Multihull Structure Thoughts

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

  1. Russell Brown
    Joined: Jul 2012
    Posts: 109
    Likes: 29, Points: 28, Legacy Rep: 25
    Location: washington state

    Russell Brown Senior Member

    I've never built a tortured ply hull, but I'm a huge fan of them. The closest I got was this study I did to learn about the process and how to design different shapes: http://www.ptwatercraft.com/Begining Tortured Plywood 1-6.pdf
    My Grasshopper motorboat is based on a pair of ancient tortured ply Tornado hulls that seem a bit flimsy but have withstood decades of abuse. I like plywood.
     
  2. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    The final on tortured ply hulls. There are many tricks that are used by people to finally torture a plywood hull to shape. When you are bending a 3 mm ply hull it can often be forced into shape slowly with some force. When it gets to 4.8 mm thick plywood some builders start to apply steam or wet blankets to assist in bending process. EG the 28 foot 4.8 mm float hull being bent in the initial jpegs used a plastic tent and 3 fry pans of water to make the plywood more flexible before bending into shape like a 28ft tornado. It has a cedar stringer, 5 bulkheads and a couple of carbon ring frames plus a layer of glass on the outside and is still about the same weight as a foam and glass hull. According to the builder it bent up pretty easily.

    The Gougeons' Adagio was 6mm and must have been difficult to bend/tension/torture. CeeBee spoken about in an earlier post (536 on page 36) tortured and bent 9 mm ply into shape just using brute force but it was “surprising easy” according to the builder. The cheap 7 mm cheap 3 ply plywood bent relatively easily for a 19 foot asymmetric proa. So it is all possible to do with thicker plywood.

    But I will quote a naval architect who has built a Tornado to plan then built a fun 20 foot cat similar to a Tornado.

    “For those afraid by a 4mm ply hull with glass and minimal framing, the tortured ply system is good for fast boats. If you use an epoxy resin with 4 to 6 % elongation so it uses the glass fibre properties to the maximum the system can take a lot before breaking, as it's able to dissipate a lot of energy along the parabolic/pressure curves of the hull. The mistake is to be afraid and to add reinforcements/stiffeners that create hard points, like deep stringers and very rigid bulkheads. In fact stiffeners have to be not too stiff. Unhappily compounded plywood on multis needs a zen state of mind, a good eye and some self confidence of your capacity. It's not an everybody's method. Some little tricks about the plywood;

    0- To be cheap is a sin. Take the best quality marine plywood: no voids, excellent gluing, good faces, regular bends.
    1- Weight each sheet. The heavier ones are the more rigid, keep them for the bow zone.
    2- Make a test of flexibility on each sheet. There are different simple methods.
    3- Eliminate the too light or too flexible, or the too rigid sheets of the compounding of the hull, keep them for other works like bulkheads, pads, stern etc...
    4- Pair them, so the bending will be identical on each side.
    5- A steam generator for cleaning carpets or a simple steamer does miracles if well used. Some use towels and boiling water.

    The second tornado was for raids was wider in the hulls and overall, with a different mast and bigger sail plan (plus gennaker). The hulls were in 3mm ply with 6oz fiberglass inside and outside, plus carbon UD strips. No bracing but stringers and ribs plus a horizontal bulkhead and crash boxes. Heavier than the original plan but very strong, and that was needed for raids and fast coastal cruising.

    And finally, another approach to tortured plywood boat building was done by Bernie Rodriguez back in the '60s. His LC-32 trimaran (page 8 nbr 116 LC 32 PDF) was designed to be built of (severely) tortured 1/4" fir ply. He built a frame and stringers then he would use 600 x 1200 6 mm flat plywood panels and torture them over the frame. In areas requiring the greatest torture he cut the plywood into smaller piece depending on the amount of compound curvature. Some were brought into place with clamps slowly over a period of a day or two. Some panels required soaking the surface of the ply with hot - almost boiling - water, allowing them to sit for a while, and then torturing them further into place. After they had been persuaded into their final shape they were allowed to dry out before they were removed, glued and permanently attached. It was remarkable the smooth shapes he could create. I don't think there were precise formulas regarding how much compound curve could be achieved. It was more the result of considerable experience with the process.
     

