Multihull Structure Thoughts

Aluminum multihulls come in a few formats most of them large (over 40 foot). In a previous post we spoke about a thick aluminum hull approach (Prometa Banana Split). We will now talk about the more conventional approach of stringers frames and thin plates. The boat we will use to start the conversation is Osram VII. A 50 x 24 foot cat that weighs 12,000 lbs. All plate is 5086H116 and all shapes are 6061T6. The bottom plate is 3/16", the lower hull sides are 5/32" and the rest of the plate is 1/8". The stringers on the bottom and lower hull sides are a hat section about 3" wide, 2" high, 3/16" wall. Most of the upper sides and deck stringers are a 'T' 2"x 2"x 3/16", with a larger section in the cabin top. Construction is jigless with panels being assembled as sub-assemblies with longitudinal stringers, then tacked together. Bulkheads are put in place at various phases of assembly. Osram 7 build logs are at Osram VII Construction Details https://ideaintegrator.com/boats/o7/const.htm which gives a lot of detail. Tony Bigras has built many home designed boats ranging from 16 foot ocean crossing catamarans Miss Cindy to real cruisers like Osram 7. A very talented guy.

 

There are many aluminum cat designs for builders. EG Bruce Roberts has a range of aluminum sailing cats from 13 meters to over 20 meters (42 66 foot). A few samples are attached of his designs and some structure photo's. Many home builders also have built aluminum cats. What interests me is how fast they can get the basic build done. EG Osram 7 structure was done in under 2400 hours. Jaydn built his 48 foot cat in a year. Photos attached. The boat structure is aluminum 5083 with 5183 1.2mm wire. 5356 fine too. All pulse mig. Extrusions are 6061, etc (round/square hollows, t-bar, solid rod). Hull 4mm, deck 4 & 3mm. 20, 16, 12mm for chainplates, etc. We did go with all alloy rudders using solid stock. I was told by a professional aluminum builder that aluminum is as easy to work as wood, but its big advantage is it is a consistent material that can be joined very rapidly. The real problem according to him is the designers need to understand how to design a structure that has appropriate strength at weld points. You also need a talented welder who really understands the characteristics of the material. Anything under 3 mm is hard to weld without distorting.

 

The final on aluminum multi's for a time. Some designers exploit aluminum advantages for unusual designs EG A 40 trimaran with aluminum hull and composite decks with foldable cross arms. Or a charter cat that is optimized for customers if not looks. But aluminum is mainly used in cat designs even when the cats were originally designed for composite construction. EG Crowther Design 85 which was a 45 x 24 foot cruising cat of about 20,000 lbs displacement. many versions were built including a 50 x 24 foot 24000 lbs displacement aluminium version. This version has a 4 mm plate hull with 37 x 37 x 3 mm T stringers on 250 mm center lines. Frames are 115 x 50 x 5 mm T sections in the hulls at 1200 mm center lines. The Deck and cabin plate is 3 mm with 75 x 4 mm with a 25 x 3 mm flange to form a T section deck and cabin frames with 37 x 37 x 3 mm T stringers. The gunnel deck connection has 100 mm x 5 mm flat bar as reinforcement. Attached is a layout plan of the 50 foot cat. Finally is a skeleton view of an aluminum cat hull frame and shell

 

We need to get back to a simpler life. I really like this guys solution to the need to shift stuff between some Canadian islands. In the late 70s he designed and built a crude sailing catamaran.
It was 31 feet long out of steeltubes (rolled 2,5 mm plates). The crossarms are 4 scaffolding planks (upright), the keel lines are 10 foot apart, hull draft is 9 inches with shallow 8 feet long keels, for grounding and lateral resistance. Masts are 2 telegraph poles 19 feet long in heavy tabernacles, hinged, lowered when needed. Sails - 5 battens, fan shape (home designed) of awning material, approximately 16o square feet each. The boat took a while to get it sailing on the wind in light to medium wind, best result with the windward sail a wee bit harder in than leeward. Seems paradoxical, beam reach was different - leeward harder. The boat steered so well with the sails alone that I could take both rudders off. Wing and wing downwind in a breeze broke some battens - which were just pine from the local building supply. Better material would be polyethylene water pipe. The guys name is Hermman Otto. This boat would have been cheap but performance would not be high but he is sailing and I bet having fun as he learns.

