Multihull Structure Thoughts

Bali catamarans introduced a few years ago (about 2021) an interesting addition. Quote “All our Catana and Bali catamarans will be equipped with a bio-membrane generator that will filter 100 per cent of the grey and black water on board our boats, so more polluting discharge. This is a major step forward” I suspect a slight problem with French English translation. They mean NO more polluting discharge.

Quote from the manufacturer. “Every new Bali catamaran is equipped with a remote submerged membrane bioreactor (BAMID), which sterilises all the waste water on board by both bacterial degradation and ultra filtration. This double process allows the discharge of totally depolluted and sterile water. Operating silently and automatically, the system discharges completely depolluted water into the sea, compatible with MARPOL certification. The system removes the need for holding tanks and the associated constraints of emptying, maintenance and periodic cleaning.”

The waste water filtration unit is in the low aspect ratio keel on the cat. This is a nice advance but for a home builder a level of complication that is not helpful. I agree with the idea but if you are getting to this level of sustainability why are you owning a catamaran in the first place. It is like the guy who built a 200 foot superyacht as the most environmentally sustainable boat in the world. It would have been better for the environment if the boat wasn’t built in the first place.

Bali cats used for charter work, this is a good feature but private owners its an added expense and maintenance item to add to the ever increasing list.

The jpegs give the idea of the location in the cat and the various components including the filters that will have to be replaced on a regular basis.

 

SAILGP has confirmed that its new high-speed T-Foils will be added to its entire international fleet of F50s in January 2025, ahead of the ITM New Zealand Sail Grand Prix | Auckland. SailGP CEO Russell Coutts said the development would be a ‘game changer,’ anticipating the league’s latest upgrade would bring faster speeds and the most competitive racing to date. Set to replace the current L-Foils in season 5, the T-Foils were designed to improve the performance and control of the F50s at high speeds. The new design eliminates the issue of negative loads damaging the foil structure and reduces loads on the board cases, reducing the amount of maintenance and repair needed.

Constructed from machined titanium and carbon, the T-Foils have thinner sections than the current L-Foils, allowing them to delay the onset of cavitation by around 11 km/h / 6 knots. When combined with the in-development rudders, the onset of cavitation will be delayed further still, resulting in increased control and higher speeds. The expectation is that the current speed record of 99.94 km/h will be broken by speeds approaching 110 km/h (59 knots).

The global racing championship has carried out testing over the past 18 months at venues around the world, with a number of teams getting first-hand experience of the new test foils in training. The Canada SailGP Team clocked 101.98kph (in a wind speed of 32kph) using the new foils while testing in San Francisco, the fastest-ever speed registered on an F50.

The jpegs give the idea of the new T foils.

 

I am still around but life is getting in the way. See you tomorrow.

 

This about a production cat that has tester using terms like floating motel and floating home. Describes the interior very nicely. The bali 4.4 is an evolution between the Bali 4.2 and 4.6 and was designed about 2020 and manufactured from 2021.

The Bali 4.4 is 44.25 x 24.25 with a weight of 29,990 lbs and a displacement of 43,210 lbs. The fixed mast carries a 818 square foot mainsail and a 452 square foot self tacking jib. The code 0 is 796 square foot. The draft over the fixed keels is 4.3 foot. The 2 engines are 40hp or 57hp Yanmars.

The accommodation is a choice of an owners cabin in one hull with 2 double cabins in the other hull or both hulls having 2 double cabins with full ensuites. The main saloon with integrated cockpit is vast with upstairs galley, much seating and entertainment galore. Literally a home away from home.

