Multihull Structure Thoughts

Do you have any details about the rig airflow calculations? What software was used & what conditions were specified?

The initial impression seems to be of a badly trimmed case (even realizing that the optimum may also be a little messy).

 

Doug. I do not know the airflow software but I suggest an email to Frederick Courouble may get some result as he is involved in many high performance sailing boats including some association with AC boats.

Part 2 of the GF 42 trimaran that Greg Lynn combined with Frederick Courouble of Courouble Design & Engineering did the design and engineering work. Greg Lynn is a pioneer in applying digital technology and computer numerically controlled (CNC) machinery in design and an advocate for the use of composites. The GF 42 design is as much art as it is a functioning trimaran and has a many complex curves in the design.

So, lets talk about how the tri is built. Start by ordering large high-density polystyrene blocks. Then find a company that has a large 3 axis CNC machine. Have an excellent accurate CAD design for the parts you want moulded. Feed the CAD/CAM into your CNC machine start it going and walk away for a good meal. Result an accurate female mold of the part you want. Now apply epoxy resin and fiberglass to Styrofoam surface to complete the female mould. After cleaning up the surface of the mold and waxing you next do conventional vacuum bagging of your carbon fibre epoxy part. Next layer is the foam which is vacuum bagged down followed by the final carbon fibre epoxy internal skin. Westerly Marine was contracted to make the CNC cut foam female tooling and produced the major parts (with the exception of some small parts) which it supplied to the builder for assembly. Some of the larger parts such as the flat backs of the crossbeams were built on conventional timber molds.

The jpegs show the complex multi-dimensional CNC foam cut outs for the forward cross beam. The complete foam forms for the main hull are on a truck in a jpeg. There are CNC molds for stairs, bunks, galley, internal floors etc. All of these items were built in epoxy foam carbon fibre. The bulkheads and shelves were built with resin infusion techniques.

The real skill in this design is the integration of the design subcomponents with the certainty of the manufactured product. All components can be manufactured separately and then literally assembled with minimum additional fitting and taping. The design and engineering process involved is large but the build time is reduced. Overall, I don’t think your going to save much money for a one off multihull, but if you are going into series production, costs will be lower. The GF 42 took months to design and 18 months to build for a 2 year project. But as all projects they have had problems, the mast came down in initial trials.

The foil jpeg is interesting as it appears to be from an actual carbon fiber composite foil from the 2013 Oracle Team USA America’s Cup catamaran. There is also another foil mold shown in the jpegs for a set of C foils but one broke in early trials.

The jpegs give the idea. There are many more jpegs and video’s at Greg Lynn YACHT https://www.facebook.com/pg/greglynnyacht/posts/ or at GREG LYNN YACHT http://glform.com/yacht/

 

Further GF 42 jpegs.

 

The SC 44 is a 44.1 x 24.6 foot cruising catamaran designed by Dr Albert Nazarov of Albatross Marine Design. The cat weighs 32,400 lbs ready to sail and displaces 38,000 lbs. The 56 foot aluminium mast carries 785 square foot mainsail, a 500 square foot genoa and a 86 square foot stay sail. The hull length to beam ratio is 9 to 1 with a prismatic coefficient of 0.65. The first jpeg shows the hull lines of the cat which is a development of proven hull shapes from previous designs. It shows a U shaped underwater profile with minimal rocker across the majority of the hull length. This is a modern shape that should have minimum wave drag across the speed range. The cat has low aspect ratio keels and spade rudders. There is an option for daggerboards available. We have a serious cruiser that has numbers that indicate it can sail well.

This boat is designed as cruising catamaran and can be built from flat panels of either foam, duracore or honeycomb sandwich. The available weight of the ready to sail cat at 32,400 lbs indicates the cat can be built strongly without having to resort to exotics like resin infused carbon fibre etc. Vinylester or polyester resin and e-glass will suffice. There would be minimal shaping required by the panels to do the majority of the upper hulls, deck and deck cabin. The hull bottoms may have to moulded separately to get the required shape. Bulkheads can be foam glass or plywood. Please again remember this is a big boat that will require a major commitment of time but Albert is a good designer and knows how to design for home builders.

The interior layout of the main cabin shows a sensible galley and dinette arrangement with hull access to several double berth cabins. The two 30 HP Yanmar engines power the cat to 8.5 knots and cruise at 7 knots.

The limited jpegs give the idea.

