Multihull Structure Thoughts

The second Banul cat we will look at is the 53 footer. The cat again is a high performance cruiser of 53 x 26.2 foot weighing 16,800 lbs and displacing 22,200 lbs. The mast is 75 foot high and 86 square foot (sq ft). The sail area is main 1075 sq ft, jib 485 sq ft, code 0 1075 sq ft with a total of 2150 sq ft. Length to beam is 14:1 and a minimum underwing clearance of 3.25 foot. Again this boat is a very high performance cruiser racer. Again the hull and deck are E-glass / Epoxy foam (Corecell) sandwich construction. Hull and deck parts moulded from female tooling to reduce fairing and save weight. Vacuum bagged and infused to control resin content, fiber ratio and improve strength/weight ratio all post cured in an oven. The mainbeam, aft beam, daggerboards, main cabin roof, rudders, chainplates and selected reinforcements are all carbon fibre construction. If you can afford it the entire boat can be built in carbon fibre. Both the 60 and 53 cost in excess of $2 million US dollars for this combination of performance and space. I know a local guy who after giving Tony Grainger his start in boat design, he had quite a few Grainger boats built then brought a Banul 60. He then said he needed more performance so he went to DNA in Holland and brought a foiling T10 trimaran, 30 knots plus and another $1 million US plus spent. Oh well, for the rest of us we may as well continue dreaming.

 

This Lavranos Marine Design is a small family ocean cruising cat. The cat is 35 x 20.9 foot weighting 6300 lbs with 775 square foot of sail. It is designed to do ocean crossings and is of low cost plywood construction and could be built by a capable of amateur with a minimum amount of labour and skill. This simple, strong, cheap boat is also aimed at being practical for local use. She is configured with two machinery and keel variations; A 4 stroke 20HP outboard on a hinging nacelle is fitted for economy, as is tiller steering, with "kick up" rudders. The other choice is that a saildrive "single cylinder baby diesel" can be fitted, as can fixed mini keels. The rig is a pair of cap shrouds and headstay triangulated, single diamond. The sail inventory is essentially a mainsail, a roller furler jib, and a storm jib. Optional: Genneker.

The hulls are 9 mm ply, underwing 12 mm ply, forward underwing 2 x 6 mm ply, bulkheads 9 or 12 mm ply. Stingers are 50 x 20 mm with some 60 x 30 mm in Meranti. Internal furniture 6 mm ply. Main cabin top build in material of choice. 87 sheets of Ockume Marine Plywood at BS1088 grade for hull and decks comprising of 6 sheets of 6mm x 1220 x 2440mm, 66 sheets of 9mm x 1220 x 2440mm, 15 sheets of 12mm x 1220 x 2500mm. It takes 150 kgs of Epoxy Resolution 816 and Cure Agents to build the boat with over 320 sq mtrs of 300 gsm cloth covering the hulls and decks.

 

With regard to which glue to use, many builders have moved over to epoxy because they think that it is the ultimate in glues. That is not the case, however. Epoxy works well most of the time but there are applications that phenolic or resorcinol glues are a better choice. I have a few words about glue selection on my website at Dudley Dix Yacht Design - Frequently asked questions - Wooden Boats http://dixdesign.com/FAQwood.htm . Additional to that info, epoxy can lose its grip on wood that is saturated with water. In contrast, the bond of resorcinol increases if the wood becomes wet. Resorcinol is my choice whenever practical for bonding important items and I prefer to keep epoxy for coatings and items that are not unlikely to become wet. Regards, Dudley Dix

I saw this quote in a blog site from Dudley Dix. I visited the site Dudley Dix Yacht Design - Frequently asked questions - Wooden Boats http://dixdesign.com/FAQwood.htm and cut down his commentary. It conveys the same message.

What is the difference between exterior and marine grade plywood?
Marine grade plywood is built to much higher specifications than exterior grade. The veneers are more carefully selected for marine grade to minimise interior voids and veneer repairs during manufacture. Both types are glued with resorcinol adhesive, which gives them the characteristic dark glue line. More care is taken in the assembly and gluing of marine plywood boards. The British Standard Specification for marine plywood is BSS 1088. It is normal to see some voids in the inner veneers if you inspect the edges of an exterior grade board. There should be no edge voids visible in a marine grade board.

