Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    One of Australia's better multihull boatbuilders / occasional designers Shawn Arbor said to me at one point that strip plank western red cedar boats needed stronger glass skins on the inside as a hit from the outside would be "pushed" through the strip planks and the inside skin would be in tension. He claimed that most of the strip plank boats he had to repair had the inside skin broken if hit hard enough. Lock Crowther explained to me that in a foam glass boat the foam compressed therefore the outside skin needed to be thicker to act as the compression face as any forces were dispersed by the foam putting less pressure on the inside skin. Ad Hoc is correct. A hard hit will puncture any structure and an unexpected load can cause a fracture in an unrelated part of the boats. Designers are worth their money.
     
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  2. Ad Hoc
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    Ad Hoc Naval Architect

    If the predominant load path creates tension on an "interior" structural member, then it requires mitigation. If, after analysis, the only way to achieve this is by using a material with a higher "design-allowable" stress, then that is one form of mitigation. Thus without actually seeing the vessels structure, its arrangement and assumed or expected design loads and various load scenarios, it is not possible to comment further on such a claim by another. It is either just an opinion based upon his "feelings" or previous failures and nothing more than just an opinion, or an opinion based upon evidence which is backed up by further analysis. Your guess is as good as mine - in the absence of any evidence.
     
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  3. oldmulti
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    oldmulti Senior Member

    Deck structures on bridgedeck cats vary from just a wing deck to a full size full headroom bridge deck cabin. In the early days full headroom bridge deck cabins only happened in 40 foot plus cats. In under 40 cats they had cabins that had roof hatches or under 5 foot headroom. The spindrift 37 had a large hatch that provided full headroom at anchor and a low profile while sailing. If this is well done it can work very well. I have enjoyed sailing in light weather with the bridge deck cabin roof hatch up allowing a nice breeze through the boat. The latest combined cockpit/cabin bridge deck cats are taking advantage of the same features. The plans below gives you the specifications for one cat.
     

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  4. oldmulti
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    oldmulti Senior Member

    For Wharram lovers. A Narai cat 40 x 19 feet 13000 lbs displacement with 600 sq foot of sail was normally built in plywood , timber with 4 solid timber crossbeams. This boat was re engineered by a builder to be basically fiberglass. The hulls which were designed as 9 mm ply were replaced 5 mm solid glass (a combination of CSM and woven rovings) with the same ply bulkheads and timber stringers. The fiberglass panels were laid up on a flat floor then cut to size and glued to the wood stringers prior to the stringers being glassed over onto the fiberglass sides. Decks were done the same way. Next came the cross beams which were redesigned to be solid glass tubes. Please remember the Narai only has 7 feet of unsupported beam between hull gunnels so these beams suit this design only. There are 4 beams spread along the hull. Each beam is 165 mm outside diameter with 9.5 mm walls. The beams are unidirectional e glass with 45/45 glass wraps around the tube about every 3 mm. Additional 45/45 wraps were added at gunnel support areas. The beams were engineered and supposedly were shown to Wharram. The boat initially worked but I do not know its longevity.
     

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  5. oldmulti
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    oldmulti Senior Member

    Reference to post 132 about Manta aluminum I beam cross beams. If your measure the top flange on the main beam it equals 3,5 square inches of aluminum. Aluminium has a safe working load after safety factors are included of about 35,000 pound/square inch. In this case the top flange can handle about 122,000 lbs of tension. The righting moment for the 50 foot Manta is 94,000 foot pounds. As with fiberglass flanges there is some consistency in the righting moment of the boat and the cross sectional area of flanges used in cross beams. I am not an engineer. I just point this out as interest. Later I will talk about tube cat aluminum cross beams. This is where we find out each designer marches to his own drum but there is still some common numbers.
     
  6. Smj1
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    Smj1 Junior Member

    There was a Hugo Myers Manta 51 built in Florida. Aluminum Ibeam crossbeams, rotating rig and cold molded hulls. The owner built a foam cored central pod later. It eventually sold and I believe the new owners put a tuna tower steering station on her and used her as a day charter cat in the Western Caribbean.
     
  7. oldmulti
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    oldmulti Senior Member

    Deck structures for trimarans are very important. A 55 foot tri that I sailed on, deck structure was good but had one flaw, it did not fully lock together the cabin sides together with an additional partial bulkhead. This was a problem because the mast lower stay's bolted to the cabin sides. The pull of the lowers was enough to move a cabin side in stronger winds. Combine this with the diagonal load a trimaran caused by having a forward bow in the water supporting the boat and the mast cap stay pulling up on the rear float cross beam. This causes the main cabin and the central hull between the cross beams of a trimaran having to handle all the twisting loads. The 55 foot tri cabin sides needed an additional bulkhead to tie together the cabin sides. The solution for this tri was not a bulkhead, but a 19 mm stainless steel rod across the cabin bolting together the lower chainplates to make the entire cabin structure work as one unit. The rod made a great towel drying rack. The center part of a trimaran has many twisting forces on it. Make sure you cabin structure is appropriate. The Bucc 33 plan below gives an idea of an older approach for a plywood tri.
     