 

Lotsa fun bits in this thread, hats off.

 

The next level of simple was done by a gary Lepak. He built Dragon Wings a 34 x 20 foot cat of 10000 lbs displacement "tube cat". The cat had a biplane rig with junk sails. The length to beam ratio on the hulls was 7.5:1 or 4 foot wide on a 30 foot waterline. The hulls were dory shaped of 12 mm plywood and 9mm plywood decks. The masts were solid fir 250 mm diameter at the deck tapering to 100 mm at the mast head. The mast was 10.7 meters long with 9 meters above the deck. The junk sail was in a hasler pattern with 8 batterns of 50 x 50 mm timber. The cat averaged 8 to 9 knots but its top speed down a steep wave was 15 knots. Gary lived aboard with his family for 8 years before selling the boat in San Francisco to a guy who said it was hard to get it to go to windward. The next boat along the same lines was China Moon designed, built in 2002 and sailed half way around the globe by Peter Hill a 38 x 19 foot 9000 lbs displacement cat that was much more refined than Dragon Wings with better length to beam and sail area to displacement ratio's. China Moon also had lighter construction and masts with a cambered Junk sail. China Moon does sail upwind. Peter Hill then built Oryx a 33 foot extended version of KD860 cat design. It has a sophisticated swing wing rig (onyx 1) then a split junk rig (oryx 2) that allows the cat good around performance. This boat has crossed oceans. The final biplane cat we have mentioned before PHA a 30 foot Tiki cat with freestanding masts on each hull that measures 33 foot high is 200 mm wide at the deck 100 mm wide at the tip and is built from 12 slats of 25 mm white pine, rounded and wrapped in 2 layers of biaxale cloth with a layer of carbon fibre from the mast base to 1 meter above deck level. PHA has a swing wing sails that allow PHA to sail well upwind and on all other points of sail. It has crossed the Atlantic. Next we will look at a few very modern versions of biplane rig cats.

 

The following are study plans for Schoning Radicalbay 8000 and 1060 design. Modern designs with carbon fibre masts. Several 8000 are raced in asia. I have spoken to a guy who has sailed the boats and says you have to think differently to sail them well, The sail and sail area has to be matched to the daggerboard position and wind strength and the course you are sailing. Reaching can be compromised having to sail up or down depending on the course. He said they are fast all rounders. The next design is Bizen 53 a 53 x 28 foot cat of 20000 lbs displacement with 15:1 hulls and a prismatic of 0.61. The sail area is 1100 square foot with 750 sq ft genoa. This will be a fast boat. One plan shows the structure of the mast support, interesting. The boat is strip plank cedar and plywood cover with epoxy. Possible home build if you have the courage. Finally the masts of a 46 Tiki biplane cat PHA Grand. Length 53 foot, 6 foot bury below deck. Mast at foot is 130 mm square, mast at deck 300 mm round, mast at head tip 100 mm. Mast is made of 50 mm wood strips around round wood bulkheads every meter or so. It has 450 gsm glass cloth all over. It is solid at the base, at deck level and the tip. Each mast weighs 610 lbs.

