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From Boat Design Wiki
UNDER CONSTRUCTION! PLEASE help and add content...
PLEASE review this for content / errors! --TerryKing 00:38, September 13, 2007 (EDT) WOW, Popular still After All These Years. Maybe wood and epoxy and wood-eating micro-organisms haven't changed much! TK 2015
Click the (discussion) tab at the top to add comments and suggestions.
This is intended for a wide-ranging set of information on the materials used to Build, Outfit, and Maintain boats.
NOTE: Terry King has collected a few papers on related subjects at: http://www.terryking.us/boatbuilding/ These include backup copies of some of the links shown below.
Much good information on Plywood comes from the Engineered Wood Association. Their website is: http://www.apawood.org/
An overview of wood for Marine applications is at: http://www.apawood.org/perf_panels/index.cfm?content=app_pp_boat
Marine Grade Plywood
The Engineered Wood Association publication on Marine Plywood: http://www.glen-l.com/wood-plywood/marine-plywood.html
Woods suitable for Boat Construction
The US Forest Service "Wood Handbook"
The US Forest Service "Wood Handbook" is one of the most complete publications about the characteristics of many species of wood.
The page with all chapter titles and individual downloads is here: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
The complete handbook (14 Megabytes!) is here: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.pdf
Here are some other documents on wood within the forum:
http://www.boatdesign.net/forums/attachments/materials/50937d1291698550-wood-thread-sources-qualities-prices-like-wood-info.pdf http://www.boatdesign.net/forums/attachments/materials/50938d1291698575-wood-thread-sources-qualities-prices-like-wood-properties.pdf http://www.thewoodexplorer.com/
Paint and Epoxy Adhesion VS Preservative Treatments
Tests by West System on adhesion of Epoxy on wood treated with Ethylene Glycol and Ethylene Glycol - Borate: http://www.maritime.org/conf/conf-reynolds-mat2.htm
Synopsis: strong (75-25%) Ethylene Glycol - Borate treatment affects adhesion badly. Less strong solutions and Ethylene Glycol alone do not significantly affect adhesion.
Dave Parnell on Paint/Epoxy VS Ethylene Glycol treatments (quote): Once the wood (treated with Ethylene Glycol alone) feels dry to the touch, then paint, epoxy and glues work fine. Borates interfere with paint, epoxy and glues.
The Engineered Wood Association: Publication "PRESERVATIVE-TREATED PLYWOOD FOR BOAT MANUFACTURING APPLICATIONS" http://www.glen-l.com/wood-plywood/boatbuilding-plywood.html Two Key Quotes:
Update: Informal discussion with West System: They have not done recent extensive tests on treated wood, but a few specific ones were OK. A few points:
--TerryKing 10:50, April 11, 2007 (EDT)
PRESERVING WOOD AGAINST DETERIORATION
Protecting Wood with Epoxy
Wood can be protected (though not "preserved against micro-organisms") by saturating the wood with Epoxy resins. If done effectively, this keeps the moisture content in the wood below that which supports fungus, and is a barrier against possible infection of the wood.
"Penetrating" Epoxies vs standard Epoxy Resins
There is some controversy about the advisability of using 'Penetrating' Epoxies, which usually have solvents to thin their viscosity. There is evidence that water can penetrate wood treated with penetrating Epoxies alone. See the following graph:
This is from WEST System. It shows that 'Penetrating' epoxies leave 'holes' behind when the solvents evaporate. But a combination of Penetrating PLUS standard Epoxy Resin does very well.
However, WEST suggests another approach: "There is a better solution to get good penetration without losing strength or moisture resistance. We recommend moderate heating of the repair area and the epoxy with a heat gun or heat lamp. The epoxy will have a lower viscosity and penetrate more deeply when it is warmed and contacts the warmed wood cavities and pores." See more at their website: WEST System (The "Using West System Epoxy" tab gets you to "User Manual")
It should be noted that while heating epoxy effectively thins it and allows better penetration, it also greatly accelerates the curing process. When using additional heat on wet epoxy, care should be taken to ensure only small quantities are done at once, and that the epoxy does not set prematurely.
