Multihull Structure Thoughts

If you are going to the trouble of erecting frames for a flat panel build and infusing your panels on a table you will save a lot of time, materials/weight and tedium if you line the mould with the material you were going to use for the table, lay all your laminate in it and infuse both sides and all the joins in one shot.

 

Kurt Hughes boats are good. He has a reasonable balance between performance and realistic load carrying capacity for home built boats. The 2 Hughes charter cats I sailed on in the US sailed well. When I spoke to Hughes in Seattle he was realistic and was interested in matching the boat to the purpose you needed. Hughes has also many building techniques available to suit your preferred approach. Some designs can be done with a cheap plywood female hull mold (based on cylinder molding) allowing a resin infusion technique for the hull. Most of the rest of the boat can be done with flat panels etc.

 

Peter Spronk created fast and distinctively elegant catamarans, built in the Caribbean, where the designer intended them to be sailed. In the 1960s he moved to the St Martin where he produced his best known boats, including BLUE CRANE, BLACK BEARD, MAHO, EL TIGRE, JOLLY MAN, RUBAYAT. PPALU, etc. His cats were generally lapstrake ply (clinker) construction on close set stringers and frames. Advanced boats for the 70’s with light weight, thin hulls, wide beams relative to length and generous sail areas. The 60 foot plus boats had 12 mm ply hulls, the smaller boats had 9 mm ply hulls. A lot of these cats are still sailing as charter boats after 40 years which gives an indication of Spronks build ability and a sensible choice of high quality building materials. Spronk boats were fast in Caribbean waters. BUT the low wing decks pounded, the fine ends pitched, the low freeboard forward caused the forebeams to hit the water not just slowing the boat but in the wrong sea state stopping the boat. The photo’s of various boats give the idea. The freeboard on his 60 foot boats is about 5 feet. Modern designs are about 10% of length or 6 foot. The underwing clearance in some photo’s appears to be about 1.5 feet. Modern cats would have 3 feet of underwing clearance for the same beam etc. Most modern cats have fuller ends. Modern cat designs have evolved a lot. The internal shot is of a 75 foot cat PPALU.

 

Rob could you explain what you mean by the materials for the table to line the mould? Do you mean strip the mould with battens? Would you share a photo of your set up with the bulkhead reinforcement in place during infusion. If that is proprietary I understand.

 

Prior to the Seacart 30 trimaran the same combination of guys/company built a 35 foot tri originally named T 35 Nexus then renamed Seagul under a new owner. Its interesting as it was built in the 90’s and as you will see it’s a fast boat. The boat is 35 x 27 foot, weight 2900 lbs fully rigged and loaded excluding crew. Sail area is 775 square foot. The mainhull, floats and deck is 450 gsm triax 12 mm airex foam 450 gsm triax. The main hull below the waterline had an additional layer of 193 gsm Kevlar inside and out. This was all done in epoxy and hand laid. The cockpit floor had an additional layer of 200 gsm cloth for some rigidity. The crossbeams were carbon fibre. The boat is still racing in 2018 and can match it with a seacart 30 on a boat for boat basis. It will beat a seacart 26. The top speed is about 24 knots, it can steady reach at about 17-18 knots and go upwind at about 9 knots in a seaway. It has to be nursed a little compared to a Seacart 30 which can handle anything. For comparison there is a 2015 10 meter Danish tri Black Marlin that is all carbon that weighs about the same. The north sails is an approximation of the boat for a sail plan quote etc.

 

For those who have an interest in understanding the basic’s of composite design for non engineers an article written in Professional Boatbuilders magazine number 105 page 32 onwards give a lot of hints about materials and design issues. But the real warnings are the failures of combinations of materials that were not tested prior to building. In one case a boat was thrown away because the material combination were tested after the structure was built and found to be “incompatible” and failed to meet strength requirements. Comments about how woven rovings work in a resin matrix are interesting and what safety factors are used etc give a better understanding of what you have think about before a build.

 

Oldmulti,

Loving your posts, thank you!

In your last, did you mean Issue 150 vs 105?

 

Thanks again oldmulti for all great posts. I think T35 as well as early Sea Cart 30 were built by Marstroem (Sweden), as well as the olympic Tornado, early M32 cats (R2AK record holder), and a few more. Although to my knowledge Marstroem ended the boat building part of their business to focus on more profitable industries, like defense. The designer of SC 30 is Mark Lombard, and the T35 was designed by one of only a few Swedish multi designers, Stefan Toernblom. Also responsible for the Gazelle, Gepard, Seaon96 and T30 designs. The latter two being quite similar to the T35 in carbon versions, both around 3000 pound ready to sail ex crew. The latest being T30 built by Pauger Carbon Ltd. All very attractive designs. The only other "swedish" multi designer I know of is Jürgen Sass, whos work I also really like. But maybe he would protest against having his fame hijacked and claimed as Swedish. Hope I'm not making a complete fool of myself by forgetting some famous multi designer. From a construction standpoint, it should be mentioned that Marstroem did pride themselves for having extreme carbonfiber to resin ratios like the SC30 baked at 120 deg C, in autoclaves at 7 bar... I think.

 

What I found some what amusing, is that it's almost pitch black inside the "cabin" of the SC30. With only minimal portlights and all black carbon without white paint "to save weight", makes it darn near impossible to find either that energybar in your backpack, or the backpack itself for that matter

 

What do people think about the large chamfer Schionning uses on the inside of the hull. Other than more space does it make the boat stiffer. I believe Schionning make reference that the chamfer is advantageous. I get that it helps spread the load in the beam, but on the other hand lots of other high performance designs do with out it. The first Gunboat 62’s come to mind and they fly a hull more than any other large cat. Thoughts?

