Dinghy - Make Your Own

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Dinghy - Make Your Own

       

Although many would say an inflatable dinghy is a necessity for the cruiser, there is much to be said for the merits of a hard dinghy. They're a pleasure to row, tow well, require no set-up, don't leak, and just plain look salty.

In some areas you can pick up a used one for cheap, but if you can't find one, want something special, or just want to know how to make your own dinghy, this page is for you.

YachtPals: Click the edit tab to edit or add more information.

Contents

• Selecting a design
• Plywood
  • Exterior grade ply
  • Marine ply
  • Joining Plywood sheets
• Lofting
• Epoxy
  • The WEST System
  • Mixing Epoxy
  • Thickening Epoxy
• Tools
• Protective equipment
• Stitch and Glue
• Painting
  • Selecting a paint
  • Preparation
  • Application

Selecting a design

Selecting a design might just be the most difficult part of the whole process.

Your first boatbuilding project should be simple and inexpensive, yet the boat needs to be useful when it is finished. The balance between these criteria is an individual decision. The Internet is a great place to start when looking for a design. A useful site for free design information is http://home.clara.net/gmatkin/design.htm.

 

Plywood

Plywood is a a man made construction material. It usually consists of an odd number of layers of thin timber veneer glued together with the grain of alternate layers running at 90° to each other. The outermost layers are termed the faces. Since these are the only visible layers, the appearance of each face is subject to standard specification. In the US, each face of a plywood sheet is described by a code letter:

• A — a high quality appearance grade veneer suitable for clear finishing.

• B — an appearance grade suitable for high quality paint finishing.

• C — a non-appearance grade with a solid surface. All open defects such as knot holes or splits are filled.

Thus a piece of plywood marked AC would have a good side and a rough side.

The adhesive used to bond together the laminations in the ply is of paramount importance to boatbuilders. There are two basic plywood glues:  Phenolic and Ureaic.  Exterior and marine grade ALWAYS use phenolic, but some "bathroom" grades do not. 

Exterior grade ply

Exterior ply, denoted by an X (as in, ACX plywood) is almost as good as marine ply.  That is to say that the standards of some exterior plywoods rivals that of some marine grades.  The issue being the wood and quality of each sheet in the laminate, and the number of laminates used.

In the Wester US, it is a great temptation to buy douglas fir ACX for the cheap price.  Unfortunately, doug fir "checks," even when glassed over.  Considering the amount of time you'll be spending, you should generally go with marine grade.

Marine ply

Marine ply is an engineering product that must comply with published standards. In the USA, the UK and other parts of the world the BS1088 standard may be used to specify marine ply. These standards detail practically every aspect of the composition and performance of marine ply. If you are going to build a seaworthy boat, or a boat that must meet survey standards, use of a certified marine ply is essential.

There are a number of quality materials available, and the cost of each will be dependent on your locale.

Joining Plywood sheets

Plywood for boatbuilding is traditionally purchased in 8 foot x 4 foot sheets. In metric this rounds out to 2400mm x 1200mm. It is apparent from these dimensions that it is necessary to join plywood sheets together to build even the simplest of boats.

Two methods are commonly used to join plywood sheets together:

• The butt joint
• The scarf joint

The butt joint is by far the easiest to make. It is formed by applying liquid epoxy to the edges of the sheets to be joined, pushing them together and glueing a strip of ply to one or both sides of the seam.

A variation is to use fiberglass tape instead of the strip of ply. Once the epoxy has cured the join is stronger than the plywood sheets themselves and the wood will fail under stress before the epoxy of the joint.

Hint: Epoxy does not stick to polyethylene. Laying down a sheet of polyethylene film, such as a disposible painters drop sheet, can save the embarrassment of glueing your work to the floor when joining sheets.

Lofting

Lofting is the process of taking the dimensions from a plan and drawing full size components on the construction materials.

Using the lofting batten
Some text is provided to YachtPals under the GNU Free Documentation License from: Do-It-Yourself/Wooden_boat at wikibooks.

Sometimes the designer provides the necessary measurements in the form of a table, but sometimes it is necessary to take the measurements directly from the drawing and scale them up to life size.

First a centre line is marked, then series of station lines are drawn on the ply at regular intervals 90° to the centre line using a set square. Station lines may be drawn every 10cm for example. The distances either side of the centre line at each station line are determined from the plans and marked on the ply as reference marks. Finally smooth lofting lines are drawn through the reference marks. A straight edge such as a steel ruler may be used for straight lines and the lofting batten is used for smooth curves. It is helpful to temporarily drive panel pins into some of the reference marks to ensure the lofting batten follows the correct curve.