Marine plywood cannot be made from anything other than resorcinol adhesive because it has the best adhesion properties to wet wood. If your plywood does not have dark resorcinol glue lines then it is not marine grade. However, remember that exterior grade also has the dark glue lines.

Can I use exterior grade plywood to build my boat?
Exterior grade can be used for certain aspects of boatbuilding but will always have a shorter life than marine grade. If you build from exterior rather than marine grade, it will deteriorate faster if exposed to the weather. The more extreme the weather conditions, the faster the deterioration. Moisture penetrating into internal voids then freezing in winter will rapidly break down the plywood structure. That means that it must either be protected from the weather by being stored indoors or it must be efficiently sealed with epoxy.

The hull and deck of a large boat must be built with marine grade. It is a false economy to do otherwise because you will have more maintenance in the medium to long term. The internal bulkheads and joinery can be built of exterior grade as long as all edge voids are filled with epoxy and all exposed edges at boundaries and openings are properly sealed with epoxy.

Must I coat my hull with epoxy or can I use polyester resin?
Epoxy resin for hull coatings is called "low viscosity 100% solids epoxy". This means that it is liquid enough to penetrate well into the surface of timber but has no solvent in it to give it the low viscosity. Solvents have to evaporate off during the curing process and, where solvents go out, microscopic pores are left which allow moisture to go in. Polyester resin is much more porous than 100% solids epoxy. Evaporation of solvents results also in reduced material volume, ie shrinkage. Polyester resin has lower bond strength to timber than epoxy has, due in part to the shrinkage of the resin as it cures.

Should I apply a layer of glass or other reinforcement in the epoxy coating?
A layer of glass fabric set into the epoxy coating on the outside of a plywood or cold moulded hull will give it improved resistance to abrasion and minor knocks. It will have little puncture resistance so it will not offer much protection from major damage. On the negative side, it increases building time considerably because of the need to fair the surface again after the glass layer is applied. A layer of kevlar or other plastic fabric in the resin aggravates this problem because it has a lower density than the resin so it floats on top of the resin rather than settling into it. That makes it more difficult to squeegee to a smooth surface and leaves more resin under the fabric and less on top to smooth the weave.

What kinds of glue are available and which should I use?
There are three main types of glue in use for boatbuilding at present. They are polyurethane, resorcinol and epoxy. Polyurethane is not preferred. Epoxy adhesive is strong and has good gap-filling properties if structural fillers have been mixed into it. It gives greatest bond strength if there is not wood to wood contact, ie the epoxy must not all be squeezed out of the joint by clamping pressure. For laminating work in producing formed beams etc, it needs to be clamped to a former until fully cured. If not, the member will straighten out due to creep of the resin and be impossible to return to the intended shape. This means that any excess glue which is not cleaned off will be hard when the piece is unclamped and can be a chore to remove if access is a problem.

Resorcinol is also strong and has moderate gap-filling properties. This two-component adhesive comprises a liquid resin and either a resin or a powder as the hardener. Accurate measurement for mixing is easier with the liquid type than with the powder type. This is the only adhesive which can be used in the manufacture of marine grade plywood, because of its superior adhesion to wet timber. Glue joints are strongest with wood to wood contact, ie high clamping pressures to squeeze the excess out of the joint. It does not creep once the initial set has taken place, at which stage it has a consistency like very hard rubber. This characteristic makes it ideal for laminated beams etc because the piece can be unclamped when the glue is still relatively soft and can be trimmed off with a chisel. It may not be suitable for laminating large areas if working short-handed. It tends to dry rapidly on the surface if spread thinly, really requiring two people working together to allow a quick enough time from start of glue application until closing of the joint.

Can I use aluminium coated posidrive screws instead of traditional bronze screws? They are far easier to use with a power screwdriver.
If building an epoxy coated cold moulded or plywood boat, aluminium posidrive screws will be fine as long as they are driven below the surface then filled flush with epoxy filler prior to applying the epoxy surface coatings. For carvel or other traditional methods of construction stick to bronze wood screws.

