Multihull Structure Thoughts

Umm we found that Epoxy carbon foam carbon with a light layer of glass on the outer surfaces to give a bit of wear protection, costed out about the same as all glass for about 1/2 the weight. Why the same cost, you use only 1/2 the epoxy and epoxy is the expensive component such is the cost of carbon these days.

We paid £ 10.80 a sqm for 200 gsm carbon and for the equivalent glass would have been around 600gsm @ £ 4.80 sqm + treble the amount of Epoxy. By memory the all up cost in Carbon was slightly more but I'll take the 1/2 the weight any day.

 

A small story of you need to understand what you are buying. I am going to leave name details out of this, as this is old history for a successful designer. The design started as a full bridge deck production catamaran of 38.5 x 21.7 foot that displaces 22200 lbs with a fractional sloop rig carrying 950 square foot in main and gennaker or 780 square foot in main and jib. The hull shape was a partial copy of a Tennant canoe stern design that had powerful desil engines to be a motorsailor. The first boat came out of the full production molds and had 2 problems. It was overweight and the bow was too fine resulting in a nose down attitude.

Modifications followed with extra length added to hull to make it 40 foot, but the forward hull still needed a more significant redesign than just a “modification”. The lack of sales resulted in the original company failing at a large financial loss and all the molds were left for over 20 years. Now the more difficult part of the story. A person brought the full set of molds and transported them to his business to build his “dream cat” part time. Even though he has full hull, deck and internal furniture molds etc, after 3 years it is not finished. The hull etc is being well built but there appears to be no modification to the hull shape that caused the bow down on the water problems.

Second issue is the weight. Now I have to be careful. I know the structure the current builder is using and I do not know if this structure is from the original design with some fabric substitution or if it is from a later designer or just the new owners build. The hull is foam glass topsides with a solid glass over the flatter bottom areas.

The hull side and around the turn of the bilge is from the inside of the hull a 300 gram csm, 600 gsm double bias, 300 gram csm, 30 mm h80 pvc foam. The outside layer is 300 gram csm, 1040 gsm quadaxial, 300 gsm csm and 300 gsm csm. Over bottom 500 mm there is no foam. The glass fabric from both hull sides overlap over the bottom of 500 mm resulting in the following solid glass layup. The layup has 2 layers of (300 gram csm), 2(600 gsm double bias), 2 (300 gram csm), 300 gram csm, 2(1040 gsm quadaxial) and 3(300 gram csm). The solid bottom is 8 mm thick.

Folks that is a solid layup with a lot of CSM (copped strand mat). The hull sides could have lighter csm and 20 mm foam. The solid bottom contains much csm for thickness (read stiffness) that will not improved strength much. The bottom will allow the cat to sit on sand but is not intended to be bullet proof. EG the Lagoon 38 near the keel line is 12 mm of solid glass.

Please understand all designers have an occasional design that does not work as required, and the designer/builder often work to correct flaws for some success. There was an attempt in this case but it the problems were significant. I admire the man’s effort to build the cat, and structurally the cat will be sound, but the launch day may not be as happy as it should be. I hope I am wrong. The jpeg was from the original designer’s brochure.

 

This is an interesting design that only appeared for a short time and was never commercialized. The Wharram Pahi 20. Yes, it did exist, but was quietly buried when the Tiki 21 proved so successful. The Pahi 20 was 20 x 9.33 foot that weighed 500 lbs and displaced 1,200 lbs. The sliding gunter rig was to carry 150 square foot rig. The hull was 2.75 foot wide at the gunnels and it had a 16.33 foot waterline.

The only details of the Pahi 20 are in the attached jpegs. When enquires were made to Wharram Design about the cat by one client, Wharram did not really answer the question, they just pointed out the Tiki 21 is the best design in this size.

The only detail available is it was a stitch and glue ply glass design that appears to be very similar to the Pahi 26. It would probably be 6 mm ply with timber ply crossbeams similar to the Tiki 21 at that design stage.