The downside is the test reports on performance. It starts like this quote: “Like most Bali models, the 4.4 is designed for boaters who want a platform for enjoying life on the water, a floating home that can be moved from anchorage to anchorage, and even from one ocean to another”. Another line is: “As soon as the mainsail was hoisted and the Code 0 unfurled, we trimmed the traveller and the sheets. With the wind oscillating between 8-15 knots on the beam, our boat speed was 7-8 knots with a few peaks at 9 knots during little gusts……When we came up to 50/55° off the true wind, boat speed barely exceeded 5 knots, leeway was noticeable, and the sail trim needed to be quite precise. In such conditions, not ideal for relying purely on sail, one or two of the engines can be used to help you maintain course.” I am quoting one of the kinder performance reports. This cat will sail and take you places but do not expect high performance of EG an Outremer.

The construction is mainly foam glass throughout. The fitout and equipment options allow you to upgrade the cat a little to suit your requirements.

This is a pure cruising charter cat that allows you to have a home on the seas. The jpegs give the idea.

 

The following is proof that you can make a cat from anything that has a reasonable hull shape. The Exe dory is a 14 foot rowing dory. Take 2 of them put a few cross beams in and you have a cat. Then put a simple rig on it and go sailing. A fun day boat for a few people.

The Exe dory is designed by Selway Fisher and is 14 x 3.9 foot with a weight 70 lbs. The waterline length is 11.3 foot and the waterline beam is 2.55 foot. The length to beam is 4.5 to 5.5 to 1 depending on crew loading. The draft is 0.33 foot. The dory is oar powered and can have a fixed seat or a sliding seat. A good little rowing craft.

The basic structure is 3 sheets of 6 mm plywood and some timber for framing, seats, gunnels and keel line. The “chines” are bog and glass tape. The Exe is a simple dory shape and relatively easy to construct.

Now if you are enthused you can construct 2 Exe hulls then buy plans for the Docat Supplementary PDF plan and you will get the additional plans to build the cross beams, strengthen the hulls, provide leeboards and a simple rig. The resulting cat is 14 x 10.75 foot with a weight of 200 lbs. The rig has a wooden mast of 10 foot and as gunter rig of 72 square foot. The draft ranges from 0.33 foot to 2.33 foot over the leeboard(s) and rudder. A low power outboard is an option. The cat can be disassembled for transport.

The cat requires 6 sheets of 6 mm plywood and a lot more timber for the crossbeams, bridge deck, leeboards, mast and rudders.

No comments about performance but it would be a fun daysailor or a limited range camp cruiser for 2.

The jpegs give the idea. The Docat is the first jpeg. The remaining jpegs are of the Exe "monohull".

 

In 1974 the Lock Crowther designed Kraken 55 was launched from a farm in NSW Australia. It was sailed across the Pacific to Honolulu in 31 days were Mike Kane named it “Spirit of America” (SOA) and went racing. He had at the time had the fastest sailing boat in the world. EG 1975 Round State Race, 240 miles where SOA was 1st in 13 hours at an average 18.5 knots. SOA also entered many other races having some success but also had small development issues which limited its success.

The Kraken 55 was 55 x 32.5 foot, weighed 12,000 lbs and had a 70 foot aluminium fixed mast that carried a 900 square foot mainsail and a 1060 square foot foretriangle. The number 1 Genoa 1400 square foot. Spinnakers up to 2500 square foot. The draft ranged from 2.5 foot to 7.5 foot over the central hull daggerboard. A 25 HP outboard was the motive power.

Now Mike Kane raced hard but there were several weaknesses in the Kraken 55 design. Submersible floats was the major weakness and eventually caused the second Kraken 55 built to capsize. The second issue with low buoyancy floats was waves slamming into the cross beam fairings which eventually were damaged on SOA. Next was a single rudder on the main hull which lifted out when the low buoyancy float went under. Third, the rig was on a flexible structure (was not torsionally rigid enough) which meant sails became full at times increasing overturning moment.

Result, SOA had some modifications done (eg SOA was lengthened to 62 foot with a fuller stern for the 1978 OSTAR) before selling the tri and turning to a Norm Cross design. Eventually Robert Underwood (of Prout fame) found SOA abandoned in France and had it rebuilt after stripping it out, steam cleaning it, having it repainted (blue tri in jpegs) etc. Robert then sold renamed Spirit to Canada who used it for a few years before it was found again tied up in Galveston, Texas since at least 2015.

