Multihull Structure Thoughts

Steel is not a material that you build multihulls from BUT there are exceptions. A school teacher in rural victoria Australia built in his spare time a 100 foot long full bridgedeck cruising catamaran which was used for charter work for about 30 years. Last seen in Queensland. 6 mm steel in hulls 4 mm steel superstructure. He did the job in about 8 years. Amazing.

Smaller steel cats have been built. A 36 footer full bridgedeck that crossed the Atlantic. This boat was so overweight it had less than 1 foot underwing clearance and could sail but was slow!

Another was a Boden designed catamaran Catrina which was 36 x 18 foot full bridgedeck displacing 25000 lbs and weighing 17000 lbs built. At least one was built and floated to lines. The multi chine hull was 3 mm plate with a 4 mm keel plate. Frames were 44 x 44 x 5 mm angle sections or 63 x 51 x 5 mm angle sections at 500 mm center lines. Decks and cabin structures were 3 mm steel with a lot of light framing (the design is no longer on the Boden site). Steels basic problem is it needs about 3 mm minimum thickness to weld smoothly unless you are a really skilled boiler maker. 3 mm steel weighs 5 lbs per square foot versus 9 mm ply at 1.25 lbs per square foot or foam glass at under a 1 lb per square foot. Also steel requires a lot of framing to support flat panels. Steel main virtue is it bends but doesn't break in most hits that would hole a plywood cat.

But maybe we can learn something from monohull boats. Brent Swain builds what he calls origami boats. Basically a tortured steel monohull. Get a large plate of 3 mm steel cut darts in it at appropriate points, add three 25 x 25 mm square stringer for 70% of the hull length, add a gunnel tube then pull the half hull into shape and weld up the dart seams. Get the 2 hull halves and weld them together. A 36 foot monohull with minimal framing is ready for internal fitout and decks. Monohull boats from 27 to 65 feet have been built this way. Home https://www.freewebs.com/origamiboats/ During the war 2 a British firm built a series of stress skin aluminum patrol boats from flat sheets of aluminium again pulled into shape and then "glued" and riveted together.

Another really interesting steel monohull was a 3 masted schooner Sundancer built by a British farmer in his 105 x 20 ft shed with a 12 foot wide door. How big was the boat? 104 x 12 foot. It displaced 78000 lbs and carried 2000 square foot of sail. It had a retractable keel. Why is interesting? It circumnavigated 3 time and could hit 17 plus knots, it averaged 11 plus knots. It was built from 6 mm steel pipes 12 foot in diameter. The ends were pulled in to form a monohull shape with a 8 to 1 length to beam ratio. Deck webs were added to the tube to provide a 12 foot wide flat deck and flat topsides for additional stability. The boat heeled a lot but was fast. It was in Australia at Ballina for a few years.

Finally a classic wharram cat of 40 foot plus had its hulls built in 3 mm steel but the rest of the boat decks etc was in plywood. A reasonable compromise if you like sailing around reefs. Steel if properly designed can form the basis of a multihull but it will not be fast unless it is large. Finally I do know of a 48 foot trimaran with a ferro cement main hull that sailed up the Australian east coast. It had steel crossarms and plywood floats. The main hull was 6 foot wide with no overhangs. Again it sailed but was slow. Len Hedges also designed a 42 foot ferro cement wide hulled tube catamaran that was partially built until the builder realized he could not sell the finished boat for anything like to money it was costing him.

 

Trip the light fandango. I have only seen 1 Tremolino sailing on Port Phillip bay but it was flying. Looks like a lot of fun. You probably already have the attached study plan for the Mark 4. From my reading of a home builder, the cross arms are 165 x 90 mm tapering to 165 x 50 mm with a top and bottom flange of 165 x 19 mm and the sides 4 layers of 25 x 12 mm in the centre tapering 1 25 x 12 mm at the end inserted into the hull. Wire stays are attached to the beam going to the centre hull.