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  8. oldmulti
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    oldmulti Senior Member

    Carbon composite chain plates done by home builders are becoming more common. They are lighter stronger and have minimal corrosion problems. Some designers do them well. Ian Farrier was one such designer. The following blog of a F36 being built February | 2015 | F36 #005 Ravenswing – Trimaran Building https://cartersboat.com/2015/02/ three quarters way down the page describes his approach to building the carbon fibre composite chain plate for the main cap shrouds for the tri. It contains carbon fibre layups for the composite chainplates. Look through his blog as it contains many hints about building a foam glass trimaran.
     

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  9. oldmulti
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    oldmulti Senior Member

  10. Angélique
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    Location: Belgium ⇄ The Netherlands

    Angélique aka Angel (only by name)

    A Dutch guy well documented his DIY build of a Farrier F-39, including vacuum resin infusion.

    After 16 years of part time building, next to his full time job and family life and holidays, she was launched at Easter 2018, and then docked at his house for further completion.

    FRAM, F-39 TrimaranFram's building story

    [​IMG]
     
    Last edited: Jul 18, 2019
  11. oldmulti
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    oldmulti Senior Member

    The following study PDF's give an overview of F36 and F39 Farrier tris. Also a 2012 study book of many of farriers designs.
     

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    Last edited: Jul 18, 2019
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  12. oldmulti
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    oldmulti Senior Member

    Modern AC hull shapes for foiling cats are heavily influenced by foils that lift the hulls out at about 10 knots of wind speed. But the hulls still have to work in displacement mode below 10 knots of wind speed so they basically have to be efficient. Notice the similarities of shape. Fairly flat bottoms, fairly full shapes in the ends, minimal rocker and have a narrow hull beam for their length. Wild Oats monohull has won line honors 9 times in the sydney hobart yacht race. It has a fairly flat bottom, fairly full shapes in the ends, minimal rocker and have a narrow hull beam for their length. Comanche is considered the fastest 100 foot monohull in the world. It appears to have a wide beam but its waterline beam is fairly narrow under all states of heel. Again it has full ends, flat bottom and minimal rocker. All of these boats have very low displacement for their size. Wild oats gains stability by the DSS foil, Comanche by wide beam. The cats pushed stability by wide hull separation. Each of these boats have very large sail area for there displacement and stability. A fast hull shape characteristics appear consistent. Yes, higher displacement boats for their length with lower sail area to displacement have rounder hull shapes to minimize wetted surface area.
     

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  13. bruceb
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    bruceb Senior Member

    OM, I have learned more construction details on this thread than everything else I have read in the past five years. Thanks!
    A question about the Twiggy fiberglass beams, did they still use the waterstays?
    Bruceb
     
  14. oldmulti
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    oldmulti Senior Member

    Bruceb, There were no waterstays on Twiggy's with fiberglass beams. The beams were stiff enough as originally designed but depended on good building as their safety factor in the design beams were lower than most other Crowther designs. The tension in the top flange of most Crowther designs are built with a tension of 19,000 to 26,000 lbs/square inch. Twiggy's top flange was built with a tension of 38,000 lbs/square inch. Unidirectional glass/epoxy (with 65% glass to 35% resin) has a tensile strength of 120,000 lbs/square inch. Normal Crowther fiberglass beams have a safety factor of about 5 to 1. Twiggy's beam safety factor is about 3 to 1. The above assumes you are using S glass unidirectionals in the beam build and a good quality epoxy. The numbers you find for the strength of resin glass unidirectionals varies according to the type of glass (E or S), the manufacturer of the glass and the type of resin (epoxy, polyester etc). The attached PDF will give an idea of the effect of fiberglass to resin ratio's on the strength of a laminate. Build using the designers specified materials and everything should be OK.
     

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  15. oldmulti
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    oldmulti Senior Member

    Whilst talking about Twiggy's foam glass beam I will talk about other features of the beams. The full cross beam comprises of a top flange, which is mainly in compression. Yes, I have spoken of tension lbs/square inch figures when comparing top flanges but that was to emphasis safety factors and glass resin combination capability. The bottom flange in a crossbeam is normally in tension. In between the top and bottom flanges is the web faces and in the case of foam glass beams a foam separator. The foam has to be good quality (corecell, airex etc 5 lbs/cubic foot 80 kg/cubic meter) but basically is a former with a low compression of 7500 lbs/sq inch and only has a shear of 160 lbs/sq inch. The web faces carry a lot of the load when combined with the foam core. In the case of Twiggy's beam the web faces are S-500 orcoweb, an aircraft quality unidirectional S glass that weights 275 gsm. The web faces near the center of the beam are 6 layers of s-500 laid at 45/45 degrees to (in short building 3 layers of biax cloth) and from mid beam to the outer end 8 layers s-500 laid at 45/45 degrees. The web faces are about 3 mm thick. Please understand this entire beams flanges and web faces glass content is S glass with epoxy, which is 20 to 30% stronger than normally available E glass. Please also study the variation of thicknesses of the glass flanges along the length of the beam as this will give a hint of load characteristics at various parts of the beam. This takes a lot of calculation to get right. These beams need to well built hopefully with vacuum bagging to get high glass to resin ratio's which again improves strength.
     

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