Now I mentioned a second Kraken 55 which was built by a farmer in Victoria Australia. It was a “standard dimension” Kraken 55 with an upgraded crossbeam structure. It was done by a home builder who employed 2 other guys. Phil Smiths “Yumi Maru” initial sea trials again highlighted several issues. The rig was pulling the main cabin apart and required a 18 mm stainless steel bolt through the main cabin to limit chain plate and rig movement. The submersible float went under and left no steering control until sheets were released. A steady 26 knots was possible prior to the float going under after which you slowed to 18 knots with little control. In Yumi Maru’s first serious ocean race a spinnaker pole guy was released, not a spinnaker sheet, resulting in a capsize. The tri was recovered and a Karken Mk 2 design was released and applied to Yumi Maru. Result was 55 x 36.5 foot, weight of 13,000 lbs and a slightly enlarged rig. The floats were 160% buoyancy, non submersible and had a longer water line with the buoyancy pushed further forward. The forward cross arm was strengthened and most of the structure lightened. A new build would also have had a lighter main hull. Yumi Maru sailed to Japan and back, did some local racing before being sold into charter work in Guam with solid decks instead of the nets and tourist seats.

The structure of both builds in Australia were polyester on Airex foam core using unidirectionals, E and S glass cloths with limited woven rovings. The decks were plywood. The cross arms were a box section with unidirectionals at each corner of the box and top and bottom unidirectional flanges.

For its time the Kraken 55 was a very fast tri and helped set a theme for future ocean racing boats.

 

This is about a promised 2025 catamaran (there will be smaller sisters). Katatec Marine Composites in the UK is marketing Calibri Catamarans which are being built in the Gulf of Southern Thailand. Katatec has brought together a team of 2 European Marine Engineers with a German Qualified Composite Technologist directing a highly skilled composite forming unit. All the senior staff have long experience in the construction of modern yachts and multihulls.

The main cat we will talk about is the Calibri X45 which is 45.2 x 24.5 foot of an unknown displacement. The 50 foot aluminium mast has a 1,238 square foot mainsail and standard jib. The draft over the fixed keel is 4.2 foot. The engine options are either diesel 2 x 40hp Yanmar Saildrives or as an option twin Electric /Diesel aux. Hybrid drives.

Hybrid drives are a fast changing technical area, but the two leading Companies by reputation. Torqeeda .. a German Manufacture & Ocean Volt .. from Finland, have proven drive systems and worldwide dealerships. In liaison with these companies Calibri can install a drive system of the most suitable type for the catamaran.

The hull shapes are inverted bell shapes with chines above the waterline to enhance the internal space of the hulls. The symmetrical fully developed flared hulls that give high prismatic coefficients and excellent performance while also providing high buoyancy reserves and good longitudinal pitch control. The low aspect ratio keels (NACA) provide upwind performance, also allowing shallow water use and the ability to beach with good rudder and propeller protection.

The accommodation is 2 double berths in each hull. One hull has 2 shower toilets the other hull can either have 2 shower toilets or a galley between the double berths. The main saloon is either seating/table with entertainment or seating/table with an upstairs galley. The attached cockpit provides additional space to the main saloon and has a semi raised area for helming and sail control. Most of the cockpit area has a roof. This design appears to be a fast cruiser.

This is a semi custom design of which the companies involved plan to build about 6 cats per year. The construction of the hull and deck are Vacuum Moulded GRP with high density foam cores. All mouldings are vacuum formed using tested osmotic resistant Isopthalic Polyester Resins with biaxial and triaxial cloth's, vacuum compression bonded onto high impact resistant cross linked cores. Below the waterline, hulls are epoxy coated for further long term protection. All main and crash bulkheads are reinforced and tab bonded to increase impact resistance.

