Multihull Structure Thoughts

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

  1. oldmulti
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    oldmulti Senior Member

    Gary. Thank you for the correction, I was working off old written articles about X5. Gary are you still doing design work or are you just having fun sailing etc now? Are there any designs you would like to tell us about?
     
  2. oldmulti
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    oldmulti Senior Member

    The next Crowther cat design started as a 33 footer but the rig was so powerful the boats hull design was extended to 40 foot to allow the rig to fully power up and still have the boat underneath it. The pod racing cat is 40 x 23.7 foot weighing 4000 lbs and displacing 4800 lbs. The 50 foot (500 mm x 220 mm) fractional wing mast carried 700 square foot of sail. The main feature of this boat was the use of 2 crossbeams and an accommodation pod longitudinally between the 2 cross beams. The accommodation pod carries the mast loads. The hulls foam glass layups are complex. Outside is 200 gsm glass cloth, 170 gsm Kevlar 20 mm H 80 PVC foam 170 gsm Kevlar in vinylester resin. There are various carbon fibre unidirectional and cloths used to reinforce the hull and deck pod structure. The deck pod structure is 12 mm H80 foam above the V bottom. The pod structure V bottom is 20 mm foam. The 2 crossbeam structures are a mast shape with a vertical strength web of foam glass and top and bottom unidirectional carbon fibre flanges. The crossbeams and accommodation pod were demountable so the boat could be trailed (moved, this is not a trailer sailor) if desired. There were several versions of this boat, all were fast but had to be treated carefully as Crowther described them as bay and coastal racers, not offshore machines. One averaged 18 knots for 12 hours during the 1999 Coral Sea Classic. The photos are of a 10 meter extended to 12 meters. The other study print is of the 33 foot version.
     

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    Last edited: Sep 26, 2019
  3. garydierking
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    garydierking Senior Member

    I'm mostly retired now but still sell plans for my sailing outriggers. Outrigger Sailing Canoes http://outriggersailingcanoes.blogspot.com/
    During the four years I worked for Rudy, I built three 44'ers, one was the old style asymmetric and two more modern performance models. Choy 44 | Choydesign https://www.choydesign.com/portfolio/choy-44/
     
  4. oldmulti
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    oldmulti Senior Member

    Thanks Gary, one question. You have several designs that have asymmetric hulls and some that have boards. From your experience of both Choy 44 boats and your own design, build experience how effective is asymmetry in leeway prevention and general sea keeping? Do you think asymmetry is of minimal value or is it effective within some speed ranges etc. What interested me about the Roy Seaman comment was at high speeds there was a negative effect but at moderate speed limited asymmetry had minimal impact. I also accept your words that Rudy said about asymmetry providing better steering.
     
  5. oldmulti
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    oldmulti Senior Member

    Now Crowther’s biggest educational design. This will be a 2 part story, the first part will describe the initial design and build, the second part will be about the boats evolution to solve some issues. Design 318 was designed as a 50 x 35.5 foot, 2 beam pod racing cat, weighing 6600 lbs displacing 7700 lbs, carrying from 1400 to 2400 square foot of sail area depending on the headsail. The mast is a 70 foot high 600 x 300 mm speaderless pre curved wing with some details in the attached Jpeg. The hulls length to beam was 22 :1. The first 2 boat hulls were built from outside 2 layers of 150 gsm carbon fibre (CF) uni at 45/45, 300 gsm CF uni fore aft, 64 kg/cubic meter Nomex honeycomb 25 mm thick, 300 gsm CF uni fore aft, 2 layers of 150 gsm CF uni at 45/45. If you want a more conventional foam glass layup you use 768 gsm triax, 25 mm H80 kliogram/cubic meter PVC foam, 768 gsm triax All options are in epoxy. The foam glass option is about 850 lbs heavier. The 2 crossbeam structures are a mast shape with a vertical strength web of foam glass and top and bottom unidirectional carbon fibre flanges. The attached Jpeg will give the idea. The construction time of the first boat was over 15,000 professional building hours. Other components like the pod, decks etc were built with thinner cores but used the same fabric materials as hulls. The first 3 boats of this design were very fast in certain conditions and had problems in other conditions which lead to each of them being modified. The modified versions had no problem maintaining 15 knot averages over 24 hours and could peak out at 30 knots. I will discuss the modification in the second part.
     