 

That may have been my boat and the original owner, if it was a fibreglass Trem.
I have 12sheets of 4mm okoume so all this info is very interesting but I'm staying focussed on glassing floats and waiting for the weather to improve [and time to overlap] for now..ha. I'm guessing after a quick read that anything that needs to be welded should be 5083, and masts are 6061[which weakens considerably with welding,.. copper content.03 or somert..] ?. I can afford 2nd hand stuff and old mast is probably my best bet for beams, apart from using my ply.. I read 5083 has greater tensile, more rigid, does it look slightly creamier .? I have 2nd hand Hobie 16 beams to double up the original. I'm wondering how they shaped 5083, frames and Spanish windlass again? Doesn't need a reply if it isn't an easy answer, just thinking aloud really, I can wade through old threads and google, thanks again for the info, very useful.

 

Hi. Aluminum is not my thing beyond it being stamped on the aluminum with a grade. Most masts are 6061 T6 but beyond that I deffer to experts. As for bending of aluminum masts be careful as it can take moderate bends relative to its diameter but mast sections being forced out of column by bridles etc can only go so far. Wood beams may be better but I have not worked out a cost etc.

Col Clifford is an Australian designer that prefers to design some of his multihulls using an aluminum frame and plywood skins. Compucraft Yacht Designs - Australia's longest serving & most experienced designer of Yachts, Catamarans & Trimarans, SAILING YACHTS, POWER BOATS, MULTI-HULLS, CATAMARANS, TRIMARANS, POWER CATAMARANS and MOTOR LAUNCHES http://www.ccplans.com.au/index866a.html?pag_id=55
His 37 foot and 43 cats are examples. They have keel, stem and gunnels of aluminum tubes 40 x 3 mm of 6060 T5. The hull framing is 25 x 1.6 mm 6060 T5 square tubes. Hull stringers and chines are 40 x 1.6 mm 6060 T5 round tubes. The good part of this approach are the range cast aluminum frame joiners used to connect the various aluminum tubes together. There is only a limited range of joiners as the boats have on have a limited range of angles designed in them. The tubes and connectors are pop riveted together. A 9 mm plywood skin is then placed over the frame and is either screwed or pop riveted to the frame. After filling and fairing a glass cloth skin then covers the entire external shell. I have only seen one of these boats in the water and cannot comment on its longevity. In areas where you cannot get good quality framing timber this may be a good framing approach even with aluminum tubing welded together or create a series of fiberglass joiners. If you like the concept speak to the designer. Next time we will discuss a 40 foot cat built with 4 mm ply skins that raced successfully in Britain.

 

Prout did some very interesting early developmental designs before producing its range of rather boring cruising catamarans. One of experimental boat was Phantom Wake, an 80% bridge deck catamaran of 40 x 19 foot that weighed 5000 lbs and displaced 6000 lbs carrying 610 square foot of sail. The hulls were asymmetric. The hulls were 4 mm ply on 19 x 38 mm stringers at 150 mm centre lines. There was a 6 mm ply shelf across the hulls at wingdeck level. The hull gunnels 600 mm higher were 19 x 70 mm. The keel was 150 x 19 mm with a T section cap of 120 x 19 mm. Frames were 6 mm ply a minimum of 110 mm wide at any point. The main cross beam box was 580 x 36 mm with fore and aft faces of 7 mm ply. The top flange was 22 x 22 mm with a 22 x 22 mm doubler over the mid 3.4 meters. the bottom flange is 22 x 45 mm for the full width. Vertical/diagonal trusses of 22 x 38 mm separate the top and bottom flanges with a 22 x 50 mm mast post in the centre. The rear beam is a 580 x 28 mm open "box" beam with 6 mm ply on the aft face. The bottom flange was 22 x 38 mm, the top flange 22 x 38 mm doubled over the centre 3.3 meter. The decks were 6 mm with 19 x 38 mm stringers at 150 mm centre lines. Cabin roof were 6 mm with 19 x 38 mm stringers at 300 mm centre lines. Underwing stringers were 19 x 38 mm at 150 mm centrelines on 6 mm ply. There was a vertical 6 mm ply panel from wingdeck gunnel to deck running the full length of the wing deck. The details were in Multihull International magazine 148. The boat raced successfully for a few years around Britain. The boat was very well built and shows it is possible to have very light skins if you have a lot of framing to support it. In the Philippines were framing timber is cheap but ply expensive they build 60 foot offshore fishing boats with 6 mm ply with stringers at 300 mm centre lines.

