Multihull Structure Thoughts

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

  1. Ron Badley
    Joined: Aug 2020
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    Location: Madeira Park, British Columbia, Canada

    Ron Badley New Member

    I'm a long time admirer of the G32. Can't help but wonder what widening the boat to even 10' would do?

    Also, I'm the guy with the somewhat modified White Wings if anyone has any questions.

  2. Clarkey
    Joined: Aug 2010
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    Location: UK

    Clarkey Senior Member

    Great Flettner rotor stuff! The Cousteau vessels didn't use Flettner Rotors but something that, while looking similar, worked in a different way.

    I have been interested in rotor sails for ages and always thought they would be great for multis - primarily due to their response to gusts. They behave in a much more benign way to sudden increases in wind speed, kind of 'saturating' as long as rotational rpm is constant.
  3. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Ron. The effect of beam on a catamaran can be modeled if you make some basic assumptions. For the purpose of this example we will assume the 32 foot cat displaces a consistent 2200 lbs and carry the same rig and sail area of 300 square foot. The hull shape will be the same. The only variable will be the beam. We will start with a 8.25 foot wide boat, then a 10 foot wide cat. In Australia you can trail a 11.5 foot wide cat in daylight and then we will do a 16 foot wide cat. Finally we will evaluate 600 lbs of water ballast in the windward hull of the 8.25 foot cat.

    Now some basics. A consistent boat weight with beam differences is not realistic but for this example it removes a variable. The water ballast version displacement is 2800 lbs to highlight the effect of 600 lbs of water ballast in the windward hull. Secondly the Bruce Number major variables are sail area and displacement, as these don’t change (except with water ballast) the Bruce Number is 1.33 and the water ballast version Bruce Number is 1.23.

    As the Bruce Number is of limited performance use, I will use the W performance, which is an artificial number based on the displacement, load water line length, beam and sail area. This is a simplified performance indicator. Higher W number means better performance.

    Now we will deal with the real variable of beam, stability. A more stable boat means you can handle a higher wind speed without capsizing. The higher the wind speed the cat can handle the faster you should go to windward or reaching. Summarizing each beams stability and windspeed the boat can handle is as follows.

    8.25 foot beam. Stability 7375 ft lbs. Wind speed 17.6 knots. W number 6.01

    10 foot beam. Stability 9342 ft lbs. Wind speed 19.81 knots. W number 6.61

    11.5 foot beam. Stability 11890 ft lbs. Wind speed 21.4 knots. W number 7.09

    16 foot beam. Stability 15875 ft lbs. Wind speed 25.82 knots. W number 8.36

    8.25 foot beam with 600 lbs of water ballast in windward hull. Stability 11200 ft lbs. Wind speed 21.69 knots. W number 5.32.

    Translation of the above. You would have to be a good sailor to notice the difference between 17.6 knots and 19.8 knots wind speed. Yes, you get slightly faster performance from a 10 foot wide cat versus a 8.25 foot wide cat. Now we come to the problem of theory versus reality. The theory says a water ballasted cat will be slower. Not reality. The 8.25 foot wide cat will do the same speed up to 17.6 knots, then water ballast will be added which allows the 8.25 foot wide cat to act like an 11.5 foot wide cat. Yes there is extra weight but there also is extra windspeed to give the power to drag the extra weight.

    An 11.5 foot wide cat can handle the same wind speed as a 8.25 foot wide cat with water ballast but the extra water ballast weight slows the boat across the higher wind speed range.

    The “best” solution is to have a wide (in this case 16 foot beam) beam with the same weight. The performance gain will be very good.

    For a G 32, I think stay with the 8.25 foot beam and use the water ballast as required. The gain of a 10 foot beam may not be worth the hassles of modifying or trailing etc. If your doing a new build and it will be moored make it wide.

