Multihull Structure Thoughts

[oldmulti]

The following Bernd Kohler designs are close sisters. The Eco 5.5 came first then people asked for a slightly larger version (every designer gets asked could I just lengthen/shorten this a little). So Bernd designed the Eco 6 which uses mainly the same materials in a slightly longer and wider boat. The Eco 5.5 is a bridge deck cat 18 x 8.2 foot weighing 650 lbs and displaces 1390 lbs. The sail area is 200 square foot. The Eco 6 is a bridge deck cat 19.7 x 9.3 foot weighing 800 lbs and displaces 1700 lbs. The sail area is 225 square foot and can have a Code 1 of 190 square foot.

The Eco 5.5 and 6 are an example of designs that for practicality, use stronger material than required in a smaller boat. 6 mm plywood can be used on a 33 foot boat. 12 mm ply underwings have been used on 37 foot catamarans that displace 7000 lbs. Result both boats can use the same materials. So what is the structure. The hulls are 6 mm ply, bulkheads 6 mm ply, stringers 20 x 25 mm, chines 25 x 35 mm. The underwing is 12 mm ply, the mast bulkhead is 12 mm plywood with timber strips top and bottom. The forward underwing and cabin roof 2 x 4mm ply and the cabin roof has external stringers. The following web site provides details of rudder and daggerboard construction I mentioned in an earlier post. How to Build Rudders or Dagger Boards | Small Trimarans http://smalltrimarans.com/blog/how-to-build-rudders-or-dagger-boards/

The Eco 5.5 plans show two rigs. A normal sloop rig and a single halyard Gunter rig. For owners who trailer the boat the short mast of the Gunter rig will be appreciated. A well designed Gunter rig with Bernds improvements is as fast g to windward as good sloop rig. The rotating aluminium mast is made from a 80mm diameter tube. The rig/mast on Eco 6 is slightly larger.

The following web address gives the Eco 5.5 list of materials. ECOpage3, pocket cruiser catamaran for the DIY boat builder http://www.ikarus342000.com/ECOpage3.htm

 

[oldmulti]

Some additional Eco 5.5 and 6 jpegs

 

[oldmulti]

Fifty Fifty is a 50 foot extreme racing 50 x 30 foot cat. It displaces 3400 lbs and carries 2 x 62 foot wing masts with interconnected rigging. The masts are 110 square foot each with a 775 square foot main on each mast. There are 2 x 500 square foot jibs or 2 x 775 square foot genoa’s. The total sail area with masts and jibs set is 2770 square foot of sail. Power to weight is of the chart. Building the boat was done at Pauger Carbon Composite, 50 km from Budapest. The aim was to compete in the 44th Kékszalag 2012 Grand Prix, the Blue Ribbon on Lake Balaton, Hungary's most important regatta. This regatta has a strong entry with over the 600 boats competing.

Unusually multihulls are only allowed to compete in the event every other year, 2012 only being the second time they have been allowed in. The winds were light averaging under 10 knots but the two masted catamaran proved her potential regularly sailing up to 3 times faster than wind speed (peaking at 24 knots) despite the team only being able to sail her for three hours prior to the race. Their winning time of 10 hours, 34 minutes and 15 seconds was a new course record. The boat was built with carbon fibre infused high temp epoxy resin with and an Airex T92 foam core that has very low resin take up. The hulls were post cured. At 3400 lbs displacement there was some very good engineering here especially with the weight of two 62 foot masts etc. PS It only takes 7 to 8 crew to control the boat and rigs while racing.

This boat was a precursor to a 35 foot biplane rig cat that was built to race in Switzerland lake races.

 

[bajansailor]

I am just wondering how much slower she would be if she only had one rig?
Although if it was one rig on one hull, then I guess she would have a preferential tack for sailing on like a proa (?)
And if the rig was on the centreline then they would need a huge amount of extra structure (hence weight and cost) to support the compression forces on the mid-span of the bridge deck.

 

[oldmulti]

bajansailor. I suspect one rig that was higher would be faster and it would be lighter. The larger one rig may carry less sail but it would be more efficient especially reaching and upwind. And very fast cats that can exceed wind speed basically are going upwind even on a broad reach. The weight saved in the rig can be used to strengthen the main mast supporting beam which will probably need a dolphin striker to support the compression loads. The additional weight will be more in the dolphin striker than extra layers in the main beam. Also some form of compression beam will be required in the bow or a central spine forward to mount the jib on. All possible and I suspect would be a faster boat for less overall cost.