'Glycol' - based wood preservatives
There has been much discussion about the "gylcol" based chemicals as a wood preservative. What is proven to be effective is Ethylene Glycol, typically sold as automotive "Anti-freeze". Here are details about two "glycols"
Name monoethylene glycol (MEG) Chemical formula HOCH2CH2OH Common Name: "automotive anti-freeze"
Details (Wikipedia): http://en.wikipedia.org/wiki/Ethylene_glycol
Excerpts: Ethylene glycol is toxic, and its accidental ingestion should be considered a medical emergency. Due to its sweet taste, children and animals will sometimes consume large quantities of it if given access to antifreeze.
Ethylene Glycol has seen some use as a rot and fungal treatment for wood, both as a preventative and a treatment after the fact. It has been used in a few cases to treat partially rotted wooden objects to be displayed in museums. It is one of only a few treatments that are successful in dealing with rot in wooden boats, and is relatively cheap.
Chemical name Polyethylene glycol Chemical formula C2nH4n+2On+1
Details (Wikipedia):http://en.wikipedia.org/wiki/Polyethylene_glycol Excerpts: Poly (ethylene glycol) is non-toxic and is used in a variety of products. It is the basis of many skin creams, as cetomacrogol, and sexual lubricants, frequently combined with glycerin. PEG is used in a number of toothpastes as a dispersant; it binds water and helps keep gum uniform throughout the toothpaste. PEG is included in many or all formulations of the soft drink Dr Pepper, purportedly as an anti-foaming agent.
NOTE: This does not sound like a toxic-to-micro-organisms wood preservative!
From the US Forest Service Laboratory: "According to the manufacturers, the polyethylene glycols have relatively low order of toxicity, Polyethylene glycol therefore has little effect as a toxic ‘material in preventing decay. This chemical reduces decay only when the wood is treated to such a high retention of chemical that the moisture absorption by the wood substance is reduced to a level below that which will support decay.
Boron-Based Wood Preservatives
General information on Boron-based wood preservation: http://www.borax.com/wood/
Boron compounds are among the most effective and versatile wood preservative systems available, offering broad-spectrum efficacy and low toxicity. Here is a paper discussing the successful use of Boron compounds to protect old wooden boats: http://www.maritime.org/conf/conf-casebolt.htm
Home Brewed Wood Preservatives
Home-Brew Water Solution of Borates: All percentages for this recipe and the others here are percentages by weight. Based on U.S. Navy spec. of 60% borax-40% boric acid (this ratio gives the maximum solubility of borates in water); 65% water, 20 %borax, 15% boric acid; 15.8% borates; borax costs 54 cents/lb. (supermarket), boric acid costs about $4/lb. in drug stores (sometimes boric acid roach poison, 99% boric acid, is cheaper in discount stores); equiv. to Tim-Bor® or Ship-Bor® at 30 cents/lb.
To make this solution mix the required quantities and heat until dissolved. The boric acid, in particular, dissolves slowly. This solution is stable (nocrystals) overnight in a refrigerator (40°F.), so can be used at temperatures at least as low as 40°F.
Home-Brew Glycol Solution of Borates: This is equivalent to Bora-Care® diluted with an equal volume of glycol to make it fluid enough to use handily; 50% glycol antifreeze, 28% borax, 22% boric acid.
To make a stable solution you mix the ingredients and heat till boiling gently. Boil off water until a candy thermometer shows 260°F. This removes most of the water of crystallization in the borax. This solution is stable at 40°F and has a borate content of 26%. With antifreeze at $6/gal. and borax and boric acid prices as above, this is equivalent to Bora-Care® at about $15/gal.
Commercially-available Wood Preservatives
Tim-Bor®: Solid sodium octaborate; dissolves in water to make approx. a 10% solution containing 6.6% borate (B2O3); about $3/lb. plus shipping.
Ship-Bor®: Same as Tim-Bor®; $19.95/lb. plus $2 shipping.
Bora-Care®: 40% solution of sodium octaborate in ethylene glycol;27% borate content; $70/gal. plus shipping.