 

Seabird was Hugo Myers 44 x 20 foot open wing deck ocean racing catamaran. It displaced 6500 lbs and carried 1020 square foot of sail. it was one of the first serious open wing deck tube cats that could seriously race across oceans. It did several transpacs holding the elapsed time record at one point. The hulls and decks were cold molded with 2 layers of 4.5 mm ply on 25 x 25 mm stringers at 150 mm centrelines under water and about 200 mm above the waterline. Frames 75 x 25 mm every 530 mm. Gunnels was 40 x 75 mm. The hulls were covered by 300 gsm glass cloth doubled below the waterline. The main and rear crossbeams were 200 x 6 mm diameter aluminum tubes with 188 x 6 mm inserts at hull intersections. A dolphin striker is on the main beam. The boat initially had 12 mm bolts connecting the beams to the hulls until one rough trip where the bolt head were being snapped off by the wracking movements. The beams deflected up to 100 mm normally but on this trip deflected up to 300 mm. The mast tube was a 270 x 3.5 mm diameter aluminum tube with plastic fairings from the mast to behind the sail track to create a semi wing mast. This boat had fine ends to help it through rough seas but it was wet, wet, wet. Several versions of the design were built in a variety of sizes for 28 to 50 feet.

 

D class catamarans were popular in the 80's and some were very advanced for their time. Imagine a Tornado blown up to 35 x 18 foot weighing 850 lbs and displaced 1300 lbs and carrying 500 square foot of sail. The hulls and decks were 300 gsm unidirectional S glass fore and aft, 450 gsm graphite at 90 degrees outside 9 mm airex inside 2 layers of 450 gsm graphite at 90 degrees to each other. A sub deck (2 layers of 450 gsm graphite at 90 degrees to each other on both sides) runs from the bow to 66% of the hull length. BH's are under the cross beams. Forward cross beam is 140 x 3.5 mm aluminum tube with a dolphin striker. The rear beam is an aluminum 140 x 2.8 mm tube. The main and rear beams were inserted into light fiber glass tubes inserted in the hulls. The mast is a wing of 14.7 meters 500 x 115 mm for the first 11 meters. the mast then tapers to 225 mm at the tip. The mast has 2 diamonds. It is built from a central box spar with 7 x 115 mm walls. 3 mm ply webs run fore and aft with light foam forming the nose. The wing skin is 450 gsm uni directional graphite over 1 mm ply. A PVC tube is glassed to the back and cut to form a sail track. This was a very fast boat that did limited coastal races in California.

 

Wow! Great info and amazing thread. Thanks! I have given a lot of thought to the seamingly huge differences in weight of multis. It's not uncommon for chunky production boats, weighing almost twice in light displacement compared to custom builds. One designer that continues to impress in this matter is Kurt Hughes. For instance his 30' half height bridgdeck cat is designed to sub 1350kg/2900pound. Sure that i with basic finish and probably excluding most items of comfort as well as 100pound+ gel coat, high gloss paint and chrome trimmings. But still. It's a design that is probably considdered as a prodiction boat with margins for amateur build standard. Easy and fast to build, with no exotic materials like carbon, but still. As a reference the early 1990s Fontain Pajot Maldives 32 is quite similar. 2 feet longer, but also 2 feet narrower, with low bridgdeck, only slightly higher with some simple windows, but still not full standing height. A stylish period correct targa top can be raised to get full standing headroom. This for it's time, space age full glass foam sandwich construction tips in at about 2000kg lightship and 3000kg design displacement somewhat unreliable numbers). Ok, that does probably include a bit more fairing, shiny gelcoat and items of comfort. But still. It is ca +50% difference despite it's "high tech construction".

