Multihull Structure Thoughts

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

  1. Bruce Woods
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    Bruce Woods Senior Member

    When you consider an A class Catamaran , 18 x 8 feet weighs the same, i'm going to call BS .
  2. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Bruce, I thought it was BS as well, then I started reading several threads. Example Small trimarans under 20'

    Randy Smyth's Trimaran

    Quotes from Randy Smyth and those who asked Randy

    Posted 02 January 2010 - 03:35 PM

    Got to see it first hand 2 years ago, lined up on the beach next to it with a Raptor. Did a quick horizon job on me. Very high SA to weight ratio, and length to width ratio. In the last Everglades Challenege, Scizzors stayed ahead of a well driven Tornado until it ran into mechanical problems.

    I asked Randy about it a year ago or so, and he shared a few stats:

    168 lbs total weight
    20' long with 20" sprit, modified A-cat hull
    18' wide OA
    26' a-class mast
    Upwind SA: 185
    Downwind SA: 337

    Randy quoted in 2011:
    Sizzor offers plenty of speed but it also is an extremely dry ride and very comfortable. When I’m sitting at the end of the hiking rack I am over 2 feet above the waves. For the 1200mi. race it has an autopilot that can operate when I nap in the comfortable hammock trampoline at reduced speed. ...

    Sizzor specs:

    LOA-20' +20" sprit
    BEAM- 18'
    MAST-26' A-class mast
    SA UPWIND-185 sq.ft.
    DOWNWIND-337 sq.ft.

    My Sizzor Ama's were new this year. They featured flat bottoms and were much wider (14" rather than last year's round bottom 8" wide ama's). My rational was the Ama's on Sizzor don't really touch the water until I'm going over 10 knots since in light air I can basically balance on the main hull. The round bottom shape is low drag at low speed but are high drag at high speed. Flat bottoms are high drag (lots of wetted surface) at low speed but low drag (think water ski) at high speed. So, these new Ama's aren't any different in light air (since they are airborne) but my high end speed is now improved.
    Construction is 1/4" 5 lb density foam built with 2" wide strips over frames (throw away) at 12" spacing. The foam strip edges are routered with a cove on one side and a bead on the other. They are glued with Gorilla glue. This forms the rounded top of the Ama's. The bottom is 1.5" thick of 1/2" x 2 lb density Home Depot blue foam with a 3/16" layer of 5 lb. density foam on the bottom. This foam bottom forms a slanted floor in the ama with the front high and the stern tapered to only about 1/2" of foam. They are glassed with 4 oz. on the inside and 4 oz. and 2 oz. Some carbon is added at the (2) carbon attachment posts. West System Epoxy (you must use epoxy with styrofoam) was use for all laminations.

    There are several other threads that have the same type of discussions about the Sizzors and its weight but they all appear to end up with similar specifications. I think its possible but it would be fragile. The 2018 version is in the jpegs and also Everglades Challenge 2018 “Sizzors” capsized, broke up in 2018 and Randy had to be rescued. Hall of Famer Randy Smyth rescued >> Scuttlebutt Sailing News

    Attached Files:

  3. oldmulti
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    oldmulti Senior Member

    This foiling trimaran is a boat I cannot verify any specifications on, I can only quote the press releases. “Hungarian Silence” is a 46.9 foot trimaran that weighs 2000 lbs, a number I doubt. An AC 45 cat weighs about 3000 lbs and has a lot less surface area than the “Hungarian Silence”. The Silence has 2 carbon fibre wing masts 36 foot high with the forward mast capable of being extended 10 foot higher, to carry an 860 square foot gennaker over a 860 square foot genoa. Each main sail and mast is about 400 square foot. There is also a jib of 150 square foot. According to the builder the rotating, wing-masted soft mainsail placed one behind the other was designed to ensure maximum aerodynamic efficiency. This is very similar to the operation of the main wing profile and flap system used in aircraft wings.

    Now the idea of the boat is to win the Hungarian blue ribbon event. Previous winners include a 50 foot biplane racing cat which is very fast in the right conditions. Silence was designed to foil at up to 35 to 40 knots maximum speed with a take off speed of about 13 to 15 knots with its foils.

    The boat took 2 years of design and 2 years to build. The entire boat is a carbon fibre foam structure using top quality materials and manufacturing approaches. The main hull is a flat bottom dory shape. The outer hulls are round bilge. The beam of the tri appears narrow and the main lift foils are 45 degree from the forward beams. The 45 degree forward foils can be lifted to allow the tri to reduce wetted surface in non foiling conditions. The rear foil is a T foil of the main hull. There are also rudders on the floats. The tri takes 5 crew to sail it.

