DUDLEY DIX YACHT DESIGN

FAQ - Wooden Boats

If you are building one of our Didi range of plywood boats, please also see the FAQ - Radius Chine Plywood Boats page for additional questions and answers particular to building with that method.

We also have a page specially for questions about stitch & glue dinghies.

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 a small dinghy 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.

Can I use fir plywood to build my boat?

Fir and pine plywood is available in marine grades from some manufacturers but consider carefully before using it if you want to build a boat that performds well and holds its value. These plywoods are heavy and dimensionally much less stable than the hardwood marine plywoods. They are seldom flat, so a fair hull can be a problem. The surface is unstable, so is subject to cracking due to expansion and contraction from changes in humidity and temperature. The only way to be sure of stabilising the surface is to cover it with glass in epoxy. Epoxy resin alone cannot hold it, so it will crack without glass if exposed to the weather.

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. This leaves the polyester resin with reduced peel strength, particularly if in combination with glass reinforcement because the increased thickness aggravates the shrinkage.

For the purposes of stitch & glass (tape & glass) jointing of plywood panels, the adhesion of polyester resin can be improved by applying a coat of resin thinned with styrene monomer or acetone to the bond area prior to making the joint.

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 adhesive is a single pack product, normally supplied in a plastic spout bottle. It cures by means of reaction with the moisture in the air. Strength is moderate but it bonds well to clean wood and expands as it cures, giving it good gap filling properties. Once cured, it works nicely with ordinary woodworking hand tools. It is the most convenient product for quick jobs or those which require small amounts of glue intermittently, like joinery. Impossible to remove from the skin, it is best left to harden, after which it can be peeled off the skin. Proprietary solvents for cleanup make it penetrate into the grain of the skin and take longer to wear off.

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 maring 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.

You advise using epoxy after the hottest part of the day to avoid bubbles forming. Gougeon say coating in cooling temperatures can sometimes produce "amine blush" - a greasy surface film . Is the answer to simply wash the epoxy off to remove the blush before subsequent coats?

Amine blush can be a problem but everything that I have read on it says that it can just be washed off with water. There have been a few questions about it recently on the rec.boats.building newsgroup recently. The bubbles can be a problem to fill because they form craters then the following coats flow away from the rims instead of covering. You end up having to sand them flat, filling each hole with thickened resin then overcoating.

Approximately what amount of springback should I allow for, if any, when laminating 5 pieces of 10mm x 25mm (3/8"x1") wood for the cabintop beams? I haven't laminated before and I am wondering if I should overbend them a bit?

It is difficult to accurately assess springback because it depends on species as well as the ratio of the strip thickness to the beam depth. Strips as thick as 10mm (3/8") should only be used for flexible species of timber, the stiffer ones needing 3-4mm (1/8"-3/16") strips for good laminating. As a general rule, the thinner the strips, the smaller the springback.

Allow for 10-12mm (3/8"-1/2") springback in the first beam which you laminate and adjust the mould as necessary for later ones. A few mm difference will not be noticed and can be adjusted by handplane after fitting.

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.

We have all experienced pain from touching dark surfaces in strong sunlight, such as walking onto a dark coloured road or roof surface with bare feet. You will very quickly burn the skin off your feet or can fry an egg on the heated surface. The temperature on the surface can be well in excess of 100 degrees Celcius.

Some boats with dark paint on their decks have wrinkled those decks from rigging loads when sailing in the tropics. The problem is not only with decks though, hulls can suffer similar problems. Glassy seas reflect the sun's rays so a hull will be attacked by the sun from above and below at the same time.

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. However, you can source good epoxies from other companies as well.

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 requirments of low viscosity and mosture 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.

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This page was updated 3 September 2016

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