BONDING FIBERGLASS TO PLYWOOD
The Engineered Wood Association publication is here: http://www.glen-l.com/wood-plywood/fiberglass-plywood.html
SCARF/BUTT JOINTS IN PLYWOOD
Dave Carnell has written about a method that avoids cutting scarf type joints. at: http://www.angelfire.com/nc3/davecarnell/epoxy.html
PAINTS and COATINGS
ADHESIVES and SEALANTS
General Notes on Glues and Goos: From various posts by Bob Smalser
Resorcinol: The marine standard. If you can get 70 degrees F or higher for an overnight cure and consistent and high clamping pressure with no gaps, you won’t go wrong using it. Likes wood at 10-15% EMC, according to Navy tests. Long open time. Repairable with epoxy. Ugly red glue line.
Marine Epoxy: The repair and restoration standard. Bonds well to a wide variety of materials, and usable in almost all flexibility and temperature conditions. Needs no clamping pressure, only contact…fills gaps well. Likes wood below 12% EMC. Repairable with itself, joints can often be broken apart for repair with using heat. Clear glue line and can be dyed to match the wood. Controllable open time with different hardeners. Although cured epoxy is effectively waterproof, it is slightly permeable to water vapour and there are reports of failures in fully saturated wood and with White Oak. Very sensitive to UV, requiring protection (UV-blocking varnishes or paints are generally suitable). Note that there are several hundred types of epoxy, and care must be taken to ensure the variety being purchased has in fact been successfully tested for the intended use.
3M 5200: A rubbery, polyurethane sealant in various colors with adhesive properties sometimes used as a glue. Fails as a glue under water saturation without high clamping pressure, and without the proper strength testing I couldn’t do here, it’s not recommended as a stand-alone marine glue. Repairable with epoxy. 5200 is an excellent general purpose sealant with many uses aboard ship, but should not be used structurally.
3M 4200: Similar to 5200, but not as strong. Used as a bedding compound or as a temporary adhesive, when it is known that the parts will have to be disassembled in future. Like 5200, it should not be used structurally.
Liquid Polyurethane: Gorilla Glue, Elmer’s Probond, Elmer’s Ultimate, and others. Versatile in temperature and bonding wet wood with moderate open time, these glues aren’t rated for below waterline use but initial use shows potential as a marine glue. Likes high clamping pressure and fits similar to resorcinol…it won’t fill gaps. Will successfully glue green wood at 30% EMC. Repairable with epoxy. Noticeable, yellow-brown glue lines.
PL Premium Construction Adhesive: This polyurethane goo shows promise as a marine glue with further testing and use. Works like 3M 5200 but cures and behaves like liquid poly. Appears to bond well to everything epoxy does, and more where epoxy and liquid poly won’t, perhaps because of a higher isocyanate content…it bonds to difficult surfaces only cyanoacrylate super glues will bond to. The only general-use glue I’ve found that will bond difficult aliphatic-contaminated surfaces. Appears flexible to temperature and moisture content with gap-filling ability, but as a construction adhesive, its open time is shorter than liquid poly. Appeared to like high clamping pressure, and unlike other glues, wouldn’t bond at all without at least some. Repairable with itself and epoxy. Glue line as in liquid poly.
Urea Formaldehyde Plastic Resin Glue: Weldwood, DAP and others. The old interior furniture standard, and in older marine applications that required well-blended glue lines. Still preferred by many, as it is a no-creep glue easily repaired using epoxy. Long open time, it needs tight fits and 65 degrees F or higher for an overnight cure…it doesn’t fill gaps. Best glue line among them all and moderate water resistance still make it useful for protected marine brightwork applications. A relatively brittle glue and UV sensitive, it requires protection….but its brittleness is an aid to repairability, as joints can be broken apart for repair. An inexpensive powder with a short, one-year shelf life. Urea-formaldehyde adhesives are gradually and voluntarily being phased out in land-based construction through health-and-environment initiatives such as LEED, but are still common in other uses.