Now a very interesting comment on Scott Brown's forum themultihull there is the following post from Steve Turner on 15th December 2007:

“Having sailed most of Wharram's designs, Pahis, classics and Tikis, in sizes from 14' to 63', I can state catagorically that the small Pahis are the worst performing of all his boats. I know that this will upset a few diehard Pahi 31 and 26 sailors, who will want to defend their boats, but with all else equal a Tiki 21/26/31 will sail rings around a Pahi of the same size. (If you think that there is no such thing as a Pahi 21, well there was, but it was quietly "buried"). I am not saying the Pahi 26/31 enthusiasts should ditch their boats, boat ownership is an emotional thing, if logic dictated our choice we would all be sailing some very boring boats. But if you are looking for good windward and light air performance from a Wharram cat, the small Pahis are not the place to start. If you can find a Classic Tane 27 in good condition, strip off all excess weight and put decent sails on it, you will have a good sailing boat. A Tiki 26 or 30 will be fast, quite weatherly and a fairly good performer in light airs. The small Pahi might just break your heart!”

I have sailed on smaller Wharram’s, they are fun seaworthy cats, but outright performance is not their design intention. The jpegs are the only known details of the design. PS Sailorman March 1981 was not produced to my knowledge.

 

Don't want to clog the thread, but you pay like +30-50$ sq. m for the foam anyway. the carbon and glass is a rounding error.

 

That was my first thought reading your post 1510 on the previous page, about the foam-glass Tiki 26 with carbon cross beams. The research done by Robert Hughes is very useful for catamaran home builders, but I wondered why someone would use expensive foam and carbon fabric on a Wharram cat, designed for ply-epoxy, with V-hulls and pronounced rocker.

Even if a Tiki 26 is not an ultimate performer, I think the choice of such a building technique was made for better longevity of the boat, better resale value and maybe larger cargo potential. Even so, imho there are many better boats (from a hydrodynamical point of view) who deserve such an investment. But as Steve Turner wrote on that forum, boat ownership is an emotional thing, and I must admit that mr. Wharram designed some beautiful catamarans, even if they are not the best at performing.

 

The Tiki 26 is a perfect demonstration of how people choose to home build boats. The Tiki 26 was chosen because over 1200 Tiki 26 plans have been sold with a high rate of completion into actual boats. The Tiki 26 (standard version) are tube cats 26 x 15.4 foot weighing 1500 lbs and displacing 3000 lbs. It has 285 square foot of sail area with a “wing sail” main and jib.

Now we will discuss over 2 posts the “structure” of various Tiki 26. The standard build is a plywood timber build with 6 mm plywood hull skins with a 45 x 22 mm mid hull timber stringers and 45 x 22 mm outer gunnel. The timber keel line has epoxy qcell gunk poured down the keel line to strengthen the keel which is then glassed over. The stem and stern posts are 18 mm ply with 18 mm ply rudders “tied on” to the stern post. It has six 6 mm plywood bulkheads, 6 mm floor and bunk tops (that are very important structural items), 6 mm ply hull decks with timber deck beams and deck stingers. The cross beams are a triangular shape with and I beam back. There is a dolphin striker on the mast beam. The beams are tied on.

Good now let’s discuss what has been built. The majority of Tiki 26’s are standard builds but there is one in England that was built from 8 mm plywood throughout. There is another Indonesian Tiki 26 that has been built with 5.2 mm hardwood plywood throughout. Both these boats have standard crossbeams. Both these boats sail well with the 8 mm ply guy saying he has reached 15 knots occasionally.

Tiki 26 crossbeams if they are not well built tend to rot which leads to replacement beams. Some builders just build different beams from new. There is full aluminum round tubes of about 150 mm diameter, one Tiki 26 with six 100 x 50 x 6 mm boxes both with dolphin strikers. Wharram does not support aluminum main beams especially if they are bolted down as the cat does not have “enough” flexibility. These cats sail OK but I cannot comment on there longevity. Next group of builders either build or replace wooden beams differently from the Tiki 26 plans. A popular variant is the Tane Tanenui beam upgrade plan available from Wharram. The narrower but heavier Tane upgrade crossbeams plans are OK for the lighter but wider Tiki 26.

The next group of crossbeam upgrades are various forms of composite crossbeams ranging from carbon fibre, PVC foam pultrusion 25 mm carbon fibre square tubes to foam with unidirectional flanges top and bottom. More on these options’ tomorrow.

Next is some people change the good standard rudder system to deeper kick up rudders. Some people have put daggerboards, leeboards or “small keels” to improve windward performance. Then we get to rigs. The Tiki 26 “wing sail” rig works very well and is cheap to construct. But some people have replaced the wooden mast with an aluminium tube (approved by Wharram). There are junk rigged Tiki 26’s. But some people have put fractional sloop rigs with battened mainsails having increased sail area to 330 square foot. There is even a carbon fibre mast Tiki 26 out there.

