Saturday, October 8, 2016

Torturing Plywood in Boatbuilding

Plywood is a great engineering material. Man has taken natures wonder engineering material, wood, and improved on the structural characteristics for use in large sheets. In it's natural form you can't cut wood into 1/4" (6mm) thickness and expect it to remain stable and strong in sizes more than a few inches in width when measured across the grain. It will split, it will twist and it will warp out of the nice flat form that you originally cut. It will be strong along the grain but relatively weak across the grain, which must be taken into account in any structural application.

When that same wood species is cut into thin veneers and they are glued together with the grain direction of adjacent layers oriented at 90 degrees to each other, each layer strengthens those on each side of it in it's weakest direction. The resulting material loses it's linear strength characteristic and strength becomes relatively similar in all directions. The material also becomes much more stable, much less likely to twist, warp and crack. This brings great benefits for designers using this material but it also introduces limitations that the designer or builder must work around. In short, the benefit of plywood is that it is flat and the drawback of plywood is also that it is flat.

Of course the wood species, the quality and number of veneers and the quality of the glue and manufacture all affect the strength and stiffness of the sheet. As a simple rule of thumb, the more veneers, the stiffer and more stable the sheet will be, when comparing sheets of the same wood species. When comparing sheets of different species, the more dense (heavier) specie will likely be the stronger one. Here is an excellent article on plywood quality.
Plywood structure. Plywood must always have an odd number of veneers, so that the surface veneers on both faces run in the same direction. This is called balanced construction and adds stability to the sheet. If it has an even number of veneers then the surface grains will run at 90 degrees to each other and result in tensions within the sheet that will cause it to warp.
Ever since plywood was first invented, man has been figuring ways to distort it into shapes other than the flat sheets in which it is formed. There are various ways to do this and understanding the nature of the material helps to figure the best way to deform it to suit our needs.

1) Wood can be stretched, within limits. When we bend it, the outer surface of the bend stretches and the inner surface of the bend is compressed. The fibres in the middle are on the neutral axis and are neither stretched nor compressed. The fact that it can be stretched means that it can be deformed in ways that are not necessarily conical or cylindrical in nature, the principles of developable surfaces normally used when designing curved surfaces for sheet materials. It can be somewhat tortured into shapes that mathematics says can't be done. But, push it too far and it will fracture, proving that the mathematics may have been right after all.

2) Wood can be softened and made more pliable if it is wet, especially if it is also heated at the same time. This is the principle used when making steam-bent frames in traditional boatbuilding, softening the wood strip in a steam box. A stiff piece of wood becomes quite soft and malleable when hot and wet, so it can be bent to shape and will hold that shape when it dries and cools. This can also be done with plywood but how do you do it with a big piece that won't fit into a steam box? I have made plywood sheets more flexible by laying them on wet grass, spraying water over the sheets then covering with black plastic and leaving in the sun for a few hours. Others have had similar success using a steam wallpaper stripper on the outside surface while gradually pulling the sheet in with clamps and other mechanical tools. If you have a sheet clamped in place and need to coax more bend into it but don't have a wallpaper stripper, spray water onto both sides of the sheet then cover with black plastic to absorb heat. Even without heat, you can wet the sheet, keep it wet overnight with hessian bags or similar, then find that it is easier to bend in the morning.
Steam wallpaper stripper can be used to make plywood more pliable
3) Wood can be bent by making saw-cuts (kerfs) across the piece on the inside of the curve. This reduces the compression loads on the inside surface and effectively moves the neutral axis locally to the mid-point of the thickness that is left after the kerf is cut. If the kerf is half-way through the wood then the bend characteristics become more like a piece of half the thickness than one of the full thickness. The more kerfs that are cut the more that the piece is softened and the smoother the curve that results. This works with plywood also but must be judiciously done. The kerfs shouldn't be cut deeper than half-way through the plywood, or the structure of the plywood will be destroyed. If you cut them so deep that only the surface veneer remains, you will almost certainly fracture that surface veneer along the kerf line. The kerfs should also not be excessively wide because they will have to be filled with epoxy to regain the lost strength after the panel is glued in place. The width of a circular saw blade or narrow router bit, so about 3mm, is a good kerf width. It gives enough width to allow the sheet to bend without the edges of the cuts closing so much that they can't be filled.

4) It is easier to deform short sheets a small amount than long sheets a large amount. If you have a panel that is 20ft long and has considerable twist to it, it will be difficult to pull the twist into that long panel if it is all in one piece. It will be easier to get that twist if the panel is fitted in three 8ft lengths that are glued to each other on the hull framing. It will be even easier if fitted in six 4ft lengths but that is only required if the twist is extreme. As example, for the Cape Cutter 19 or Cape Henry 21, with lots of twist in the bow and not much twist aft, the panel would be best fitted as 4ft lengths in the bow, changing to 8ft lengths further aft.