 

Niclas. I will do a post on design structure versus weight differences on multihulls soon. The best example of the way designers think is done by Dudley Dix with his Didi 38 foot monohull which is molded ply on a timber frame. The boat for racing (across the Atlantic on several occasions) has a 12 mm skin (2 layers of 6 mm ply). The exact same boat and frame structure has a 18 mm skin (3 layers of 6 mm ply) for cruising. Paul Gartside, a very experience designer, recently wrote "the determinations of structural scantlings is an imprecise business ... there are no fixed rules". The reason for writing this thread is to show what designers have done and get others to share their experience on how successful its been. Even the great Rudy Choy on Alley cat (or patty cat?) 44 x 18 foot 7500 lbs racers built the boats with 12 mm ply BH's with no edge framing. A 12 mm ply bow BH fractured during a Transpac race, allowing the 6 mm inner tunnel hull skin to flex and fracture. The lack of edge framing on the BH gave insufficent support to the BH. The outside hull skin was 9 mm ply. The variations in structural design by designers for a given displacement is amazing.

 

Strider 24 is an example of a boat with variable structure for the same boat. I normally would not describe the details of a design still being sold but there is so much detail about this boat on even Woods site it should be OK. Strider is a 24 x 16 foot open wing deck cruiser racer weighing 1700 lbs, displacing 2400 lbs with 270 sail area. The hulls started as ply on timber frame. The hull has 6 mm ply BH's with 25 x 50 mm timber edging, 25 x 50 mm inner keel, 25 x 37 mm external keel, 25 x 37 mm gunnel and ?? stringers. The decks are 6 mm ply with stringers etc. The beams are box timber which I will describe in a later post. Now the interesting part. The flat part of the hull skin can be 4 mm ocume ply(racer), 5 mm ply (if you cannot decide), 6mm ply (cruiser/racer). The double molded bottom is 2 layers of 3 mm ply. Some ply boats are covered with 200 gsm glass to waterline other boats covered all over. If you want a strip plank version the hull can be as light 200 gsm cloth 9 mm western red cedar 200 gsm cloth or as a foam core of 10 mm Airex or Klegcell with 600 gsm glass cloth on either side. Richard Woods is a pragmatic designer who sells across the world. Many builders cannot get good 4 mm ply but can get average 6 mm ply so they in build 6 mm ply. Also the weight difference between a 4 mm ocume ply boat that is only painted but no external glass would be very close in weight to a foam glass boat. You choose your purpose and build accordingly. If you are a careful racer 4 mm ply with no glass will be great, if you are a lazy cruiser and crash into things 6 mm ply may not be enough. Part of the reason why boat structures vary so much.

 

White Knuckler was a californian open wing deck racing cat 46 x 25 foot that displaced 7000 lbs and carried 1600 square foot of sail. the hulls were advanced for there time with 3 layers of Orcon s500 uni directional glass with 1 layer fore aft and the other 2 layers laid at 60 degrees to each other on the outside 19 mm airex foam and 2 layers of Orcon s500 uni directional glass laid at 45 degrees inside. The resin was Vinylester. There were 22 bulkheads at 600 mm centrelines inside the hulls with no stringers. Cross beam sleeves in the hulls were built from vinylester resin and unidirectional s glass. S glass is a lot stronger than normal e glass and vinylester resin is stronger than polyester resin nearly matching epoxy resin. The mast was 60 foot high putting a lot of pressure on the main 250 x 19 mm aluminum tube crossbeam that had a vertical web in it. A 15 mm solid rod dolphin striker was also installed below the beam. The boat has a 80,000 foot pound righting moment and could carry 1600 sq foot of sail in 22 knots of wind which is the reason for the massive main beam. I suspect the main beam could have been lighter with current knowledge.