 

Chingogan produced 40 foot and 52 foot cats in the late 90’s. Peter Coram built these boats well and several were sold. We will focus on the 40 footer. They were bridge deck cats 40 x 23.5 foot displacing 16500 lbs with a sloop rig of 1000 square foot area. They were foam glass and built in molds. They were good performance cruisers. The boats came in a survey version and a private owners version. The survey version hulls had from the outside a 450 gram csm 1150 gsm triax 19 mm foam 375 gram csm 868 gsm triax. The private owner version has 225 gram csm 751 gsm triax 19 mm foam 225 gram csm 568 gsm triax. That is quite a difference in hull weight although it would take about the same time to layup. The low aspect keels are 14 mm solid glass on a plywood structure with a 19 mm ply sacrificial layer on the bottom. The underwing is the same as the hulls version of layup. The main crossbeam is a 20 mm good quality plywood web with 2 layers of 760 gsm triax on either side in epoxy. The top and bottom flange s are each 225 mm wide x 12 mm deep solid unidirectional e glass in epoxy.

 

As a comparison of the previous cat. Crowthers Design 226 42 cat was a 42 x 23 foot cat of 13200 lbs displacement carrying 1050 square foot of sail area. It is a bridge deck cat. The hulls could be built from the outside with 1150 gsm triax 20 mm foam 760 gsm triax OR if you wanted a lighter hull layup 760 gsm triax 20 mm foam 300 gsm Kevlar. There was a 300 mm wide 900 gsm unidirectional pads under all bulkheads. The decks are 706 gsm triax outside 20 mm foam and 650 gsm biax on the inside. Same concept of boats between the previous Chingogan and 226 but different designers had multiple layup options to “suit” what the boats were going to be used for and what builders were capable or able to build with. Each of these boats have sailed long distances performing very well. If you find a good one second hand and you will have a good boat.

 

Woodenboat magazine Sept/Oct issue has a good article on the strength of scarfs from 4:1 slopes to 20:1 slopes versus solid timbers. You will have to read the article online but it says a 12:1 scarf deflection equals solid wood, an 8:1 scarf deflects to 50% of solid wood before breaking and a 4:1 scarf deflects only 33% of solid wood before breaking. This is a bit of a revelation. The Strength of Scarf Joints https://www.woodenboat.com/strength-scarf-joints

 

The following is a work of art more than a Val 3 30 foot Newick trimaran. It was built in Canada. The Lacquered finish would not survive in Australian conditions without full time maintenance. The details like genoa/jib track supports are amazing. It looks cold molded and very well done. The hull lines should be studied as they were at the end of a long line of development by Newick. The owner says the boat sleeps one uncomfortably. The mast is wood also.

 

For those who cannot decide if they are coming or going the proa Madness CLC should fill the bill. Its 30.5 x 20 foot with a displacement of 2000 lbs. 300 square foot of sail. Most of the main hull and float is 6 mm ply taped together with glass. Keel is 150 x 50 mm. The daggerboard case sides are 9 mm ply. Decks are 6 mm ply. According to owners this is a fast fun boat. Video’s indicate fast speeds in light to moderate winds. A very fast daysailor with limited accommodation. The study plans give an insite into the build.

 

Proa Madness further study plans.

 

Wanna segue into the Aspen powercat? It uses proa hulls, too.

 

The L20 was designed was finalised in 2017. This is a very aggressive day sailing trimaran design at 20 x 20 foot weighing just 880 lbs with a Tornado rig. If the designer has got the proportions correct this will be a fast boat. The Vaka (mainhull) panels requires 6 sheets (3 per half-side) of 3/16″ [5mm] thick marine plywood. The location of the scarf joints need to offset from each succeeding panels, in order to attain a stronger hull. Each scarf requires a feathered length of 4″ [102mm]. Glass tapes hold the hulls together. The floats are 5 mm ply also. The cross arms (aka) have 5 mm webs with bulkheads with 25 x 25 mm timber on the sides. The top flange is 51 mm wide x 29 mm laminated timber. The bottom flange is 100 mm wide 19 mm thick laminated timber. The bottom panels of the cross arms will be the last part to be installed. The interesting part of this design is it will be rig limited. A Tornado is 10 foot wide and lighter. The L20 will have twice the righting moment when sailed hard. The rig will be at the limit of its capability in stronger winds.