I want to paint my hull a dark colour. Have you any comments or recommendations on this please?
I recommend that you stay away from dark colours as long as you are working with epoxies that cure at ambient temperatures. Epoxy resins can benefit by increasing in strength from the post-curing process that occurs under increased heat, up to a point. That point is the Heat Deflection Temperature (HDT) and varies for different resins. For resins that cure at ambient temperatures the HDT is likely to be around 70C. Temperatures in excess of the HDT will soften the resin and seriously weaken it, allowing it to stretch under whatever load is being applied at the time. That means that the structure will deform if the epoxy is working as an adhesive or is in a laminate. If the epoxy is in a coating then it will sag or run. Any paint that overcoats the epoxy will craze. The solution is to stay away from dark colours. The darker the colour, the bigger the problem.

What epoxies can I use for wooden boatbuilding?
Suitable epoxies are available from most suppliers. Without being able to test them myself, I have to go by the manufacturer's product description the same as you would. Recognised brands are WEST, System Three and MAS; I have had good results from all of them. You need a low viscosity epoxy that does not include any solvents. A laminating epoxy normally qualifies. It must be a type that suits the temperature in your workshop or building location, to give a 20-30 minute pot life. For most locations that is probably a fast hardener in winter and standard hardener in summer but closer to the equator you may need a standard hardener in winter and slow hardener in summer. In the tropics, you will need a hardener formulated for high temperatures and high humidity. Each supplier must tell you which hardener you need for your temperature range.

The need for low viscosity is so that the epoxy will be drawn into the fibres of the wood, for better bond and long-term durability. A thick epoxy will sit on the surface with little bond strength but a thin epoxy (like runny honey) will bond itself to the fibres below the surface. A low viscosity epoxy also results in a smoother surface, with less work required in finishing.

If you will build your boat outside of a controlled environment (heated/cooled factory building) then you must add moisture tolerance to the required qualities of the epoxy. There will be times that you are caught out by the air cooling down before your epoxy has its initial cure. If the epoxy is not moisture tolerant then the cure will stop and you will be left with a coating of gummy gel that has to be completely removed before a replacement coat can be applied. To test an epoxy for moisture tolerance, apply a coat to a sheet of plywood, plastic or metal. While it is still tacky and at regular intervals until it is hard, run water over the surface to see how it reacts. If it still cures without problems, you have a good epoxy for your purpose.

The epoxy brands that have been developed for boatbuilding will normally fit the requirements of low viscosity and moisture tolerance. If you choose an epoxy that is used by some other industry that does not have the same requirements then it may not be suitable. In that case I suggest that you test some samples before using the materials on your boat.

 

Some additional jpegs of the Proteus 106 build. The bulkheads are 12 mm ply. Cab roof 4 mm ply 25 mm foam 4 mm ply.

 

Graeme Delaveau is a New Zealand designer of cats and tris mainly in plywood with timber stringers, crossbeams etc. The designer, first boat was Nick Cruz, so he named this tri Nicky Cruz 7.6. This build had the following mods: 1) longer floats to enable a straight beam across the rear of the cockpit and 2) the float sheer height raised. Nicky Cruz 7.6 Sports Weekender is 25' x 19' 3'' foot weighing 1600 lbs, displacing 2200 lbs. Sail Area 345 square foot on a 130 x 90 mm section, untapered so if I want to go to a square top later the rig will be stiff enough. Construction is basic 6 mm ply over frames and stringers glassed over. The wooden box beams have 2 timber lamination's top and bottom of beam with timber separating blocks every 500 mm, ply fore and aft faces. The rear beam is about 240 x 350 mm with top and bottom lamination's of 2 layers of 240 x 50 mm. The fore beam is deeper with a heavier lamination on the top flange.

The owner of the yellow boat reported with his current rig he could carry full working sail in 20 knots upwind comfortably, and with a little judicious feathering of the main she seems comfortable in 25 knots of wind. The interior has a small galley bench and seat with 2 narrow side bunks. A nice cheap to build tri.