 

Use sheet material to line the frames, then lay the glass, foam and glass in it and infuse all in one hit. No filleting or tabbing and much less fairing. This is what we do with Intelligent Infusion, but with simpler shapes. No pics of the bulkhead landings as without the other information in the plans, they will lead you astray. Plus, I don't have any.

 

Slingshot. Please refer to Rob’s web site for further information and details about his build techniques. I seem to remember on another page there was a photo of a panel to be infused that had additional reinforcement etc. Start at Custom 20m/65′ – NORWAY – HARRYPROA http://harryproa.com/?p=726 Slingshot. Tony Grainger was the first person I know who put chamfer panels in a cat. He had 2 builders each building an early Grainger 35 foot cat design, one put a smooth curve between the hull and bridgedeck the other guy took a short cut and put in a flat chamfer panel. When the boats sailed up to Queensland from Melbourne with Grainger onboard one of the boats Grainger found the flat panel chamfer boat had a smoother motion in a seaway and as a result was slightly faster. Many designers have used the idea since. Yes its adds space, reduces wingdeck/crossarm stresses etc in performance cruising cats. Grainger has evolved the approach into his tri’s as well as his cats.

Bluebell, professional Boatbuilder issue 105 Professional BoatBuilder - 105 - Feb-Mar 2007 https://pbbackissues.advanced-pub.com/?issueID=105&pageID=34

Niclas thanks for the additional information. They are all good boats and I like your comment on all black boats interiors. I am still researching the T30 design but it looks fun.

 

Ajen Koorevaar the creator of RTM-Worx infusion software and owner of Polyworx made the following statements. “Vacuum infusion has an advantage over prepreg in the amount of resin is not fixed but adapts to the geometric constraints of the fabric and core type, this makes its very possible to build light without compromise”. Koorevaar mixes and matches materials for each project and tests infusion trials prior to shooting the real thing so he can match the infusion process with his expectations of the flow process and then tests a test the panel for strength and stiffness and for destruction testing. Koorevaar “Infusing carbon is more difficult than infusing e glass. This has to do with the size of the filliments. Much attention must be paid to the layup and core selection and consumables. We have a lot of experience with infusion of solid carbon laminates ranging from less than 1 mm to over 20 mm and carbon foam or carbon balsa sandwiches in one shot. There is no standard recipe that works in every case, we need to take the material properties and understand the physics of flow resin”. Koorevaar explains infusing epoxy is easier than polyester because epoxy chemical shrinkage takes place in liquid stage so the only shrinkage you get from epoxy in the solid stage is from cooling down. Post curing also helps in improving strength. PBB 104 page 13 for more details.

Next in Professional Boatbuilder 104. Tests were done on stiffening stringers on new 12 meter rescue power craft for Sweden. It was found that a premade stringer that was then attached to eg a flat hull bottom then bogged and glassed on was weaker by a factor of 6 than a premade stiffener with a premade flange that was able to be glued onto the foam sandwich skin. The final design used was a foam former attached to the bottom and then fully glassed in epoxy or vinylester insitu with a vacuum bag or infused. which had similar test results as the full stiffener flange model. Also they found unidirectionals on top of the stiffeners added strength. Its applicability is eg underwing stringers.

Whilst you’re in PBB 104 read page 50 to 57 “More on core and reinforcements”. Its analysis on Kevlar as a building material is not kind but explains it with understandable reasons. Professional BoatBuilder - 104 - Dec-Jan 2007 https://pbbackissues.advanced-pub.com/?issueID=104&pageID=53

 

Professional BoatBuilder - 99 - Feb-Mar 2006 https://pbbackissues.advanced-pub.com/?issueID=99&pageID=107 has a great article on the difference between a single skin structure and a sandwich structure in simple terms, initially. It gives an understanding as to why stiffness is as important as strength to eg a hull design and clearly indicates why some single skin boats have too much strength to get the hull thickness up to a required thickness for stiffness. By implication, you can understand why CSM has been used for so long. It adds cheap thickness without much strength. It also shows why resin infusion and vacuum bagging effects of lower resin ratio’s can make a solid skin thinner and stronger but also reduce its stiffness due to the single skin being thinner. The result is sandwich structures are much more effective on a strength to stiffness to weight ratio for almost all types of boats, especially sailing multihulls.

High Modulus’s Richard Down Honey information again on comparative shear strength of laminates and cores. E-glass polyester with a 55% glass to resin ratio and biax cloth has a shear strength of 3190 psi. E-glass epoxy with biax cloth laminate as above has a shear strength of 4205 psi. 9 lbs/cubic foot end grain balsa has a shear strength of 411 psi, 6 lbs/cubic foot PVC foam has a shear strength of 230 psi, 4 lbs/cubic foot PVC foam has a shear strength of 120 psi, 4 lbs/cubic foot Nomex has a shear strength of 125 psi. Translation a shear failure between glass and core may not be at the join line, it may be just a core failure irrespective of the resin type. Don’t worry about the strength of a core though even if it is only 120 PSI shear. It means it will only require a load of 17,000 lbs spread evenly over a square foot to shear a square foot of polyester glass away from its core.

PS in 2002 Route De Rum Nomex core OMRA trimarans had problems with sever slamming loads. The boats had to go through winds up to 70 knots and the boats had structural core failures. Although Nomex was theoretically strong enough it did not perform as required due to Nomex being to stiff. The designers of the tri’s went back to PVC foam cores that were more flexible and could absorb the slamming loads.

 

The advantage I see with laminating on a flat table and then moving to the frames is the flat table is used many times over for bulkheads, cabin house, deck panels, and furniture.