Sorry about the limited jpegs but more can be seen at their web site. Luxury Sailing Catamarans | Calibri Catamarans https://katatec.com/design

 

A home builder decided to extend a USA built flush deck Cabin Sib constant camber 32 footer to a 39.6 foot Cabin Sib. How? Saw the tri in half at its fattest point insert a 6 foot section in the main hull and do a similar action on the floats and add an additional rear crossbeam.

The original Cabin Sib 32 foot design by John Marples is 32 x 24.4 foot with a weight of 3,200 lbs and a displacement 4,200 lbs. The 40 foot fixed aluminium mast carries a 526 square foot main and foretriangle. The maximum sail area suggested is 613 square foot. The draft when the centreboard is down is 5.4 foot. The suggested outboard power is 10 HP that produced about 6.5 knots.

The structure is constant camber panels of 3 layers of 3 mm WRC or ply with 9 m plywood bulkheads and 9 mm ply decks. The wood box beams are 200 mm deep and 210 mm wide. Where crossbeam go through the main hull there are doublers that build the hull thickness up to 27 mm. This would be a relatively fast boat to build but it has one down side, accommodation.

I suspect this is part of the reason the original 32 foot to be extended to 39.6 foot and had a small cabin roof added to the flush deck. The 24.5 foot beam of the tri was not increased nor was the basic hull shape so the displacement would be about 5,300 lbs. The original rig was maintained and left in its original position but chain plates were made composite and moved aft. An additional rear cross beam was added. Also, a wind driven self steering system was added.

The additional hull structure grafted in could have been constant camber panels or eg 2 layers of 4.8 ply panels curved to shape. The decks could be again 9 mm ply. Mr Marples designs strong boats so the structure could handle the 1,100 lbs displacement increase, especially if the rig was not significantly increased.

The tri in the 32 foot and 39.6 foot formats crossed the Atlantic under sail so the boat is very sea capable. The lengthened hulls were claimed to improve windward performance with the same rig.

The interior of this tri is basic with a main hull width of 4.45 foot and a small area of 6 foot headroom. Result a toilet forward, a double bunk then the main cabin with a very basic galley on one side and a seat on the other side. The cockpit is next with an additional aft cabin added. The interior is improved but this is still a fairly basic (but very sea capable) 2 person long distance cruising tri.

The jpegs give the idea. The first jpeg is a flush deck Cab Sib 32. The following drawings are of the original Cab Sib 32 design. The following jpegs are of “Twilight” sailing, interior and rebuild jpeg.

 

Cab Sib 32 extended to 39.6 foot. These are mainly reconstruction jpegs.

 

The award for the most outrageous proposal I have seen. The following catamaran “Galileo 2” is a proposal that has a price tag in the original article of 450 Billion (not Million) British Pound catamaran designed by a team of German designers from Beiderbeck Design. I suggest the concept is very much a marketing tool as there is no person on earth who is rich enough to pay for this concept. Realistically this cat could be built for 450 Million British Pounds or less if required.

So, to this mythical design. The cat is 656 x 260 foot of an unknown displacement. It is a power catamaran. The cat is capable of travelling 19,000 miles at 22 knots with its onboard fuel tanks. The catamaran will be powered by a methanol drive system and will ‘pioneer geothermal energy’ in yachting, the designers said.

The catamaran is designed to ‘stretch the limits of what is possible’ and has an enormous ‘harbour area’ below the main deck. There is 250 foot of dock/pier for the support boats to tie up to. When required the support boats can be lifted on board. The ‘fully equipped harbour’ between the two hulls has room for up to 25 ‘toys’ such as jet ski’s, submarines, and even smaller boats. If guests need to board the boat while it is already at sea, there will also be a helicopter platform with room to store the aircraft and a place to refuel.