    This is an interesting concept but we need someone to find out if this boat successfully raced and flew. Also if anyone could verify any of the boat specifications it would be good. Silence was launched in February 2019 so it may have raced last year. The jpegs give a better idea.

    Attached Files:

  4. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Look at those little shark fins on the rudder foil!
    Odd they are pointing up.
    Lots of vertical stabilizers on board.
  5. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    I think in non foiling conditions, only the outrigger's rudder are used and the mainhull rudder is retracted to lessen drag. The opposite in foiling conditions !
    BlueBell likes this.
  6. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    In this video recorded in July 2018 during the Blue ribbon race on lake Balaton, one can see Hungarian Silence during 4 s, from 1:33 to 1:37. That year, the winner was Christophe Peclard on the cata Safram. An Hungarian cata with 2 masts was 3rd, Fifty Fifty

    Safram : Kékszalag 2018 - Safram remporte la Kekszalag sur le Lac Balaton
    Fifty Fifty (here at the Bol d'or 2016) : Bol d’Or Mirabaud
  7. oldmulti
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    oldmulti Senior Member

    Thank you Dolfiman for your research on Silence and associated race boats in the Blue Ribbon.
  8. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    After a few complex tris we go back to the simple life. The Tridory 15 is a 15 x 11 foot plywood tri that weighs 190 lbs and carries an aluminium rotating 20 foot wing mast with a 105 square foot main and 40 square foot jib. The main hull has a 6 to 1 length to beam ratio. This tri is a performance day sailor more than a racer. The designer, Malcolm Barnsley of Nimbus boats, says “Performance of this boat is excellent in flat water and quite testing conditions in the Solent (20-25 knots wind speed). On a reach the tri readily gets up to 12 knots, although it gets pretty wet at these speeds!”

    The specification for the Tridory 15 was for a lightweight, fast boat that would normally carry 2 people, but could be sailed safely by 1 person. The main hull has a good cockpit with storage areas forward and aft. The daggerboard is offset to one side of the main hull to minimise any intrusion into the cockpit.

    The construction is stitch and glue with 4 mm plywood on the hulls below deck level. Decks are 6 mm plywood (probably 2 layers of 3 mm ply on fore deck). There are 4 bulkheads with light framing and a transom to provide the flat bottom main hull its shape. There are gunnel and keel stringers in the main hull but the chines are stitch and glue with glass tape inside and out.

    The cross arms are about 90 mm in diameter aluminium tubes that fit into fiberglass molded down tubes that go into the floats. The cross arms are attached to the main hull by a rope lashing to the gunnels of the main hull. The cross arm half tubes meet in the centre of the main hull where they are welded together under the mast step for the forward beam. The aft beam is also welded together with a traveller on top.

    A simple to build, reasonably fast, fun design. The only downside is the need to assemble and disassemble the cross arms each time you trail the boat. Just had a thought, if a 10 foot wide tornado can be trailed by tilting it, this tri could be trailed on a tilt trailer without the need to disassemble it. I will have to do some drawings. The jpegs give the idea. The PDF is a clearer version of the study plan.

    Attached Files:

    Last edited: Aug 31, 2020
  9. oldmulti
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    oldmulti Senior Member

    The Pandora 8.50 is a sports cat that looks vaguely familiar. The designer is Francois Perus from Perus Yacht Design who used to work for Tony Grainger (think Livewire 28). The Pandora 8.50 is 27.9 x 16.5 foot that weighs 1900 lbs and displaces 3250 lbs. The 40 foot rotating mast carries 312 square foot main, 150 square foot jib and a 400 square foot gennaker. The length to beam is about 12 to 1. The production boat has low aspect ratio keels that draw 2 foot. Daggerboards are an option but the at performs well with keels.

    The Pandora 8.50 is not a full racer, but is a high performance cruiser racer with a moderate rig that is easy to sail without scaring people onboard. The cat can match a F18, but the crew stay dry. The cockpit design make all the controls easily reachable. The Pandora 8.50 offers accommodations inside the hulls and outside thanks to the half solid bridge. There are 4 single berth, a folding chart table with 2 seats, a small galley and a chemical toilet inside, to allow you to sail away for a few days.

    The Pandora 8.50 is built as a PVC foam sandwich with e-glass and is fully infused with vinylester resin. The cross beams are full carbon epoxy for stiffness and low weight. The rudders are carbon fibre. There is a semi bridge deck pan. The cat can be disassembled for transport by container or on a trailer. Cranes are required for assembly.

