GUIDE TO RADIUS CHINE PLYWOOD CONSTRUCTION
TABLE OF CONTENTS
BACKGROUND
THE DESIGN
LITERATURE
WHICH GLUE TO USE
EPOXIES GENERALLY
FASTENERS
PLYWOOD
GENERAL ORDER OF CONSTRUCTION
BULKHEADS - SETTING OUT
BULKHEADS - CUTTING & ASSEMBLY
BULKHEADS - JOINERY CLEATS
BULKHEADS - UNDERCOATING
BUILDING STOCKS
SETTING UP BULKHEADS
GENERAL NOTE RE LONGITUDINALS
BACKBONE AND STEM - SOLID TIMBER TYPE
BACKBONE AND STEM - PLYWOOD
DROPKEEL BOX
SHEER CLAMPS
FLATS FOR HULL SKIN
HULL STRINGERS
LAMINATED FLOORS - Designs with solid timber backbones
I-BEAM - Designs with solid timber backbones
BEFORE STARTING THE SKIN
INTEGRAL TANKS
HULL SKIN - SHEET AREAS
HULL SKIN - RADIUSED AREAS
CENTRELINE DETAIL - SOLID TIMBER BACKBONE
CENTRELINE DETAIL - PLYWOOD BACKBONE
TRIMMING THE STERN
HULL FAIRING
TURNING THE HULL
DECK AND CABIN CONSTRUCTION
MULTI-CHINE CABIN STRUCTURES
DECK JOINERY
INTERIOR JOINERY
WINDOWS
DECK HARDWARE
MOUNTING WINCHES ON SLOPING COAMINGS
BALLAST KEEL
STERN TUBE AND P-BRACKET
PLUMBING, ELECTRICS ETC
GENERAL
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BACKGROUND
Marine plywood was once the most popular material for boat construction by amateurs. It was knocked aside by ferro-cement, which thankfully did not hold the No 1 spot for very long. Now most amateurs buy a GRP hull and deck to fit out while those who want to build from scratch themselves generally choose steel.
A few years ago I built a new boat for myself. Having built two offshore boats in plywood before, I had a reasonable understanding of the strengths and weaknesses of the material and the details normally used with it. In the design of it I set out from the beginning to improve on existing principles where possible and, in some cases, to introduce some new ones. After starting construction I heard of other designers or builders, in far-off places, also working on plywood based projects so it seems that there is some popularity being regained by this material.
Aside from my three offshore yachts, all of which have been in the 33-38ft range, I have also built dinghies and a catamaran. All have been of plywood in one form or another. The catamaran, of tortured plywood with stitch-and-glue, was the first design that I ever drew, in the early '70s. The dinghies were hard chine stitch-and-glue boats, also my own designs. The first keelboat was a van de Stadt multi-chine and was followed by another to my first keelboat design.
I am not hooked on plywood as a material, I just like working with wood and find plywood to be a convenient and economical medium to produce a boat. I have built keels from steel but have never built a steel boat. I always feel relieved to get back to working with wood.
Most people believe that wooden boats have rot problems. What has slipped by them unnoticed is that most of the wooden boats which they see are pretty old, often from long before the days of GRP. Many far newer GRP boats are in worse condition, with major osmosis, gelcoat cracking or delamination. They see also that in a GRP boat the wooden parts deteriorate first and start to rot so the material is blamed rather than the installation.
A GRP hull will not deteriorate through bad ventilation but the timber bulkheads and joinery will if not protected from damp stale air. Most amateur built GRP boats that I have been on have a stale smell to them but the wooden ones generally are fresh. That is one of the principles for preventing rot - ventilate it well.
Another good principle is to coat it so that water cannot get to the timber. Here epoxy saturation coatings are the secret, sealing the timber in and moisture out. Most wooden boats afloat today were built before epoxy resins were introduced as marine coatings. Think how long they would last if coated with epoxies when new.
With regard to chines, they can look good if carefully placed, or they can ruin the look of a hull. Some people prefer them to a round bilge hull, feeling that chines give character. Others hate them. What cannot be disputed is that they generally pull down resale value.
Chines can improve the performance of a planing hull (and generally do) but they hurt the performance of a displacement hull. My earlier 33ft yacht was a multi-chine with the chines placed for good seakeeping. This worked well because she seldom slams and is a delight to sail. Downwind in heavy airs she flies and, in planing mode, will do 18 knots. Upwind, in flat water she is very fast but lumpy water knocks her performance due to increased turbulence.