    Attached Files:

  4. peterbike
    Joined: Dec 2017
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    Location: melbourne

    peterbike Junior Member

    RonB, I would be interested to hear about your/any whitewings.
    What changes, before & after changes, future changes...... ?
    thank you.
  5. Bruce Woods
    Joined: Sep 2007
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    Location: perth

    Bruce Woods Senior Member

    Excellent summation .
    The Elephant in the room is windage.
    Narrow boat = low frontal area = low aero drag.
    Shuttleworth wrote some great articles on this subject.
  6. rob denney
    Joined: Feb 2005
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    Location: Australia

    rob denney Senior Member

    Re biplane rigs:
    A biplane rig keeps the centre of effort low. However, it is nearly twice the price of a larger single rig and does not reach the light air up high. A better solution is a single, unstayed rig, in one hull. either way, make sure your cat tacks well in a seaway with reduced sail (most don't) and/or that you have a robust rudder arrangement that allows you to sail backwards when you get caught in irons without a jib to back.
    Pete Hill called in for a chat en route to Brisbane. He was selling his boat and wanted to build a Harryproa as his boat did not perform well enough, particularly upwind. The problem is apparently not the rig as Pete wanted one on his Harryproa. He eventually sold it and decided to cruise the Arctic, for which a Harry is not suitable. The guy who bought his boat was also disappointed, has since bought a 40' Harry in NZ.

    Re cognitive bias:
    Which part is cognitive bias? You won't hit something? (in which case why bother strengthening your daggercase and hull?); you won't come to a sudden stop if you hit a whale, waterlogged tree trunk or container?; or that coming to a stop at 19 knots won't be painful for someone not holding on?

    Cleaning a daggerboard is really easy. You pull it out, clean it up, antifoul it and put it back in. All good.

    "Pulling it (them, actually) out, cleaning and antifouling" is significantly more work than not having to do any of it. And you didn't mention cleaning and antifouling the cases.

    The majority of high speed boats have surface piercing foils. 40+ year old examples don't really cut it any more.

    I was crew when it set the Gladstone record. More than 15 knots average up the coast, less getting out of Moreton Bay and up the Gladstone river. 3 of us on deck working hard, the boat was on the edge of pitchpoling or capsizing most of the time. 30 minutes on the helm was long enough. Top speed 27 knots, according to the gps, although no one saw more than 25, a common occurrence with gps 'top speeds". No way a 40'ter would come close to similar performance while cruising.

    Vac bagging is a good way to save weight but with the money you put into making plastic waste you could just make a slightly larger boat, or save money for more fuel, or buy lithium batteries, or something. I loved bagging in one way but it did slow me down.
    Bagging wet laminates on flat panels is daft. Infusion on a table uses less resin (far less overall 'plastic waste" than adding twice the optimum resin amount to your boat by hand laminating, or adding it, then throwing it away with bagging), has no joins and is quick and easy. 2 of us have been easily producing an 8.5m long panel every 2 days for the last couple of weeks. These are solid glass, would have been even quicker with foam core. No mess, no gloves, no excess resin in the job, less than a household garbage bin of waste per week, with the potential to recycle or reuse a significant part of it. eg, the heaviest part of the waste is the infusion medium, which enables the resin to flow across the laminate. We are using old shade cloth which a local orchardist was going to throw out; the vac bag is nylon, we use it for mould lining and are investigating how to turn it into shipping boxes; the tacky tape comes away clean if the bag is torn off the right way, and can be rolled/heated and reused; the spiral plastic for the resin to travel along can be arranged so there is no resin left in it, making it reusable.

    Re ply being good for home boat builders:
    You are welcome to your opinion, but it would carry some weight if you explained why, and how it compares to the advantages I listed for infused foam/glass.
  7. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    A quick talk about the magicians of the Gougeon brothers. They initially built a series of day sailing tri based on their DN iceboat experience. Very light weight with very efficient rigs. In a 1960’s One of a Kind regatta a 25 x 14 foot tri they built sailed in 1 to 3 knot winds that did not move any other boat. Notice the cross-beam structure which is very light and strong for its weight. (Jpeg 1) Next came Victor T in 1969 (jpeg 2), another very light day sailing tri that had 2 mm ply hulls articulated floats and weighed less than 400 lbs.