The Titebond Family of Aliphatics: Convenient. No mixing, just squeeze. Short open times, fast tack, and short clamping times. Fast, and an acceptable long-grain layup glue…in heated, commercial shops, I’ve had rough-cut Titebond panel layups in and out of the clamps and through the planer inside of an hour. Flexible in temperature and to a lesser extent in moisture content, but the bottled glue can freeze in unheated shops. A flexible glue, it has been reported to creep under load, sometimes several years after the joint was made. The latest “Titebond III” appears to be a stronger glue than its two predecessors. Difficult glues to repair, as they won’t stick to themselves and no other glues will except cyanoacrylates, which are too brittle for general use. Epoxy and fabric aren’t bonding to aliphatic glue lines in marine strip construction, compounding repair difficulties. While not definitive, the new PL Premium appears to bond well to Titebond III residue and is worth pursuing by those repairing old white and yellow aliphatic joints.
Cyanoacrylates: Better known as Superglue. They bond tenaciously to plastics and ceramics of many sorts, set in a matter of seconds and reach full cure very quickly. Activated by water vapour, they are good for quick repairs to many small items. Cyanoacrylates are brittle and rarely develop the full strength of the material being glued; they are well suited as a temporary fix but are not a structural adhesive for marine use.
Oh. Yeah... Metal.. Thanks to Matt for some excellent material. Others: Click the  and help out!
Steel, at approximately 7800 kilograms per cubic metre, is probably the heaviest material in structural use on boats. The term "steel" includes a wide variety of alloys, all of which have iron as the primary component.
The somewhat misnamed "stainless" steels are a special class of alloys high in chromium and nickel. In aerobic conditions (ie. exposed to oxygen) the chromium forms a thin layer of chromium oxide, nearly impervious to gases, that protects the metal beneath from corrosion. The protective effect of this oxide layer gives the alloy its name.
Stainless is a popular choice for fittings, railings and equipment to be used above the waterline, as well as hose clips and engine connections. It is also commonly used for propellers and other components of the running gear. It is difficult to maintain the composition of the alloy near a weld, and welded stainless steel is rarely used for structural purposes.
These alloys are remarkably prone to corrosion damage in anaerobic conditions; lacking oxygen, the protective oxide cannot form and the metal is even more vulnerable than ordinary mild steel. Stainless cannot be used in areas where stagnant water will become trapped, and care must be taken to ensure that crevices and gaps between components are well sealed.
There are several grades of stainless steel; MatWeb has an exellent database of property and composition sheets for the various grades. The quality of a stainless fitting can be easily judged with a small magnet; the magnet will not stick as hard to better grades as it will to bad ones.
Stainless steel is not a good choice for underwater use in wood. Without oxygen it is subject to crevice corrosion. Bronze is a better choice for this.
The term "carbon steel" encompasses most of the steels commonly used in boatbuilding. These are mostly iron, with small amounts of carbon but with only miniscule amounts of other elements. Although attempts are being made to harmonize the nomenclature used to classify them, carbon steels are still designated under several different systems and it is worthwhile to use a service such as MatWeb or AZoM to narrow down your options according to the particular set of properties you require.
In general, the various grades of carbon steel are used for heavier craft, especially working boats, where durability and strength are paramount. They are also the material of choice for most modern large ships. Steel is very strong for the volume of material used, but is relatively heavy and so is most commonly seen in vessels for which speed is not the top priority.
Steel, being primarily iron, is remarkably prone to corrosion. Its oxides are not protective, and their presence can accelerate the corrosion process. Many paint systems are available for steel to provide a protective barrier against corrosion; however, none are permanent and a steel vessel will generally require regular repainting if it is to be held to the same standards of appearance as a wood, aluminum or composite hull.
Copper and Alloys
Aluminum is the lightest metal in common marine use, weighing only 2700 kilograms per cubic metre. It is very resistant to corrosion damage when used properly, and is quite strong for its weight. Aluminum is neither as strong nor as stiff as steel; however, its much lower density means that an aluminum structure is lighter than a steel one of equivalent strength even though the aluminum plates will be thicker than steel ones.