 

Niclas. I will start the comparison of hull skin structure versus weight with multihulls of about 15,000 lbs displacement. I will not be saying boat names if the designer is still selling plans for that boat. 3 Australian cats Bagatelle 43 x 25 bridgedeck cat 15000 lbs 4 layers 330 gsm S cloth outside 15 mm foam 3 layers of 330 gsm S cloth inside polyester resin BH's and some ring frames. A 46 x 23 cat 14000 lbs bridge deck cat 756 gsm triaxle, 200 gsm uni under the waterline outside 30 mm airex foam 300 kevlar inside main BH's only epoxy. A 44 x 25 bridgedeck cat 15000 lbs 600 gsm double bias 12 mm western red cedar (WRC) 600 gsm uni direction of 80% vertical 20% horizontal epoxy resin internal furniture and shelves. 2 New Zealand cats. A 45 x 22 bridgedeck cat 14000 lbs 756 gsm triaxle 19 mm WRC 756 gsm triaxle epoxy resin with BH's and some ring frames. A 45 x 26 bridge deck cat 15400 lbs 400 gsm double bias 12 mm WRC 400 gsm double bias epoxy resin with BH's and some ring frames. 2 US cats 45 x 26 bridge cat 13500 lbs 1156 gsm triaxe 25 mm corecell 1156 gsm triaxe below waterline 1156 gsm triaxe 20 mm corecell 1156 gsm triaxe above the waterline in epoxy BH's and some ring frames. Horizon 48 48 x 25 16000 lbs bridgedeck cat 700 gsm triaxe 17 mm WRC 700 gsm triaxe with BH's and some ring frames in epoxy. As you can see for about the same displacement it ranges from EG 12 mm WRC with 400 gsm either side to 19 mm WRC with 756 gsm either side. The "strength"/ stiffness difference between these 2 hull structures is large. All these boats have many miles of ocean sailing without a problem.

 

Some strange building approaches have been used by some famous designers. EG Rudy Choy designed a 50 x 23 bridgedeck cat that weighed 16000 lbs called Glass Slipper had its hulls built by making a male mold. The mold was then sprayed with 4 lbs/cubic foot polyurethane foam. The foam was then faired down to 50 mm thick. 2 layers of 685 gsm woven rovings was laid over the foam and then faired again with polyester and filler. The hull was pulled of the full male mold and glassed on the inside with 2 layers of 685 gsm woven rovings. The rest of the boat was ply timber. It lasted for years BUT low density polyurethane foam with open cells tends to crumble and soak up water over time. Airex, corecell etc are closed cell foams. In 1958 Choy, Brown and Seaman also produced Foamy 24 x 10 cat of 1800 lbs displacement built of strip of 18 mm styrofoam covered of with epoxy glass. The boat did inshore and coastal sailing for years. Malcolm Tennant was creative at using odd materials. He built main and secondary BH's on some large cats from EG 6 mm ply 20 mm styrofoam 6 mm ply. He also on his smaller boats (under 30 ft) did flat panels from the internal diamond cardboard cores of cheap doors that were soaked in epoxy resin for a short time then had EG 3 mm ply put on either side. and Ruel Parker a USA mono designer uses blue styrofoam as a core material in a ply foam sandwich for some 30 foot plus monohulls. Another australian designer use the high density version of blue styrofoam as a core material used in the foam glass crossbeams of a 28 foot racing trimaran. Its still racing after 30 years. In a 35 cat the internal furniture was built with 330gsm glass cloth 12 mm blue styrofoam 330 gsm glass cloth. The seats started to collapse in 2 years and other components cores started to fracture. In the 70's 2 of Hedley Nicol Wander trimarans were cold molded from 2 layers of packing case timber (about 5 mm ply of junk meranti plys) glued together with who knows what. Both these boats are still sailing today. Jay Benford designed and built a 12 foot long keel boat that displaced 860 lbs of ferrocement (wire matrix plus cement) of 5 mm thickness. The boat sailed well. There is a global competition among uni students to build concrete canoes and race them. They use glass Woven Rovings and cement and are under 3 mm thick. Now some general warnings. Styrofoam and some polyurethane foam react to polyester resin, they require epoxy for building. Blue styrofoam come in a variety of densities, the heavier the better but it MUST be used in low shear environments.