    These boats inspired Adaigo which was built in 1970. Adagio was design and built by the Gougeon brothers as an example of wood epoxy construction. The boat is 35 x 24 foot weighing 1200 lbs and displacing 2700 lbs with 640 square foot of sail. The main hull and floats were 6 mm 3 ply Okume tortured ply. The main hull had 12 x 19 mm ribs every 100 mm from bow to the main beam then 19 x 19 mm ribs every 300 mm from the main beam aft. Float gunnel was 2 layers of 22 x 45 mm.

    Now Adaigo was considered a little heavy so they developed a light weight version, “Cake Walk”, in cooperation with an owner. The tri that was built is 34 x 26 foot that is claimed to be 600 lbs lighter than Adagio. Umm? Then I started to see the lack of a cabin space, low free board, small floats and the structure of the crossbeams. Maybe a 700 lbs boat weight may be possible. Lets look at the cross beam. The cross beam has 3 mm ply across the majority of the beams (doubled at the main hull) with 12 x 19 mm framing about every 150 mm centre lines. This is a very time consuming build. The tri needs a crossbeam repair which the current owner has requested someone to do. No volunteers came forth. The cost of this work would be large. The jpegs of the beams give the idea.

    In each case these tris are fast and long lasting. Adagio is winning Great Lakes races after 50 years. All are of these tris are ply timber epoxy structures with virtually no metal (that means no screws, nails etc) in them. I hope the owner of “Cake Walk” has the boat up and running again as it would be a fun boat.

    Attached Files:

  8. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Thanks for that Old Multi. The front on shot of Cakewalk makes me think of that Tennant design tri - Green Demon? that had a very thin main hull too. The Gougeons have allowed us to play with lovely wooden boats using materials that are lighter (like cedar rather than hardwoods) and have timber boats with the maintenance of glass. Great contribution to boatbuilding and design.
  9. Gary Baigent
    Joined: Jul 2005
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    Location: auckland nz

    Gary Baigent Senior Member

    You're thinking about the Tennant Demon Tricyle, Catsketcher. Which is no more, unfortunately.
    These designs in tensioned ply glass, carbon are also very light boats.

    Attached Files:

  10. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    A short one about a tri designed by John Patterson that have threads pleading for the design to be published. There were 3 of the 21 footers built and John is willing to let the design plans be published if anyone has a copy. John will not provide any builder support if plans are found. Why is the tri lusted after? Look at the jpegs, a trailable trimaran that has sailed it over 2,000 Great Lakes miles, 6,000 ocean miles and been trailed over 15,000 road miles.

    The ”Little Boat” is 21 x 16 foot and can be folded to 8 foot wide for trailing. John made a statement that the tri weighs about 500 lbs, a number I doubt, the displacement is approximately 1600 lbs based on the main hull plan in the jpeg. The reason I doubt the weight figure is a Corsair Pulse 600 racing tri weighs 990 lbs with a displacement of about 1600 lbs. The wing mast of the “Little Boat” is 27.5 foot high with approximately 160 square foot main and wing and 95 square foot foretriangle. A kick up spade rudder, daggerboard, and wing mast made for great performance up wind or down.

    The main hull length to beam is 9.5 to 1. John designed his larger tri’s with 200% buoyancy in the floats. I am assuming a similar floats ratio based on the jpegs. “Little Boat” had a porta-potty under a single berth forward, small galley, center cockpit, and “double” berth aft. The maximum head room is 4.25 foot. John lived aboard for over four months at one time as he cruised around the Caribbean.

    John gave up his boat design business in 2001 and built a 44 x 31 foot tri “Buddy” which weighed 9800 lbs and was built from 600 gsm 45/45 e glass 15 mm WRC 300 gsm 45/45 e glass in epoxy. John knows how to design and build light boats. John sailed around the Caribbean for at least a decade.

    The structure of “Little Boat” is unknown bar the crossarm structure was complicated and hard to build. It was a variation of Farriers but at the time it was designed Farriers patents were still in force.

    The first Jpeg is the actual hull lines of the main hull of the 21 foot tri. The lines give you enough to develop your own version if you have the desire. Sorry about the enlarge quality of the hull lines, google search Patterson 21 trimaran images and you should get a clean version of the lines on your PC. But I reiterate, if anyone could find a set of plans or a partial plan many people would be very happy. An excellent design, that from a person who has sailed on the tri, can sail very well and according to John Patterson can handle 40 knot plus wind conditions (not sailing just towing an anchor drogue).