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  6. Slingshot
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    Slingshot Junior Member

  7. garydierking
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    garydierking Senior Member

    The last two Choy 44s were symmetrical with boards. I don't have enough experience with catamaran asymmetry to judge its effect. Most of my work has been with single outrigger proas in the last 20 years. There is a danger with extreme asymmetry in these canoes where at higher speeds the normal weather helm turns into lee helm and can cause a jibe if you aren't paying attention. I only have one asymmetric design offered now and the asymmetry is much more subtle. Sailing without foils in small outriggers simplifies things and can give great confidence when in shallow water or around coral reefs. When paddling, asymmetry balances the tendency for the canoe to turn towards the outrigger and I think it was traditionally used for that as much as lateral resistance. The lateral resistance is also enhanced with the rocker shape that is especially apparent in Marshallese designs.
     
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  8. oldmulti
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    oldmulti Senior Member

    Part 2 of Design 318 50 foot 2 beam accommodation pod racing cat. The first boat of this design was named Raw Nerve. It was a carbon fibre nomex build. On an early delivery voyage the boat when moored blew onto a beach and bounced on the bottom in fairly mild conditions. On the next leg of the trip a hull broke away from the crossbeam. A court case ensued everyone blamed everyone else. I spoke to a builder of the initial boat and he also rebuilt Raw Nerve his words were “we rebuilt it with the same materials and design we just changed the fabrication and build method”. This may give a clue of what went wrong. Next issue came when the boat sailed in rough water especially upwind when the forward beam slammed into waves slowing the boat. The result was the first 2 boats were lengthen, Raw Nerve to 56 feet the second to 58 feet allowing higher speeds in rough water. Most of the added length was in the bows. There is a university paper written about slamming loads on multihulls which did an analysis of Design 318 slamming problems. https://espace.curtin.edu.au/bitstr...5859_Grande K 2002.pdf?sequence=2&isAllowed=y

    Design 318 was superseded by later designs but 318 showed the potential of light long thin hulls with big rigs. The structure of these boats proved to be reliable if correctly built, but please be advised you really need to understand how to fabricate Nomex sandwich structures and use proper post curing of the carbon fibre epoxy structures. A later version of a Crowther 50 foot racing catamaran triple 8 was designed in 2006. It was full carbon and again very fast topping at over 27 knots in New Zealand. A few photo’s will give the idea. The PDF is when triple 8 was for sale and contains many good photos and line plans.
     

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  9. oldmulti
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    oldmulti Senior Member

    The next boat is Crowther design 294, a 56 x 27.5 full bridgedeck cruising catamaran capable of global travel. The boat weights 30,000 lbs and displaces 36,000 lbs on 10.5:1 length to beam hulls and carries 1500 square foot of sail on a 70 foot conventional triple spreader aluminium mast. The big difference between this pre 2000 design and todays 55 foot cats is the load carrying ability. The big French cruising cats can carry a 10,000 lbs load. Result the French cats have fatter 8: 1 length to beam hulls requiring more sail to get anywhere near the same speed as design 294. The cat could be powered by 2 x 100 HP engines giving 12 knots or 2 x 200 HP engines giving 20 knots. The boat was designed for ease of construction so only had a single layer of glass on either side of a core in the majority of the boat. EG the hulls were 1121 gsm triax e glass 20 mm H80 PVC Foam 768 gsm triax e glass in vinylester. The decks are 1121 gsm triax e glass 20 mm H80 PVC Foam 576 gsm biax e glass in vinylester. The underwing is 1121 gsm triax e glass 40 mm H80 PVC Foam 768 gsm triax e glass in vinylester. The boat had full foam glass secondary bulkheads and the main beam bulkheads had 25 mm H100 PVC cores with unidirectional e glass top and bottom flanges. These boats were design to travel long distances and had simple comfortable layouts with all sail controls back to the main cockpit. At least 3 of these cats were built by professional builders (at a similar time) with an average time to build the shells of 5000 hours and a fully fitted boat about 9000 hours. A one off home builder could add 4000 hours to that. Stuart Bloomfield did an update to the design and allowed a 9000 lbs payload and made several other improvements. A good boat.
     