 

The C329 trimaran nicknamed the Tardis is an aluminium trimaran of 29.5 x 20 foot that can fold to 8.2 foot. The tri weighs 7700 lbs and can displace 10500 lbs. It carries 620 square foot of main and genoa on a 38 foot spreader mast that has rigging to the main hull only. The main and float hulls and decks are aluminium with the cross arms of carbon fibre. It accommodates 6 people in 2 double berths and a saloon table that converts to a double. Headroom of 1.91m to 1.87m. A shower-toilet cabin. The interesting part of this boat is its folding system. The cross arms attachment to the float is on a track. As the float moves toward the main hull the cross arms rotate with the float end of the cross arm sliding forward or aft on the tracks in the float deck. A neat solution which keeps the floats parallel to the main hull as it folds. I have no details of any of the boats structure etc. This is a practical cruiser that has room and a reasonable carrying capacity. The jpegs give more detail.

 

Bernd Kohler designed a personal multihull cruiser for the least amount of work and cost. The boat is a cross between a catamaran and a proa. Unequal hulls like a Proa but sails like a catamaran. He calls it a CATAPROA®. The craft is 13.75 x 7.5 foot and can be compressed to 5.4 foot. Its empty weight is 200 lbs and displaces 630 lbs. It carries 88 square foot of sail in a “crabclaw” rig (actually a Sunfish rig). A one or two person boat for raids and treks. The cabin looks tiny from the outside but is adequate. Most surprising is the handling quality, which was even better than expected. Coming about and gibing is like a very good dinghy.

To follow the low cost approach he used a Sunfish sail and parts of the rigging. The mast is a carbon tube with a diameter of 50mm. To shift the pressure points (CLR) of the hulls to the correct position in relation to the CE of the sail from one tack to the other, the daggerboard is canted forwards and backwards. The system works to perfection.

Construction is the time proven plywood/glass/epoxy composite system. The construction is a mixture of systems. No “stitch and glue” (too time consuming when you look at the steps necessary). The hulls are taped together and then fillets and glass strips are added on the inside. The sheer line is conventionally built up with stringers. The boat was built in around 180 hours. A first time builder would require more time. The total weight of the boat including the rig is 89 kg. The mast hull weights a mere 22 kg. The hulls sides, bulkheads, decks, cabin are 4 mm Ocume ply. The keel panel, transom etc are 6 mm ply. Rudder housing, beam support etc is 8 mm ply. 18 x 18 mm gunnels and other wood supports. 20 x 25 mm stem. The exterior is covered with 100 gsm cloth in epoxy. Aluminium cross beams are 70 x 3 mm square tubes. The window is 3 mm polycarbonate. The jpegs give an idea of the Catproa.

 

The following 2 Roger Hill cats are close cousins. The 10.35 meter model is a semi production cat made from flat panels. The 10 metre is a ply home build design that has been built in several locations. The 10.35 is 34 x 18.75 foot displacing 10,200 lbs and carrying 690 square foot of sail.. The 10 is 33 x 18.75 foot displacing 8800lbs and carrying about 650 square foot of sail.

The 10.35 can be obtained as a complete composite kit. The 10.35 was commissioned by an Australian builder who has orders for several boats destined for a charter group in the Whitsunday islands. The internal layout is offered in 2 versions, a ‘normal’ private yacht and the charter version! An option of centerboards or shallow fixed keels is also available, the later being cheaper and easier to build but not giving the same sailing to windward ability and shallow draft capability. The construction system is detailed completely around a composite kit set, pre made and pre cut flat panel process that will enable even the most unskilled amateur builder the opportunity to build this design as specified. The hull shape is a simple double chine around the bottom and there is a knuckle in the topsides that will add stiffness and reduce the panel size for ease of handling while under construction. The entire project can be built the right way up and there is minimal secondary bonding on the outside of the hull which will reduce the time and cost of the more normal fairing and painting process.

The 10 meter is a plywood timber construction with single chine hulls. The design has a simplified but effective structure to simplify building. Construction is almost totally out of sheet ply materials with foam cored panels for the cabin top, bridge deck. and 2 primary bulkheads, there is a minimal amount of ‘solid wood’ required in areas such as the stems and connecting some panels where access is prohibited for coving and taping. A ‘beaching’ skeg is detailed that will allow the cat to take the ground without damaging the bottom and simplifies the steering with fixed, under hull rudders (and for prop protection should a small sail drive unit be fitted instead of an outboard). As is normal for all cats that are expected to sail well to windward centerboards are fitted but are not intrusive to the layout nor difficult to build and fit. The rig and sail plan is simple and the deck layout has all the sailing controls lead aft to the cockpit for ease of handling and safety. The jpegs give some detail.

