in your light foam method. Also look at Rob Denny's Harryproa site and see how he builds cheap fast female moulds for vacuum infusion of his hulls. As you said you have a few years to think about it, please do some experiments, play with boat shapes and you will probably come up with a good solution.
     
  7. oldmulti
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    oldmulti Senior Member

    This one is about plywood as a hull material. Plywood is generally available everywhere at a reasonable price but the quality varies according to the plies intended purpose. For this discussion I will assume its at least exterior ply with marine grade glue lines. Marine ply is preferable for longevity. The problem with marine ply construction is the way designers deal with practicality versus the global structural requirements. EG a 15 foot boat can be built with 3 mm ply but humans stand on the ply. 6 mm ply handles human weight better than 3 mm ply. But 3 mm ply can handle the water forces and structural requirements. A very clear example of this is John Marples Sea Clipper 16 that has 9 mm ply decks because its easier to build than 6 mm ply with many stringers/ deck beams decks. So I will talk about what is required for global structural strength and a degree of practicality.

    Multihulls 10 – 18 foot 6 mm bottoms, 3 or 4 mm sides, 4 or 6 mm decks. Minimal stringers (19 x 19 mm if required), frames with epoxy fillets every 3 feet.

    18 – 24 foot 6 mm bottoms, 4 mm sides, 6 mm decks. Stringers 19 x 25 mm every 300 mm, frames 6 mm ply with 19 mm x 50 mm edging every 3 feet. 200 gsm glass on external surfaces.

    24 – 33 foot 6 mm bottoms, 6 mm sides, 6 mm decks. Stringers 19 x 32 mm every 300 mm, gunnels and chines 30 x 50 mm, 6 or 9 mm ply frames with 19 mm x 65 mm timber edging every 3 feet. 300 gsm glass over external surfaces.

    34 – 50 foot 9 mm bottoms, sides and decks. Stringers 19 or 25 mm x 50 mm every 300 mm, 9 or 12 mm frames with 19 x 75 mm edge framing every 3 feet. 300 gsm glass on external surfaces.

    After 50 foot the design of the structure of a ply multihull becomes very dependent on displacement and intended use. I have seen 60 foot boats with 12 mm ply skins and another with 18 mm ply skins with similar stringer and frame structures. The “strength”/stiffness of the 18mm ply hull is literally double that of the 12 mm ply hull. Both boats have had long lives.

    Finally, tortured ply cats or tri’s are in a different league. 10 to 18 foot cats 3 mm ply with 50 mm Styrofoam horizontal panels forward and foam acting as “deck beams” on thin hulls.

    18 – 28 feet 4 mm ply (good quality 3 ply) with 330 gsm glass on outside with foam glass ribs and or stringers on the inside with bunk tops etc acting as reinforcing.

    28 - 36 foot 6 mm ply with ribs, stringers, bunk top stiffeners with 330 gsm glass on the outside. All the tortured ply boats are LIGHT displacement racers, occasional cruisers. These boats are not meant to be full time cruisers.
     
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  8. catsketcher
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    catsketcher Senior Member

    I like ply a lot. I have used it in all my multis. I think it is great until it needs stringers. Then use foam or cedar instead.
     
  9. oldmulti
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    oldmulti Senior Member