I will discuss the Tiki 26 fiberglass hull variations that have been built by production and home builders tomorrow.

Many people love their Tiki 26 but the 3 main reasons they appear to be sold is “there is not enough room inside, I feel like I am camping all the time”, next “I thought the Tiki 26 was meant to be trailable but it takes 3 or 4 hours to set up or break down with a couple of people, I want something smaller or can fold” and rot has gotten to cat somewhere.

What does all mean. I complement Wharram hull shape and rig as all of the above boats perform reasonably to very well. A standard Tiki 26 rarely weighs the advertised 1500 lbs with some builders saying their cat weigh 2000 lbs. This is home building. Many people pour more epoxy gunk along the keel line, use heavier glass fabrics to cover things, upgrade the timber dimensions to “add a bit more strength” etc. Tiki hull shapes can handle overloads but be warned moderately built crossbeams can break in overloaded boats. The jpegs give some idea.

 

My understanding of the Tanenui beam upgrade is to bring the Tiki style Wingsail rig and increased beam over to the Tanenui ?

 

Redrueban the JWD Tiki style Crossbeams & lashings upgrade can have a Wingsail mast on it. The Tane standard beam is 12.75 foot, the Tiki style crossbeams can increase it to about 15' or 16' foot according to information from JWD.

Now to the fiberglass hull variations of the Tiki 26’s. They came in 3 forms. The Imagine catamaran foam fiberglass built in the early 80’s onwards. These molds were sold to the US in the 90’s and the Tiki 8 meter was manufactured. Again, the boat was manufactured in foam glass with internal molds for furniture and bulkheads. The final variation are home built foam glass or solid fiberglass hulls. Each has a variation of crossbeams which ranged from the original timber cross beams to foam glass crossbeams to carbon fibre crossbeams.

The Tiki 8 meter is 26.3 x 15.3 foot that weighs 2500 lbs and displaces 3300 lbs according to BoatSmith Inc. Wharram claim a weight of 1650 lbs with a displacement of 3300 lbs. The sail area is a fractional sloop rig with 330 square foot of sail area. Folks, some Tiki 26 real sailing displacements end up nearer 4000 lbs when actually cruising. This is not recommended but survivable.

I do not know the hull structure of the Imagine Tiki 26 glass foam build but it has three 9 mm plywood bulkheads with a glass stringer line at mid hull level. The build of Tiki 8 meter is Deck and hull has a gelcoat a layer of 225 gram csm, a layer of 1708 (600 gsm biax with 225 csm mat backing) fabric mat then a 10 mm divinycell foam with a 225 gram csm followed by a 1708 fabric mat inside. All done in vinylester. Hull halves joined by 5 layers of 1708 mat fabric. Deck is constructed using the same layup. The internal structure is fiberglass moldings. The cross beams are made of Divinycell foam and the tops and bottoms have two layers of 3/4 oz CSM, a layer of 24 oz unidirectional glass fibers and a layer of 10 oz biaxial carbon. The fwd and aft faces have twice this. Beam blocks are made of Coosa board. Some rudder parts are made from Coosa board (more on that product in a later item) and glass.

A solid glass version of a Tiki 26 by a home builder hull structure is gelcoat 450 gram csm, a 600 gsm biaxial fabric and a 450 gram csm (about 4 mm thick). The 450 gram csm is doubled below the waterline (about 4.8 mm thick) and the top 100 mm of the hull at gunnel level. An additional 18 x 45 mm gunnel timber is placed in the hulls. The 6 plywood bulkheads, bunk tops and floors are very structural in this build. Also, the curve of the hull shell (about 25 mm over 1000 mm) helps stiffen the structure. This build was going to have a ply deck and cabin covered with 200 gsm cloth. The hulls weighed 495 lbs each which is not much heavier than ply/glass hulls. Thin you think? Remember 4 mm tortured ply works on 28 foot racing cats.