The two main joining methods for aluminum are welds and rivets. In general, aluminum boat structures are welded when the plates are thicker than approximately 3 mm; thinner structures are more often riveted. There has been considerable research into chemical bonding techniques for aluminum construction, but welds and rivets continue to dominate the field.
Light sheet aluminum, rivetted, is a common material in small utility and fishing boats. Although the investment in production tooling to form the material is significant, the resulting craft can be built by the thousands and can be inexpensive and durable.
Heavier welded aluminum construction is a popular choice for utility craft, workboats, and sport fishing boats that are expected to endure abuse, rocks and poor maintenance. It is also a popular material in fast offshore powerboats, and is sometimes seen in sailing and cruising yachts designed for the open ocean.
Aluminum can be a difficult material to weld; conventional arc welding equipment and gas-flame equipment is generally not suitable. A MIG welder is a minimum, but computer-controlled MIG units as well as specialized (and expensive) TIG equipment yield better results where weld quality is critical. A problem with welding alloy is the considerable loss of strength of the material in the weld zone even when properly welded.
For marine use, aluminum is generally made as a flat sheet or plate material. Although it can be coaxed into shallow compound curves, construction is easiest and cheapest if the individual segments of the hull are developable. A skilled crew can form remarkably fair compound curves from aluminum, given appropriate training and equipment; this level of skill, however, does not come cheap.
Aluminum in its pure form is neither strong enough nor durable enough for structural use. For most applications it is alloyed with small amounts of other metals.
The common wrought alloys are designated by a four-digit number that describes the impurities used. The addition of a "0" to the end of an alloy number indicates a cast alloy. An additional code after the number may indicate heat treatment or work hardening. A very thorough but readable description of the different alloy properties is available from the AZoM database.
In marine applications, the 3000-series (manganese-doped) and 5000-series (magnesium-doped) are the most common. Alloys 3003/3103 and 5052/5251 are commonly found where the marine atmosphere is a concern. Marine structural aluminum (ie. for hulls) is most commonly of the 5083 and 5182 alloys. The MatWeb database contains a wealth of data on individual grades and their particular properties.
Aluminum's remarkable resistance to corrosion damage is a result of the unique properties of its native oxide. Aluminum oxidizes rapidly when exposed to oxygen; however, the oxide itself is extremely hard and durable (sapphire is simply crystalline aluminum oxide), as well as an electrical insulator and virtually impenetrable to air. Once the thin protective oxide has formed, further corrosion of the metal below is nearly impossible.
The forming and maintenance of the passive oxide is a natural process and so no surface treatment or painting is required for aluminum. However, the oxide may be damaged, or fail to form altogether, under certain conditions. Aluminum does not fare well in anaerobic conditions, such as cracks or seams where stagnant water can collect. Aluminum is also anodic to most other metals and so will be the first to corrode in the presence of a galvanic couple with any more noble material. Electrolysis from stray electric currents will either protect or rapidly corrode aluminium depending on the polarity of the applied voltage. It is imperative that zinc or magnesium anodes be used on aluminum parts that are expected to remain in seawater for any length of time; without sacrificial anodes the aluminum itself will be vulnerable to galvanic corrosion if any other materials are nearby.
Painting / Surface Preparation for aluminum
Thanks to Jimbo for this overview: The trick with bonding to aluminum is the preparation of the aluminum, more so than the selection of the adhesive. The thing is that aluminum is always corroding. You have to remove the corrosion, then you have a few minutes or hours to STOP it. Then you can make a permanent bond to it.
You can remove the corrosion mechanically or chemically, or a combination of the two methods. If there is visible corrosion, you probably should start with mechanical methods. Scotch-Brite non-woven 'surface preparation' discs are a favorite for this, as well as the 3M bristle discs and the nylon cup brush. All are available for various size hand grinders to match the size of the work.
After the grinding step you may be able to go straight to the corrosion stop step, which is the chromate conversion (Alodine), but most likely you will need to use an acid cleaner/etchant first. Apply the etchant by either a spray bottle, garden sprayer or a chip brush. Scrubbing with scuff pads helps. Never let the etch dry out! If it does dry before rinsing, etch again! After etching, rinse thoroughly and make sure water 'sheets out' over the entire surface to be bonded with no 'breaks' or little pits or dimples in the film of rinse water on the surface. This means the surface is truly clean and corrosion free. Then you can apply the chromate conversion coating. For a long narrow surface, a clean white rag is the ideal applicator, but you could also use a spray bottle or a garden sprayer or chip brush.