I have been successful with my radius chine steel designs in giving amateur builders a way of producing a "round bilge" hull for themselves out of sheet steel for no more effort than building a multi-chine, improving resale value and performance en-route. They are now widely accepted by people who want relatively heavy boats but the performance orientated sailor (myself included) was still left on the dock.
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I set out to produce the benefits of radiused chine construction but in plywood, ie I wanted to produce a hull mainly from sheet plywood but which would look like a round bilge hull. I did not want it to just be a "soft-chine" design such as I and various other designers have drawn in the past. I also did not want to rip plywood sheets into strips and double-diagonally plank the hull. That defeats the purpose of using a sheet material and one could then just cold-mould the hull.
I also decided from the start to use myself as the guinea pig so that any risk would be my own not a client's. The benefit was that any lessons learned during construction were turned into improvements in the design.
THE DESIGN
I started with the same principles that worked for my steel radius chine designs. I soon found that the bow sections and forefoot which work well on a steel boat, using a constant chine radius from bow to stern, do not work on the plywood hull because it is too shallow. I had to rethink it and developed a method which uses a constant radius over most of the length but tapering to a tight radius at the forefoot. It produced a good looking hull shape but some strange looking stringer paths. People looking at the hull when in framing thought that I had gone mad but were impressed by the final results.
Developing the concept further, later designs have the same construction and shape benefits but a less radical look on the drawings and in skeletal form.
The radius takes up about 1/3 of the total hull surface area. That means that 2/3 of the hull can be skinned at a fast rate because it is applied in sheet form. On the 38, the panel widths vary from a narrow wedge for the forward bottom panel to a little more than a full sheet for the topsides at the bow. They vary with the size of boat but are generally wider than found in multi-chine designs so there is less wastage. The radius is also not particularly slow to do because it is only about 1m girth at its maximum so there is minimal spiling necessary for a good fit, unlike cold-moulding which requires lots of spiling.
Before describing the construction in detail, a few points of interest to the whole project would be useful.
LITERATURE
There are some good books on the construction of timber/epoxy boats and they can give pointers to assist you in your project. My recommendation would be to use the WEST system book "The Gougeon Brothers on Boat Construction" as your boat building bible. It may conflict in some minor ways with my statements here but my opinions have been formulated by my experiences working as an amateur builder in typical amateur conditions, not as a professional in a factory.
WHICH GLUE TO USE
I have worked with 3 basic types of glue and find particular uses for each. They are resorcinol, epoxy and polyurethane .
Resorcinol I find to be best for structural joints which have a large contact area, particularly for laminating. Clamping pressure can be released when the glue is still a little soft and easy to plane for cleanup.
Epoxy is best for structural fillets but I do not like the paste type epoxies because they are normally brittle and do not penetrate into the timber, bonding only to the surface. I use a low viscosity epoxy thickened with wood flour and carbosil to achieve the consistency needed for the particular application, relatively thin for bonding and thick for fillets.
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If laminating with epoxy take note that it must be well cured before release because even epoxy which feels hard will creep under load if not cured. I found this out when I released a laminated floor after 24 hours in cold weather and found an hour later that it had straightened considerably and then couldn't be pulled back into shape. I have not had this experience with resorcinol so would recommend that it be used in preference to epoxy for laminating. The same applies to scarphing of structural members because the joints will burst if loaded up before full cure.
Whether bonding with resorcinol or epoxy, particular attention must be paid to end grain areas, which soak up adhesive. Allow it to soak in then apply more just before assembly.
Polyurethane is the most convenient because it is single part and "grows" as it sets, thus filling any gaps in the joint. It is readily cleaned up with a chisel, plane or sandpaper but is not as strong as epoxy or resorcinol so I keep the polyurethane for interior joinery. Beware though, it does not adhere well to painted surfaces so you must sand back to clear timber.
EPOXIES GENERALLY
Epoxy coatings must be applied to the hull and deck skin and all associated structure to ensure that they are sealed. The material used must be of low viscosity and 100% solids, ie it must contain no solvents to evaporate off during curing. Pot life needs to be in the 20-40 minute range to give time to work with it while not having an unduly long time before being tack-free.
If working out of doors or in any location where condensation is a possibility, another factor must be taken into account in the choice of epoxy. If using covers which are removed for working on your boat, you will be caught out by damp in the form of dew on occasions. This applies particularly with the first coat, which must be applied after the hottest time of day so that the cooling timber draws the resin into itself rather than blowing bubbles through the resin if the timber is warming up. This means that you will only be applying the first coat during the afternoon and a winters day will mean slower setting of the resin, earlier sunset and earlier dew. If your resin has no moisture tolerance built into it then the dew will turn it into chewing gum forever after.