    Bulkheads in hulls are generally a set and forget but let’s look at options. Malcolm Tennant in his early years used the concept of cheap polyurethane foam with 3 mm ply either side for some bulkheads. He also in his smaller boats used the cardboard used in house doors soaked in epoxy between 3 mm ply panels. A cheap mans nomex. Most older home built plywood multi’s have some form of pure plywood bulkheads. Bulkheads that don’t support crossbeams on plywood boats are generally the same thickness as the plywood hull skin. These bulkheads generally have epoxy fillets in smaller sizes (up to about 20 feet) or timber edging above that size. The timber edging is generally about 19 mm thick by about the thickness of stringers plus 50%. Cross beam supporting bulkheads are 50 to 100% thicker depending on how the crossbeam is built. Now we go onto foam glass hulls. If there is plywood bulkheads it depends on how you attach the ply bulkhead to the hull skin. First lay a glass tape of 450 or 600 gsm under the bulkhead edge on the hull then do a good fillet of epoxy under the edge and on either side of the bulkhead and a glass layer at least equal to the inner skin thickness then a 6 mm (to 28 foot) 9 mm (to 40 foot) 12 mm (to 50 foot) should be good enough. This is for non cross beam bulkheads. Cross beam bulkheads are a separate discussion. Next is Durakore or Duflex bulkheads. Great material for this task, in most cases lighter and stiffer than a ply bulkhead and premanufactured. On smaller boats Durakore can be cut out and taped in but above 25 foot a glass light layup on both faces will be required. For foam glass boats with foam glass bulkheads, non cross beam bulkheads should have the same foam thickness and glass layup on either face of at least the inner skin hull glass thickness. If it is the forward crash bulkhead the skin should match the outer hull skin glass layup on either face. For any foam glass (or durakore, duflex) bulkhead that has cutout (eg doorway) the foam around the cut out should be routed out to a depth of 12 mm at least and replaced with solid glass unidirectionals. Alternatively as you layup a foam glass bulkhead put plywood insets around the cut out of the thickness of the foam and at least 40 mm width. The jpegs of bulkheads all came from the same 40 foot boat. Yes he has Duflex, ply and foam glass bulkheads in the same boat. It was both structural requirement and an evolution of build process over the boats construction process. The final jpeg is the all carbon 53 foot Fujian bulkhead structure.
     

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    Last edited: Oct 15, 2019
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  10. bajansailor
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    bajansailor Marine Surveyor

    Thank you for all of your excellent and most informative posts on here OldMulti - already it is a veritable book on proper boatbuilding, and I am sure that by the time you have finished it will become an encyclopaedia. I have bookmarked it some time ago to ensure that I can always find it in future.
     
  11. oldmulti
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    oldmulti Senior Member

    For those who would like to have variety in their multihull design the following web site may be of interest. Boat plans https://coastfish.spc.int/en/publications/posters/boat-plans This site gives you access to about 33 boat plans for fishing boats for islanders. These boats are mainly set up for simple hand line fishing and are mainly sail driven with low powered outboards . Some are full designs others are not fully specified designs. The plans were originally funded by a UN off shoot and the designs were meant to be simple for easy construction. But over time the boats became more advanced. It ranges from many plywood sailing tacking proa’s to 2 aluminium power catamarans and the odd monohull power boat. Do not expect high tech here but they may give some ideas and clues on various construction approaches etc. Most of the designs have been done by professional designers. Also some of these designs could be converted to fun day sailing boats. Attached are 3 sample plans, each PDF is about 1 meg.
     

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  12. oldmulti
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    oldmulti Senior Member

    Another Grainger design that had great success was design 101 produced in 1997 which was known as Turning Point. The boat is a tube cat 32 x 20 foot, weighing 2900 lbs and carrying a sail area of main and genoa of 635 square feet. It had 3 aluminium cross beams. Turning Point is a very close relation to Mad Max. Mad Max (now Ullman Sails) which is basically the same hull but was built with carbon fibre skins. She beat a Seacart 30 Morticia across the line in 5 of the 7 races at Airlie Beach Race Week 2015 and in all six races at Hamilton Island Race Week 2015.

    The hulls and cabins on Turning Point were 200 gsm 90/0 8 mm WRC 200 gsm 90/0 done in epoxy. An additional layer of 400 gsm 45/45 is laid on the inside hull glass at crossbeam points. This is a racer hull structure and would not be recommended for any form of cruiser. Gaboon ply 6 mm is used for bulkheads. Gaboon ply is also used for bunks, floors etc. Daggerboards are WRC core 500 mm wide by 2.7 meters long with 10 layers 400 gsm uni from top of board down 2 meters with each layer shorter by 100 mm and the entire board is covered with a 400 gsm 45/45 layer.

    Crew fitness is a critical factor when sailing Turning Point or Mad Max well/hard. They are 32 x 20 feet so just getting from side to side, front to back when it’s rough and the windward hull is up in the air is hard. Winching sails requires a lot of strength, even with just a jib according to Mad Max’s skipper.
     