    The design was test sailed for a year before being put into production by a company Catamarans – Corthinx after a French company did not get it into production. The jpegs give the idea. This would be a good coastal cruiser.

    Attached Files:

  10. oldmulti
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    oldmulti Senior Member

    The Pacific proa featured here was a very early design study by Jim Antrim in 1997. This proa was not built as far as I know but it is an interesting design study. The proa is 72 x 40 foot displacing approximately 22,000 lbs. Main hull length to beam is 17 to 1. The mast is 55 foot in the sloop option carries about 1100 square foot of sail area. The mast is 65 foot in the cat rig option carries 1200 square foot of sail. All these numbers are approximate, excluding the length over all.

    Why is this pacific proa of interest? For the same reason Cimba, Jzerro etc are. Pacific proa’s with “accommodation pods” to leeward are a lot more stable than people think. The first jpeg is the stability chart for this proa. Notice the proa is still self righting from 90 degrees. The second jpeg showing the cross section shows the centre of gravity and centre of buoyancy when the leeward pod is immersed as would be the case if the proa was overpowered. Result, up to 90 degrees the proa can self right if overpowered. Very few tris or cats can come back from a 90 degree capsize.

    If a Pacific proa is caught aback (tries to capsize over the windward float) the righting moment of the proa is much higher than if the proa capsizes in the other direction. Result a pacific Proa with a well designed “accommodation pod” can actually be a pretty stable craft across a wind range. Now I warn you, Pacific proa’s have their own issues and characteristics that need good seamanship, but they can be made viable cruisers.

    What issues? The rig design can be tricky. If you have a sloop rig with wires holding the mast up, it may not have a lot of rigging support on one side of the mast, which means the mast has to be handled with care. Going downwind you can be caught aback putting your rig under pressure again. If you have a freestanding rig there are less issues but may also have less upwind performance. The more experienced proa folk can talk about this.

    The other Pacific proa issue is you have to build a fairly large proa to get a reasonable amount of accommodation. The proposed 72 footer here would probably only have the accommodation of a 40 foot cat.

    The upside of a 72 foot Pacific proa is, it should be fast, and I mean seriously fast across the wind range. Pacific proa’s tend to be light for their length, do not need large amounts of sail area to drive their lighter weight. Pacific proa’s have minimal wetted surface in light to moderate winds when they are in effect sailing on one hull (with a minimal amount of float immersion). The main hull they are sailing on tends to also have a high length to beam ratio which results in lower hull and wave resistance.

    I have no idea of the intended construction approach but a chined hull indicated this proa may have been plywood. This style of boat could be built in any light weight build method. Possibly a very fast, fun boat if you could afford the mooring fees. Sorry about the limited jpegs but as I said this was an early design study.

    Attached Files:

  11. oldmulti
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    oldmulti Senior Member

    Bucket List is a Harry Proa which indicates what can be done when you apply clear thinking and ruthless logic to boat design you can create solutions that will surprise you. Lets start with the basics. The Bucket List is conceived as a very fast racer. So whats different? Try a 36 x 19.5 proa that has in Mark 1 version a 1120 lbs all up weight. When this version went sailing Rob Denny decided it was to heavy and conceived the Mark 2 version of Bucket List that grew to 40 foot and weighs 670 lbs. The Mark 2 version hull structure already exists. (And yes, I did say 670 lbs weight for a 40 foot proa.) The Mark 2 version is designed to be a foiler with T foils on the rudder foils. Part of the reason for the reduced weight in the Mark 2 version.

    The Mark 1 mast was 55 foot long and is built in 3 sections that can telescope into each other for “reefing of the mast and sail. Being able to telescope means that it will not be drag when reefed. Added advantage is that it can be lowered to get under bridges etc. The bottom mast section weighed 32 lbs, mid mast 26 lbs and top mast weighed 25 lbs. The mast jpegs give the idea. Sail area is about 500 square foot in both versions.

    The build of Bucket List Mark 1 and 2 is foam carbon fibre with pure carbon fibre mast and beams.

    So what is the major difference between the Mark 1 and Mark 2 versions? Look at the lee hull. Mark 1 had a full length full size 36 foot lee hull. The Mark 2 version has the lee hull chopped up into 2 x 11.5 foot hulls, joined by a 19.5 foot x 150mm diameter carbon tube. The windward hull in the Mark 2 version cut down to half the height and a cockpit added. The beam clearance is reduced, but once foiling, will be higher than before. The rudders and foils will be mounted on the hull sides so they can kick up. The comparison of the Mark 1 main hull weight was 395 lbs, the new Mark 2 main hull weight is 125 lbs.