 

The following trimaran is a dutch home build. The tri is a day sailor with a small cabin for storage or covering your head. The tri is 18.4 x 12 foot displacing 800 lbs. It has a day sailing cat rig EG Hobie. The main hull is 3.25 foot wide, the float hull is 1.2 foot wide. The main hull, floats and decks are 6 mm ply covered externally with 280 gsm glass cloth and epoxy. All chine seams are taped. Bulkheads and cockpit floor are 6 mm. The really fun part of this design is the folding cross arms. They are timber. The outer portion of the 3.25 foot cross arms 40 x 100 mm, the inner end are 40 x 180 mm. On the ends of the cross arms is glued a length of plastic gas pipe with an internal diameter of 8 mm. This pipe is glassed onto the cross arm with 4 layers of 280 gsm cloth intertwined with 5 layers of uni directional carbon fibre for about 200 mm either side of the beam. A 6 to 8 mm bolt is put through the pipe and connects to 6 x 50 x 250 mm stainless steel connecting plates. The connecting plates are bolted to 200 x 200 50 mm blocks attached to bulkheads in the main hull and floats. The timber cross arms after all the end connectors are complete is covered with glass cloth. This boat sails well and performs its job as a day cruiser very well, a mini Dragonfly type tri. A good concept.

 

The following is about a 1994 tri designed by Bruno Fehrenbach. The tri was designed and built for the formula 40 racing. Its 40 foot long and looks about 30 foot wide. Pure guess, but I would think it displaces about 7000 lbs. A later owner then hired Merfyn Owen to redesign the crossbeams. She is constructed of 12 mm strip plank western red cedar with 2 x 250 gsm UD E glass either side and epoxy. The beams which were timber and ply box beams reinforced with carbon fibre. The decks and bulkheads are plywood. After completion Spirit then "Spirit of England" went on to pursue a long and successful racing career.

In 2010 she was sold to her current owner Jason Gard and underwent a mini refit. Her plywood float and ama decks were replaced with carbon. All the bulkheads were replaced using carbon ring frames in place of the plywood. All the work we did not only replaced what was there but it also increased the strength. Her rigging and electrical wiring was replaced after which she set sail to Australasia via Panama and the Pacific. Since then she has sailed more than 20,000 nm or halfway around the world competing in numerous races and regattas and is now enjoyed as a simple and fast cruiser. In Australia another full refit and rebuild of the interior and cockpit was done. Everything has been removed, paint sanded back to timber and glass in most areas and then it's been redesigned and rebuilt out of 60HD foam with a layer of 400 gsm double bias on each side. The boat has minimal accommodation with only 2 bunks one either side of the daggerboard case. The boat had a water maker installed in the Caribbean. The water maker or desalination unit has proved to be one of the most impressive things we have. It uses very little power (16 amps) and produces about 25 lts an hour of fresh water. It's incredibly simple and there is a lot of information on the net about the units.

The reason for the interest is the amount of effort to keep this boat going. Its had cross beam redesign and rebuild, its had float decks replaced, its internal plywood bulkheads have been replaced by carbon foam structures. Its entire interior has been stripped and replaced. Its cockpit has been stripped and rebuilt. Its had a complete rig replacement and a complete set of sails replaced. The boat was still winning serious races in 2015. The accommodation is sketchy at best for a 40 footer. The “upgrades” have cost in excess of $150,000 and months of personal effort by the owners. The reward they have sailed half way around the world with 200 mile days are normal. Peak cruising speeds is 16 – 18 knots. Racing the boat goes 20 knots plus.

 

This boat may be known to a few of you, the Stealth 15. The Stealth 15 is a plywood foiling cat that was developed by Barry Marmion with his son Brad. Barry was involved in the building of several of Australia’s early C class cats and understands how to build and develop very fast day sailing cats. The 15.5 foot ply hulls were built in Barry’s carport using 3 mm plywood with timber stringers as shown in the jpegs. The boat is 15.5 x 8 foot carry about 140 square foot of sail. He glued 3 broken sections of A class carbon mast together and rigged it with a 2nd hand cut down A class main to end up with a boat costing $ 4,500.