    Attached Files:

  3. bregalad
    Joined: Dec 2010
    Posts: 113
    Likes: 8, Points: 18, Legacy Rep: 36
    Location: Georgia

    bregalad Senior Member

    I was good friends with the owners who commissioned Bernie to build he LC-32 as a replacement for the 25' Piver Dart 'Amistad' that Bernie had built and sailed in the OSTAR. I was working near where Bernie was building the new boat and gave him a hand on a couple occasions. He would wet towels with scalding water and place them where needed .... then slowly persuading the plywood into place. Some of the curves were pretty severe but I never saw him destroy a section while working it into place.

    Part of the deal was that Bernie would be able to use the boat in the '72 OSTAR before turning it over to the owners. Unfortunately he didn't get it finished in time to do sea trials and sail it over to England. Instead he sailed it single-handed to Puerto Rico and left it moored for the winter in front of his parents house on the southwest of the island. The owner and I went down the following spring and sailed it back from Puerto Rico to NY.

    As a racer-cruiser it was a fine boat. The owners and their infant daughter moved aboard and went cruising south.
    Bernie offered plans, but to my knowledge there were no other boats built.
     
  4. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    A minor 0ne to address a few issues. The following Professional Boat Builder web sites are interesting. This is an article in Professional boat builder magazine about a stepped catamaran hull shape. It an interesting idea that has been tried in a 60 foot biplane French cat but was not racing successful.
    Take a Free Copy - Professional BoatBuilder Magazine https://www.proboat.com/2019/12/take-a-free-copy/

    Also in the same PBB source is an article on a 25 foot, 5000lbs 3D printed powered patrol boat at:
    The Biggest 3D Printed Boat - Professional BoatBuilder Magazine https://www.proboat.com/2019/10/worlds-biggest-3d-printed-boat/

    Next is a Tennant design that is of interest because the skipper was the first solo sailor in The Race to Alsaka. Not bad for a 23 x 14 foot cat slightly extended aft and having a fixed cross beam structure.
     

    Attached Files:

  5. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    While we are on Tennant boats, his most famous sailing design is the Great Barrier Express a racer cruiser catamaran that was developed in the 70’s and has over 300 built so far. In its original version it was a home build cold molded design of 28 x 16.5 foot weighing 2400 lbs displacing about 3300 lbs carrying 410 square foot of sail. This version was built with 2 layers of 5 mm ply as a monocoque hull and deck with minimal framing, a 25 x 50 mm gunnel and a 25 x 100 mm keel. The main bulkhead is 10 mm ply with the aft bulkhead 7 mm ply. Where the deck has a heavy curve it was 3 layers of 2.5 mm ply. The cross beams and mast were aluminium sections. Pilkington boats started to produce foam glass versions with the initial boats being glass with a coremat core. Pilkington then produced the GBE 111 which was 28 x 17.5 foot weighing 2400 lbs carrying 450 square foot of sail. Basically the same hulls but the moulds were widen by 25 mm. This version had a 300 gram Ultimat which is a Flemings Industrial Fabric with a 300 gsm biax stitched to a 150 gram CSM laid either side of a 6 mm divinycell core for the majority of the hull and deck. Again the cross beams and mast are aluminium alloy. The fore beam is a 100 mm diameter tube with the main beam 150 x 114 mm section. The mast is 149 x 99 mm weighing 2.45 lbs / foot. The boom is 80 mm diameter.
     

    Attached Files:

  6. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    After the L 7 trimaran item and its pultrusion I beam cross beam I went searching for information about I beam Pultrusions and came across the following document. This is only for those who have a solid ambition of understanding fiberglass materials and characteristics. The Composites Design Manual 3rd edition is by JA Quinn and is good. Its just under a 3 meg PDF but it not only contains material information but design information and characteristics of composites and various companies pultrusion sections etc. It can be found at

    Design Manual 3rd Edition - Composites



    composites.ir › wp-content › uploads › 2018/03



    If this does not work google the title and the PDF should show up.

    The attached paper honeycomb core tests were run for Douglas aircraft corporation to test resin impregnated paper honeycomb cores and there ageing over 5 years. Reason for the interest is early in Tennant career he suggested that a cheap “core” material was a cheap house door internal paper core dipped in epoxy resin skinned with thin ply on either side would act as a strong panel for underwings, bunk panels, furniture etc on small boats. The document did some longevity testing on plywood and aluminium panel covered paper cores. An unprotected (no resin) paper honeycomb core loses 65% of its strength over 5 years. A resin protected paper honeycomb core holds over 95% of its strength. This is a cheap mans version of a Nomex core (page 38 number 566) similar to what we have talked about before. If you look on the web you will find companies that sell paper honeycomb cores of various cell sizes.
     