Let the conversion coating dwell on the surface for a few minutes. Again, never let it dry out! Then rinse off thoroughly. A visible straw yellow to tan coating should remain. You now have about three days to bond to this before it hardens up too much and nothing will stick to it.
Now I prefer to apply a coat of Mil Spec yellow epoxy primer, let that dry for 12-24 hours and then do any bonding over the primer while the primer is in 'green cure' and accepts epoxy very well. But you could just bond right after the aluminum drys off from the last rinse.
The etch will burn you and the conversion coating is toxic, so wear gloves and a respirator, especially if you spray it on .
Suppliers / Trade Names: Alodine 1201 is the trade name for the surface treatment from Henkel. The Etchant from Henkel is Alumiprep 33. These are the products featured in the Awl-Grip Application Guide.
Eldorado Chemical (now PPG aerospace) also sells one called Doradokote. Eldorado offers several etchants, my favorite being their AC-5 for heavily corroded aluminum and their AC-12 for general etching and brightening.
PPG automotive offers a conversion coating as DX503 and the etchant as DX533.
DuPont sells them as 225S for the cleaner/etchant and 226S for the conversion coating.
DuPont has published documents detailing use of these products here: http://www.performancecoatings.dupon...19290_225S.pdf http://www.performancecoatings.dupon...19291_226S.pdf
You can use these instructions for the other company's products since they are all similar, with the exception that some company's etchant may be stronger and need more dilution.
The dilution ratio of the etchant is not at all critical and can be varied according to need. On surfaces that have been recently well abraded, a weaker etch is preferred over a strong one. Conversely a dull, tarnished aluminum surface can be brought up to readiness for the conversion coating by using stronger etchant, also known as brightener.
Composites, or Fibre-Reinforced Plastics
A composite material combines two or more distinct materials into a new material, possibly having more desirable properties than any of its components would on their own. In the case of boats, the term "composite" usually refers to some form of fibre-reinforced plastic. The most popular material for production boats today is the fibreglass-polyester composite, although more sophisticated composite materials are becoming increasingly common.
The resin in a composite part is usually a thermoset polymer of some form. It penetrates between the fibre reinforcements as a liquid, then after a few hours cures to a solid. Once solidified, the resin cannot be melted and reformed as most common plastics can; it becomes permanently fused to the fibres and will form a matrix that locks the entire composite together. Most common resins can be lumped into three categories based on chemistry; they are listed below in order of increasing quality and cost.
These are the cheapest resins, and are used to make the majority of production fibreglass boats. They adhere well to fibreglass, but tend to be slightly porous and prone to osmotic blistering if left immersed in water. The liquid resin is cured by the addition of a very small amount of a chemical catalyst, and they are very sensitive to good mixing and proper temperature while curing. Polyesters are a good choice for production boats that must be inexpensive and are built in controlled conditions. They tend to be more trouble than they are worth when used by the home builder.
These have a less porous and more water resistant structure than polyesters, and are somewhat stronger. They are also more expensive. Vinylesters and similar resins are often used in the outermost layers of a hull to improve resistance to blistering and osmotic damage.
Epoxies are the 'creme de la creme' of composite resins; they are generally stronger and more waterproof than the cheaper resins. The chemistry is very well known and many suppliers have tailored different blends for specific applications. They are less toxic and easier to use in liquid form than the other resins, and so are generally ideal for home building despite their somewhat higher cost. Epoxies are the only resins commonly used with the more advanced fabrics, and can (with appropriate additives and fillers) be used for bonding, sealing, etc. as well. Many are vulnerable to ultraviolet light if not properly protected, however.
Several types of reinforcing fibre are in common use; the most common are fibreglass, Kevlar and carbon. Suppliers list them by the weight of the material (usually sold as a roll of cloth or mat), eg. "10 oz per square yard" or "250 gram per square metre". Lighter weights are easier to work with on curves; heavier ones are used to build up thickness quickly.