If you have no prior experience of the properties of the resin which you are planning to use then first coat a piece of scrap plywood with resin. Wait for it to become tacky then pour water over it and leave it wet to see what happens. Some resins will cure even if totally immersed in water. Other will harden, but with a milky surface which will either disappear by itself or can be sanded off. Some just stop curing. If your sample falls into this last category, reject it unless you are sure that moisture will not be a problem.
Some builders precoat all of their timber before assembly but I prefer to coat afterwards. My reasoning is that I have found epoxy to bond far better to timber than to itself (even after sanding and solvent cleaning) so bonding onto precoated timber is likely to result in a considerably weakened joint unless the bonding is done within 12 hours of the application of the coating. The alternative is to mask off the bonding surfaces prior to precoating.
Different hardeners can be used to give faster or slower setting rates to suit warm or cold weather. However, the hardeners which give slower setting produce stronger and more resilient products so I prefer to use one hardener and try to adjust the resin temperature to speed up or slow down setting.
In warm weather it is important to keep your resin and hardener supplies in a cool spot. The pot with which you are working can also be stood in a bowl of ice water to dissipate heat as you work. In cold weather the supplies should be kept in a warm spot and, if there is sunlight, it is amazing what a difference some black plastic sheeting over the supplies will make to the workability and setting times. If your location suffers extremes of temperature then you should probably work with two different formulations to give satisfactory results in both extremes.
FASTENERS
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A note re the use of screws and ring nails generally would not be misplaced. Screws can be used almost anywhere in the boat to pull gluing surfaces together smoothly and evenly by one person. Ring nails are a less costly alternative and are quicker to do but have disadvantages. In many situations there is too much recoil in the structure so it simply bounces away when the nail is struck, requiring a second person with a sledge hammer or weight held on the opposite face to prevent the recoil.
Even pressure is also more difficult to achieve with ring nails because maximum pressure is instantaneous. The nail is struck into the timber then the glue moves afterwards, relieving the pressure. In contrast, a screw applies pressure progressively and you can apply as much or as little pressure as you want, squeezing out as much glue as you feel necessary at the time. If working single-handed, it is best to save the ring nails for attaching plywood to heavier members or where the structure is already stiffened by other plywood.
Note that throughout the boat the fasteners are working along with adhesives to produce a strong structure. All joints are to be glued unless specifically described as not being glued.
Staples can be used in many places to provide temporary fastenings until the adhesive has set. They must be used to assist with inter-sheet bonding when laminating two or more sheets of plywood together for cabintop, foredeck, hull radius etc. Some builders remove them before doing epoxy coatings, others leave them in - the choice is yours.
PLYWOOD
As a general rule, the more you pay for your plywood the better the quality is likely to be. As a minimum it must be WBP, ie water and boil proof and normally graded as "Exterior" plywood. It should also be fabricated to a recognised standard (and marked as such) or you will be taking the word of the manufacturer only.
If using WBP then voids will be found in the edges of inner veneers. Fill these with epoxy when fitting. It is best to check the edges of sheets when buying and to reject any which have excessive edge voids. Overlaps of inner veneers should also be rejected because, although the surface will have been pressed and sanded flat during manufacture, it will swell, producing a bump on the surface.
The species of wood used for the plywood will greatly affect its strength, weight and other characteristics. Gaboon (Okoume) plywood is light so it is best suited to interior construction and the skins of performance boats. Mahogany plywood is stronger, harder and heavier so it is a good choice for the skin of a more cruising oriented boat. The heavier, denser timbers absorb less resin, which tends to stay on the surface and bond onto the timber rather than into it as happens with the lighter timbers which benefit more from the epoxy.
The construction of the plywood will greatly affect its stiffness. The more veneers that there are the stiffer it will be. That is great for flat surfaces but bad news if it has to be bent much. For the radiused section of the hull use 3-ply or you may not be able to bend it easily around the radius. Gaboon plywood is also generally better for doing the radius because it is more flexible. The stringers in this area are more closely spaced than elsewhere in the hull so the radius is plenty strong enough even if you are using a stronger plywood elsewhere in the hull.
Be wary when buying your plywood. Some manufacturers or suppliers will try to sell you products which are not quite what they would like you to believe. For example, different grades of Gaboon ply are unlikely to vary much in weight from each other because they are made from the same species. If one grade is appreciably heavier it probably has inner veneers of some other more dense species of timber.
GENERAL ORDER OF CONSTRUCTION