By Sherry Johnson Guzy, Director of Marketing Communications
LL Johnson Lumber Mfg Co & Johnson's Workbench.
Our specialty hardwood lumber business has been in my family for over 110 years. My great-grandfather, L.L. Johnson, who started out as a nomadic-lumberjack in the early 1900s established the business in 1909. My brothers and I grew up immersed in the world of hardwoods. As a result, we have a strong fascination and appreciation for the wooden boat industry.
Plywood History Might Surprise You
Plywood has a rich history traveling back to the Ancient Period, 1500 BC. Early Egyptian tombs contained remnants of plywood used primarily in furniture making. Fast forward to the Modern Era where the 20th century saw the development of superior water-resistant glues – and plywood became a viable resource in the boat building industry. During World War II, boat builders made Navy PT boats, Army assault boats, and lifeboats of water-resistant plywood.
Today, Marine plywood is manufactured to the highest quality, with select woods and glues used to make them ideal for boat building and other marine uses. However, it is essential to understand the trade-offs with the many different options and price points available to support a wide range of marine applications.
The Species Is Important
To discuss the marine plywood options, we called on Luke Westholme, President of Wolstenholme International, the largest stocking importer of FSC certified plywood for use in the furniture, cabinet, and boat building industries.
Luke, explains that the best marine plywood species choices can range from the top of the line European Okoume marine in certified or non-certified, to the marine Meranti panels. It's well-known and often specified in trade journals and magazines that European Okoume marine plywood is the best choice of plywood for boat construction since Okoume offers a significant amount of durability without adding too much weight.
There is always that trade-off with any marine plywood. Durability usually means high density. For this reason, Okoume, Sapelle, and some specific types of Shorea (Red Meranti, for example) are the most commonly used in boat building. All of these species offer durability without adding excessive weight.
Marine plywood, such as Hydrotek, produced to European standards, offer more veneer layers and uniform in species throughout the panel (Red Meranti). These panels are less expensive than the European Okoume marine plywood, offering more exceptional durability and hardness; however, they are heavier than the Okoume plywood sheets.
Sapele marine plywood is not as commonly available in the US market. The cost of a sheet of Sapele is almost double that of Okoume plywood. Sapele offers a much harder face veneer than the Okoume plywood, which, while durable, is considered soft.
Certified vs. Non-Certified
Marine plywood is regulated so that the glues and materials used can withstand severe conditions. Five or more veneer layers are bonded together with a waterproof adhesive to withstand heavier loads and repel moisture throughout. The glues used for this process need to be able to withstand boiling in water for several hours without de-laminating.
As mentioned earlier, marine ply is available in certified and non-certified. Increasingly, boatbuilders are requesting that the plywood they buy adheres to the British Standard 1088 (BS1088) or British Standard 6566 (BS6566) known to be the strictest standards for marine plywood manufacturing.
Historically, Lloyd's Register of Shipping was a specifically maritime organization. In the late 20th century, it diversified into other sectors, including oil & gas, process industries, nuclear and rail. Today, only about one half of imported marine-grade Okoume plywood carries the Lloyds certification. Marine plywood mills that are certified by Lloyds pay a fee in excess of $10,000 to have inspectors from Lloyds visit their manufacturing facility regularly and test current production to ensure that it meets the British Standard 1088.
Some mills will produce marine plywood that is Lloyds certified, and some mills will create a panel that is "made in accordance with" the Lloyds standard. Lloyds does not currently certify the British Standard 6566. However, some marine plywood manufacturers still used the standard as a guideline. About 90% of the Okoume plywood we carry is BS 1088 Lloyds certified, while the Hydrotek sheets are BS6566.
The critical point of the British Standard 1088, is that no core gaps greater than 0.3mm are allowed. Other vital aspects of the standard are the thickness of the face veneers -- they must be at least 1.0mm after sanding. Face thickness is always the costliest part of plywood manufacturing (as the nicest veneers are used for faces), and the BS1088 standard ensures your plywood will have a full-thickness face. The British Standard 6566, is not as common as the BS1088 standard, both here and in Europe. However, as the BS6566 standard is still more stringent than the American PS-1-95 standard for marine Fir plywood, some companies continue to offer this grade.
The Different Faces Of Marine Plywood
Marine plywood panels receive two alphabetical ratings that range from A to D. The first letter rates the quality of the front-facing side of the plywood sheet. The second letter rates the quality of the sheet's backside. A single panel can have an "A" face and a "D" back.
A-grade plywood has few imperfections and is free of small knots and defects. B-grade plywood is the second-highest grade given to plywood material; this type of plywood has tiny knots or nicks. The more economical varieties of plywood are those with the C and D grading. Plywood sheets with this grade contain many knotholes, voids, and repairs, and are usually left as is.
According to our resident plywood expert Jacob Bruner, the slicing process consists of several techniques: rotary slicing, quarter slicing, plain slicing, and rift slicing, to name a few. Each of these methods will produce a very distinctive type of grain pattern depending upon the species selected.
Marine plywood manufacturing utilizes primarily rotary and plain slicing. Okoume and Hydrotek (Red Meranti) are Rotary cut (ROT). These two species peel exceptionally well as the trees are typically quite large in diameter -- a critical factor in producing both core & face veneers that are free from knots or voids. Rotary is the process of peeling the entire log and turning it while shaving the surface as it turns to produce Rotary Cut veneer. This process is the only cutting method that is capable of creating whole-piece face veneers. A rotary cut will yield veneer with a broad grain pattern.
Sapele and Teak, however, are usually plain sliced. Plain slicing a log occurs when a log is only turned a few inches at a time when slicing it, versus a constant turn and peeling method used for Rotary Cut veneer. Plain Sliced veneer produces a cathedral grain pattern similar to that presented in plain-sawn lumber. Plain Sliced veneer is cut along the growth rings, meaning there will be more than one veneer piece glued together to make a veneer layer in a sheet of marine plywood. Due to this, the best use for Sapele and Teak is in boat cabinets and on decks, mainly for decorative applications. Okoume and Hydrotek are suitable for submerged applications.
As you can see, not all marine plywood is the same. An excellent resource for information is the lumber yard staff. Don't hesitate to ask detailed marine plywood questions to help ensure you are getting the best product for your particular application.