 

[oldmulti]

The final Bernd Kohler design for now is the Eco 7.5. It is a bridgedeck cruising catamaran that filled the slot between the ECO 6 and KD 860. The cat is 24.5 x 14.75 foot and weighs 1650 lbs it displaces 3100 lbs. It carries 305 square foot of sail in a sloop rig. There is a Gunter rig option for those who have to pass under low bridges etc. The hulls L:B is 9:1, in short a reasonable cruiser.

Its build specifications are similar to the Eco 5.5/6 but with some mild upgrades. EG The planking for the chine hulls has a constant radius and is tortured plywood in some spots. The hulls are 6 mm plywood and have biaxial glass 340 gr/m2 and one layer 200 gr/m2 glass fabric. The roof is 2 layers of 4 mm ply and forward under wing is likely to be 3 layers of 4 mm ply. The under wing is 12 mm ply. The attached sample PDF plan gives more details of the structure. Many copies of this cat have been built globally and owner reports I have read indicate it is a good boat that sails well and is capable of coastal passages.

When you look at some of the build photo’s please remember the boat has a reverse bow and the hulls are built upside down.

 

[oldmulti]

Wingmasts have been around for years and are on a lot of high performance boats from small cats to fast cruiser EG Eagle 53 cat mentioned earlier. But the evolution is still going on with EG Beneteau has developed a reefable wingsail for their cruiser racers. The market is still not ready for the concept but Beneteau is not hiding the fact they can sell it.

But one of the most interesting developments is the inflatable wingsail. This wingsail depends on air pressure to hold its shape and appears, according to tests, to function well as a sail. But the real trick is it is reefable. The sail has effectively 3 separate air chambers, combined with a mast that has 3 sections in it and the mast can expand or contract according to the what portion of the sail that is pressurized. Each air chamber has its own valve which can be controlled from deck level. There is an air pump which runs continuously to pressurize the sail. The air pressures in the sail are low. The whole rig is relatively light and can withstand a small hole in an air chamber and still operate. Why is this interesting? Think about a multihull “capsizing”. With a large inflatable wing sail which is in effect a giant flotation device that will not let you capsize beyond 90 degrees (assuming the mast is strong enough). You have a lot better chance of righting a boat from 90 degrees than fully inverted. You get a win win, a wingsail and an anti capsize device in one.

 

[oldmulti]

Whilst we are talking about wingsails here are a few more items to contemplate. Professional Boat Builder magazine issue 14 page 11 has information on wing spars including Gold Coasts 53 foot charter cat spars. Professional BoatBuilder - 14 - Dec-Jan 1992 https://pbbackissues.advanced-pub.com/?issueID=14&pageID=11 Wingmasts were also discussed on page 5 of this thread with plans for Stressform wing masts. The additional information is in jpeg one which is how land yachts build their wing masts that carry 100 square foot of sail at speeds up to 140 kilometers per hour (80 MPH). Basically a blue Styrofoam core with a 3 mm plywood web with 12 mm aluminium rods at either side. The whole thing is wrapped in 2 layers of 200 gsm cloth. These masts take a lot of abuse and at 80 mph at lot of pressure. The next 2 jpegs are of large “wing” masts designed by Eric Spondberg. The first is a carbon fibre version and the second is a timber version. The timber wing mast version is for a 110 foot (yes 110) steel charter boat. The final jpeg is of a 40 foot cat that had 2 freestanding wing masts. The boat scared the owner in its first few sails and he cut 12 feet of the top of each mast. The boat then sailed 2000 miles up the Australian coast before being put up for sale. Rob Denny built the carbon fibre masts to the owner’s requirements.

 

[oldmulti]

The final one on masts for a while, the diagram below is an interesting comparison of wooden masts. I understand most people who read this are likely to have aluminium or something more exotic for their rig but this diagram conveys a lot of information. A. a solid mast is heavy for its stiffness/compression. B. the thinner the wall the larger the diameter required to maintain the same stiffness/compression. C. different shapes can have an influence on weight and stiffness/compression. D. a plywood sided mast can be lightest weight but it has one of the largest cross sectional area. The table down the bottom on round masts is also informative. As the wall thickness is decreased to reduce weight of the cross sectional area the mast diameter must increase to maintain the same stiffness/compression capability. Like all things in yachting masts are a compromise based upon the design needs. With mast design leave it to the professionals unless you have a big budget or a small boat where you can use a known quantity like a beach cat rig. The original source for the diagram came from Hollow Masts for Small Sailboats https://www.pdracer.com/mast/hollow-mast/ if you need a clearer image.