 

The following is a concept design. There is no great detail but it has many conceptual details that may invoke an idea. There are some other features that will provide questions. EG the half forebeam. The engineering on that forebeam would have to be spectacular. Assume the cat is about 40 x 24 foot of unknown displacement and sail area.

The cat is known as Sunbeam Concept one. The person who drew the cat was doing a series of design studies for various products. The motorbike should also be studied, look at the ‘steering’ arrangements. This cat was one of the concepts. It has a very bold shape that ‘hides” the height of the full headroom bridgedeck. The cockpit is unique providing an uninterrupted view and access to the mast, winches etc. The bike storage is a nice touch but I hope those bikes are built from full 316 stainless steel or rust may be only an ocean away.

The stairway from the wing deck to the cockpit is interesting and would be entertaining in a seaway without hand holds. The foils and rudders indicate a hope of high performance as does the tall rig. But I suspect the forward rake on the mast may need a little refinement. The internal layout looks Ok.

Please review these drawings as a concept design. There are some good ideas in its bold shape and cockpit concept. Good for some ideas. Reality will return tomorrow.

 

Reading the Searunner Construction Manual i became a fan of mr. Jim Brown designs and toughts - thank you for sharing the links to download his Among the Multihulls vol. 1 and The case for the Cruising Trimaran. I used to be a fan of Jim Wharram boats, now i am fan of two Jims!

Nice to read about the proas of Russel Brown too - i follow some news from his proas news since a old article in a sailing magazine about the first Jzero. At this times i was building Wharram Melanesia and really interested on proas. And i am really surprised about Tim Mann proas too.

Huge amount of good references here in this post. Thanks for you work, OldMulti.

 

The Trika 540 is a “kit” or plan daysailing folding trimaran designed by a German who wanted a fun tri for the family. The Trika 540 is 17.75 x 10.75 foot and weighs 154 lbs with a 15.7 foot aluminium or carbon mast that carries 84 square foot of sail area. The length to beam is 9.2 to 1. The folding system is very effective and its folds to 4.25 foot. The initial floats could displace 180 lbs but was found to lack buoyancy. In the Trika 540.2 the upgraded standard float design now displace 225 lbs. One builder upgraded the floats to displace 330 lbs.

The designer commented about floats “The Trika 540 it is not especially designed for high waves and – as many small boats – it likes longer waves better than the short choppy waves, but if reefed in time and combined with the new 100 liter amas (225 lbs displacement) I think it needs not fear comparison with its competitors.” This tri is a bay or lake sailing fun machine suitable for light to moderate winds and smaller waves.

The construction is basically plywood. The hulls are mainly 4 mm plywood. The bulkheads and frames are 9 mm plywood. 12 mm plywood is used in the crossarms. The hulls have taped glass seams. The gunnels are timber.

The jpegs show the details of the cross arms and the ‘post’ slot in the floats. A fun design which the jpegs give a good idea about.

 

The following is about a young designer Mikko Hörkkö of Viitodesign in Finland. He entered the Millennium Yacht Design Award 2018 design awards and won the “Beginners” category, Section B: THE PEOPLE-ORIENTED BOAT. The boat that won is a coastal high performance racer cruiser catamaran of 26.5 x 16.5 foot that weighs 2700 lbs. The 43.5 foot rotating wing mast carries 400 square foot of sail. The length to beam of the hulls is 12 to 1. The deep narrow dagger boards draw 6 foot with underslung spade rudders. The above numbers indicate a peak speed of 20 knots with near wind speed sailing across a lot of the wind range.

The mast and foils are carbon. I am assuming the remainder of the cat is foam glass as there is no details of its construction. The weight would allow a EG 600 gsm biax 12 mm pvc foam 400 gsm with vinylester resin type of layup on the hulls and decks. The beams look like foam glass structures that fit into slots which would allow the cross beams that can be removed to allow the cat to be transportable. The steering system is interesting and relatively simple. My only concern is the cats draft of 3.1 foot with its underslung rudders. There appears to be no kick up or withdrawable rudders for sailing in shallow water. The rudders could be made kick up with a bit of thought. You can tell the designers that live in a deep water world versus designers who live in shallow water worlds by the keel depths and rudder configurations.

The accommodation is limited with space for a loo and small galley. Two small berths could be fitted. The headroom is only 4.25 foot which limits its internal practicality. This boat is focused on performance. The helm position and cockpit arrangements allow the cat to be controlled by a short handed crew. This is the next step up from a beach cat for a young or young at heart person.

The jpegs give the idea. I hope someone has built this boat as it looks like fun.