Once the lofting process is complete the lofting lines are used as cutting guides when the components are cut from the sheet.

Epoxy

Epoxy is the material that makes wooden boatbuilding possible for the average person. It is used as an adhesive, a structural component, and to seal the wood to stop it absorbing water.

Wikipedia has an article about epoxy at http://en.wikipedia.org/wiki/Epoxy.

The WEST System

The Wood Epoxy Saturation Technique, or WEST system, is a technique for sealing a wooden boat against the ingress of moisture. A thin layer of liquid epoxy is applied to the wood like paint. Some of the epoxy is absorbed into the grain and the pore structure of the wood while the epoxy is a liquid and when it hardens the wood is effectively protected by a moisture proof barrier.

The WEST System is a generic boatbuilding technique, but also a registered trade name for a brand of epoxy products manufactured under licence from Gougeon Brothers, Inc. USA. The Gougeon Brothers developed the epoxy coating of the original WEST system in conjunction with DOW Chemicals in the late 1960s. Since that time epoxy systems, including the WEST System brand, have undergone significant development. Today there is a wide range of manufacturers offering epoxy systems for wooden boatbuilding. A web search using the term boat building epoxy will return thousands of references, tailoring the search for your own location should return more than enough local epoxy manufacturers to choose from.

A word of warning however. Not all epoxy formulations are suitable for sealing a wooden boat against moisture. Marine coating epoxies generally do not include organic solvents or thinners. They may be described as high solids epoxies. The reason is that organic molecules are much larger than water molecules. When organic solvents evaporate from the liquid epoxy during curing the departing molecules may leave microscopic tunnels that allow much smaller water molecules to enter. This reduces their effectiveness as a moisture barrier and makes them unsuitable for use as a marine coating.

Mixing Epoxy

The most critical part of the epoxy process is mixing the correct amount of hardener with the epoxy resin.

Stored separately the hardener and the resin will stay in liquid form. When mixed together the hardener acts as a catalyst and induces the resin to polymerise, or form cross linkages between its molecules. As these bonds between the molecules form the viscosity of the mixture increases and eventually, after a few hours, it solidifies. The reaction continues for several days afterward so it is important not to apply excessive stress to the material for at least 24 hours after application.

The rate at which the polymer bonds form is dependant on the chemical properties of the hardener used and the ambient temperature. The reaction that forms the bonds is exothermic which means heat is generated each time a new bond is formed. The hotter it is, the faster the bonds form and the hotter it gets, so new bonds form even faster. The heat usually escapes to the surrounding air by conduction so it is important to spread the epoxy out in a tray rather than keep it in a pot after mixing. The greater surface area of the tray allows more heat to escape and increases the pot life, the time when the mixture is still thin enough to apply effectively.

Warning in extreme situations, such as mixing and leaving a large quantity of epoxy in an insulated container like a polystyrene cup, the mixture may become hot enough to ignite. If your mixture rapidly increases in temperature spread it out in a tray immediately or take it outside and put it where it can burn safely.

The pot life of a batch of epoxy is generally about 45 minutes. Once a batch has reached the end of its pot life it is no longer possible to apply it, so it must be discarded. It makes sense to mix up small quantities of epoxy at a time. Only mix up as much as you can comfortably apply during the pot life or you will waste a lot of expensive epoxy.

Polyethylene containers such as those used by some takeaway food outlets are excellent mixing pots for epoxy. Epoxy will not stick to polyethylene so it is easy to peel out the thin layer of hardened epoxy next day and re-use the container.

It is important to follow the manufacturers directions about mixing ratios exactly. Adding more or less hardener to the mixture will not affect curing time or pot life, but it will reduce the strength of the bond.

It is also important to use the type of hardener the manufacturer recommends for the ambient temperature of your workshop. Bote Cote recommend the following hardeners for use with their epoxy system:

Fast Hardener: For use in southern Australian climates and in winters, has a pot life of 45 minutes and a thin film set time of 5-6 hours at 14-16 deg.C. Can be used at temperatures as low as 8 deg.C.

Standard Hardener: Suitable for warmer conditions and southern summers, has a pot life of 45 minutes and a thin film set time of 5-6 hours at 20-22 deg.C.

Tropical Hardener: For hot climates such as northern Australia where temperatures rarely fall below 30 deg.C. and there is consistently high humidity. Has a pot life of 45 minutes and a thin film set time of 4-5 hours at 30 deg.C.