 

[oldmulti]

C class cats have always been on the leading edge of design. The basic design is 25 x 14 foot with 300 square foot of sail. The weight of the boats vary. The old all timber boats were 550 lbs the modern nomex carbon fibre boats are under 400 lbs. The hulls are much lighter in modern C class cats but the rigs are a lot more complex full wings which are heavier than the aluminium mast, light Dacron sail cloth, old C class versions. Lady Helmsman an early C class hulls were 2 layers of 2.5 mm ply diagional reinforced with 3 mm ply around the daggercase area and a 150 mm wide glass bandage around the hull at cross beam attachment points. A 21 x 19 mm stringer at turn of bilge. Three 50 mm Styrofoam frames at 900 mm centreline in the bows. The deck is 4.5 mm supported by 6 mm ply deck beams. The fore and aft beams are M. 160 with the main beam having a 400 mm deep dolphin striker with 7.5 mm wire. The next generation came about the time of Miss Nylex which was the first full solid wingsail and composite hull C class. The boat was 500 lbs and had a 6 mm balsa sandwich hull with 330 gsm of glass on either side. It still had aluminium beams.

The next generation started in the early 90’s with foam carbon fibre hulls, carbon fibre cross beams and very sophisticated multi element wings. These boats started to become lighter (under 500lbs) but the wing weight was increasing due to the multi elements. These hulls are PVC foam with epoxy glass of 270 gsm Carbon Fibre. In the late 90s the design of C class were becoming very refined with EG Cognito not only being very well designed but had significant developmental testing done as they refined the building process. We are talking female moulds, Nomex cores, carbon fibre skins and crossbeams. The aerodynamics of the hulls was also being designed in. The rigs are really advanced with fewer elements but having more flexibility allowing greater control across the wind range. The following article in Professional Boat builder issue 39 page 30 gives details. Professional BoatBuilder - 39 - Feb-Mar 1996 https://pbbackissues.advanced-pub.com/?issueID=39&pageID=33 Cognito took 4500 hours to build and many hours to develop to its full potential.

The next generation was experimented with about 2006 but was not fully developed until about 2013. The foiling C class cat. These C class cats required a rethink. The hull structure had to be reinforced around the dagger/foil case as it now has to support 70% plus of the boat weight. The bows are becoming finer with narrower decks to pierce through waves not ride over waves. The transom structures need to be reinforced to handle the some very high load and control forces. The aerodynamics of the hole structure is now considered as the entire boat is often 2 feet in the air. The hulls are less than 200 gsm carbon fibre on either foam or nomex with fully integrated crossbeams and almost all fittings are carbon fibre structures. These boats are approaching 350 lbs of weight. These boats need very good sailors to control them.

Even student groups are trying to develop C class cats eg. Rafale II carbon/epoxy C-Class hydrofoil catamaran http://www.jeccomposites.com/knowledge/international-composites-news/rafale-ii-carbonepoxy-c-class-hydrofoil-catamaran The cut away diagram is of that boat. Finally there is a PDF written by Steve Killing about the transition development of C class cats in 2007. The non foil boats were ending and the foil cats were being developed. This I one story of that transition.

 

[oldmulti]

Famed sailboat aerodynamics researcher C. A. Marchaj published this startling graph in his research paper Planform Effect of a Number of Rigs on Sail Power.

Most startling is the extraordinary performance of the crab claw sail, which demonstrates its superiority to a Bermuda mainsail right from the close-hauled condition. Its superiority increases when the boat bears away, and on reaching, with the heading angle 90 degrees, the driving force coefficient of the crab claw is about 1.7, whereas that of the Bermuda rig is about 0.9. That is, the crab claw rig delivers about 90% more driving power than the Bermuda rig.
~ Sail Performance

Marchaj theorized that the crab claw develops lift in a different way than the standard Western Bermudan. It operates in what is called *vortex lift* mode, which creates powerful spinning tornadoes of air off the leading edge. The spinning vortexes are zones of intense low pressure, and thus lift is created. If you wish to learn more, Marchaj’ book Sail Performance (Adlard Coles Nautical 1996) is highly recommended.

His tests showed that the crab claw was not the only traditional rig that could outperform the Bermudan on some courses. A lateen was superior to windward, and the gunter, sprit, and lugsail were all superior overall to the Bermudan.