 

A young designer Mikko Hörkkö of Viitodesign in Finland did the following design after his DRC 8. The DRC10 is 33.1 x 20 foot cruiser racer catamaran that weighs 3500 lbs. The mast is a 50 foot wing mast carrying approximately 600 square foot of sail area. The deep narrow dagger boards draw 6.5 foot with underslung spade rudders that draw 3.25 foot. The above numbers indicate a peak speed of 20 knots with near wind speed sailing across a lot of the wind range.

The main design element is the monohull-inspired cockpits. The cockpits have two steering wheels connected through the aft cross beam. In the back, the cockpits open to the centre to give the helmsman an enhanced sense of speed. Removable safety bars with lifelines keep the helmsman safe. There’s seating for three in the front of the cockpits.

The two cabins are entered through sliding hatches. The port hull has a galley, dining area and a single berth. The dining area can be converted to a second single berth. The galley has 6 foot standing headroom. The starboard cabin has storage space and a toilet. A double berth can be added by opening a hinged panel and adding mattresses. The aft portion of the hull has a technical space.

The construction is not specified but I am assuming a foam glass design with PVC foams, e glass biaxial cloths with vinylester resin. The cat is demountable judging by the cross beams which appear to be able to push into slots in the hulls. The accommodation is showing in the jpegs and appears to be very practicable for a sports cat.

Again, a nice design for fast fun and coastal sailing. The web site for the DRC10 DRC10 http://viistodesign.fi/portfolio/drc33/

 

This is a short one because the main part has a copyright associated with it. Mike Waters of Smalltridesign.com who has designed the W17 and W22 tris has a good web site that covers his mainly performance day sailing tris with some cruising capability. Mike writes well thought out articles on various aspects of tri construction, use and practical aspects of tri ownership. His web site is at Small Trimaran Design | Home Page https://smalltridesign.com/index.html

He has a new design which he is developing for a Canadian client, the W19. No great details are available, but on the following web page is some detail of the center board and the centre board case. The page is Chesapeake Light Craft is now offering quality plywood kits for the W17 https://smalltridesign.com/whats-new/FG-LOG%20article%20for%20W19.html

This page shows an interesting approach to making a fiberglass base of the center board or dagger board case. This case base can then be inserted into the hull as a relatively strong and waterproof case base to a hull structure. This is better to trying to build a board case and attaching it to the hull then trying to seal the case hull interface with layers of glass. Also, Mike shows how to build a board in a simpler way but is structurally stronger. Good thinking.

I am fully supportive of cedar core boards with fiberglass coverings. Both cedar and foam glass boards break off if they hit something solid but foam glass boards have more issues. Any time I have spoken to people who have had homebuilt foam fiberglass boards they have had problems over time. The main issue is the fracturing of the board faces which either leads to a notch failure or water ingress from hitting something which allows water ingress to the glass structure.

The attached jpegs are just some random centreboard case drawings and do not reflect the quality of Mikes solution. The daggerboard repair is of a foam glass board from a 50 foot cat. The final peg is of a top and bottom insert used in a smaller cat (similar to what Mike is advocating) for the top and bottom of a fiberglass case.

 

Comar Yachts Comet Cat 37 has been designed by Marc Lombard. The C Cat 37 is 36 x 19.15 foot weighing 11,200 lbs and displacing 17,700 lbs. The 47 foot mast carries 495 square foot mainsail, 355 square foot genoa, 460 square foot Code 0 and 600 square foot gennaker. The cat can have a 3 to 7.3 foot draft with centre boards or 4.25 foot with low aspect ratio keels. The hull width is 5 foot at the gunnels, the waterline beam is about 3.25 foot which gives a length to beam ratio of about 11 to 1. The underwing clearance is 2.1 to 2.3 foot.

The C Cat 37 is an ocean performance cruising catamaran that according to initial calculations was capable of 12 to 13 knots under sail (I suspect peaks of 18 knots and 8 knot averages). To enhance performance Marc Lombard concentrated the weight centrally with engines, plant equipment, anchor locker, mast and even the life raft (under a cockpit seat) centralised.

The majority of the structure is sandwich construction with quadraxial fabrics using resin infusion techniques with epoxy and vinylester resins. The boat has been designed to be unsinkable with 4 watertight compartments, 2 forward and 2 aft bearing bulkhead pass-through for the passage of pipes and cables in order to ensure absolute waterproofness. The crash box in the bow, consists of a sacrificial part of the hull that in the event of a collision at high speed, breaking, absorbs the impact avoiding structural damage.