Most commonly seen in small-volume supply shops are the "cloth" forms of the reinforcements. These are exactly what they sound like- relatively thin fabrics, woven from small yarns (bundles of fibres) in a fairly tight pattern. They generally drape well and are easy to work, but are tedious to use when trying to build thick layers. A variant of cloth known as a "woven roving" has much wider, thicker yarns and a looser weave, and is heavier- thus it is easier to build thickness quickly, but harder to handle tight corners.
The fibres of both cloth and roving fabrics are interwoven; thus, they are bent up and down slightly at each crossing. This prevents the fibre from developing its full load capacity until the part has already flexed somewhat. Greater strength can be obtained with straight, non-kinked fibres- the "unidirectional" and "multidirectional" stitched fabrics. These are available in an amazing number of varieties and must be specified according to the particular application and load direction.
Another very common form of reinforcement is the chopped-strand mat, or CSM. This is a sheet of short, random strands, either loosely held together by stitching or temporary adhesive, or sprayed on by chopper gun. It is by far the weakest structure, used primarily because it is a cheap way to build thickness. CSM's main value is as a very thin layer just below a finished surface (to prevent print-through of the cloth texture) or between layers of heavy roving (to provide extra short fibres to help interlock the heavy layers).
Fibreglass comes in many types and grades, and is by far the most popular reinforcement. Home builders will most frequently use E-glass cloths or rovings; mass production builders often prefer heavy E-glass rovings and CSM in order to reduce labour costs. Specialty builders often gravitate to the stronger, lighter laminates that can be produced using directional cloths and S-glass.
Fibreglass is, generally speaking, compatible with all common resins. The fibres of an individual fabric are usually coated with various chemicals to improve the resin-fibre bond; it is important to ensure that the coating on your fabric is compatible with the class of resin you are using.
The garden variety "E-glass" is a readily available and inexpensive variety, and can be had in woven cloth, woven roving, unidirectional or multidirectional forms from many suppliers. Various types of E-glass are used in most boats. A much stronger, substantially more expensive form of fibreglass is known as "S-glass" and is competitive with the cheaper aramid and carbon fibres in many respects.
The design and construction of fibreglass laminates is something of a black art, at least as far as the average person is concerned. There are many textbooks at the 2nd- and 3rd-year university level that are suitable for learning the art of laminate design. Learning the construction process is mainly a matter of experience, patience, and trial/error experimentation, although there are also many good books to help the newbie.
Aramid (Kevlar, Twaron)
Kevlar and Twaron are trade names for complex aramid polymers (there are other such materials, but these are the most commonly seen). In the case of boat hulls, it is drawn into fibres, which are woven into a cloth and used like fibreglass is. Kevlar is also used for rigging, in which case the fibres are twisted or braided into ropes and cables. Kevlar is substantially stronger than fibreglass, and far more durable. It is most commonly used in kayaks and canoes that must be light enough to portage, but also durable enough to hit rocks without taking damage. Designing with Kevlar is somewhat more complicated than designing with fibreglass; the material is more expensive as well as harder to work with, and since the polymer chains are not isotropic the structural geometry is usually more complicated to analyze. When under tension in the direction of the polymer chains, Kevlar is considered a high-modulus (ie. stiff) fibre; in compression or perpendicular to the fibre direction it is much more flexible and has very different properties.
Kevlar is sometimes used with vinylester, but best results are achieved with an epoxy matrix. The cured laminate is difficult to cut or sand cleanly; the fibres tend to clump up in little fuzzy balls along the cut, rather than leaving a clean edge. The material can be somewhat sensitive to UV light exposure over time, becoming somewhat more orange than yellow with exposure, and a UV-protective coating is generally a good idea.
Carbon fibre is essentially long, ultra-thin wires of pure carbon, with a microstructure generally close to that of graphite. When used with suitable epoxy resins, it has a strength-to-weight ratio as much as four to five times better than steel, making it arguably the lightest and strongest engineering material in common use. It is also among the most expensive; the bare fabric often costs upward of $150 USD per kilogram.