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  13. catsketcher
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    catsketcher Senior Member

    Making a daggerboard using this method can create a strong board. One problem is that you tend to make a nice fair cedar blank and then put glass on it. This will make the board thicker at the exit point of the hull. To get around this you have to know how thick each laminate will be and then remove some cedar where the unis will go. This will make a straighter board that slides better in the case.
     
  14. oldmulti
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    oldmulti Senior Member

    Bernd Kohler is a talented designer who mainly focuses on ply epoxy boats from 11 foot to 40 foot. These boats are intended for home building. One of his more successful designs is KD 860, a 28 x 18.5 bridge deck cat that weighs 4000 lbs and displaces 6700 lbs. The main and jib are 400 square foot. The length to beam is 11:1 and the prismatic coefficient is 0.57. This is a performance cruiser.

    The 8 mm plywood shell is covered with light glass epoxy with 25 x 38 mm stringers at 400 mm centre lines and 8 plywood 8 mm bulkheads. I am guessing here but the underwing is likely to be 12 mm with 32 x 50 mm stringers at 800 mm centre lines. The main mast cross beam and rear crossbeam bulkheads are 12 mm ply with 450 gsm 45/45 on either side with top and bottom flanges of timber. The cabin roof has stringers run fore and aft. The rudders are NACA 4412 sections. The blades are 1470 mm long and 250 mm wide. The skins are 4 mm ocume ply placed in a V mould of 52 degrees then epoxy is poured into the bottom of the V to 15 mm deep. A 20 x 20 mm spar with 2 layers of 400 gsm uni directional on the length of the spar. The spar is placed 78 mm back from the bottom of the nose V. The tail pieces of the ply are pulled together to meet a tapered 25 mm piece of timber to form a foil. The entire outside is covered with 2 layers of 240 gsm cloth.

    There have been many KD 860’s built globally. The photo’s and diagrams will show the evolution of the design over time. The main was a pin head and now is a square head. The front windows in the cabin have been made larger and roof raised forward. The design originally had flat triangular panels for leeway prevention but the latest design version has a central daggerboard. Several interior main cabin layouts have been done. The owners of this boat are generally very happy as the boat is a good all rounder.
     

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  15. oldmulti
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    oldmulti Senior Member

    A few technical bits. An interesting article on MEKP catalyst ratios with polyester resins is in Professional Boatbuilder Magazine issue 1 page 6. Professional BoatBuilder - 1 - Oct-Nov 1989 https://pbbackissues.advanced-pub.com/?issueID=1&pageID=9 part 2 of the article is here Professional BoatBuilder - 2 - Dec-Jan 1990 https://pbbackissues.advanced-pub.com/?issueID=2&pageID=8 Basically get your ratio’s correct of 1 to 2% MEKP catalyst depending on temperature. On a hot day don’t cut your catalyst below 1% to get longer working times as the polyester resin may never fully cure. It better to take a day off or do other things. Page 12 has the temperature versus gel time chart. Also in PBB magazine number 1 there is an article on vacuum bagging.

    There is an article on vinylester resins in PBB number 6 page 12 Professional BoatBuilder - 6 - Aug-Sep 1990 https://pbbackissues.advanced-pub.com/?issueID=6&pageID=12 which explains its strength and water resistance advantages over polyester.

    There is an article in PBB number 9 page 38 Professional BoatBuilder - 9 - Feb-Mar 1991 https://pbbackissues.advanced-pub.com/?issueID=9&pageID=38 on installing core materials in a hand layup and vacuum bagging situation that has a few good hints.

    An article on why fiberglass breaks is in Professional BoatBuilder - 13 - Oct-Nov 1991 https://pbbackissues.advanced-pub.com/?issueID=13&pageID=35

    This covers the reason fiberglass fails due to some bad design or build practices and how you may be able to detect some of the failures. There is also an article on secondary bonding on page 67 that may be of interest.

    Article on post curing of laminates is in PBB number 14 page 47. Professional BoatBuilder - 14 - Dec-Jan 1992 https://pbbackissues.advanced-pub.com/?issueID=14&pageID=47

    The article shows the effect of post curing on the strength of a laminate in all resin types.
     
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