Brad and Barry decided to design and retro fit foils to the Stealth. To construct the moulds Barry had 8 plugs (4 for each foil) made up using a router machine at RMIT, and then took moulds of the plugs. The 1st set foils had 10 layers of 200 gsm carbon and an expanding foam core, they weren’t up to the stresses and broke under the keel. The 2nd foils of consisted of 15 layers of 200 gsm carbon fibre with end grain cedar core They are still good a bit heavy (5 kg) each. The materials for moulds and foils cost around $ 2,500.

The development process was not easy due to the fact that Stealth was not built for foiling. Breakages include 1 set of foils, 4 rudder blades, 5 rudder boxes and Barry had to reinforce the main bulkheads. From the jpegs you can see it is a viable foiler that can do what was intended. I do not know if plans are available.

 

This is the start of a small series on structures and some material choices. There could be in a simple design a minimal build approach in EG timber. Hull design requires 2 things global strength and mostly panel stiffness. Global strength is about handling longitudinal loads, knocks against wharfs, daggerboard, rudder forces etc Panel stiffness is about how much a panel deflects before it loads to the point of fracture or failure. Certain very stiff materials like carbon fibre can only elongate about 1% before they come under stress. The result is most designers design a panel to deflect a maximum of 1% of the boats length. Depending on the material EG Polyester e glass foam you could go to 1.5% of the hull length in panel deflection.

In most cases Panel stiffness is the harder to achieve in a design. Panel stiffness is dictated by the loads and by the distance between frames and bulkheads. So the wider the spacing of the internal framing the thicker the panel needs to be. It is not impossible to imagine designing a boat with no internal support structure that needed 1" thick panels to get sufficient stiffness. It would be counter to all modern design, but it's possible. Brown Marples constant camber boats are examples of what is possible in reduced amounts of framing and thick timber skins. EG minimal stringers and bulkheads in a timber boat. A few jpegs of a 25 mm thick timber hull are below.

What I think gets lost here is that skin thickness is all about strength. It does nothing else for a boat in normal conditions. And frankly boats don't need much strength. They just aren't loaded much that way and fiberglass is incredibly strong. What boats generally need, is stiffness, and stiff and strong are very differently things. Dyneema rope has little stiffness, but is very strong. They are not the same thing at all.

The reality is that for most boats the required stiffness is a couple of orders of magnitude more important than its strength. And the best way to get the required stiffness is to add thickness. Within reason it doesn't even matter what adds that thickness, more roving fiberglass, a core, CSM, carbon fiber, timber, etc. none of it matters, the important question is how thick is it.

Now let’s get some perspective on what is thickness. The extreme is a single skin on EG a 64' Nordhavn power trawler that has a bottom of solid glass over 101 mm thick. That is 126 plies of laminate. According to calculations on another site the 101 mm thick solid fiberglass weights the almost the same as 24 mm steel. Large commercial ships have (700 foot) have double bottom shell structures built from 10 mm steel. The Nordhaven and steel ship may be bullet proof but they are not rock proof.

A “light weight” 77 footer monohull was built of 25 mm solid carbon which is stronger than 10 mm of solid steel used in many 700 ft long ship hulls, and is an overkill for hull areas not subject to any point loading, like ballast keel attachments. Even a Beneteau 50 mono that has no core has a 13 mm single skin, taken from 300 mm in front of the keel. A Lagoon 38 has a 12 mm single skin on its hull bottoms.

All of these single skin boats are excessively strong and could be built from CSM and still be structurally strong enough and most of them would still have sufficient stiffness to handle panel deflection requirements. The down side of these single skin dimensions. Weight!!!

Next we will discuss composite cored hulls.

 

As you have shown before in this marvellous thread, there has been a wide variation in laminates on similar sized boats. One thing that is also important is fatigue. A typical example is the Laser. When new Lasers are nice and stiff with their single skin bottom and foam cored decks. However after a few seasons of hard use the boats go soft, on the bottom and top.