Metric conversion hint: go to google.com and type "20C in F" in the search box.
This neat trick works in converting many other units too. (you can try out the example here)

Some manufacturers also supply a hardener accelerator. For Bote Cote the addition of 2% accelerator doubles the reactivity (half the pot life) while 4% makes it react 4 times as fast. The accelerator is not a replacement for hardener, it just speeds the process. This can be useful when you have a supply of tropical type hardener but want to use it with epoxy on a cold day in winter.

Whatever epoxy system you choose it is important to follow the manufacturers directions exactly when mixing.

In my case the mix ratio was two parts of epoxy resin to one part hardener. While it is possible to mix epoxy to the correct ratio using measuring cups, or by weight, I decided to purchase measuring pumps when I bought my epoxy.

Epoxy system and fibreglass tape

Two hand operated pumps were required, one for the epoxy resin and one for the hardener. The pumps were inexpensive, fit directly on to the bottles, and I have never regretted the decision. Mixing was simply one squirt of hardener to every two squirts of epoxy resin. I highly recommend measuring pumps to anyone intending to build a wooden boat.

Once the epoxy and hardener has been dispensed in the correct ratio it is essential to make sure it is thoroughly mixed. Stir the mixture carefully for several minutes with a flat, disposable stirrer. Some thin plywood offcuts trimmed to size are ideal.

When the epoxy is thoroughly mixed many people transfer it to another container before application, even if the quantity is too small to worry about it exotherming in the mixing pot. The reason for this is no matter how careful you are with mixing, there will always be some unmixed epoxy resin or hardener adhering to the sides of the pot, or right down in the corners. If a single drop of this unmixed resin finds its way on to your work it will result in a sticky spot that will never cure properly. Murphy's law dictates the sticky spot will occur in the worst possible part of your surface. Pouring the mix into a clean container and giving it a final stir with a clean stirrer is cheap insurance against unwanted sticky spots in the epoxy.

Thickening Epoxy

Properly mixed epoxy is a very runny liquid. It is ideal for painting on to wooden surfaces, but sometimes its low viscosity can present a problem. Building up a fillet in the join between a horizontal and a vertical surface is impossible without the addition of a thickening agent.

Epoxy manufacturers sell a variety of thickening agents. Bote Cote supplies the following fillers that can be used to thicken epoxy in various situations:

High Strength Filler: A high density glass fibre and particle filler for maximum strength glueing applications such as catamaran beams, mast bulkheads, load carrying transoms, etc.

Glue Filler: A low cost blended filler used for most glueing applications and also for making all fillets.

Fairing Filler: A low density, easily sanded microsphere based filler, which spreads like cream to a perfect feather edge. Used for filling and fairing.

Most manufacturers supply their own versions of these filler products.

Some manufacturers also supply wood flour for thickening epoxy.

Once the epoxy has been thoroughly mixed, simply add your chosen thickener a little at a time, stirring continuously, until the mixture reaches the desired consistency. It makes sense to use as little thickener as possible consistent with the application. Every particle of thickener in the mix is a particle that does not form polymer bonds with the surrounding epoxy. The less thickener in the mix, the stronger the material will be when it hardens.

Tools

Tools useful in constructing the boat included

• Power drill
• Power jigsaw
• Power plane
• Random orbit sander

• Handsaw
• Hand plane
• Hand sanding block
• Clamps (lots of them)
• Lofting batten (a long, straight, flexible strip of wood to aid in drawing even curves)
• Set square
• Steel ruler
• Tape measure

Protective equipment

• Particle mask
• Safety glasses
• Disposable plastic gloves
• Close weave work clothing

A particle mask is essential. There is a lot of sanding in boatbuilding and the dust it generates is not good for your health. Wood dust itself is bad enough, but when you add epoxy dust and fibreglass dust you will definitely want to keep it out of your lungs. You can buy disposable paper particle masks quite cheaply at hardware stores.

Safety glasses are highly recommended. Personally I wear prescription glasses so I don’t have to worry about special safety glasses. People fortunate enough not to need spectacles should consider purchasing a pair of safety glasses before using chemicals like epoxy and hardener.

Disposable plastic gloves are also a good idea when you are mixing or applying epoxy resin. Some people rapidly become sensitised to epoxy and develop dermatitis. Disposable plastic gloves prevent skin contact from occurring in the first place and avoid potential health problems. Disposable plastic gloves are available cheaply at major supermarket chains.