The “Galileo 2” would have on board, room for entertainment, lounge areas and a unique ‘fold-down’ restaurant for its 38 guests to experience as well as have a 5300sqft open-air pool, and an ‘outdoor amphitheatre style’ cinema. There will be an onboard hospital, just in case of emergencies. Up to 75 crew will be required to keep the craft operational including expert chefs, cinema attendants, bar staff and many more.

No mention of the structure but I strongly suspect it will be a steel hull and underwing with an aluminium and quite a bit of glass deck area. The engineering of structure will be matched by the electrics, plumbing, engine installation and control systems. The $50 million plus design engineering fees will just be the start.

The jpegs give an idea of a dream.

 

An Australian company called AML3D is providing equipment, software and feed stock for 3D printing of metal parts. They also manufacture parts as required. #D printing of metal parts is not new but this company has an innovative lead in the business. The companies products from one manufacturing 3D print can produce items in aluminium, titanium, copper alloy’s, nickel alloys, steel and stainless steel using appropriate feed stock.

The other significant advance is that the metal heating and laying down of layers of metal is handled at the nozzle used for laying up the materials. Result the 3D printer can be compact and light compared to a 3D printer that requires a heated enclosure. In simple terms you are MIG (TIG etc) welding each layer to the underlying layer.

The US Navy was so impressed that in May 2024 it sought funding to have 1 of these 3D Metal actively integrating additive manufacturing (3D printers) on each aircraft carrier to produce metal parts to minimise down time on ship systems. The parts that can be manufactured can be large, we are talking feet long and 2000 lbs of weight.

The interest here is the same technology could be used to manufacture difficult hull shapes in aluminium, steel or titanium. Yes, it would take time but robots work 24 hours per day and are capable of printing a frame structure as well as a shell at the same time.

I will quote from AML3D information: “WAM® revolutionises the production of medium to large-scale and exotic material parts by combining an electric arc with certified welding wire feedstock in a free-form manufacturing environment.

AML3D's patented Wire Additive Manufacturing (WAM®) process fits into the Direct Energy Deposition (DED) category, specifically DED-arc. Unlike many other metal additive manufacturing methods, WAM® enables the printing of metal parts in an open free-form fabrication environment using localised inert gas and certified wire feedstock. This localised gas shielding eliminates the need for an enclosed chamber, unlocking the potential for larger component build sizes. The certified wire feedstock is melted by an electric arc, and the molten metal is deposited onto a substrate, layer by layer to create a three-dimensional part.”

Technology is advancing quickly and combined with designers using CAD/CAM we may find boats being manufactured faster and more accurately.

The jpegs give a hint but look at the website if you want more detail: ARCEMY® | Wire-arc Additive Manufacturing Solutions https://aml3d.com/arcemy

 

Kurt Hughes occasionally does some interesting conceptual designs that get translated into real boats and start a new idea line. As an example, is the following power trimaran. It started out as a 32 foot idea, was built by a legendary boat builder Andre Cocquyt as a 35 footer with a different style of external finish. The next conceptual version of the design was a 50 foot semi planning concept.

Let’s start at the conceptual 32 foot power trimaran. It was proposed to be a foam/glass build with a 30 hp outboard that gives 20 knots empty and with a 45 hp outboard give 20 knots under a full load. Kurt tried to make it look radical but be functional. Opening doors on each side of cockpit with steps aft to motor.

Andre Cocquyt modified the the looks of the 32 foot design and extended to 35 foot but kept the same concept. The tri is powered by 2 x 50 HP 4 stroke outboards. This version did 23.4 mph on the second test run and cruises happily and quietly around 3,400 rpm doing 14 mph. With engines in idle forward, she does 6.5 mph. Some trim mods and a swim platform were added. After further development the tri did 24 knots (27 mph). The majority of the build is foam fiberglass.

The final conceptual version is a 50 footer which was conceived to do 50 knots. This version has a semi planning main hull shape. There is little detail of this version as the conceptual design was done on a low budget so no estimate of the outboard power required. Again it would be a foam glass build.