You get something similar in very light laminates with larger boats. We had Newspaper Taxi in our family for a while and she was very soft after 20 years. So when designing strucures you also have to deal with the loads caused by the action of the boat and ensure that these do not exceed the fatigue limits of the boat - unless you are a racer and don't care if the boat is thrown away.

So the best way is to increase the laminate strength so that the laminate is stressed less by the same load. This greatly increases the cycles required to reduce the laminate strength. Some laminates, like choppy polyester are more susceptible to fatigue than epoxy and timber.

 

Catsketcher again brings up a very valid point about the fatigue life of a hull, deck structure etc and how some boats are ”over built” to allow for this fact. I fully agree. But there is a limit to what is required in over building. In the Nordhavn 64 trawler case the boats 101 mm thick fiberglass hull is an absolute overkill. The Nordhavn 64 displaces 185,000 lbs (82 tons) could live with a 60 mm layup and still be strong/stiff enough. The Lagoon 38 weight is 16000 lbs and displaces 22500 lbs with a 12 mm bottom. The Prout 45 displaces 22500 lbs has a 9 mm solid bottom. A Prout Snowgoose 37 weights 12000 lbs and displaces 18000 lbs has a 7 mm thick solid bottom. All of the cats, if they have low aspect ratio keels, will have heavier layups (up to double) in and very near the LAR keel. As you can see the Lagoon is very solid and I would almost say its overbuilt.

The Beneteau 50 is 28000 lbs displacement and the hull is built with a reasonable safety/fatigue factors. EG Westerly 490 29000 lbs is 15.5 mm thick, Swan 48 displacement 31,000 lbs Kevlar, uni glass is 13 mm thick.

We now move on to cored hulls. If panel stiffness is more a function of thickness than strength then how do we obtain thickness without weight? By using a core of lighter material than say polyester e glass which weighs about 100 lbs per cubic foot. If a core of 6 to 25 lbs per cubic foot can replace the polyester e glass, the overall hull weight will be reduced. If we have a core then the fiberglass skins can be designed for the global strength factors required. The skins of a fiberglass cored sandwich should be as far away from the neutral axis (approximately the centre line of the sandwich) as practical. Now we have to introduce a bit of reality. When you design a cored panel solution that can meet the panel stiffness requirements and the skins to meet the global strength requirements you need to allow for 3 things. Knock resistance, fatigue life and marginal build technique. Result, in reality panels are dimensioned as a compromise between bending strength of panel and point loading properties requiring somewhat thicker skins. Core density selected for required compressive properties of those point loadings whether it is foam, balsa, honeycomb or western red cedar cores.

So you can design a race boat that meets the bending strength and global strength requirements that cannot be run aground, hit a wharf or last any more than 10 races without becoming soft or fail. These boats can be as low as 3000 lbs displacement. A more realistic boat is a Shockwave 37 cat that displaces a maximum of 4000 lbs has a polyester resin hull of 330 gsm cloth, 12 mm airex, 330 gsm cloth doubled over the bottom. This boat could have been (and in later Super Shockwave models were, with 18 mm foam) built in epoxy and have 165 gsm kevlar style 248 which is 43% lighter and twice as strong than e glass. So effectively it has the same strength as 330 gsm e glass. These boat have to be treated very carefully. You do not run it aground or hit wharfs without thinking about repairs. For what are “cruisers” the hull layup for a lighter cruiser racer is 450 gsm 45/45 e glass polyester 18 mm foam 450 gsm 45/45. More realistic 37 foot cruisers of a weight of 11000 lbs have outside 600 or 850 gsm triax 15 or 18 mm foam 445 or 600 gsm 45/45 hull structures. Even the heaviest layup on a hull here has a thickness of 1 mm of glass on an above the waterline skin. Below the waterline where most hull layups are doubled the skin thickness is about 2 mm. Really “heavy” 38 foot cruisers or production boats weighing 14,000 lbs have from the outside 1175 gsm triax e glass 20 mm foam and 850 gsm 45/45. That is the equivalent, if the bottom glass is doubled on the outside of just under 3 mm thickness of external glass.