    The end result was a weight saving of the Mark 2 over the Mark 1 is about 40% (500 kgs to 300 kgs). The Mark 2 has an optimized rig, less than half the windage and no component longer than 19.5 foot or heavier than 90 lbs. The sail area on the 3 piece, 16m (1.5m of which is “bury”) telescoping mast drops to about 485 square foot. The jpegs give the idea. This boat is would be seriously fast across a wind range and would need a lot of attention to sail especially in foiling mode but, hell, you would be having a lot of fun. The web site is The “Bucket List” Prototype Building Blog – HARRYPROA

    Attached Files:

  12. oldmulti
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    oldmulti Senior Member

    For a little more mind bending thoughts, the French have released this in the journal "Nature" (very serious science stuff). Only the French would want to 'float underwater".

    "At risk of stating the obvious: boats float on top of water. Yet French researchers have seemingly inverted gravity and made a boat float on the underside of a levitating layer of liquid.

    Make your own jokes about the antipodes.

    The research into unusual buoyancy found that tiny plastic boats are able to float upside down on the interface between air and liquid. Not only that, gas bubbles in the lower half of the liquid layer don’t rise, they sink. The mind-bending results have been published in the journal Nature.

    When placed above a less-dense medium, such as air, liquids fall with gravity due to a destabilising effect called the Rayleigh–Taylor instability.

    As the liquid forms into drops, it displaces the air underneath, and falls to the bottom. However, this can be overcome by vibrating a container vertically – the movement keeps the bottom of the liquid layer flat, preventing drops from forming and thus unable to displace the air underneath. The result is a levitating liquid layer unable to fall, and an air layer beneath which is compressed, but unable to rise.

    This is not a new phenomenon; it’s been known about and used for a while. But until now, no one has studied how that affects buoyancy on the bottom interface."

    I now know how Australians can sail down here.
  13. Burger
    Joined: Sep 2017
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    Burger Junior Member

    Which gives rise to yet another money-making idea that I'll be too lazy to follow up on: Aquarium-like decorative tanks with a realistic model sea floor glued to the roof, the amazing French vibrator keeping the water at the top, an upside-down model Spanish galleon or Orma 6o floating on the bottom edge of the water, model cloudscape with sunrays at the bottom. Prices start around $10K.

    Oldmulti: Could you check your archives for an Aussie trimaran called Shanda? About 38', canoe stern, equal height schooner rig, and I seem to recall Shawn Arber was the designer/builder, long ago. She's been anchored off Airlie Beach for the last few weeks. Beautiful boat, and the rig looks very well-thought out for cruising.
  14. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re Shanda, here are a couple of photos which I found on Google - she does look rather fine.

    Shanda Australia 1.jpg

    Shanda Australia .jpg

  15. oldmulti
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    oldmulti Senior Member

    Shanda is a 42 x 30 foot tri designed and built by Shaun Arber in 1975. Shanda, when rated in 2000, weighed 12600 lbs ready to race without crew or liquids. Shanda has a schooner rig with aluminium 40 foot (?) masts carrying two 215 square foot mains and a 365 square foot genoa. The tri was conceived as a fast, easy to handle cruiser.

    Shanda won the 308 nautical mile Brisbane to Gladstone (BTG) race on corrected time in 1978, 1981 and 2001. Shanda has done a BTG in 33 hours, a 30 foot Grainger tri did the same race in 26 hours and a Farrier 28 did it in 25 hours. Shanda rating was initially .7732, Cliffhanger a 42 foot light weight cruiser racer tri had a rating of 1.0 at the same time. Shanda has a big daggerboard which helps it go to windward. The schooner rig was more effective than many thought possible and a schooner rig rating advantage was large. Shanda did not have a high top speed but could do good averages. The crew Shaun had were very good and they understood how to get the best from her. The tri has a canoe stern which when I spoke to Shaun, I got the impression it was done for looks and seakeeping more than performance.

    The tri was built in cold molded ply on stringers with ply decks and timber crossarms. For a 42 foot tri the 12600 lbs weight indicates a well designed and built boat. The room inside was surprising as it had a fairly fat main hull with big overhangs on the main hull. There was a good saloon with galley, several wing bunks and an aft cab with a double. The cockpit was small but effective.

    It still is a very good boat and is much loved by those who have owned it.

    Attached Files:

    rogerf and bajansailor like this.
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