The Bermudan with a small jib tested very well close-hauled, second only to a lateen in Lift to Drag (L/D) ratio. The modern sport of yacht racing has developed around the triangular race course which favours windward sailing efficiency. The Bermudan tested very poorly on off wind courses however. It has also been pointed out that very fast sailboats, such as racing multihulls or ice boats, create so much apparent wind that they are always sailing close-hauled, and therefore, it’s the L/D ratio that really counts, not the max Lift Coefficient.

Further tests were done in the attached PDF which compared various pacific island rig types for the best performance. Again the crab claw was best. The jpegs are a subset of the information. A summary from the authors is as follows “To understand the sailing performance of traditional canoes in Oceania, we replicated ten sail rigs and tested them in a wind tunnel. Measurements of lift and drag forces demonstrate substantial differences in their performance. At low heading angles, from about 30° to 80° off the wind, three sails (Massim, Ninigo, Santa Cruz) are remarkable for their higher efficiency. Three other sails (Tonga, Hawaii, Tahiti) are remarkable for their lower efficiency from heading angles of about 90 to 130°. In between, four more sails (Arawe, Micronesia, Vanuatu, Marquesas) have roughly similar performance to each other. The ranking of these sails is followed by a description of their distribution with inferences on historical evolution of canoe rigs.”

 

[oldmulti]

Vaka 990 Proa is a 32.5 x 18 foot cruising proa with float accommodation or if the proa is a sports version the float has water ballast. Its displacement is 1900 lbs carrying 400 square foot of sail. The boat is constructed of west epoxy plywood construction. The bulkheads and main hull are 9 mm ply with floats 6 mm ply. (36 sheets 2.0 X 1.25 mtr 9 mm. 16 sheets 2,5 x 1.25 6mm). The "Pod" is habitable and acts as an anti-capsize device and the boat has 4/2 berths, a WC, limited galley with storage. The jpegs give an idea of the Vaka 990 Proa. The PDF is a detailed study plan of the boat and its structural components. The second proa is a racer cruiser designed for the race to Alaska. This is aimed at performance plus. Page 14 of this thread has study prints of the proa Madness which is the same size. Each should be capable of sailing very well for good sailors. These boats are not set and forget boats, they need a constant watch to ensure not being backwinded etc which would threaten the rigs on Madness and the Bieker Proa. As Russell Brown proved with Jzerro, pacific proas can be very fast if well designed and built.

 

[catsketcher]

Thanks (again) for another great series of posts. I am interested in the stability curve for the Bieker proa. It must be one of the very few multis that doesn't drop off consistently after max moment. I guess that is the effect of the pod coming into play.
I got to go out on Russel Brown's proa in 2001 or so but we only went out for a motor. Beautifully built boat.

 

[oldmulti]

Catsketcher. I got to speak to Russell when he was in Monty's yard in Queensland. A nice guy. He was modifying his proa by putting uni carbon fibre on the cross arms to minimize the "vibrations" caused by the float bursting thru wave tops. Asked what he would change, he said nothing beyond making the main hull 45 foot long and a chine hull bottom. A really good sailor and a fun boat for those capable of sailing it.

 

[oldmulti]

The attached Gruit PDF on composite construction is a bit technical but contains many gems of information about the relative strengths of resins, fabrics and composite construction approaches. The document tries to explain both in simple and occasionally with technical information the relative vices and virtues of each product and approach. EG The relative tensile strength of S glass and HS carbon are the same. Its just S glass elongates 3 times more than HS carbon before failure. The same applies to compressive strain where S glass compresses 3 % compared to HS carbons 1% before failure but they both fail under about the same pressure. The real difference between the 2 is S glass can take 6 times the impact pressure of HS carbon fibre. Translation HS carbon may be strong, but because it doesn’t elongate as well a S glass, it fractures more easily. Another aspect is the curing rate of resin versus its strength. In each case of polyester, vinylester and epoxy in a EG resin/fabric matrix, the tensile strength is 25% higher if it is post cured for 5 hours at 80 degree centigrade than if it is left at 20 degrees centigrade for 7 days. The discussion on fabric and core materials gives very good in sites into the virtues and/or vices of the different materials to allow you to make a more informed decision about there selection. EG SAN foams are able to elongate more and are tougher than PVC foams although they have similar strength characteristics. And there is differences between grades of PVC foams beyond strength. Some have greater resistance to styrene attack than others etc. A very informative document. Please read.