    Attached Files:

    BlueBell and Corley like this.
  7. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    The Sailish cat was originally conceived as a Race to Alaska design but can be used as a cruiser racer for more general purposes. The open bridge deck cat is 28 x 19 foot weighing 1570 lbs and displacing 2220 lbs racing and 3000 lbs in cruising mode. It carries 365 square foot in main and fore triangle but can add a 400 square foot screacher. The length to beam of the hulls in cruising mode is 12.6:1. This cat will be fast. The cat is built from 26 sheets 6mm plywood, 4 sheets 9mm plywood, 50 x 25 mm 150 meters, 38 x 25 mm 120 meters, 75 x 25 mm 10 meters, 75 x 38 mm 2 meters with 45kgs minimum epoxy and 1kg wood flour or similar filler. Glass is 20kgs min 200g glass cloth (biaxial +/-45deg recommended). Why the interest in this boat? It is because Richard Wood wrote an interesting comment about its structure in relation to a question asked about making the boat lighter.

    Richard Woods wrote “The article says that if your solid plywood hull is less than 9mm it probably isn't worth using composite as the weight saving is minimal. "On panels less than 3/8", a composite panel may not have a significant weight difference over plywood" But it costs 4 times the price. You can make very light solid ply hulls that also have good abrasion and impact resistance. If you use solid 6mm ply then a sandwich of 3mm-foam-3mm will be heavier and less "tough" with respect to abrasion as the outer skin is only 3mm. To save weight you'd need to come down to 2mm ply each side (if available). That's pretty thin! You'd need to be a good, if not excellent boatbuilder to build that way. I do not recommend using balsa as a core on any boat.

    The Salish hulls are made from 6mm ply (4mm topsides optional). Not sure of the real weight, as opposed to predicted.

    Years ago we built a foam sandwich 24ft Strider catamaran and used 60kgs of glass for the whole boat - 2 hulls, 2 deck, 4 bunk bases. My wife and I could easily lift a complete hull. Yet the whole boat weighed 500kgs (I think) in racing trim. The safety gear required for day racing weighed 90kgs! I weighed everything on that boat. The rules stated "a stove" but it didn't say it had to work. So, I had an empty propane cartridge, that saved 1lb. The boat is still sailing 25 years later

    So, saving some weight in the hull will save only a very small percentage of the all up weight. It is probably better to have a carbon mast and rope rigging as that will save weight, reduce pitching and be a stiffer mast. Carbon sails of course, even if you throw them away after 1000 miles.” Richard Woods of Woods Designs
     

    Attached Files:

    BlueBell and bajansailor like this.
  8. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    Simpson’s Ground Effect catamaran is a semi bridgedeck catamaran aimed at a higher performance whilst still providing some reasonable accommodation for a 29.5 x 19.5 foot catamaran. The cat displaces 5650 lbs and carries a 395 square foot main and 165 square foot jib. As with most Simpsons the hulls are strip plank WRC with plywood bulkheads and plywood timber cross beams for the main and aft cross beams. The fore beam is an aluminium tube. This design has good performance (not high due to its weight) and are strong. Many Ground Effects have been built and most have been built to plan which indicates Roger got it correct first time. The jpegs etc give a clearer indication.
     

    Attached Files:

  9. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    The Simpson 10.2 is one of his most popular designs being a full bridge deck cruising cat of 33.5 x 19.33 foot displacing 8000lbs with a 460 square foot main and 250 square foot jib on a fractional rig. The original version had transom hung rudders but later options included inboard skeg rudders with fixed minikeels. Several early builders added transom scoops/steps and small doghouse extensions to get full headroom in part of the main saloon. The construction is strip planked cedar with Bruynzeel plywood bulkheads and cabin, then covered with epoxy and glass. Alternatively you can do a foam glass version. The main mast cross beam is 90 x 19 mm timber structure sandwiched between plywood faces. The 10.2 performs quite well with one owner commenting on the speed as a fair bit quicker than the Seawind 1000. Being only a 33'6'' hull it does tend to dig a hole at around eight knots. That being said our record speed is 20.7 knots on a southern ocean swell. Light and stripped of weight makes a huge difference and the cat can exceed 8 knots easily. The jpegs are of 2 different 10.2’s.
     