We have mentioned Robert Hughes carbon fibre cross beams for his Tiki 26. The mast beam was 158 x 122 mm inside diameter with a 3.9 mm wall thickness. In the centre of the beam at the mast point there was an additional 9 layers of 200 gsm carbon cloth up to 1 meter long on the top (and bottom) of the beam for additional reinforcing. Inside the beam under the mast is a 40 mm carbon fibre tube to separate the faces. The ends of the beams at the hull attachment points were reinforced with extra carbon fibre wraps. The mast beam can take 40,000 lbs of down thrust with 8 mm of deflection. The rear beam was 158 x 122 mm with 3.3 mm walls. The owner wanted this size of beams but C-Tech said the beams could have been 138 x 94 mm of the same build and be strong enough.

There are even more variations than this but you get the idea. You can successfully build a boat from many materials and approaches. A designer does a structure that suits their experience and more importantly they can defend in court. That does not mean it is the only approach. EG A Tiki 26 now could be home built with 400 gsm biax inside 9 mm divinycell foam and 600 gsm biax outside (doubled below the waterline) in vinylester vacuum bagged. CSM not required. PS this would be conservative. There are 32 foot designed cats out there with 3 mm thick solid round bilge bottoms. The jpegs give some idea.

 

Just a short note (I hope) to highlight why this thread is so immensely valuable:

Boating in my country is still in it's infancy, due to restrictions during the communist era. (In those times boat ownership was almost completely forbidden, only professional fishermen or party leaders were allowed to own boats, others had to get a very hard -to-obtain party's approval). So today there is a huge lack of tradition and knowledge in this field in my country, from monohull powerboats to multihull sailboats.

Most of the Romanian boaters choose to buy second hand boats from Western Europe, but the sailors minority is beginning to look at new trends like multihulls or monohull racer-cruisers. Production multihulls are bought by rich people (new boats) or by people interested in a charter or diving business (used or even auctioned hurricane damaged boats). For the "normal" boater, the only way to obtain his dream boat is by building it himself (until recently nobody built production multihulls here).

In other ex-communist european countries (Poland, Russia, Ukraine) there are local designers, specialised almost entirely on monohull amateur-built sailboats, and most of these countries builders choose to work with them, because these designers know very well the local conditions and resources. However on multihull design, the internet resources are the only choice, and quite often these are oriented to the mature markets. So for a prospective est-european builder it's hard to make a decision when he feels overwhelmed by the complexity of the build and the difficulty to obtain the required materials.

On the other hand, any builder wants the maximum of results from his building endeavour, so maximum performance, internal space, standing headroom (where possible) and trailerability are top requirements. Being a family business, the resulting boat must satisfy all family members. From my experience, builders in east-european countries get their knowledge from internet forums published in their native languages, and quite recently some started to attend worldwide forums.

I learned these things by participating in the build of a significant number of self-built boats, among which there were 2 multihulls, both from Richard Woods designs - Saylon 20 and Elf 26. There were also online suggestions for Wharram cats, but the final choices took into consideration the maximum boat for the effort involved. The reason for this was the valuable content from mr. Woods website, where cat hydrodynamics are treated in detail, and also the many discussions on boatdesign.net on multihull performance (for example Terho Halme's Catamaran parameters thread).

I think there is an immense advantage for builders from lack-of-tradition countries to skip the mistakes made in the early age of multihull design. This thread is not only a multi structure oriented discussion, but it contains great history, hydrodynamics, seaworthiness, manufacture know-how, and most important, it offers a wide perspective and gives great confidence to anyone contemplating building his own multihull. I learned from this thread more than in many years of web surfing on multihull design and construction. Words are not enough to thank all who contributed here, especially @ oldmulti.

 

I remember the boats being built and talked to the designer about it when I visited him. I asked him about the bustle at the stern as it reminded me of S and S monohull yachts - not known for their ability to slide along at 20 knots. He went cold with my query. As it happened the design was a flop with the contorted aft sections creating drag. Tennant's power cats had a different bustle with a canoe style underbody over a flat aft section - much like the idea Schionning uses.

Another large problem was the location of the centre of bouyancy. It was way too far aft. The centre of accomodation was further forward. This meant that when the boat was overloaded (all boats get overloaded) it pitched nose down. The big bum and skinny bows were a problem. One owner put large bulbs on the bows to get the CB further forward but it was not a total success. The design was sadly a pig and the innovations - contorted aft sections, very full stern sections and fine bows, and aft location of CB were not normal design practices. An overweight version of this build will be an even poorer cat than the modified one I know of. Bummer but if it looks different it can be a dud.