Kurt Hughes is an adventurous, very competent designer who is lucky enough to get some good builders to build his designs. If he trusts the builder, he lets them have a little artist licence.

The first 3 jpegs are of the conceptual 32 foot design, the next 3 jpegs are of the Andre Cocquyt 35 foot build and the final jpegs are the study of the 50 foot semi planning proposed trimaran.

 

The following is an edited version of Kurt Hughes recommendations for resins combined with a bit of my experience, I will do foams tomorrow. The article also included comments on plywood and timber but will focus on resins now.

For plywood and timber if you are planning on any multihull above a limited daysailor use quality branded plywood and clear straight grain timber virtually knot free. It depend on where you live in the world and what is available in your local area, but do not compromise and ask your designer what ply or timber you plan to substitute. If the boat is a long term proposition use epoxy saturation and epoxy glues with a light glass cloth on exterior surfaces.

For resins for foam fiberglass construction they range from polyester low strength to high strength polyesters waxed or unwaxed that can range from brittle to flexible formulations. These are the cheapest with often lower strengths. But remember polyester resin has a stretch to failure of about 1%. E-glass has a stretch to failure of about 6%. The result is a highly tension loaded part could become a necklace of fiber with polyester chunks on it. More than that, in the marine environment, polyester leeches out into the water as it bonds with the water molecules. It loses strength its entire life. After several years it will have about 20% the strength that it started with. Also don’t forget gel coat blisters. With polyester, it is not if it will get blisters, but when it will.

The next step are Vinylester resins has stretch to failure and other mechanical properties similar to those of epoxy, at a lower cost. There many successful boats done with vinylester. It is tougher to get right though for a backyard builder. The initiator and catalyst proportions must be accurate to fractions of a percent of volume, and must change during the day as a function of temperature change. Vinylester is more “waterproof” than polyester resin and in normal circumstances not blister like polyester resin boats. Again, check the version of vinylester as there is formulation variations some more suited to the marine environment than others. Also vinylester has a shorter shelf-life than epoxy so if you have a long project you may not be able to buy in bulk.

Next is the considered “best” for marine use, epoxy resins. Now there is a vast range of epoxy formulations ranging from low strength to high strength, a range of flexibilities and specialised epoxies for various functions such as resin infusion, high temperature oven curing, varying cure times etc. Understand your build requirements and capability, understand the glass fabrics and there characteristics and select an epoxy that will suit these. I have seen a guy build some epoxy glass panels that did not appear to “cure” until a temporary oven was built and the panel sat in 80 plus degrees centigrade for 20 hours. Only then did the builder found he had the wrong formulation of epoxy for his style of building. The big downside of epoxy is its cost, it is the most expensive, the upside is there are a lot of manufacturers of epoxy and it is a competitive market.

For home building both polyester and vinylester need accurate mixing of the resin and hardener for the temperature of the day where as most epoxies are a fixed weight or volume ratio within a temperature range. Easier to do but still requires accurate mixing. The upside of epoxy is that it continues to cure over time and increasing temperature. Both polyester and vinylesters reach near maximum strength on the initial full cure and start to decline in strength after that. Polyester declines the fastest especially if it is constantly under flexing loads due tio insufficient rib support of to thin a foam in a foam glass panel.

Final comment. If a designer specifies the type of resin, glass and foam do not substitute another product without consulting the designer as you may significantly alter the structural characteristics of the vessel you are building for what may appear to be a 3% cost saving.

 

Back in the day, 1978 or so, the business I worked for was pioneering VE in spa tubs and pools, we used VE behind the gelcoat with a 225gm csm. This was post cured at a moderate temperature. The big thing I remember though was the 225 litre drum of VE had a copper line in it so dry air could be bubbled through it every couple of days, this supposedly kept the resin fresh, I’ve never seen or heard of it since.
2c

 

Today we will talk about foam (and a little about timber, ply and balsa) cores, again part Kurt Hughes information and part my experience being discussed. Start comment, cores vary in shear, compression and “workability”. You need to understand why and what you are using and if it is compatible with the fabric and resin system being used. To give the most extreme example I have seen of bad building was a guy who used polyester resin and CSM over a plywood hull. It lasted less than a year before the fiberglass started to break away from the ply leading to leeks and rot. We will be talking about boats above day sailors here.