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  10. oldmulti
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    oldmulti Senior Member

    A slight deviation. I commented about design 294 being potentially faster than a French catamaran. The Crowther design 294, a 56 x 27.5 foot displaces 36,000 lbs on 10.5:1 length to beam hulls and carries 1500 square foot of sail on a 70 foot mast. Let’s look at the Lagoon 52 a “performance orientated” cruising catamaran. The Lagoon is 52 x 28 foot displacing 57,500 lbs on 7.2 : 1 length to beam hulls and carries 1720 square foot of sail on a 78 foot mast. The Lagoon may have a more powerful rig but it has a heavier boat to drag around on fatter hulls. The Lagoon can average 200 miles per day but peaks at about 220 miles per day. People now want accommodation, air conditioning, washing machines, dish washers, powered winches, hybrid engines, very big battery banks etc and most importantly walk around double beds. As long as a boat can get from A to B “faster” than a monohull these people are happy. Unfortunately, modern mono’s are developing very quickly and are often as fast as a full displacement cruising catamarans. The Crowther 294 will sail faster and be cheaper to “home build” than a than the Lagoon 52 type of boat. But any conversation with a partner will result in a lot of pressure to own a Lagoon 52 over design 294. I can assure you big accommodation and adequate sailing performance outsells moderate accommodation and high sailing performance.
     

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  11. oldmulti
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    oldmulti Senior Member

    Lock Crowther entered the Round Britain and Island race in the 80’s with a british guy in a 45 foot racing catamaran. The boat was run down by a trawler on the final leg of the race. Both men survived. The boat was an extension of a 40 foot racing cat “John West” later converted to Windswept when its pod cabin was expanded. The 45 foot racing catamaran was 45 x 29 foot and displaced 6500 lbs and carried 1200 square foot of sail. The hull had from the outside 300 gsm 5800 glass cloth, 165 gsm 2800 kevlar, 20 mm H80 PVC foam, 165 gsm 2800 kevlar. The decks were outside 300 gsm 5800 glass cloth, 165 gsm 2800 kevlar, 15 mm H80 PVC foam, 165 gsm 2800 kevlar. The hulls had bulkheads every 900 mm. The beams were a box with D front and top and bottom unidirectional flanges. The boat was fast but was superseded by the 2 beam models which were superseded by conventional 2 and half beam versions.
     

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  12. oldmulti
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    oldmulti Senior Member

    The final Crowther catamaran is a racing cat design 304 that preceded the 2 beam cats design 318 and 325. This racing catamaran is 60 x 40 foot catamaran that weighs 12100 lbs and displaces 13300 lbs. Sail area of mainsail 1970 square foot, jib is 630 square foot, genoa 1800 square foot. This is ORMA 60 foot tri power type rig. The mast is a spreaderless 800 x 400 mm and 92 foot high which has both solid carbon fibre on its 4 main axis and foam carbon in lower stress area’s. The hulls length to beam was 16.2 :1. The draft is one third of the hull waterline beam. The hull bottoms were built from outside 2 layers of 150 gsm carbon fibre (CF) uni at 45/45, 300 gsm CF uni fore aft, 64 kg/cubic meter Nomex honeycomb 30 mm thick, 300 gsm CF uni fore aft, 2 layers of 150 gsm CF uni at 45/45. If you want a more conventional foam glass layup you use 768 gsm triax, 30 mm H80 kliogram/cubic meter PVC foam, 300 gsm Kevlar, 600 gsm unidirectional s glass in some area’s. All options are in epoxy. The foam glass option is about 850 lbs heavier. If you build the foam glass version you basically have only 500 lbs payload capability. This boat needs a really competent builder who understands nomex fabrication and post curing of big structures. The deck topside and pod structures are slightly lighter layups with some thinner cores. The crossbeams are box beams with fore and aft webs of carbon foam with top and bottom flanges of 600 and 900 gsm unidirectionals. The build time was estimated at 17000 hours for professional boat builders. I do not know if this boat was built but it has all the characteristics of a very fast boat. This is the 60 foot cat version of a 60 foot racing tri.
     