All lines are led back to the cockpit, including reefing lines, which can be directly controlled from the wheelhouse, without being obliged to leave the cockpit to steer or reduce the canvas. Even the jib sheet, the traveller and mainsheet are within reach of the helmsman. There is also an optional security system already in use on ocean-going catamarans racing, through software, reads any critical structures of the catamaran and automatically loose the sails.

The double helm aft makes the cockpit free of clutter and is large and with a sliding door to the main cabin gives a feeling of open space in the saloon. The kitchen is located between the cockpit and the saloon. There can be an owner’s version with 3 double berths and 2 heads. Other options are available.

The jpegs give the idea of the cat and the initial male mold build of strip plank cedar. Not a minor task. A good design which should perform very well.

 

An old racing tri is featured today. Originally built as a 53' trimaran for Pascal Hérold in 1990 to a Nigel Irens design in the Seatec yard in La Trinité sur Mer. In 1992 the tri was shortened to 50 foot for the 50 foot offshore racing class. The tri is 50 x 42 foot displacing 15,000 lbs and carries a 71 foot fixed carbon fibre mast with twin spreaders. The mainsail is 900 square foot with a 635 square foot foretriangle allowing a 1000 square foot code 0. The rig is a cutter with 3 furling headsails with a 2000 square foot spinnaker. The power to weight ratio is very good and with the overall beam providing good stability and slim waterlines I think even with a 30 year old design this tri could go close to 30 knots in peak speeds.

The main hull length to beam is 12 to 1 with floats having length to beam of 14 to 1. The main hull shape is slightly wider than modern 50 foot tris that displaces 3000 lbs less. The float shape on modern 50 footers is more catamaran like with fuller sterns as modern 50 footers are sailing more on their floats than older style 50 footers that kept their main hulls in the water more.

Big tris like this have a lot of stability but do not be fooled, they can be capsized very easily if not well handled. Old 50 footers only had a rudder on the main hull which can lift out or ventilate if driven hard. No steering, no control, release sheets NOW or practice swimming. Remember a 55 foot racing tri called Yumi Maru which capsized off the NSW coast in the Sydney to Brisbane race. It was similar to the racing tri being discussed and had control issues due to the rudder lifting out when pushed hard. This is part of the reason most modern racing tris have rudders on floats.

The build of the Irens 50 foot tri was PVC foam carbon fibre epoxy and Kevlar. To last in racing for 20 plus years indicates the initial build was very good.

The “accommodation” is limited to say the least. But on any serious very high performance tri you will not have much real accommodation. If you want space and some privacy you are going to sacrifice some performance or buy eg a big Gunboat.

The jpegs give the idea.

 

The M6 is a performance day sailing catamaran designed by the ACdesigngroup. The cat is 19.5 x 9.5 foot that weighs 340 lbs with a 28.5 foot mast that could be carbon fibre or aluminium that carries a 158 square foot main and a 51 square foot jib and a 187 square foot gennaker. A Hobie 18 or 20 rig would work just as well. The hulls have a length to beam of about 12 to 1 with a fine bow shape which will help upwind as there appears to be no centre or daggerboards. I still think this cat will sail reasonably well upwind because of the fine bow and the cat has enough beam/stability to allow you not to need to trapeze (optional).

The hull and decks are 4 mm plywood with 6 mm plywood bulkheads. The hulls and decks are covered by a 200 gsm cloth in epoxy. The crossbeams are about 100 mm aluminium or carbon tubes or mast sections. The hull shapes are V bottom with a single chine. The thin ply will be able tortured into shape and all the ply panels are joined by taped seams. The prodder is able to carry the jib base and the gennaker for reaching or downwind work. The rudders are cedar glass with aluminium rudder head boxes.

This is a fun concept of slightly below a serious racer but enough performance to have some fast days on the water for 2 or 3 people. The jpegs giving the idea. There also is a 16.5 foot version available at the web site which is https://aacdesigngroup.wixsite.com/acdesigngroup/multi-6-en

 

Tiki 38 have been discussed before but this specific cat “Songlines” has a unique rig. A standard Tiki 38 is 38 x 20.5 foot that weighs 6600 lbs and displaces 11,000 lbs and carries 573 square foot of sail in the standard schooner gaff wingsail rig. In the standard rig the masts are 32 foot high 150 x 3.5 mm aluminium tubes with rigging from the mast head to deck with no spreaders.