    Attached Files:

  10. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    The final Simpson cat for a while will be the Simpson 11.88 one of his later flared hull full bridge deck cruising cat. I do not know if any were built but I think quite a few plans would have been sold. The cat is 40 x 23 foot weighing 10800 lbs and carrying 975 square foot of sail in a fractional rig. The hulls are strip plank or foam glass. Similar sized Simpson boats hulls are 460 gsm 70/30 biax 19 mm wrc 460 gsm uni doubled from waterline to waterline. The main mast cross beam webs are either 9 or 12 mm ply depending on the design sandwiching 19 mm thick timber. The bottom flanges are 3 19 x 90 mm timber on top of each other. The top flange is 3 19 x 42 mm timbers on top of each other. A matrix of vertical and diagonal 19 x 90 mm timbers act as trusses. The forward centre wing frame has 19 x 90 mm timbers sandwiched between 9 mm ply faces. The decks are 9 mm ply with 45 x 19 mm stringers at 300 mm centre lines, the curved cabin tops are 2 layers of 6 mm ply. I have sailed on a Simpson 12.01 which could go well for a cruiser and was a strong boat. The jpegs will give a guide to the boat. The structural jpegs are indicative but not guaranteed to be the 11.88 structure.
     

    Attached Files:

  11. Bruce Woods
    Joined: Sep 2007
    Posts: 132
    Likes: 10, Points: 18, Legacy Rep: 90
    Location: perth

    Bruce Woods Senior Member

    I know of three that were built in Western Australia.
    Lally Cat, Whale Song and Blue Yonder??, also a 4th was built for the charter trade by slicing the hulls length ways and adding a wedge to increase displacement.
    Not sure if any of the three mentioned were built exactly to plan however.

    Pretty sure Roger started building one under his house. Not sure what happened to that one.
     
  12. Burger
    Joined: Sep 2017
    Posts: 17
    Likes: 2, Points: 3
    Location: Australia

    Burger Junior Member

    Roger's book "Simpson on Boatbuilding" is a gem, regardless of what design or material you're building. AFAIK still available from Boatcraft Pacific.
     
  13. oldmulti
    Joined: May 2019
    Posts: 1,001
    Likes: 504, Points: 113
    Location: australia

    oldmulti Senior Member

    The Coral Cove 31 catamaran is designed by Mike Waller. It is described as a budget ocean cruiser. The cat is 30.9 x 19 foot displacing 7800 lbs with a maximum payload 2200 lbs. The sail area is 540 square foot for inshore or 480 square foot if the cat is intended for offshore work. Both are fractional rigs with aluminium masts. Mike provides alternatives for some deck gear to save money, EG rope tie downs for the rig instead of metal turnbuckles. This cat with a 28 foot waterline and a 9:1 length to beam ratio is about the minimum viable long term offshore cruiser to carry the payload required. The cat is plywood and timber with in the initial design a box section main mast crossbeam. After many were built with bridge deck cabins a Mark 2 option appeared with a full panel crossbeam. The box section cross beam allowed the components of the boat EG hulls to be built separately and assembled closer to the water. The hulls are 9 mm ply with a layer of glass over a ply bulkheads and timber frames and stringers as shown in the jpegs. The box section main beam has 18 mm ply webs on either side of timber top and bottom flanges with vertical and diagonal timbers between the webs and flanges. The rear beam is a similar structure but longer. I have only spoken to one person who has sailed on a 31 and he said its sails well but is not a racer.
     

    Attached Files:

  14. Russell Brown
    Joined: Jul 2012
    Posts: 109
    Likes: 29, Points: 28, Legacy Rep: 25
    Location: washington state

    Russell Brown Senior Member

    Friends of mine circled the globe with their two kids on a Simpson catamaran. Pretty sure it was the 10.2 design. They built it in their back yard in Oakland and I saw them in Mexico and the South Pacific. I thought it was a hell of a good boat.
     

  15. YoungGrumpy
    Joined: May 2012
    Posts: 60
    Likes: 4, Points: 8, Legacy Rep: 34
    Location: New Jersey

    YoungGrumpy Junior Member

    First,
    all this knowledge is incredible, Greatly appreciated! Thanks!
    now it seems like I just caught the homebuilding bug, it is not full-blown infection yet, just satisfying the curiosity...
    I got the plans of the Fish & Chips from Team Scarab (Aside from being a great deal, thanks for reminding!). I am waiting to see what they've got on the foam/glass option...
    At first glance, the board solution almost surprised me. I would think a simple Daggerboard would be easier!? Then it is the size being on the smaller side? (at about 80 cm long looks like to me)
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.