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  13. oldmulti
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    oldmulti Senior Member

    The Conser 47 was originally designed as a kit catamaran but was productionised in the US. John Conser designed the boat after being involved with the Myer 44 Seabird racing cat. About 9 were built before a series of problems lost the moulds to South Africa in 2002 where they were damaged. The boat was 47 x 24.5 foot weighing 12000 lbs and displacing 15000 lbs. It had a 56 foot mast of the deck carrying 1060 square foot of sail. The cat had narrow 13:1 hulls and low forward freeboard 1100 mm and narrow stern of 300 mm to not overpower the bows. The Conser 47 had a low underwing clearance of 600 mm. One boat has circumnavigated. It Bruce number was 1.48 which is good. One boat carried a carbon fibre wing mast.

    These boats were fast, “Paragon II”, a Hawaiian Conser 47 charter cat holds the speed record of 31 knots. Hull # 6, "SHEARWATER" a long-distance cruiser, sailed at 8-10 knots with a maximum speed of 20 knots. 240 plus miles a day are easily achieved. Her construction is Vinylester resin and triaxial fiberglass knitted cloth and Corecell foam core, vacuum bagged. Her bulkheads are constructed in the same way and integrally bonded to the hull and deck. This construction process yields a very light weight, and very strong and durable hull. With her light weight, close sheeting angles, long narrow hulls and twin daggerboards, she is capable of good speed and very good windward performance, much closer than with most other cruising cats.

    The owner of another Conser 47 Runaway said “I have a Conser 47 and have sailed 40,000 miles on her. Although the bows are narrow the do not bury but are wave piercing. The have almost no bow wave. David just sailed around the world single handed in another Conser 47. His biggest problem is slowing her down. Weight is an issue for top performance (30+ potential) but even heavy she will sail 20+ but it takes more wind to plane. His boat is in great shape. In cruising form we have won many ocean races Caribbean 1500 Bermuda Cup Around Long Island Bahama Rally. We point to 35 deg but Ron in Barbados point to 25 deg with the wingmast and daggerboards.”
     

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  14. oldmulti
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    oldmulti Senior Member

    Bullfrog verbatim is Crowther’s most famous racing trimaran. It was 40 x 35 x 5600 lbs weight. The boat carried 915 square foot of sail on a 58 foot 210 x 150 mm mast. This boat would match it 50 foot racing cats. Bullfrog held the Solo Tasman record and won 32 consecutive ocean races. Ian Johnston (who built and sailed the boat) will describe Bullfrog (later Verbatim, Australia’s Child).

    “One of the first safe, strong, predictable, seaworthy ocean racing multihulls in the Southern Hemisphere. She could be raced to windward in a gale and several times we sailed donuts around maxi boats in those conditions. But we had to be careful when power reaching, she could go tremendously fast but because she was only 40 foot long she could get a bit mixed up in uneven waves (longer boats eg 60 foot, can be long and lean) but Bullfrog because she was shorter needed to be a bit fuller when the amas were well immersed to carry the weight. She could develop a lot of drag when she scooped heavy water onto the forward beam fairing. We solved the problem when we installed the second mast (after the first year) raking it well aft, this generated lift and solved the problem. You must keep the runners on so the forestay can be kept very tight as this allows very close windward performance.

    Lock Crowther gave us the line drawings, bulkhead designs and strength calculations based on foam sandwich. I came up with the strip planked 19 mm balsa construction method above the waterline because to get foam sandwich stiff enough to withstand the tremendous forces generated it would be either to heavy or not durable. The 19 mm balsa planks are in short lengths isolated with epoxy and each piece is selected by its density, (stressed areas heavy, light timber towards the ends of the vessel). So if one area is damaged it will remain isolated, the inner skin was 6oz (197 gsm) kevlar for stress and impact resistance and the outside skin 2 layers of 6oz (197 gsm) unidirectional e glass with the fibre oriented to maximise torsion strength in the centre of the boat and bending forces forward (I used the impressive strength along the grain as part of the strength calculations unlike foam which has no strength). Below the waterline on the main hull core was 12 mm WRC. The crossbeams were 50 mm square of unidirectional glass flanges top and bottom of a web with D front fairings. Bullfrog has done over 120,000 miles without structural problems. With a lot of money, I would replace the rig with exactly the same design in carbon fibre, fibre stays, change the shape of the forward beam fairing and increase the size of the main hull bulb bow.”

