Below: Joubert Marine Plywood - an example of some of the finest boatbuilding plywood you can buy. This is 6mm Okoume plywood manufactured in France to the BS 1088 standard. Each sheet has the Lloyds of London stamp of approval.
I was recently involved in a discussion on the Wharram catamaran forum regarding the cost of building a boat and the question of whether or not it is necessary to use "marine" grade materials in the construction. (View discussion here)
This is something Ive been meaning to address here anyway, as the cost of most boatbuilding materials are going up and probably will continue to do so. In the often long and difficult decision making process that proceeds the beginning of a new boat construction project or the refitting of an old one, cost is certainly an issue for most of us. Although we would like to be able to nail down a number before ordering the first piece of wood or gallon of epoxy, accurately estimating the cost of a project is difficult, even for professionals, as there are so many variables. This is especially true for boat projects larger than canoes, kayaks or the simplest of skiffs. Cruising boats with interior accommodations, auxiliary power and sailing rigs can vary widely in cost depending on the care and thought taken in the selection of each and every component.
Since the hull (or hulls in the case of catamarans and trimarans) is usually the first part built in a new boat construction project, many builders are tempted right from the beginning to save money on hull materials. Plywood and epoxy composite is perhaps the most common type of wooden boat construction chosen by first time builders, especially since there are so many intriguing designs available for all types of vessels using some form of this construction. Many new builders incorrectly see the plywood as a major component of the boat, (after all, it is a plywood boat, right?) but the reality of it is that in most modern designs that rely heavily on epoxy for the assembly, sheathing and fairing of the hull, the plywood cost is a small percentage in relation to other materials. In fact on a boat like the Tiki 26 catamaran that I am building, the plywood cost is in the range of 10 to 12-percent of the the complete boat, in sailaway condition. And this percentage is based on using the good stuff - BS-1088 Okoume marine ply from Joubert, as pictured above.
The temptation to save money by purchasing non-marine plywood can be quite strong, especially when a new builder prices marine plywood for the first time. But the cheaper alternatives are, unfortunately, miserably inadequate in most cases, depending on the design being built. The Tiki 26 is a good example, as it is a cruising size boat, but being a catamaran, depends on strong but lightweight materials for good performance. The hulls, decks and cabins are all built of just 6mm (or 1/4-inch) thick plywood to keep the total boat weight in the range of 1500 lbs. Yet, this is a sailboat designed to go offshore and cope with the conditions that can be expected in that environment. The design is well proven for its intended use, and of course, the thin plywood is reinforced with well-engineered interior stringers and the epoxy joinery and sheathing methods employed in its construction, but still, 6 millimeters is a thin skin. It would not do to have core voids or cores with lots of knots or partial panels of a lesser grade wood in the interior of a plywood hull skin that thin. To get the full strength required by the design, a quality marine plywood with cores as good as the face veneers is clearly the best choice.
Although one can occasionally find good-looking exterior plywood at Home Depot or the local independent building supply vendor, the quality is never consistent from bundle to bundle and even varies a lot with individual sheets. Cut into most of this plywood and you will find even greater inconsistencies in the cores. Having built many cabinets and other residential carpentry projects over a period of years, I have seen the quality of almost all commonly available plywood decrease - from cabinet grade birch to exterior BC yellow pine. Its hard to find plywood that is adequate for even the roughest construction these days, much less any that would be good for a boat hull.
But having said this, there are exceptions and there are types of boats that are designed to allow for a margin of poorer quality materials. A type that comes to mind are the shallow draft cruising designs of Reuel Parker. His cold-molded construction method is designed to allow layers of lesser grade exterior plywood to build up the hull to the required thickness. There is a margin for these poorer grade laminates in such hulls because of the much greater overall thickness of the hull skin, particularly on the bottom.
This drawing below, scanned from Parkers book, The New Cold-Molded Boatbuilding: From Lofting to Launching
The other case for non-marine plywood is in the construction of small, simple skiffs that are inexpensive in terms of both time and money invested, and can be viewed as somewhat disposable after a few years of use. In this case you might want to save some money by using a good exterior grade plywood, sealed well with epoxy. If the boat is maintained and kept out of the weather when not in use, it could last a very long time. Ive built two Phil Bolger dinghies this way, using exterior grade 1/4" Luan plywood, the type sold as "underlayment" for flooring in my local building supply. At about $10 per sheet, its cheap and can sometimes be of surprisingly good quality.
Heres one of the dinghies - the Bolger "Nymph" design - which makes an excellent rowing tender for a cruising boat. I built this one with cheap Luan right from the plans given in Dynamite Paysons Build the New Instant Boats
What it comes down to in choosing construction materials for a boat is the intended use of the vessel and also the size of your investment in time and materials. For a dinghy like the one shown above, its optional as to whether you want to use BS-1088 Okoume at more than $60 per sheet or exterior Luan for about $10-15 per sheet. The time to build the dinghy is only a relatively small investment of evenings and maybe a couple of weekends. If you get a few years of service out of it, your time and money will be well spent.
If you are considering building a cruising-sized vessel from wood composite construction, however, especially a lightweight multihull, the time and hard labor required will more than offset the difference in plywood costs. Not many people can look at such a boat as "disposable," so consideration must be given to insuring the vessel has a long life and retains a decent resale value throughout that life. The best way to do this is to use high quality materials throughout and to document this use for a possible future buyer. The ease of working with top quality marine plywood is another factor that makes the whole project more pleasant and saves a lot of labor on fitting and finishing plywood parts. A well-built and properly maintained wooden boat built this way of excellent materials can be expected to last a lifetime and beyond. Anyone willing to put a couple thousand hours of their life into the construction of a cruising boat should expect nothing less.
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