    John Patterson spoke to Jim Brown in the following pod casts on Outrigger media. The information page is:

    About Trimarans | OutRig Media

    One of 2 podcasts is at the following, the first 5 minutes John talks about “Little Boat” :

    Nautical Lore Capercast 40: BLIND SAILOR? (Part 2) | OutRig Media

    Attached Files:

    Last edited: Aug 10, 2020
  11. Russell Brown
    Joined: Jul 2012
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    Location: washington state

    Russell Brown Senior Member

    That "Little Boat" is really cool. I saw it on a forum a while back and have been trying to find it since. Buddy is about the prettiest cruising tri ever. Do you have access to info about Buddy?
  12. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Russell. John Patterson 44’ live-aboard tri “Buddy” was for sale (recently probably still up on boat sale site) and according to John: It’s “Fast and beautiful. I've lived aboard my 44' cruising-racing trimaran “Buddy" for 17 years. Buddy is well maintained and loaded with all the proper gear to take you on your next adventure. Rig is a true cutter, mainsail, stay sail and genoa can all be set at the same time. Dagger board forward and low aspect ratio fenced keel insures great upwind performance. If you have owned a cruising catamaran and didn't like the performance then this would be a great boat for you. It sleeps 6 in 3 cabins, queen size aft, double forward and 2 singles in the main cabin.” The structure is above and “Buddy” carries a 54 foot carbon mast and 1326 square foot of sail in the main and genoa. The pictures are from the sale site and were on page 36 of this thread. There supposedly is more pictures of the 21 foot tri on Tom Henry's facebook site which I have not investigated. You may be able to get access to them.

    Attached Files:

  13. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    An old design that can give some insights into light weight building. The Kraken 25 was an early (1962) Lock Crowther day racing tri. It is 25 x 14 foot weighs about 470 lbs depending on the builder. It carries a 300 square foot rig on a 30 foot aluminium mast of 115 x 89 mm weighing 1.33 lbs/ft. The floats are below 100% displacement as the tri is intended to be sailed with 2 people on a trapeze to help stability.

    There have been several built globally and are fun machines for those who own them. But they are a little difficult to build as they cold moulded hulls. Apologies to Corley who has been rebuilding a Kraken 25 for 10 years (whilst he runs a business, has 2 new daughters and builds his 40 foot Hughes tri). Corley on his blog(s) describes the task and issues well. He has had to rebuild the hulls among other upgrades.

    So what is the structure? A cold moulded main and float hulls of 2 layers of 2.5 mm western red cedar over a “simple” mold. The hulls should be sheathed with 200 gsm cloth in epoxy outside. The keel lines are glassed with 400 45/45 e-glass in epoxy. The main hull gunnel strips are 32 x 19 mm. The main hull cross beam bulkheads and crash bulkheads are 4.8 mm ply with 19 mm timbers attached. There are deck beams every 450 mm and the deck is molded as well. The bulkheads in the floats are 6 mm ply. The cross beams are 72 x 22 mm top and bottom flanges with 4.8 mm web faces. The fairings on the beams are 4.8 mm ply. The rudder cheeks and centreboard case are 9 mm plywood.

    The structure has proven to be structural sound over many years as some boats are over 30 years old. The jpegs tell part of the story. The PDF has the design lines of the main hull. Is it possible to get the float lines as well, if anyone has the plan sheet. The beautiful molded wood K25 tri with folding beams is from Texas. Yes the hull shapes and rig could be improved and you will probably get more speed but as it is a fun machine.

    Attached Files:

  14. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Kraken 25 hull and float lines found. I should have looked harder.

    Attached Files:

  15. redreuben
    Joined: Jan 2009
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    Location: South Lake Western Australia

    redreuben redreuben

    Hi Guys,
    I communicated with John Patterson by email about Little Boat several years ago now, John indicated at the time that the drawings if still in existence were in a shed under 6' of snow somewhere on mainland USA, he also indicated at the time that his eyesight was failing badly and his cruising days were limited. Thats the last I heard from him.
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