    The tri could do 250 miles/day cruising and 350 miles/day racing. It peaked at 30 knots. Again from Ian. “Our speed to windward was very good, the sheeting angle on the jib was 7 degrees, very tight forestay, and we used the speedo as the only instrument, our normal speed was 13 knots and this is where a trimaran is at its best--the leeward hull which displaces twice the weight of the boat stays firmly in the wave train bursting through the wave tops while the main hull is very light, and being narrow, is able to leap from one wavetop to the next flying over the troughs and the landings are soft. The windward ama is waving about in the air, but the ride is quite soft, there is almost none of the lurching motion of a catamaran. We used to feel safe and comfortable (but bloody wet) we occasionally went past big monos doing this, pointing just as high and going faster. We needed to wear goggles and dry suits under our foul weather gear”.
     

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    Last edited: Sep 30, 2019

  15. oldmulti
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    oldmulti Senior Member

    A sailing hydrofoil catamaran was designed by Mark Pivac and constructed in Western Australia by Windrush Yachts. The 12 m (40 foot) long Spitfire is a catamaran with a pair of aerodynamic crossbeams connecting the widely-spaced hulls. These also support a raised cockpit and storage area on the centreline of the craft. Three retractable hydrofoils are fitted, comprising a pair of surface-piercing units outboard of the hulls and somewhat forward of the centre of gravity, and a combined rudder and fully-submerged hydrofoil mounted in an inverted ‘T’ configuration on the centreline aft. Twin masts mounted on the demi-hulls support a pair of ‘soft wing’ sails. The design modelling indicated a maximum boat speed is achieved while reaching at about 110 degrees to the true wind. For this heading, a speed of around 32 kn was predicted for 15 kn of wind. Take off to foil-borne operation should occur at around 10 to 12 kn, which can be achieve in 10 kn of wind. Once on the foils, Spitfire will accelerate to approximately 25 kn in 14 kn of wind. The boat meet most of its predictions. In only its third outing off the coast of Perth the hydrofoil catamaran lifted its top speed to 30 kn, on a day with only 18–25 kn winds. Mark Pivac expected Spitfire to reach a maximum speed (without ballast) of approximately 35 kn in 25 kn of wind or less. With water ballast tanks fitted, the estimated top speed is above 40 knots. The three hydrofoils are placed in such a way that the two main foils support most of the weight of the boat, and the rear foil mainly provides balance and trim adjustment. Surface-piercing main foils are used as they offer several advantages. Firstly, as the boat’s speed increases, more lift is generated on the submerged portions of the foils and this, in turn, causes the foils to lift themselves and the boat further out of the water, thus maintaining equilibrium between the weight and lift forces on the boat. This leaves less drag producing foil surface area in the water, which is perfect for good performance. Secondly, the foils are inherently stable. If the boat rides too high, there will be less foil area in the water and hence less lift generated and the boat’s weight will force it back down. Conversely, if the boat is riding too low, the additional submerged foil area will generate more lift and raise the boat. The foils, rudder skins and shear webs are predominantly constructed of unidirectional and double-bias carbon fibre prepreg. The shear webs were formed over foam and plywood and are bonded between the two skins. Spitfire’s foils can be retracted to enable shallow water operation and beaching. The centrelines of Spitfire’s hulls are located 8 m (26 foot) apart, giving the boat excellent heeling stability both on and off the foils. Despite travelling at high speed the ride is reported by the crew to be smooth and quiet, even in relatively choppy 1.2 m seas. The hulls are of GRP sandwich construction with low density balsa core and E-glass skins. Bulkheads are of 50 mm PVC foam core, with a combination of glass and carbon fibre skins. For each of the hulls, the two hull halves were constructed in a mould and then glassed together. The laminate was wet out with vinyl ester resin using a vacuum bag infusion process, allowing this task to be completed in less than an hour for each hull half. The owners lost interest in the boat and it was sold to NSW. I do not know what happened after that.
     

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