Close weave work clothing with long sleeves and long trousers is highly recommended. If you are considering more than just one boatbuilding project it might be wise to invest in light cotton overalls. I made the mistake of sanding fibreglass reinforced joints wearing only a pair of shorts. This was very comfortable at the time in the hot Queensland summer, but very uncomfortable for over a week afterwards when the glass fibres in the dust made their presence felt. Ever since I have worn cotton drill work clothes with sleeves down regardless of the temperature!

Stitch and Glue

Stitch and glue is a technique for constructing complex three dimensional objects using plywood and epoxy resin.

The stitch and glue method was developed by the first television "do it yourself" (DIY) expert, Barry Bucknell, in about 1964. Wikipedia has an article about stitch and glue at http://en.wikipedia.org/wiki/Stitch_and_glue.

The stitch and glue technique

First a row of small holes about 10cm apart is drilled close to the edge of one of the mating pieces of plywood. A matching hole is drilled in the other piece adjacent to each of these holes as construction proceeds and the two pieces are stitched together using either copper wire or plastic cable ties.

Once the mating pieces have been stitched together a fillet of epoxy is applied to the joint, glueing the pieces together. In the photo on the right a piece of waste plywood with a radius sanded on to one corner is being used to spread thickened epoxy into an even fillet.

If you look closely at the photograph you will notice the edges of the mating plywood pieces are darkened. This is because the edges were wetted out with a light coat of liquid epoxy before the thickened epoxy of the fillet was applied. The unthickened epoxy wicks into the grain of the plywood allowing greater adhesion. Since both the thickened and unthickened epoxy cure together they form a single polymerised structure of great strength. In fact the joint formed is stronger than the plywood itself. Once cured the wood will break before the epoxy of the joint if sufficient force is applied.

When the epoxy fillet has cured the stitches are removed. The holes filled and the addition of fibreglass tape and more epoxy provides the required level of structural strength to the joint.

Painting

Painting a wooden boat is more than just a cosmetic effect. The paint forms an essential part of the boat's structure.

If a wooden boat is not sealed against moisture it will absorb water, the wood will swell up and severe damage can occur.

Most wooden boats today are sealed against water using a thin coat of epoxy. Epoxy does a terrific job sealing out moisture but is susceptible to damage from ultraviolet light (UV). Unprotected epoxy will be damaged by the UV rays in sunlight and start to deteriorate in a single season.

Fortunately most exterior grade paints offer good protection against UV light as well as added protection against moisture ingress.

Selecting a paint

Selecting a surface finish for your boat is a complex task. If you go down to your local hardware store and ask for a paint that is good over epoxy you will most likely be greeted with a blank look. Very few paints specifically state they can be applied to epoxy. The good news is that most paints will stick well to epoxy despite the lack of formal specification. Success depends on surface preparation however.

People often change their minds about surface finish during the construction of the boat. They start out with a vision of the boat painted in solid covers, but during the construction process the beauty of the timber is revealed, especially in the areas where unthickened epoxy has been used. Epoxy forms a smooth, clear, shiny surface that enhances the grain of timber. Its too bad it can't just be left as-is but unfortunately it breaks down quickly when exposed to UV light. Epoxy must be protected by a coat of UV stabilised paint or varnish if it will be exposed to sunlight.

Some surface finishes that have been suggested for marine use are:

• standard-type marine enamels
• silicon/alkyd types
• epoxy paints
• one or two-part urethanes
• Linear polyurethane

Each of these surface finishes has its own advantages and disadvantages. For my first project I selected a latex type exterior acrylic house paint. It was relatively inexpensive, formed a smooth medium gloss finish and adhered well to the WEST treated plywood of the boat.

Some manufacturers recommend the use of etching primer before the application of their product. This ignores the fact that cured epoxy is chemically inert. Etching primers, which rely on a chemical etching action, have no effect whatsoever on cured epoxy.

The only way to achieve good paint adhesion on cured epoxy is to lightly sand the surface prior to application, making millions of microscopic scratches on the surface. This facilitates mechanical keying of the paint to the surface, reducing the chance of peeling.

Preparation

Before doing any finishing work or applying finishes, allow the last coat of epoxy to cure 5 to 7 days if the ambient temperature is above 20°C, even longer at colder temperatures. (There is a lot of waiting in all aspects of boatbuilding)

Some epoxy systems suffer from amine blush, a greasy substance that forms on the surface during curing. If your chosen epoxy system suffers from this problem, wipe down the work with a water/ammonia dampened rag or sponge prior to sanding. This will eliminate the greasy amine blush, make sanding easier and minimize clogging of the abrasive paper.

Application

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