5) You can't easily deform a sheet so that one edge has convex curvature (curved outward) and the opposite edge has concave curvature (curved inward). The stresses in the sheet will likely damage it. The builder of the first Didi Mini to be skinned emailed me to say that he had found it impossible to skin the side panel forward of the mast; the sheet simply could not be forced into the required shape. It has convex curvature at the deck to add reserve buoyancy and concave curvature at the bottom in the form of a hollow bow waterline, to improve wave penetration and increase speed. I told him to cut the sheet vertically so that it was two sheets 4ft long instead of one sheet 8ft long. He emailed back that the problem disappeared and he could easily skin that area.

6) Narrow panels are easier to twist than wide ones. If you have a long and narrow panel that is twisted and another that is the same length and twist but 5x the width, the narrow one can possibly be fitted in one length but the wide one will need to be broken down into shorter lengths to make it easier to fit. My radius chine designs generally have considerable twist in the bottom panel toward the bow. There is no problem twisting this panel because it is narrow in that area.

7) Plywood will take on a set if it is distorted and held in the distorted shape for a few hours. If it won't pull all the way into the shape that you need, don't force it too far, you will break it. Pull it in with clamps, levers and Spanish windlass as far as it will go, then leave it. If you were to unclamp it a day later you would find that it doesn't want to lie flat because it will have taken on some of the bend due to stretching of the wood. Instead, go back 2 or 3 times in 24 hours and pull it in some more. Eventually you should be able to pull it all the way to where you want it.
Ian Allen, building a Cape Cutter 19, used clamps and levers with
Spanish windlass to pull in the bottom panels in the bow. He used
an 8ft length and would have found it easier in two 4ft lengths.
If you are building a boat that has a skin panel that has a large amount of twist that is mostly concentrated in one area then you are almost certain to need one or more of these methods to get the plywood to conform to the hull shape. You can combine various methods from the list, to supercharge the process. Softening with water/heat or steam, combined with kerfs and fitting narrow sheets will allow plywood to take on considerable twist. Once you have managed to pull that panel and it's partner on the other side to the form that you need and they will hold most of the twist when released, then it is time to glue it into place. Don't consider getting one side done and glued in place before starting to twist the other side, you will seriously limit your options for clamping any edges where the two panels meet. You will also load the framework on one side and possibly pull it out of alignment so that the completed boat is asymmetrical. It is best to prepare the one side then set it aside while you prepare the other side, then glue both on in parallel.

To see our boat designs, go to our main website or our mobile website.

Labels: , , , , , ,

Wednesday, February 24, 2016

Fibreglass Boats from Plywood Plans

There are many, many options available to build a boat from plywood plans. If you want to build a small fiberglass boat as an amateur, the choices are much more limited.

There are ways to go about it and occasionally someone does. Phil Coveny bought plans for our smallest design, the 8ft Dixi Dinghy. It is a plywood 3:1 dinghy intended as a yacht tender or a child's first boat. It just happens to also be a fun little boat for older children to sail (read that as Dad or Grandpa).

Phil wanted to build his boat from sandwich fiberglass for various reasons that included an available supply of the needed materials. He built her with perforated 9mm foam with a density of 80kg/cu.m, glassed both sides with 9oz E-glass fabric in polyester resin. You can replace the polyester resin with epoxy for a more durable boat but at higher cost. He worked with the foam panels in much the same way as he would have with plywood except that, instead of doing stitch-&-tape joints, he used a hot-glue gun to do joints, then filled the joints with polyester putty before glassing over the whole boat, inside and -out.
Bare foam hull with joints glued then filled with epoxy putty
Closer view of the perforated foam, with glued and puttied joints.

Hull has been glassed and seats are being bonded in.
Interior view of finished boat, with wood rubrail and rubber bumper.
Bow view of completed boat.
Phil sailing his new tender. This rig is from another boat that Phil built, a bit smaller than the Dixi rig.
Phil's boat came out at 65lb (29.5kg), somewhat heavier than he had hoped, primarily because he didn't use a squeegee to remove excess resin from the laminate. Removing excess resin binds the glass fabric closer to the foam, reduces resin usage, reduces weight, increases strength and improves the finish. If you don't have prior experience with glassing then it is worthwhile to practice the technique before you start to glass the hull.

If your motivation to build a small sandwich fiberglass boat is to build it lighter than with plywood, you are unlikely to achieve what you want. Modern okoume plywood produces a very light boat (about 48lb for the Dixi Dinghy) and the foam sandwich fiberglass boat will need very light skins (max 6oz for the Dixi Dinghy) to have similar weight to plywood.

Our other plywood dinghies can also be built in the way shown in this post but they are only detailed for plywood construction.