 

Boats between about 20 and 24 feet appear to be of interest to many so I will focus on design of hull skins for this size. Again some boats will be unnamed due to designs still being sold. Tornado 20 x 10 foot 500 lbs displacement 200 gsm uni outside 3 mm WRC 200 gsm uni inside all in epoxy. A 20 x 8 foot cabin cat of 1100 lbs has 450 gsm double bias 8 mm wrc 450 gsm double bias in epoxy. A 20 x 8 production cabin cat of 1800 lbs displacement has 450 gsm double bias 3 mm corecell 450 gsm double bias in polyester. The boat has 12 mm plywood crossbeam bh's. Trikini a 20 x 17 cabin tri main hull was 270 gsm cloth, 450 gram copped stand mat (CSM) 9 mm airex foam 450 gram copped stand mat, 270 gsm cloth in polyester or a solid glass version of 3 layers of (300 gsm of CSM and 200 gsm glass cloth) all in polyester. A 21 x 12 foot glass version of a popular ply cat displacing 1650 lbs has a 225 gsm CSM, 1708 biaxe outside 9 mm divinycell 1708 biaxe inside in vinylester resin. A 23.5 x 16 foot tri mainhull displacing 1700 lbs has 330 gsm cloth 9 mm WRC 330 gsm cloth in epoxy resin. All these boats are bay/coastal sailers (except the Tornado). Each has BH's and some vertical foam ribs and in almost all cases the ply equivalent boat (of the same design) is lighter. I dont know the exact cut off point but after about 25 feet foam sandwich probably is lighter than plywood.

 

Trimaran crossbeam structures are an interesting area. Many famous designers have had a broken trimaran cross beam in their past EG Derek Kelsall wrote a book about a 35 ft racing trimaran FT and its cross beam troubles. But in this post we will talk about the crossbeam changes in design for a Twiggy 32 x 29 foot 4200 lbs displacement tri with 680 square foot of sail. The boats righting moment is 55,000 foot lbs. The original design had a plywood wood box beam with water stays, The beam is 255 x 380 mm box with 12 mm ply fore and aft faces. Top and bottom faces 9 mm ply. In each corner of the box is 5 layers of 19 x 45 mm laminated timber to a 45 mm high x 95 mm horizontal flange that goes full width of the beam. Two water stays of 8 mm 1x19 SS wire are attached to each side of a beam from just beside the float to 500 mm down the main hull. The beams were strong enough but were heavy. The twiggy the beams were upgraded to foam glass beams which were several hundred of kilo's lighter. The foam glass beam has a 25 mm divnylcell foam web with 5 layers of 500 gsm cloth on each face. The top and bottom flanges are 25 x 40 mm of uni directional glass in the centre tapering to 25 x 15 mm in the ends of the beam. When doing these unidirectional flanges DONT lay them up in one go. They need to be progressively laid up over 24 hours as one thick lay up builds up to much heat as it cures and often fractures the layup. Also each uni directional layer need to be tensioned to ensure the fibres are straight. Finally use epoxy in the flanges as it is "stronger" than polyester etc. On the front of the beam is a triangle of 12mm foam triangle with 2 layers of 500 gsm on each face. The triangle acts as a torque box. Attached is a photo of the original beam and a plan of the glass web form the web.