A New Zealander wanted something a little different, a biplane junk rig Tiki 38. Not to unusual as there are several biplane junk rig Wharrams around. But this man’s variation is in how he mounted his masts. Most biplane masts are hull mounted with the mast base supported at deck and keel level. This is the “bury” of the mast which is normally 10 to 15% the length of the free standing mast.

The builder of Songlines thought differently. He did not want to put the mast base in the hull as it would interrupt the internal accommodation etc. So, he placed the freestanding masts ontop of the second cross beam. This creates the small problem that a freestanding mast requires some support structure above its base to hold the mast up. Songlines builder built 2 support arms that attach to the second and forward cross beams. The support structure starts at 14% up the mast providing the required bury. The jpegs show the structure.

Songlines has sailed many miles including going through a Pacific storm that sweep away part of its deck structure, so the mast arrangement has worked in many conditions.

Songlines masts are 32 foot high and according to one report are 200 x 6 mm aluminium tubes. The sails are “standard” junk rig. No reports on performance.

A Tiki 38 build (which Songlines is besides the mast) is basically plywood timber with 9 mm plywood hull skin, 9 mm ply lower bulkheads and 12 mm ply upper bulkheads. The stringers are 45 x 19 mm. The keel ply 18 mm plus 9 mm either side for a total thickness of 36 mm. The cross beams are 140 x 70 mm top and bottom flanges with a 36 mm web (there is more to them than this) that are “tied” on with low stretch rope. There is also a 18 mm bolt through the beam bulkhead joint.

Please do not be under the impression a bigger ply Wharram is easier to build than a full foam glass bridgedeck cat. They both take about the same time for the same quality of finish. Choose your preferred build method first the decide on the design options.

Jpegs give the idea. The first 3 jpegs are of a standard Tiki 38 the rest are Songlines.

 

Spirited designs have an entry level cruising cat design called Seascape. The Seascape is 19.5 x 8.5 foot that weighs 1870 lbs and can carry 770 lbs for a total displacement of 2640 lbs. The 26.5 foot aluminium fixed or rotating mast carries 253 square foot of sail area. A Hobie 16 or 18 rig could be substituted. The design according to the designer has “underwater hull design is aimed at efficiency with a reasonable payload capacity enabling extended cruising with coastal open water capabilities”. The beam of the boat can be modified to an 8.2 foot to allow for local trailing rules.

This is basically an open boat suited warmer waters and outdoor living. The open cockpit design is spacious enough for groups of up to 6 to 8 people with comfortable seating around the table for 5 adults. There are 2 berths below along with seating and a spot for a portable toilet. There is plenty of locker space and an outdoor BBQ for cooking. The layout could be adjusted to allow for some internal cooking and washing facilities but please understand you are dealing with a 19.5 foot long boat. The headroom below is 5.25 foot at best. This cat is practical without being luxurious.

The structure is based on simple surfaces and shapes in this design. Only flat panels are used in the construction with no complex shapes to contend with. Material options include all Gaboon plywood and timber, plain PVC foam glass or Duflex (choice of balsa or foam core) glass. If a kit system is chosen this boat is very quick and easy to assemble with all parts cut to size. Cats of this size generally 6 mm ply shells with 200gsm cloth covering in epoxy. The underwing can be 9 or 12 mm ply. In foam glass you would probably be building in 10 mm PVC foam with 400 gsm e glass either side.

This is a relatively simple cat to put together that does not depend on complicated folding systems to be trailable. It lowers the cost and build time. The downside is the need to handle these cats with care in stronger winds and bigger waves as they do not have the stability of folding designs.

But if you desire more “internal” comfort and a bit better motoring performance there is the “Stow-Away” 6.5 again a Spirited Design which is an off shoot of the Seascape. This cat is 21.3 x 8.2 foot that weighs 2460 lbs and displaces 3450 lbs. It carries the same rig as the Seascape. The big differences in the design are the longer bridgedeck and the outboard power. The Seascape has a 10 HP outboard that does 8 knots. The Stow Away has a 40 HP outboard that does 20 knots peak speed. The hull shape has been modified to emphasis power speed over sailing speed. The Stow Away can still sail quite well but its concept is more a power sailor.

The structure has been upgraded (as has the weight with an extra 770 lbs of materials) to handle the extra loadings and weight of the outboard but it is still a relatively simple build. The internal accommodation arrangement provides an additional double berth and a pop top option that allows the galley and dinette to be on the bridgedeck.

Take your choice of sail over power or power over sail. Good concepts. Jpegs provide a guide of the Seascape with the last 2 jpegs of the Stow Away.