 

The Pahi 63 has been mentioned before. The standard model is 63 x 28 foot with a weight of 18,000 lbs and a displacement of 28,000 lbs. The rig is a schooner wing sail rig with cutter jibs, the masts are 55 foot high and are 225 mm in diameter in wood. The hulls have a length to beam of 17:1 on its waterline length of 51 foot. The hulls have trimming centreboards.

The standard Pahi 63 has wooden crossbeams “flexible tied on” with 6 wraps of 12 mm 16 braid rope. You may just find a 25 mm solid stainless steel rod through the centre of the end of the beam and hull bulkheads that helps locate the cross beams.

Now we will talk about a variation of the Pahi 63 which is being developed for the Seamercy charity organization. I do not know who is doing the design work. The cat is 63 x 28 foot with a 55 foot waterline and a 12 to 1 length to beam. The hulls have low aspect ratio keels. The rig is appears to be a Bermudan mainsail schooner with jib.

The crossbeam deck structure is a more integrated unit although it could still be disassembled but I doubt if it would be done after initial assembly. The accommodation is much more cruiser like than the standard Pahi 63. The structure is unknown but I will bet 9 mm plywood will not be used to build the hulls. This cat is more likely to be foam glass etc.

An interesting variation. I just wonder why a standard Pahi 63 which is a reasonable cheap low tech simple cruiser is trying to be “converted” into a higher performance cat with a modern rig and increased accommodation. Seamercy comment is “A proven design by James Wharram, the Pahi 63 is a low tech, cost effective and culturally sympathetic design, sensitive to the local area of operations (Polynesian inspired) whilst being suitable and kitted out for expedition sailing and for larger volunteer groups. It’s conceivable that Sea Mercy could be operating multiple Pahi 63s which stay in country year-round, delivering health clinics and economic development training programs in the dry season, and standing by for limited first response disaster relief in the wet season.”

Folks, this is where Wharram type “marketing” overpowers logic. The “low tech, cost effective and culturally sympathetic design” could be any 50 foot plus French/South African production charter type cat or tri that will probably cost less, have multiple cabins and the capacity to carry the stuff required for their charity work. Just because the "improved Pahi 63" has a canoe stern and shaped bow tops does make it a substitute for a Polynesian log catamaran with a crab claw rig.

PS I like the standard Pahi 63 as a design for its simplicity, lack of all mod cons and effective rig. But its designer listed 4000 hour build time is for most home builders, optimistic to very optimistic.

 

The following is an interesting experiment in a 16 or 24 or 32 foot proa or trimaran rigs. Confused? The vessel is the Gary Dierking designed Wa'apa. This design comes in 8 foot segments. Being a Proa in its basic form the 8 foot bow and stern segments (which are the same shape) form a 16 foot proa. If you want a 24 foot proa you insert a 8 x 1.75 foot basic box between the bow and stern segment and form a 24 foot proa. One person wanted a 32 foot boat and inserted two 8 foot segments. So, for this item we will stick to the 24 foot version.

A 24' Wa'apa proa is 23.75 x 11 foot (the trimaran version has a 14 foot beam) and weighs 275 lbs fully rigged. Each 8 foot hull section weighs around 60 lbs. The hull construction is basically 6 mm plywood (can be good 4 mm ply on sides) and timber gunnel and chines. The hulls have timber framed plywood bulkheads at joint points and a timber stem/stern. The hull segments are bolted together with 8 mm stainless steel bolts. The crossarms are timber and can be solid or a laminated structure depending on the type of boat and size you have chosen.

Now let’s talk about the rigs. Yes, I said rigs. It can have a Lanteen shunting rig of 84 square foot, a Lanteen tacking rig of 128 square foot, a Bermudan mainsail and jib rig of 147 square foot, a Bermudan mainsail schooner rig of 208 square foot and Gary’s latest experiment a 100 square foot cambered junk sail rig. There was also a Gunter ketch rig in one trimaran version. PS the mast(s) can be stayed or unstayed. Mast height can be from 13 to 17 foot depending on the rig type chosen.

This is a very flexible design. The 24 foot version can carry 2 or 3 adults with a some kids. And its final great virtue is you pack the majority of the boat up into a 8 x 4 x 4 foot package for storage or trailing.

Plans are available from Duckworks, Gary or by purchasing Gary’s excellent book “Building outrigger sailing canoes modern construction methods for three fast beautiful boats”. There are 3 separate designs in the book with variations of each design.