So which would you prefer 12 mm of solid glass or a “heavy” from the outside 3 mm glass 18 mm foam 2 mm glass. The 12 mm bottom is twice as heavy as the foam glass structure but the panel stiffness of the solid 12 mm would “flex” more (relatively in real world there would be minimal difference). The global strength of the 12 mm solid bottom would be superior BUT it is not required to be twice as strong, less than 50% of the strength of a 12 mm solid bottom is required to meet a 38 foot cats needs. The advantage of the 12 mm solid is it can take knocks well BUT it will still break if it hits rocks etc.

This is getting to long. Next the types of cores and how water can effect any core, followed by the effects of framing, stringers and other reinforcements.

 

We have looked at core materials in a sandwich structure before. Sorry if I repeat a few bits of information, The difference between foam cores (Airex, Corecell etc) and Western Red Cedar (WRC) often depends on $ and availability but occasionally on a "strength" issue. Lets look at WRC versus Corecell versus end grain Balsa. I will assume polyester resin for Corecell foam and end grain Balsa. WRC is assuming epoxy. In all cases if the glass core bond is epoxy the strength characteristics are significantly improved. EG Balsa shear strength in polyester is 550 PSI, with epoxy the shear strength is 1300 PSI. PS Balsa and foams structural characteristic depends on its density.

WRC weights about 22 lbs/cubic foot, Corecell from 4 to 9 lbs/cubic foot, Balsa from 7 to 17 lbs/cubic foot. In many hulls the core is EG foam may be 18 mm or WRC may be 15 mm thick. The weight of a WRC core will be a least 3.66 times the foam core. In a 1000 sq ft hull that adds 1000 lbs in core weight alone. WRC is "stronger". Depends what you mean by stronger. WRC compression strength is 240 pounds/square inch (PSI), Corecell foam 165 PSI, end grain Balsa is 800 PSI. Shear strength WRC 70 PSI, Corecell foam 191 PSI, end grain Balsa is 550 PSI. In short glass skins will peel from WRC easier than it will peel from Corecell and both will peel easier than end grain Balsa. If hit hard WRC breaks, foam deforms and "bounces back", end grain Balsa does do limited deformation. In short Corecell (or Airex, Divinycell etc) is a better material under most circumstances. BUT you say WRC cores can have lighter glass so the total skin weight will be less. For similar hulls the glass skins on a WRC hull will be about 70% of a foam core hull. So the glass skins on WRC will be about 300 lbs lighter than a foam core. Translation a Corecell etc will be about 700 ls lighter than a WRC hull. BUT the real world now interferes. Foam absorbs more resin due to the “open” surface cell structure, foam often needs more filler in a home built boat than WRC. If a foam glass boat is built in a female mould or as a flat panel under a vacuum bag or resin infusion the panels are marginally lighter than a hand layup. According to some designers the difference in real world weight between any of the cores/fiberglass layups is under 15% if designed to meet the same structural requirements. Compared to a solid glass hull all the cored structures are a lot lighter (30 t0 50%).

Timber (WRC) only stretches about 1% before it breaks. Foam can stretch over 6% before it breaks. Why is this important? Because carbon fibre CF only stretches 1% before it breaks. Glass stretches 5 to 6% before it breaks. So if your going to associate materials its better to have e glass and foam together or have timber, balsa and CF together. But in the real world $'s and weight come into the equation. Also another factor is longevity. CF maintains 50% of its strength after 10 million cycles, fiberglass only maintains 22% of its strength after 10 million cycles. In simple terms imagine a hull going thru a cycle each time it goes over a wave.

Now we come to the vexed question of what happens when a core gets wet due to a fracture in the glass. This really depends on how the fracture occurs. If it is a localised single point hit that in effect punctures a hull the whether is foam, WRC or Balsa water will only effect the core in that area. BUT and it’s a big but if the hull damage extends over a larger area breaking the surface core bond line allowing water to seep into many areas both Balsa and WRC core can rot over a larger area. Result many people are wary of Balsa cores especially below the waterline. So you have a choice foam can soften over time due to heat, fatigue cycles etc, Balsa can rot if water can travel over a large are inside the skin and WRC is probably the most stable but is the “heaviest” core glass structure.