Visit our website to see our dinghy designs or our full range.


Labels: , , , , , ,

Wednesday, December 30, 2015

Amateur Boatbuilding Coming Alive Again

All things boating have taken a hammering over the past decade, with most people suffering to a greater or lesser extent through the financial woes that flooded the world. Initially, amateur boatbuilding seemed to stay quite strong, while sales of production boats went into a deep slump.

After amateur boatbuilding also started to weaken, it was interesting (but distressing) to observe the progression of the slowing sales around the world. First to go were the industrialised countries, led by USA and UK, then it spread to less developed countries. It is said that when America sneezes the world will catch the cold. When we lived in South Africa we could watch that phenomenon and knew that what was happening financially in USA, whether up or down, was likely to be mimicked in South Africa within a year or two.

The one market that seemed to stay remarkably buoyant was Russia, where amateur boatbuilding seemed to stay strong. Then war and politics stepped in, the Russian economy took a dive and sales of plans to amateurs in Russia also fell.

Boats are not an essential part of life for most people. For some they are transport or a work implement, for others they are a home but for most people their boats are toys, sporting equipment, or somewhere between, certainly not high on the list of essentials when money becomes tight.

Now we seem to be coming out of that slump. The US economy has been showing signs of life for a year or two and that now seems to be filtering down far enough that amateur boatbuilders are again starting new projects. If the normal pattern pertains, the upswing will spread out to the rest of the world as well.

Over the 35 years that I have been designing boats professionally, I have drawn mostly boats that are suitable for amateurs to build. This was not an intentional path in my design career but, in retrospect, was probably the most natural and logical one. My roots are in amateur boatbuilding, when I built a plywood 36ft cruiser/racer in my in-laws' garden, based on a design by Ricus van de Stadt. My own first full design was the CW975 for a 32ft plywood racer/cruiser, which I also built after it won a design competition. The third design that I drew was the steel Pratique 35 cruiser for an amateur-builder friend. Next was the Coquette 39 plywood racer/cruiser commission, also for an amateur. Only after that did I receive my first commission from a professional builder for a production GRP boat.
My own 2nd big boat project, the CW975 "Concept Won", in my garden.
This depth of involvement with amateur boatbuilders and designs has resulted in a long string of commissions for amateur builders, mostly for plywood. My own third build big-boat project, in the form of the radius chine plywood Didi 38 "Black Cat", and my well-publicized voyaging in her, have reinforced that trend. For the past 30 years I have always had at least a year of work waiting in  line. That has now expanded to a 2-year backlog and I have had to turn away new commissions to keep the backlog somewhat manageable.

As the slump deepened, the average size of boats that people chose to build slipped lower, until we were selling only dinghies and trailer-sailers. Now that average size is starting to creep upward again, as builders move back into bigger boats. The most durable design that we have is the Argie 15, which has developed a good following that keeps it going strongly through thick and thin.
Argie 15 built by Andrey Borodikhin in Moscow, Russia.

In trailer-sailers, the little gaffers, in the form of the Cape Cutter 19 and Cape Henry 21, are the ones that keep going. These lapstrake plywood boats are more complicated projects than a simple stitch-&-glue boat but they are just so pretty that they have attracted a good following. They have also proven to be quick in the light stuff and capable in the rough stuff, so they have grown a reputation for being desirable boats. That resulted in the upward expansion of this design range with the Cape May 25.
Beautifully-built Cape Cutter 19, built by Sergio Vianna of Curitiba, Brazil.
Newest in this series, and already generating much interest even before completion of the design, is the Cape Charles 32. Although not yet on our website and pricelist, drawings have already gone out to the first two builders. This one is a coastal and offshore cruiser, with shallow fixed keel and external ballast, whereas the smaller sisters have steel centreplates and internal ballast. We should have a web page for this design in a month or two. Until then, please email me for info on plan price etc.
Accommodation layout of Cape Charles 32 cruiser.
It is not only cruisers that are growing in popularity. One of the commissions that is waiting for attention is a bigger sister to the radius chine plywood Didi 950, at 11.4m (37ft). I have also been asked multiple times to draw a Didi concept to the Class 40 Rule but am not able to fit in the design in a reasonable time, so have turned down the commissions. Meanwhile other larger boats in our radius chine plywood range are also starting in various countries. This includes the first in that series, the Didi 38, with two new builds starting in Asia.
Didi 950 radius chine plywood racer/cruiser.
We are also experiencing increased interest in steel cruising designs, which have been very dormant for the past 10 years. Overall, amateur boatbuilding is looking a lot more healthy now than it has for a long time. Now we all need the politicians in all of our countries to play nicely together. If they do that then life should improve for all of us. Lets all go play with our boats.

To see more of these and our many other designs, visit or

Labels: , , , , , ,