Gary’s latest rig, the cambered junk rig, came about because he was “deterred from some expeditions because of the limited reefing capabilities of traditional Oceanic (lanteen) sails. The Chinese junk rig is famous for being the quickest and easiest to reef, but it has also gained a poor reputation for windward ability in modern adaptations…. The junk sail is traditionally cut as a flat panel and this worked well with old style stretchy fabric which would form an airfoil shape when the wind started blowing. When polyester fabric was used in a flat cut sail, no shape was produced and neither was much lift.” So, Gary choose to use an Arne Kverneland (very talented Junk Rig guy) inspired cambered junk sail made from heavy duty Polytarp. The individual barrel shaped panels of the sail (with about 5% camber cut in) are sewn together at the batten locations. The battens are two pieces screwed together through the fabric and are intentionally very stiff. The result is a sail with the majority of its surface area having a 5% camber. Gary reports “The wind speed averaged 8-10 knots with some higher puffs. I got the boat speed up to 8.4 knots a couple of times in the puffs.” The boat goes to windward well but does not tack quickly due to its flat no rocker keel line.

An interesting design that you can give you years of fun and experimentation. The jpegs give some idea including a catamaran version. The PDF's are small and give rig options.

 

This is about solid glass multihulls but I warn you, this is general information only. The design of any multihull is combination of factors of both local and global loadings. Each part of a structure needs to be designed based on the assumed loads that are going to be placed on it. In the old day’s “designers” would just add thickness (read weight) of CSM fiberglass with little rovings to a hull until it would bounce of rocks. Modern designers are dealing with weight versus “strength” to improve performance and load carrying capacity. Result there needs to be a greater understanding of the laminates, resins, fabrication techniques and most important panel design with supporting framing or stringer infrastructure.

Now we get to real issue. You can have a thick flat solid fiberglass panel without ribs or bulkheads that will weight twice what a curved framed solid fiberglass panel for the same stiffness. If you need some guidance on this topic I will point you at a good old book for composite construction. Be warned this is a 400 page 20 meg PDF download if you choose that option. Marine design manual for fiberglass reinforced plastics : Gibbs & Cox : Free Download, Borrow, and Streaming : Internet Archive https://archive.org/details/marinedesignmanu00gibb/mode/2up

The difference between an older design and modern solid glass design are interesting. In mono hull land an old CSY 44 cruiser had CSM heavy polyester 50 mm thick hull at the keel and 25 mm hull at the turn of the bilge with minimal framing support. A modern Dufour 50 has a fabric heavy vinylester 6 mm thick hull in the topsides with furniture and framing support. In mono power boats an older Bertram 28 foot has a 25 mm hull at the keel line and 12 mm hull at the turn of the bilge. A recently built 140 foot, 6000 HP game fishing boat has a 15 mm to 12 mm thick carbon fibre, kevlar fabric epoxy hull with significant framing and stringers support. In short, the skin thickness is only a third of the story. The shape, fabrics, resins and framing support need to be understood to be of any value. From this point on assume the solid fiberglass numbers will be for round bilge or curved fiberglass hull panels with some framing (and/or stringer) support.

Let’s start with the MacGregor 36 a 4000 lbs displacement tube catamaran. Its hull/deck is between 4.8 and 6 mm thick with areas around beam mounts and centreboard case up to 9 mm thick. A Prout Scamper 26 foot cat of 4000 lbs displacement has 5.5 mm of glass in its topside and 7 mm of glass on its bottom and around the LAR keels. Heavenly Twins 26 foot cat that displaces 9000 lbs has 6 mm top sides with 9 mm bottoms. A solid glass Tiki 30 that displaces 4500 lbs has 5.5 to 6 mm glass hull panels. Woods Wingsong 30 footer that displaces 5200 lbs has about 6.5 mm thick hull side panels. A Prout Ranger 31 that displaces 7000 lbs has 8 mm of solid glass over most of the hull wing deck structure. The modern Rush 10 33 foot day charter cat that displaces 5600 lbs has a 5 mm thick hull/deck structure with a lot of ribs. A later Prout Snowgoose 37 that displaces 12000 lbs has a 7 mm thick topside going to a 14 mm thick sides on the low aspect ratio keels. This is not an extensive list but some realistic sized examples.