So, let’s start with ply, timber and balsa cores. Some designers use a ply core then do a light cloth in epoxy on either side of the ply for the shell or deck. Good if done as a surface covering or when you use a thin plywood with glass as a strengthener. It gets a bit of a problem when a designer says use 9 mm plywood or a 9 mm foam core. The ply version will be strong but be EG 25% heavier as ply can be 6 times heavier than foam as a core. In Australia good quality ply costs more than direct import foam cores.

Timber cores EG WRC or Paulownia strip plank cores or Balsa with biax or triax cloths on either side in epoxy can produce light strong hull and deck shells. You need to ensure the underwater portions are well sealed to negate water ingress but if done successfully you will have a strong long lasting boat. To quote Kurt Hughes: “One client is using Paulowina wood for core in place of balsa as it costs much less. It is heavier than balsa at 15 to 19 pounds per cubic foot (balsa averages 11 lbs/cuft), but has better structural properties. The bending modulus is double that of balsa. I don’t find a shear strength printed, but I’m confident it is much better than balsa. It would need to be hot coated with a tie-coat like bare balsa must be. It looks very interesting.” Balsa does not absorb impact as well as the foams do.

On to foam cores. Styrofoam is for model boats only. Kurt Hughes: “Several times a year I get asked if urethane foam is a good substitute for PVC or SEN. It is not. To get adequate shear strength, Scott Lewett at Structural Composites tells me that the urethane core needs to have about double the density of the PVC or SEN to do the same job. What’s more, it crumbles much more easily under repeat load. It is called being friable. Your boat could see a lot of repeat load. Friable core is thus not acceptable.” Small day boats only, not ocean going vessels. On to the serious foams.

Corecell and Divinycell are the best of the foam cores but are expensive. China now produces very similar products at a far lower cost if you can directly import a reasonable volume of product (EG 200 square meters or x cubic meters). If the total structure is designed well and built appropriately the fiberglass skin can fail before the core fails when the boat hits something. These foam products are very good when used properly. Bu again there are many densities of foam and material preparations. There EG 80 kilogram, 100 kilogram, 120 kilogram weights of foam each has stronger surface shear and flexibility differences. Again follow the designers specifications. Don’t substitute a 120 kg foam 18 mm thick when the designer wants 80 kg 12 mm foam, you will double your core weight and your cost of core material. It may appear to be light but 1500 square foot of it can add weight.

Next level are products like the original Nida-Core which is a polypropylene honeycomb used as a core in sandwich panels. It comes covered with a non-woven polyester scrimm that is thermofused to the cells. This covering helps bonding properties and keeps resin out of the cells. The density is 4.8 pounds per cubic foot. The shear strength ranges from 70 psi to 130 psi. That gives a shear specific of 14.6 to 27. It is available with foam filling. Corners cannot be easily faired into a radius. Nidacore has superior sound deadening properties to foams or balsa of similar densities. Other products like Polycore or Plascore are similar but have different physical properties depending on the core material and any foam fillers. Some of these products are sold as premade flat panels.

Many other core materials have been tried but the above are the materials that have stood the test of time. But please remember that if a designer specifies a material resin combination, please follow the instructions. I know 26 foot tri’s that have 600 gsm skins on 12 mm foam. I also know 60 foot ocean racing tri’s that have 600 gsm skins on 20 mm foam. The difference is the 26 footer has 600 gsm biax e-glass with vinylester skins. The 60 footer has 2 x 300 gsm unidirectional high strength carbon fiber using high strength epoxy that was cooked for hours in an autoclave. The strength difference between the 26 foot and the 60 footer hull shells is vast.