Carbon is the stiffest of the common reinforcements and has the highest tensile strength. It is, however, much more brittle than Kevlar or fibreglass; carbon parts tend to 'give' very little before shattering, even though they may withstand much higher loads than other fibres. Designing with carbon is more difficult than with the other fibres; firstly, the material is very expensive and so it is essential to avoid waste, and furthermore, careful consideration must be given to the failure modes. Fatigue conditions in carbon are difficult to analyze. There are thousands of grades of carbon, made by varying processes and suitable for different uses; a full discussion is beyond the scope of this article.
Carbon is generally not used by the amateur builder or repairer due to its high cost and the difficulty of working it. It is commonly seen in high-end racing craft as well as more expensive kayaks and canoes. Lower grades are often used cosmetically, and the visual effect of an expensive carbon part can often be achieved by using a single light, cheap carbon layer followed by a high-gloss finish.
In many cases, a composite part can be made lighter and stiffer by building it as two thinner composite layers, with a stiff but light core in the middle. In general, it is unwise to use cored construction below the waterline of a boat; if a significant amount of water enters the core over time, the boat can generally not be repaired.
If a cored construction is being considered, vacuum bagging of the laminate is strongly recommended. Especially on highly curved areas, it is very difficult (if not impossible) to get a good bond between the core and the skin without vacuum bagging. Production boats with cored hulls should be surveyed with extreme care as damage to the core, while potentially fatal to the boat and her crew, is generally invisible except to the trained surveyor and his instruments.
End-grain slabs of balsa wood are a popular core material. Although not cheap, balsa has been in use for decades and techniques for using it successfully have been developed.
A less common but also with a high strength to weight ratio is to use the strip plank method with balsa strips. This method is necessarily limited to quite small boats because of denting problems - something that end grain balsa is much less prone to. Though impressively low weights can be achieved. Balsa Strip Canoe
Honeycomb materials, generally based on aluminum or aramid papers, are often used as cores in aircraft structures. They are becoming more common in large racing yachts and are finding their way into smaller craft. Honeycombs are difficult to use with wet resins, as they can soak the resin out of the skin laminates. They are more common with the pre-impregnated, heat cured reinforcements used in aerospace work. Honeycombs are, if properly engineered, probably the lightest and strongest of the cores, but are expensive and somewhat difficult to use correctly.
There are a huge number of foam cores that pretend to be suitable for boat use. Many are prone to water damage, deteriorate easily, break down in sun or heat, etc. It is recommended that you test small samples of any foam you may be considering for a boat, using at a minimum the following criteria.
- Let a sample sit in water (add a bit of dye to the water) for a few days. If it soaks up water into more than the first few cells, reject it.
- Press your thumbnail into it. If you can make a significant dent, reject it.
- Place a piece of scrap foam between two cinder blocks, in sunlight, to form a bridge. Put some weight on it. If it warps substantially in the sun under the weight, it will do the same in your boat hull on a sunny day, even with the skin laminates.
If you are considering foam cores, make sure to check several suppliers and several brands. Core-Cell is often considered the best, but other suitable ones may be available in your area.
A disconcerting trend in production boats of recent decades is the use of "mystery materials" as the hull. Such boats are sometimes marketed as being built of "advanced composites", a sure warning sign that something is amiss. These materials are generally fibrous putties, applied by spray gun in a high-volume factory. Engineering data on them is scarce to nonexistent. Boats built of these materials often have a single layer of fibreglass cloth on the inside of the hull to give the illusion of fibreglass construction.
The motivation for using such materials is usually cost. They are cheap and can be used by unskilled workers. The resulting boats look and feel like fibreglass hulls, and are similar in stiffness. They are, however, very prone to fatigue, water ingress and impact damage. David Pascoe has an excellent article on the damage that can be inflicted on such materials with remarkably little force.
In short, any new or unknown material for which good engineering data is not readily available is probably not a suitable choice for any aspect of boat construction. At present, no spray-applied or form-in-place putty exists which is suitable for use as a core material in a boat hull.