Earlier hulls were mainly CSM with one layer of woven rovings. Later glass hulls have more biaxial fabrics with minimal CSM but because they are thinner with the same weight of glass. Thinner glass panels require more framing stringer support to maintain panel stiffness. Also how you put the layers of CSM and fabric together will result in different stiffness characteristics. A layup of 300 gsm cloth, 600 gram CSM, 300 gsm cloth will bend less than a 300 gram CSM, two 300 gsm cloth, 300 gram CSM layup. Same materials different results.

Most modern multihulls are of foam glass construction with minimal solid glass panels. Result is there are few modern larger examples of solid glass multi’s. The jpegs are of the boats.

 

When trolling through a Boatsmith web site I noticed they had built a set of Tiki 46 foot catamaran crossbeams with Coosa Panel, divinycell and glass instead of the “standard” timber crossbeams to overcome any potential rot problems. So, what is Coosa Panel?

Coosa panel is a high-density, polyurethane panels impregnated with fiberglass fibers to provide additional strength and stiffness. Coosa panels come in Bluewater 26 (blue), Bluewater 20 (black), Nautical 24 (red), Nautical 20 (green) and Nautical 15 (white). The number represents the density per cubic foot. All the panels are rot roof, waterproof and depending on the density/type stiffer than plywood of the same thickness with Coosa panel being 40% to 60% lighter than plywood. We will mainly discuss the Bluewater 26 and Nautical 20 from here on.

Coosa Bluewater and Nautical panels are structurally different. Bluewater panels contain both continuous-strand fiberglass and woven roving fiberglass. Nautical panels contain only continuous strand fiberglass (read CSM not woven roving). For any equivalent density, a Bluewater panel will generally be stiffer than a Nautical panel. The panels can be obtained in 8 x 4 foot and up to 5 x 12 foot with thicknesses from 6 mm to 50 mm. Bluewater panels cost more than Nautical panels and depending on your country/location can be cheaper than marine grade plywood or PVC foams.

Coosa is produced by the vast Penske organization, whose major income is from trucking, probably developed it for large truck and semitrailer sides.

The attached PDF’s will give more detail on the structural characteristics. The important part for this thread is that Coosa Panels can have polyester, vinylester or epoxy resins with any type of CSM or fabric glass laid over it. In short it can replace a PVC foam or plywood panel. Coosa is used in several powerboat builders as a structural component for transoms and/or decks. All sounds good rot proof, lighter than plywood and can be obtained at a cheaper price in some places, but here is where the fun begins.

When I looked at the Tiki 46 crossbeams I said why? I can justify the vertical web in Coosa panel but the top and bottom flanges have 3 layers of Coosa board with unidirectional fiberglass flanges top and bottom finally wrapped in 1708 biaxial glass. Why have Coosa panel in the top and bottom flanges? Delete the Coosa panel and just add some additional unidirectional glass on the top and bottom flanges directly onto the divinycell core either side of the Coosa web. The same applies to most areas on a multihull. A 26 lbs/cubic foot Coosa panel with glass either side will be heavier than a 5 lbs/cubic foot divinycell panel with the same glass on either side.

Again, this comes back to design. In deck applications Coosa panel may be justified for its heat resistance and its ability to have thinner glass on the Coosa board as it can handle knocks or point loadings better. In power boat transoms Coosa panels can handle the point loadings and vibrations better than foam glass and will not rot like a plywood core.

The jpegs show the Tiki 46 cross beams, some Coosa panels and the PDF’s with structural specifications.

 

I have great respect for D Halliday of Boatsmith FL. If I had to guess the reasons for using coosa vs glass; you would effectively be able to use the coosa as a stay in mould which would reduce build times.

In my build, I made only a few moulds; less than 10 discounting the station jigs. The trouble with moulds for one off construction is obvious. If you need to make more than one or say ten; a mould can payback. Or if you need two or more absolute replicas, a mould is useful, even required. Moulding single parts is also slow. If you want to make ten pieces that can be made sans mould, and they don't need absolute precision, you might be able to build all ten in a few days versus making one each day over ten days, which is two weeks.

My two cents. Dave is a good builder. The ONLY way a builder can survive is to NOT spend too much time on low value items like moulding for low returns.

perhaps I misunderstand...really enjoy the thread...that word csm...I used mat backed tapes in my build; they do not snake and wetout on a table well, then rolled into place; you can cut them to any dimension easily, but I calculated the penalty and it is close to 300 pounds...a bit nervous and that is just bonding tapes